Topics
Contents Lasers and Laser Optics, 196 Article(s)
Spectral programmable mid-infrared optical parametric oscillator
Junrui Liang, Jiangming Xu, Yanzhao Ke, Sicheng Li, Junhong He, Yidong Guo, Yang Zhang, Xiaoya Ma, Jun Ye, Xiao Li, Jinyong Leng, and Pu Zhou

A spectral programmable, continuous-wave mid-infrared (MIR) optical parametric oscillator (OPO), enabled by a self-developed high-power spectral tailorable fiber laser, was proposed and realized. While operating at a single-wavelength, the maximum idler power reached 5.53 W at 3028 nm, with a corresponding pump-to-idler conversion efficiency of 14.7%. The wavelength number switchable output was available from one to three. The single idler was tunable in a range of 528 nm (2852–3380 nm). In a dual-wavelength operation, the interval between two idlers could be flexibly tuned for 470 nm (53–523 nm), and the intensity of each channel was controllable. Triple-wavelength idler emission was realized, meanwhile exhibiting spectral custom-tailored characteristics. Furthermore, we balanced the parametric gain through the pre-modulating broadband multi-peak pump spectra, enabling a 10 dB bandwidth adjustment of the idler emission from 20 to 125 nm. This versatile mid-infrared laser, simultaneously featuring wide tuning, multi-wavelength operation, and broad bandwidth manipulation, has great application potential in composition detection, terahertz generation, and speckle-free imaging.

Photonics Research
Jul. 15, 2024, Vol. 12 Issue 8 1593 (2024)
Detector-integrated vertical-cavity surface-emitting laser with a movable high-contrast grating mirror
Minglu Wang, Hongling Peng, Chenxi Hao, Xuyan Zhou, Wanhua Zheng, and Anjin Liu

In this paper, we present a detector-integrated vertical-cavity surface-emitting laser (VCSEL) with a movable high-contrast grating (HCG) mirror in an n-i-p-i-n manner. The detector-integrated VCSEL with a movable HCG can achieve three functions, including wavelength tuning, power monitoring, and resonant-cavity-enhanced (RCE) photon detection. Currently, the device can achieve a wavelength tuning range of 27 nm at room temperature when the suspended HCG is driven by the reverse-bias voltage. The n-i-p structure located at the upper part of the device can serve as an intra-cavity photodiode to monitor the output power due to the defect absorption. The RCE photon detection function of the detector-integrated VCSEL with a movable HCG is measured, and it has a peak responsivity at about 926 nm. This detector-integrated VCSEL with a movable HCG will be useful for sensing and imaging.

Photonics Research
Jun. 27, 2024, Vol. 12 Issue 7 1437 (2024)
Burst-mode pulse generation in passively mode-locked all-fiber green/orange lasers at 543 nm and 602 nm
Qiujun Ruan, Jinhai Zou, Chunna Feng, Tingting Chen, Hang Wang, Zhipeng Dong, and Zhengqian Luo

We report on the experimental realization of, to the best of our knowledge, the first green and orange passively mode-locked all-fiber lasers. Stable mode-locking in the burst-pulse status is achieved at the wavelengths of 543.3 nm and 602.5 nm. The figure-9 cavity comprises the fiber end-facet mirror, gain fiber (Ho3+:ZBLAN fiber or Pr3+/Yb3+:ZBLAN fiber), and fiber loop mirror (FLM). The FLM with long 460 HP fiber is not only used as an output mirror, but also acts as a nonlinear optical loop mirror for initiating visible-wavelength mode-locking. The green/orange mode-locked fiber lasers with the fundamental repetition rates of 3.779/5.662 MHz produce long bursts containing ultrashort pulses with the 0.85/0.76 GHz intra-burst repetition rates, respectively. The ultrashort intra-burst pulses stem from the dissipative four-wave-mixing effect in the highly nonlinear FLM as well as the intracavity Fabry–Perot filtering. Long bursts of 22.2/11.6 ns with ultrashort pulses of 87/62 ps are obtained in our experiment. The visible-wavelength passively mode-locked lasers in an all-fiber configuration and burst-mode would represent an important step towards miniaturized ultrafast fiber lasers and may contribute to the applications in ablation-cooling micromachining, biomedicine imaging, and scientific research.

Photonics Research
May. 31, 2024, Vol. 12 Issue 6 1231 (2024)
Short-term prediction for chaotic time series based on photonic reservoir computing using VCSEL with a feedback loop
Xingxing Guo, Hanxu Zhou, Shuiying Xiang, Qian Yu, Yahui Zhang, Yanan Han, Tao Wang, and Yue Hao

Chaos, occurring in a deterministic system, has permeated various fields such as mathematics, physics, and life science. Consequently, the prediction of chaotic time series has received widespread attention and made significant progress. However, many problems, such as high computational complexity and difficulty in hardware implementation, could not be solved by existing schemes. To overcome the problems, we employ the chaotic system of a vertical-cavity surface-emitting laser (VCSEL) mutual coupling network to generate chaotic time series through optical system simulation and experimentation in this paper. Furthermore, a photonic reservoir computing based on VCSEL, along with a feedback loop, is proposed for the short-term prediction of the chaotic time series. The relationship between the prediction difficulty of the reservoir computing (RC) system and the difference in complexity of the chaotic time series has been studied with emphasis. Additionally, the attention coefficient of injection strength and feedback strength, prediction duration, and other factors on system performance are considered in both simulation and experiment. The use of the RC system to predict the chaotic time series generated by actual chaotic systems is significant for expanding the practical application scenarios of the RC.

Photonics Research
May. 31, 2024, Vol. 12 Issue 6 1222 (2024)
Perovskite quantum laser with enhanced population inversion driven by plasmon-induced hot electron transfer under potential shift polarization conditions
Yong Pan, Lijie Wu, Yuan Zhang, Yihao Zhang, Jie Xu, Haixia Xie, and Jianguo Cao

The hot electron transfer resulting in fluorescence enhancement is significantly meaningful for theory and experiment of the study on photoelectric devices. However, the laser emission based on direct hot electron transfer is difficult to realize because of the low transfer efficiency. To achieve a laser with a new-generation mechanism based on hot electron transfer, the photoelectric co-excitation is proposed for improving the efficiency of hot electron transfer. The lasing behavior at 532 nm is realized with a threshold of 5 kw cm-2 and 1 μA, which can be considered as the hot electron transfer resulting in population inversion enhancement. Meanwhile, the lasing output power is 0.3 mW. The hot electrons transfer process was described via the transient absorption spectrum according to the improved ground-state bleaching and excited-state absorption signal in device ON. Through comparison with the optical pump only, the quantum efficiencies of hot electron generation (HEG) and hot electron transfer (HET) were increased ∼31% and 31%, respectively. Most importantly, a triple gain mode coupling device including local surface plasmon, hot electron transfer, and array oscillation was presented. Two modes of population inversion enhancement are proposed. This study can provide theoretical and experimental reference for the research of hot electron lasers and devices.

Photonics Research
May. 01, 2024, Vol. 12 Issue 5 986 (2024)
Direct amplification of femtosecond optical vortices in a single-crystal fiber
Changsheng Zheng, Tianyi Du, Lei Zhu, Zhanxin Wang, Kangzhen Tian, Yongguang Zhao, Zhiyong Yang, Haohai Yu, and Valentin Petrov

Spatially twisted light with femtosecond temporal structure is of particular interest in strong-field physics and light–matter interactions. However, present femtosecond vortex sources exhibit limited power handling capabilities, and their amplification remains an ongoing challenge particularly for high-order orbital angular momentum (OAM) states due to several inherent technical difficulties. Here, we exploit a straightforward approach to directly amplify a femtosecond optical vortex (FOV, OAM=-8ℏ) by using a two-stage single-crystal fiber (SCF) amplifier system without pulse stretching and compression in the time domain, delivering 23-W, 163-fs pulses at a repetition rate of 1 MHz. The spatial and temporal features are well-conserved during the amplification, as well as the high modal purity (>96%). The results indicate that the multi-stage SCF amplifier system is particularly suited for direct amplification of high-order FOVs. The generated high-power femtosecond OAM laser beams are expected to help reveal complex physical phenomena in light–matter interactions and pave the way for practical applications in attoscience, laser plasma acceleration, and high-dimension micromachining.

Photonics Research
Dec. 14, 2023, Vol. 12 Issue 1 27 (2024)
High-power distributed feedback lasers based on InP corrugated sidewalls at λ∼2 μm
Yongqiang Sun, Yunfei Xu, Jinchuan Zhang, Fengmin Chen, Junqi Liu, Shuman Liu, Quanyong Lu, Ning Zhuo, Lijun Wang, Fengqi Liu, and Shenqiang Zhai

We report a high-power single-mode InP-based 2 μm distributed feedback (DFB) laser with a second-order buried grating and corrugated sidewalls. A second-order semiconductor grating is used for in-plane feedback and vertical out-coupling. The corrugated sidewalls are used to eliminate higher-order transverse modes. For the DFB laser with a 2 mm long cavity and 15 μm wide ridge, the maximum continuous-wave edge-emitting and surface-emitting single-mode powers at 300 K are up to 81 and 42 mW, respectively. A single-lobed far-field radiation pattern with a low divergence angle of approximately 8.6° is achieved by a device with a ridge width of 15 μm. The single-longitudinal-mode emission wavelength of the fabricated laser can be adjusted from 2003.8 nm at 288 K to 2006.9 nm at 313 K without any mode hopping. Robust single-mode emission with a side-mode suppression ratio of 30 dB is achieved under all injection currents and temperature conditions.

Photonics Research
Editors' PickJul. 27, 2023, Vol. 11 Issue 8 1390 (2023)
Photonic integrated spiking neuron chip based on a self-pulsating DFB laser with a saturable absorber
Yuechun Shi, Shuiying Xiang, Xingxing Guo, Yahui Zhang, Hongji Wang, Dianzhuang Zheng, Yuna Zhang, Yanan Han, Yong Zhao, Xiaojun Zhu, Xiangfei Chen, Xun Li, and Yue Hao

We proposed and experimentally demonstrated a simple and novel photonic spiking neuron based on a distributed feedback (DFB) laser chip with an intracavity saturable absorber (SA). The DFB laser with an intracavity SA (DFB-SA) contains a gain region and an SA region. The gain region is designed and fabricated by the asymmetric equivalent π-phase shift based on the reconstruction-equivalent-chirp technique. Under properly injected current in the gain region and reversely biased voltage in the SA region, periodic self-pulsation was experimentally observed due to the Q-switching effect. The self-pulsation frequency increases with the increase of the bias current and is within the range of several gigahertz. When the bias current is below the self-pulsation threshold, neuronlike spiking responses appear when external optical stimulus pulses are injected. Experimental results show that the spike threshold, temporal integration, and refractory period can all be observed in the fabricated DFB-SA chip. To numerically verify the experimental findings, a time-dependent coupled-wave equation model was developed, which described the physics processes inside the gain and SA regions. The numerical results agree well with the experimental measurements. We further experimentally demonstrated that the weighted sum output can readily be encoded into the self-pulsation frequency of the DFB-SA neuron. We also benchmarked the handwritten digit classification task with a simple single-layer fully connected neural network. By using the experimentally measured dependence of the self-pulsation frequency on the bias current in the gain region as an activation function, we can achieve a recognition accuracy of 92.2%, which bridges the gap between the continuous valued artificial neural networks and spike-based neuromorphic networks. To the best of our knowledge, this is the first experimental demonstration of a photonic integrated spiking neuron based on a DFB-SA, which shows great potential to realizing large-scale multiwavelength photonic spiking neural network chips.

Photonics Research
Jul. 20, 2023, Vol. 11 Issue 8 1382 (2023)
Open-ended exploration of ultrashort pulse lasers: an innovative design strategy for devices based on 2D materials
Qing Wu, Gang Zhao, Haibin Wu, and Meng Zhang

Ultrashort pulse lasers have vital significance in the field of ultrafast photonics. A saturable absorber (SA) as the core device to generate ultrashort pulses has innovative design strategies; the most interesting of which is the integration strategy based on 2D materials. This review presents recent advances in the optoelectronic properties of 2D materials and in the way the materials are prepared, characterized, and integrated into devices. We have done a comprehensive review of the optical properties of materials and material-based devices and their current development in the field of fiber lasers and solid-state lasers. Finally, we offer a look at future applications for 2D materials in ultrafast lasers and their prospects.

Photonics Research
Jun. 23, 2023, Vol. 11 Issue 7 1238 (2023)
Dynamics of a dispersion-tuned swept-fiber laser
Duidui Li, Guolu Yin, Ligang Huang, Lei Gao, Laiyang Dang, Zeheng Zhang, Jingsheng Huang, Huafeng Lu, and Tao Zhu

In this paper, we studied the dynamics of a dispersion-tuned swept-fiber laser both experimentally and theoretically. By adding a dispersion compensation fiber and an electro-optic modulator in the laser cavity, an actively mode-locked laser was obtained by using intensity modulation, and wavelength sweeping was realized by changing the modulation frequency. Using a high-speed real-time oscilloscope, the dynamic behaviors of the swept laser were investigated during wavelength switching, static-sweeping cycle, and continuous sweeping, respectively. It was found that the laser generates relaxation oscillation at the start of the sweeping mode. The relaxation oscillation process lasted for about 0.7 ms, and then the laser started to operate stably. Due to the nonlinear effect, new wavelengths were generated in the relaxation oscillation process, which is not beneficial for applications. Fortunately, relaxation oscillation disappears if the laser starts up and operates in the continuous sweeping mode, and the good sweeping symmetry between the positive sweep and negative sweep increases the application potential of the laser. In addition, the instantaneous linewidth is almost the same as that in the static state. These results describe the characteristics of the laser from a new perspective and reveal, to the best our knowledge, the intensity dynamics of such lasers for the first time. This paper provides some new research basis for understanding the establishment process of dispersion-tuned swept-fiber lasers and their potential application in the future.

Photonics Research
May. 19, 2023, Vol. 11 Issue 6 999 (2023)
Hundredfold increase of stimulated Brillouin-scattering bandwidth in whispering-gallery mode resonators
Guoping Lin, Jingyi Tian, Tang Sun, Qinghai Song, and Yanne K. Chembo

Backward stimulated Brillouin scattering (SBS) is widely exploited for various applications in optics and optoelectronics. It typically features a narrow gain bandwidth of a few tens of megahertz in fluoride crystals. Here we report a hundredfold increase of SBS bandwidth in whispering-gallery mode resonators. The crystalline orientation results in a large variation of the acoustic phase velocity upon propagation along the periphery, from which a broad Brillouin gain is formed. Over 2.5 GHz wide Brillouin gain profile is theoretically found and experimentally validated. SBS phenomena with Brillouin shift frequencies ranging from 11.73 to 14.47 GHz in ultrahigh QZ-cut magnesium fluoride cavities pumped at the telecommunication wavelength are demonstrated. Furthermore, the Brillouin–Kerr comb in this device is demonstrated. Over 400 comb lines spanning across a spectral window of 120 nm are observed. Our finding paves a new way for tailoring and harnessing the Brillouin gain in crystals.

Photonics Research
Editors' PickMay. 16, 2023, Vol. 11 Issue 6 917 (2023)
Widely tunable continuous-wave visible and mid-infrared light generation based on a dual-wavelength switchable and tunable random Raman fiber laser
Han Wu, Weizhe Wang, Bo Hu, Yang Li, Kan Tian, Rui Ma, Chunxiao Li, Jun Liu, Jiyong Yao, and Houkun Liang

Nonlinear frequency conversion of wavelength agile and high-power random fiber lasers can provide a promising way to generate continuous-wave (CW) visible and mid-infrared (MIR) light with unique properties such as the continuous modeless spectrum, low temporal/spatial coherence, and high temporal stability. Here, we report a dual-wavelength switchable and tunable random Raman fiber laser (RRFL) based on a phosphosilicate fiber that has two Raman gain peaks for the first time and demonstrate its superior capability to generate widely tunable CW visible and mid-infrared light via nonlinear frequency conversions. By using the combination of a tunable pump and two tunable gratings in Littrow configuration that can provide separated point feedback for the two Stokes wavelengths corresponding to silica- and phosphorus-related Raman peaks, the spectrum of an RRFL can be flexibly manipulated for the aim of nonlinear frequency conversions, including single-wavelength tunable emission at the 1.1 μm or 1.2 μm band for second-harmonic generation (SHG), dual-wavelength simultaneously tunable emission at the 1.1 μm and 1.2 μm bands for the sum-frequency generation (SFG), and dual-wavelength separation tunable emission for difference-frequency generation (DFG). As a result, with the combination of SHG and SFG in a periodically poled lithium niobate crystal array, we experimentally demonstrate the broadest tuning range (560–630 nm) of visible light generated from an RRFL, to the best of our knowledge. The tunable MIR light in the range of 10.7–12.3 μm is also demonstrated through DFG of an RRFL operating in separation tunable dual-wavelength emission mode in a BaGa4Se7 (BGSe) crystal, which is the first realization of >10 μm CW DFG in the BGSe crystal. We believe the developed dual-wavelength switchable and tunable RRFL can provide a new compact, robust, and cost-effective platform to realize broadly tunable light in both the visible and MIR regions, which can also find potential applications in imaging, sensing, and temporal ghost imaging in various spectral bands.

Photonics Research
Editors' PickMay. 01, 2023, Vol. 11 Issue 5 808 (2023)
Direct generation of 3.17 mJ green pulses in a cavity-dumped Ho3+-doped fiber laser at 543 nm
Tianran Li, Ziyu Wang, Jinhai Zou, Jinfen Hong, Qiujun Ruan, Hang Wang, Zhipeng Dong, and Zhengqian Luo

High-energy pulsed lasers in the green spectral region are of tremendous interest for applications in space laser ranging, underwater detection, precise processing, and scientific research. Semiconductor pulsed lasers currently are difficult to access to the so-called “green gap,” and high-energy green pulsed lasers still heavily rely on the nonlinear frequency conversion of near-IR lasers, precluding compact and low-cost green laser systems. Here, we address this challenge by demonstrating, for the first time to the best of our knowledge, millijoule-level green pulses generated directly from a fiber laser. The green pulsed fiber laser consists of a 450 nm pump laser diode, a Ho3+-doped ZBLAN fiber, and a cavity-dumping module based on a visible wavelength acousto-optic modulator. Stable pulse operation in the cavity-dumping regime at 543 nm is observed with a tunable repetition rate in a large range of 100 Hz–3 MHz and a pulse duration of 72–116 ns. The maximum pulse energy of 3.17 mJ at 100 Hz is successfully achieved, which is three orders of magnitude higher than those of the rare-earth-doped fiber green lasers previously reported. This work provides a model for compact, high-efficiency, and high-energy visible fiber pulsed lasers.

Photonics Research
Editors' PickFeb. 27, 2023, Vol. 11 Issue 3 413 (2023)
Controlled generation of picosecond-pulsed higher-order Poincaré sphere beams from an ytterbium-doped multicore fiber amplifier
Kunhao Ji, Di Lin, Ian A. Davidson, Siyi Wang, Joel Carpenter, Yoshimichi Amma, Yongmin Jung, Massimiliano Guasoni, and David J. Richardson

Higher-order Poincaré sphere (HOPS) beams with spatially variable polarization and phase distributions are opening up a host of unique applications in areas ranging from optical communication to microscopy. However, the flexible generation of these beams with high peak power from compact laser systems remains a challenge. Here, we demonstrate the controlled generation of HOPS beams based on coherent beam combination from an Yb-doped multicore fiber (MCF) amplifier. Using a spatial light modulator to adaptively adjust the wavefront and polarization of the signals seeded into the individual cores of the MCF various structured beams (including cylindrical vector beams and first- and second-order vortex beams) were generated with peak powers up to 14 kW for ∼92 ps pulses.

Photonics Research
Jan. 23, 2023, Vol. 11 Issue 2 181 (2023)
Self-synchronized temporal-spectral characterization system for revealing ultrafast fiber laser dynamics
Yulong Cao, Zhenghu Chang, Qiang Wu, Jingsheng Huang, Laiyang Dang, Ai Liu, Yiyang Luo, Ligang Huang, Wei Huang, Lei Gao, and Tao Zhu

Due to the electronic bottleneck limited real-time measurement speed of common temporal-spectral detection and the particle-like nature of optical soliton enabled nonrepeatable transient behaviors, capturing the ultrafast laser pulses with unknown times of arrival and synchronously characterizing their temporal-spectral dynamic evolution is still a challenge. Here, using the Raman soliton frequency shift based temporal magnifier and dispersive Fourier transform based spectral analyzer, we demonstrate a self-synchronized, ultrafast temporal-spectral characterization system with a resolution of 160 fs and 0.05 nm, and a recording length above milliseconds. The synchronized nonlinear process makes it possible to image full-filled temporal sub-picosecond pulse trains regardless of their arrival times and without extra pump lasers and photoelectric conversion devices. To demonstrate the significance of this improvement, a buildup dynamic process of a soliton laser with a complex breakup and collisions of multisolitons is visually displayed in the spectral and temporal domains. The soliton dynamic evolution processes observed by our characterization system are in one-to-one correspondence with the numerical simulation results. We believe this work provides a new multidimensional technique to break the electronic bottleneck to gain additional insight into the dynamics of ultrafast lasers and nonlinear science.

Photonics Research
Jan. 23, 2023, Vol. 11 Issue 2 173 (2023)
Multi-wavelength random fiber laser with a spectral-flexible characteristic
Sicheng Li, Jiangming Xu, Junrui Liang, Jun Ye, Yang Zhang, Xiaoya Ma, Jinyong Leng, and Pu Zhou

In past decades, multi-wavelength lasers have attracted much attention due to their wide applications in many fields. In this paper, we demonstrate a multi-wavelength random fiber laser with customizable spectra enabled by an acousto–optic tunable filter. The operating wavelength range can be tuned from 1114.5 to 1132.5 nm with a maximal output power of 5.55 W, and spectral channel tuning can also be realized with a maximal number of five. The effect of gain competition and the interaction between Raman gain and insertion loss are also analyzed. Furthermore, the output spectra can be ordered by radiating appropriate radio frequency signals to the acousto–optic tunable filter. This work may provide a reference for agile shape spectrum generation and promote multi-wavelength random fiber laser practicability in sensing, telecommunications, and precise spectroscopy.

Photonics Research
Jan. 23, 2023, Vol. 11 Issue 2 159 (2023)
High performance visible generation of Ho3+-doped all-fiber lasers
Shuaihao Ji, Xuexian Lin, Bo Xiao, Zhongyu Wang, Xiuji Lin, and Zhiping Cai

Direct generation of visible frequency from a compact all-fiber laser while preserving high output characteristics has been a subject of research in laser technology. We investigated the high output performance of all-fiber lasers based on Ho3+-doped ZBLAN fluoride glass fiber especially operating in the deep-red band by pumping at 640 nm. Remarkably, we achieved a maximum continuous-wave output power of 271 mW at 750 nm with a slope efficiency of 45.1%, which represents, to our knowledge, the highest direct output power recorded in an all-fiber laser with a core diameter of less than 10 μm in the deep-red band. Additionally, we successfully developed a 1.2 μm all-fiber laser pumped by a 640 nm laser. We extensively investigated the correlation between these two-laser generation processes and their performances at 750 nm and 1.2 μm wavelengths. By increasing the pumping rate, we observed an efficient recycling of population through a highly excited state absorption process, which effectively returned the population to the upper laser level of the deep-red transition. Moreover, we determined the optimized conditions for such lasers, identified the processes responsible for populating the excited state energy levels, and established the corresponding spectroscopic parameters.

Photonics Research
Nov. 24, 2023, Vol. 11 Issue 12 2121 (2023)
Dual-mode distributed feedback quantum cascade laser based on stacked 3D monolithic integration for on-chip multi-channel gas sensing
Xiyu Lu, Yanjiao Guan, Pengchang Yang, Shan Niu, Yu Ma, Lijun Wang, Ning Zhuo, Jinchuan Zhang, Shenqiang Zhai, Fengmin Cheng, Shuman Liu, Fengqi Liu, and Junqi Liu

To facilitate the development of on-chip integrated mid-infrared multi-channel gas sensing systems, we propose a high-power dual-mode (7.01 and 7.5 μm) distributed feedback quantum cascade laser based on stacked 3D monolithic integration. Longitudinal mode control is achieved by preparing longitudinal nested bi-periodic compound one-dimensional Bragg gratings along the direction of the cavity length in the confinement layer. Additionally, transverse coherent coupling ridges perpendicular to the cavity length direction are fabricated in the upper waveguide layer to promote the fundamental transverse mode output when all ridges are in phase. Stable dual-wavelength simultaneous emission with a side-mode suppression ratio of more than 20 dB was achieved by holographic exposure and wet etching. The entire spectral tuning range covers nearly 100 nm through joint tuning of the injection current and heat-sink temperature. High peak power and beam quality are guaranteed by the parallel coherent integration of seven-element ridge arrays. The device operates in a fundamental supermode with a single-lobed far-field pattern, and its peak output power reaches 3.36 W in pulsed mode at 20°C. This dual-mode laser chip has the potential for in-situ on-chip simultaneous detection of CH4 and C2H6 gases in leak monitoring.

Photonics Research
Editors' PickNov. 24, 2023, Vol. 11 Issue 12 2113 (2023)
Spiking information processing in a single photonic spiking neuron chip with double integrated electronic dendrites
Yahui Zhang, Shuiying Xiang, Xingxing Guo, Yanan Han, Yuechun Shi, Xiangfei Chen, Genquan Han, and Yue Hao

Dendrites, branches of neurons that transmit signals between synapses and soma, play a vital role in spiking information processing, such as nonlinear integration of excitatory and inhibitory stimuli. However, the investigation of nonlinear integration of dendrites in photonic neurons and the fabrication of photonic neurons including dendritic nonlinear integration in photonic spiking neural networks (SNNs) remain open problems. Here, we fabricate and integrate two dendrites and one soma in a single Fabry–Perot laser with an embedded saturable absorber (FP-SA) neuron to achieve nonlinear integration of excitatory and inhibitory stimuli. Note that the two intrinsic electrodes of the gain section and saturable absorber (SA) section in the FP-SA neuron are defined as two dendrites for two ports of stimuli reception, with one electronic dendrite receiving excitatory stimulus and the other receiving inhibitory stimulus. The stimuli received by two electronic dendrites are integrated nonlinearly in a single FP-SA neuron, which generates spikes for photonic SNNs. The properties of frequency encoding and spatiotemporal encoding are investigated experimentally in a single FP-SA neuron with two electronic dendrites. For SNNs equipped with FP-SA neurons, the range of weights between presynaptic neurons and postsynaptic neurons is varied from negative to positive values by biasing the gain and SA sections of FP-SA neurons. Compared with SNN with all-positive weights realized by only biasing the gain section of photonic neurons, the recognition accuracy of Iris flower data is improved numerically in SNN consisting of FP-SA neurons. The results show great potential for multi-functional integrated photonic SNN chips.

Photonics Research
Nov. 20, 2023, Vol. 11 Issue 12 2033 (2023)
Universal dynamics and deterministic motion control of decoherently seeded temporal dissipative solitons via spectral filtering effect
Zilong Li, Huanhuan Liu, Zimin Zha, Lei Su, Perry Ping Shum, and Hairun Guo

Temporal dissipative solitons have been widely studied in optical systems, which exhibit various localized structures and rich dynamics, and have shown great potential in applications including optical encoding and sensing. Yet, most of the soliton states, as well as the switching dynamics amongst, were fractionally captured or via self-evolution of the system, lacking of control on the soliton motion. While soliton motion control has been widely investigated in coherently seeded optical cavities, such as microresonator-based dissipative solitons, its implementation in decoherently seeded systems, typically the soliton mode-locked lasers, remains an outstanding challenge. Here, we report the universal dynamics and deterministic motion control of temporal dissipative solitons in a mode-locked fibre laser by introducing a scanned spectral filtering effect. We investigate rich switching dynamics corresponding to both the assembly and the disassembly of solitons, revealing a complete and reversible motion from chaotic states to soliton and soliton-molecule states. Significant hysteresis has been recognized in between the redshift and blueshift scan of the motorized optical filter, unveiling the nature of having state bifurcations in dissipative and nonlinear systems. The active soliton motion control enabled by filter scanning highlights the potential prospects of encoding and sensing using soliton molecules.

Photonics Research
Nov. 20, 2023, Vol. 11 Issue 12 2011 (2023)
Intracavity-dynamics-based optical phase amplifier with over tenfold amplification
Mingwang Tian, and Yidong Tan

The relative phase change between two light fields can be used as a fundamental parameter to measure the physical quantity causing this change. Therefore, amplifying the relative phase change becomes attractive to improve the measurement resolution. Phase amplification using a many-body entangled state (NOON state) is a well-known method; nevertheless, the preparation process for a high-number NOON state is difficult and sensitive to optical loss. Here, we propose and experimentally verify a concise phase amplification method with a tolerance of about five orders of magnitude for optical loss. The method is based on the optical-feedback-induced intracavity harmonics generation effect to amplify the phase change by 11 times, which is comparable to the highest level of about 10 experimentally reached in NOON states. Furthermore, the 20th intracavity harmonic is generated when the reinjected photon number increases, indicating that 20 times phase amplification is attainable. The proposed method has a prospect for precision measurement applications.

Photonics Research
Oct. 26, 2023, Vol. 11 Issue 11 1892 (2023)
Significant enhancement of multiple resonant sidebands in a soliton fiber laser
Tianqi Zhang, Fanchao Meng, Qi Yan, Chuanze Zhang, Zhixu Jia, Weiping Qin, Guanshi Qin, and Huailiang Xu

Resonant sidebands in soliton fiber lasers have garnered substantial interest in recent years due to their crucial role in understanding soliton propagation and interaction dynamics. However, most previous studies and applications were restricted to focusing on only the first few low-order resonant sidebands because higher-order sidebands usually decay exponentially as their wavelengths shift far away from the soliton center and are negligibly weak. Here we report numerically and experimentally significant enhancement of multiple resonant sidebands in a soliton fiber laser mode-locked by a nonlinear polarization evolution mechanism. The birefringence and the gain profile of the laser cavity were shown to be critical for this phenomenon. Multiple intense resonant sidebands were generated whose maximum intensity was more than 30 dB higher than that of the soliton, which is the highest yet reported, to our knowledge. To accurately predict the wavelengths of all high-order resonant sidebands, an explicit formula was derived by taking the third-order dispersion effect into account. The temporal features of multiple orders of resonant sidebands were characterized, which all exhibit exponentially decaying leading edges. This study provides insight into understanding the properties of high-order resonant sidebands in a soliton laser and opens possibilities for constructing multi-wavelength laser sources.

Photonics Research
Oct. 16, 2023, Vol. 11 Issue 11 1847 (2023)
Spatiotemporally reconfigurable light in degenerate laser cavities
A. Bartolo, N. Vigne, M. Marconi, G. Beaudoin, L. Le Gratiet, K. Pantzas, I. Sagnes, A. Garnache, and M. Giudici

We show that a III-V semiconductor vertical external-cavity surface-emitting laser (VECSEL) can be engineered to generate light with a customizable spatiotemporal structure. Temporal control is achieved through the emission of temporal localized structures (TLSs), a particular mode-locking regime that allows individual addressing of the pulses traveling back and forth in the cavity. The spatial profile control relies on a degenerate external cavity, and it is implemented due to an absorptive mask deposited onto the gain mirror that limits the positive net gain within two circular spots in the transverse section of the VECSEL. We show that each spot emits spatially uncorrelated TLSs. Hence, the spatiotemporal structure of the light emitted can be shaped by individually addressing the pulses emitted by each spot. Because the maximum number of pulses circulating in the cavity and the number of positive net-gain spots in the VECSEL can be increased straightforwardly, this result is a proof of concept of a laser platform capable of handling light states of scalable complexity. We discuss applications to three-dimensional all-optical buffers and to multiplexing of frequency combs that share the same laser cavity.

Photonics Research
Editors' PickSep. 28, 2023, Vol. 11 Issue 10 1751 (2023)
Reflection sensitivity of dual-state quantum dot lasers
Zhiyong Jin, Heming Huang, Yueguang Zhou, Shiyuan Zhao, Shihao Ding, Cheng Wang, Yong Yao, Xiaochuan Xu, Frédéric Grillot, and Jianan Duan

This work experimentally and theoretically demonstrates the effect of excited state lasing on the reflection sensitivity of dual-state quantum dot lasers, showing that the laser exhibits higher sensitivity to external optical feedback when reaching the excited state lasing threshold. This sensitivity can be degraded by increasing the excited-to-ground-state energy separation, which results in a high excited-to-ground-state threshold ratio. In addition, the occurrence of excited state lasing decreases the damping factor and increases the linewidth enhancement factor, which leads to a low critical feedback level. These findings illuminate a path to fabricate reflection-insensitive quantum dot lasers for isolator-free photonic integrated circuits.

Photonics Research
Sep. 27, 2023, Vol. 11 Issue 10 1713 (2023)
Experimental demonstration of coherent photonic neural computing based on a Fabry–Perot laser with a saturable absorber
Dianzhuang Zheng, Shuiying Xiang, Xingxing Guo, Yahui Zhang, Biling Gu, Hongji Wang, Zhenzhen Xu, Xiaojun Zhu, Yuechun Shi, and Yue Hao

As Moore’s law has reached its limits, it is becoming increasingly difficult for traditional computing architectures to meet the demands of continued growth in computing power. Photonic neural computing has become a promising approach to overcome the von Neuman bottleneck. However, while photonic neural networks are good at linear computing, it is difficult to achieve nonlinear computing. Here, we propose and experimentally demonstrate a coherent photonic spiking neural network consisting of Mach–Zehnder modulators (MZMs) as the synapse and an integrated quantum-well Fabry–Perot laser with a saturable absorber (FP-SA) as the photonic spiking neuron. Both linear computation and nonlinear computation are realized in the experiment. In such a coherent architecture, two presynaptic signals are modulated and weighted with two intensity modulation MZMs through the same optical carrier. The nonlinear neuron-like dynamics including temporal integration, threshold, and refractory period are successfully demonstrated. Besides, the effects of frequency detuning on the nonlinear neuron-like dynamics are also explored, and the frequency detuning condition is revealed. The proposed hardware architecture plays a foundational role in constructing a large-scale coherent photonic spiking neural network.

Photonics Research
Dec. 21, 2022, Vol. 11 Issue 1 65 (2023)
Spectrally programmable Raman fiber laser with adaptive wavefront shaping
Yanli Zhang, Shanshan Wang, Mingzhu She, Yunjiang Rao, and Weili Zhang

Raman fiber lasers (RFLs) have broadband tunability due to cascaded stimulated Raman scattering, providing extensive degrees of freedom for spectral manipulation. However, the spectral diversity of RFLs depends mainly on the wavelength flexibility of the pump, which limits the application of RFLs. Here, a spectrally programmable RFL is developed based on two-dimensional spatial-to-spectral mapping of light in multimode fibers (MMFs). Using an intracavity wavefront shaping method combined with genetic algorithm optimization, we launch light with a selected wavelength(s) at MMF output into the active part of the laser for amplification. In contrast, the light of undesired wavelengths is blocked. We demonstrate spectral shaping of the high-order RFL, including a continuously tunable single wavelength and multiple wavelengths with a designed spectral shape. Due to the simultaneous control of different wavelength regions, each order of Raman Stokes light allows flexible and independent spectral manipulation. Our research exploits light manipulation in a fiber platform with multi-eigenmodes and nonlinear gain, mapping spatial control to the spectral domain and extending linear light control in MMFs to active light emission, which is of great significance for applications of RFLs in optical imaging, sensing, and spectroscopy.

Photonics Research
Dec. 14, 2022, Vol. 11 Issue 1 20 (2023)
Low-threshold and narrow-emission random lasing in a self-assembly TiN nanoparticle-doped carbon quantum dot/DCM nanowire composite
Jing Lyu, Xinyu Zhang, Lei Cai, Li Tao, Weifeng Ma, An Li, Ye Tian, Yunsong Yin, Denan Kong, Wen Yi, Xianshuang Wang, and Ruibin Liu

The random lasing in quantum dot systems is in anticipation for widespread applications in biomedical therapy and image recognition, especially in random laser devices with high brightness and high monochromaticity. Herein, low-threshold, narrowband emission, and stable random lasing is realized in carbon quantum dot (CQD)/DCM nanowire composite-doped TiN nanoparticles, which are fabricated by the mixture of carbon quantum dots and self-assembly DCM dye molecules. The Förster resonance energy transfer process results in a high luminescence efficiency for the composite of carbon dots and DCM nanowires, allowing significant random lasing actions to emerge in CQD/DCM composite as TiN particles are doped that greatly enhance the emission efficiency through the plasmon resonance and random scattering. Thus, sharp and low-threshold random lasing is finally realized and even strong single-mode lasing occurs under higher pumping energy in the TiN-doped CQD/DCM composite. This work provides a promising way in high monochromaticity random laser applications.

Photonics Research
Sep. 01, 2022, Vol. 10 Issue 9 2239 (2022)
Generation of single solitons tunable from 3 to 3.8 μm in cascaded Er3+-doped and Dy3+-doped fluoride fiber amplifiers
Linpeng Yu, Jinhui Liang, Shiting Huang, Jinzhang Wang, Jiachen Wang, Xing Luo, Peiguang Yan, Fanlong Dong, Xing Liu, Qitao Lue, Chunyu Guo, and Shuangchen Ruan

High-power tunable femtosecond mid-infrared (MIR) pulses are of great interest for many scientific and industrial applications. Here we demonstrate a compact fluoride-fiber-based system that generates single solitons tunable from 3 to 3.8 μm. The system is composed of an Er:ZBLAN fiber oscillator and amplifier followed by a fusion-spliced Dy:ZBLAN fiber amplifier. The Er:ZBLAN fiber amplifier acts as a power booster as well as a frequency shifter to generate Raman solitons up to 3 μm. The Dy:ZBLAN fiber amplifier transfers the energy from the residual 2.8 μm radiation into the Raman solitons using an in-band pumping scheme, and further extends the wavelength up to 3.8 μm. Common residual pump radiation and secondary solitons accompanying the soliton self-frequency shift (SSFS) are recycled to amplify Raman solitons, consequently displaying a higher output power and pulse energy, a wider shifting range, and an excellent spectral purity. Stable 252 fs pulses at 3.8 μm with a record average power of 1.6 W and a pulse energy of 23 nJ are generated. This work provides an effective way to develop high-power widely tunable ultrafast single-soliton MIR laser sources, and this method can facilitate the design of other SSFS-based laser systems for single-soliton generation.

Photonics Research
Aug. 26, 2022, Vol. 10 Issue 9 2140 (2022)
Octave-spanning frequency comb generation based on a dual-mode microcavity laser
Ting Wang, Ji-Liang Wu, Xu-Cheng Zhang, Yang Shi, Yue-De Yang, Jin-Long Xiao, Da-Ming Zhang, Guan-Shi Qin, and Yong-Zhen Huang

Octave-spanning optical frequency comb (OFC) generation has achieved great breakthroughs and enabled significant applications in many fields, such as optical clocks and spectroscopy. Here, we demonstrate octave-spanning OFC generation with a repetition rate of tens of GHz via a four-wave mixing (FWM) effect seeded by a dual-mode microcavity laser for the first time, to our knowledge. A 120-m Brillouin nonlinear fiber loop is first utilized to generate wideband OFCs using the FWM effect. Subsequently, a time-domain optical pulse is shaped by appropriate optical filtering via fiber Bragg gratings. The high-repetition-rate pulse train is further boosted to 11 pJ through optimal optical amplification and dispersion compensation. Finally, an octave optical comb spanning from 1100 to 2200 nm is successfully realized through the self-phase modulation effect and dispersion wave generation in a commercial nonlinear optical fiber. Using dual-mode microcavity lasers with different mode intervals, we achieve frequency combs with octave bandwidths and repetition rates of 29–65 GHz, and demonstrate the dual-mode lasing microcavity laser as an ideal seeding light source for octave-spanning OFC generation.

Photonics Research
Aug. 19, 2022, Vol. 10 Issue 9 2107 (2022)
Repetition rate locked single-soliton microcomb generation via rapid frequency sweep and sideband thermal compensation
Runlin Miao, Chenxi Zhang, Xin Zheng, Xiang’ai Cheng, Ke Yin, and Tian Jiang

Dissipative Kerr solitons (DKSs) with mode-locked pulse trains in high-Q optical microresonators possess low-noise and broadband parallelized comb lines, having already found plentiful cutting-edge applications. However, thermal bistability and thermal noise caused by the high microresonator power and large temperature exchange between microresonator and the environment would prevent soliton microcomb formation and deteriorate the phase and frequency noise. Here, a novel method that combines rapid frequency sweep with optical sideband thermal compensation is presented, providing a simple and reliable way to get into the single-soliton state. Meanwhile, it is shown that the phase and frequency noises of the generated soliton are greatly reduced. Moreover, by closing the locking loop, an in-loop repetition rate fractional instability of 5.5×10-15 at 1 s integration time and a triangular linear repetition rate sweep with 2.5 MHz could be realized. This demonstration provides a means for the generation, locking, and tuning of a soliton microcomb, paving the way for the application of single-soliton microcombs in low-phase-noise microwave generation and laser ranging.

Photonics Research
Jul. 22, 2022, Vol. 10 Issue 8 1859 (2022)
Lanthanide-doped nanocrystals in high-Q microtoroids for stable on-chip white-light lasers
Limin Jin, Zhuo Liu, Yuqi Zhang, Yunkai Wu, Yilin Liu, Huachun Deng, Qinghai Song, and Shumin Xiao

The plentiful energy states of lanthanide (Ln3+)-doped nanomaterials make them very promising for on-chip integrated white-light lasers. Despite the rapid progresses, the Ln3+-based white upconversion emissions are strongly restricted by their low upconversion quantum efficiency and the color stability. Herein, we combine the CaF2:Yb35Tm1.5Er0.5 nanocrystals and the high-Q microtoroids, and experimentally demonstrate the chip-integrated stable white-light laser. By optimizing the sizes, density, and distributions of Ln3+-doped nanocrystals, the Q factors of Ln3+-doped microtoroids are maintained as high as 5×105. The strong light matter interaction in high-Q microtoroids greatly enhances the upconversion emission and dramatically reduces the laser thresholds at 652 nm, 545 nm, and 475 nm to similarly low values (1.89–2.10 mJ cm-2). Consequently, robust white-light microlaser has been experimentally achieved from a single microtoroid. This research has paved a solid step toward the chip-scale integrated broadband microlasers.

Photonics Research
Jun. 14, 2022, Vol. 10 Issue 7 07001594 (2022)
Deep reinforcement with spectrum series learning control for a mode-locked fiber laser
Zhan Li, Shuaishuai Yang, Qi Xiao, Tianyu Zhang, Yong Li, Lu Han, Dean Liu, Xiaoping Ouyang, and Jianqiang Zhu

A spectrum series learning-based model is presented for mode-locked fiber laser state searching and switching. The mode-locked operation search policy is obtained by our proposed algorithm that combines deep reinforcement learning and long short-term memory networks. Numerical simulations show that the dynamic features of the laser cavity can be obtained from spectrum series. Compared with the traditional evolutionary search algorithm that only uses the current state, this model greatly improves the efficiency of the mode-locked search. The switch of the mode-locked state is realized by a predictive neural network that controls the pump power. In the experiments, the proposed algorithm uses an average of only 690 ms to obtain a stable mode-locked state, which is one order of magnitude less than that of the traditional method. The maximum number of search steps in the algorithm is 47 in the 16°C–30°C temperature environment. The pump power prediction error is less than 2 mW, which ensures precise laser locking on multiple operating states. This proposed technique paves the way for a variety of optical systems that require fast and robust control.

Photonics Research
May. 26, 2022, Vol. 10 Issue 6 06001491 (2022)
Ultra-broadband flat-top quantum dot comb lasers
Jing-Zhi Huang, Zi-Tao Ji, Jia-Jian Chen, Wen-Qi Wei, Jia-Le Qin, Zi-Hao Wang, Zhi-Yuan Li, Ting Wang, Xi Xiao, and Jian-Jun Zhang

A quantum dot (QD) mode-locked laser as an active comb generator takes advantage of its small footprint, low power consumption, large optical bandwidth, and high-temperature stability, which is an ideal multi-wavelength source for applications such as datacom, optical interconnects, and LIDAR. In this work, we report a fourth-order colliding pulse mode-locked laser (CPML) based on InAs/GaAs QD gain structure, which can generate ultra-stable optical frequency combs in the O-band with 100 GHz spacing at operation temperature up to 100°C. A record-high flat-top optical comb is achieved with 3 dB optical bandwidth of 11.5 nm (20 comb lines) at 25°C. The average optical linewidth of comb lines is measured as 440 kHz. Single-channel non-return-to-zero modulation rates of 70 Gbit/s and four-level pulse amplitude modulation of 40 GBaud/s are also demonstrated. To further extend the comb bandwidth, an array of QD-CPMLs driven at separate temperatures is proposed to achieve 36 nm optical bandwidth (containing 60 comb lines with 100 GHz mode spacing), capable of a total transmission capacity of 4.8 Tbit/s. The demonstrated results show the feasibility of using the QD-CPML as a desirable broadband comb source to build future large-bandwidth and power-efficient optical interconnects.

Photonics Research
Spotlight on OpticsApr. 29, 2022, Vol. 10 Issue 5 05001308 (2022)
High-performance distributed feedback quantum dot lasers with laterally coupled dielectric gratings
Zhuohui Yang, Zhengqing Ding, Lin Liu, Hancheng Zhong, Sheng Cao, Xinzhong Zhang, Shizhe Lin, Xiaoying Huang, Huadi Deng, Ying Yu, and Siyuan Yu

The combination of grating-based frequency-selective optical feedback mechanisms, such as distributed feedback (DFB) or distributed Bragg reflector (DBR) structures, with quantum dot (QD) gain materials is a main approach towards ultrahigh-performance semiconductor lasers for many key novel applications, as either stand-alone sources or on-chip sources in photonic integrated circuits. However, the fabrication of conventional buried Bragg grating structures on GaAs, GaAs/Si, GaSb, and other material platforms has been met with major material regrowth difficulties. We report a novel and universal approach of introducing laterally coupled dielectric Bragg gratings to semiconductor lasers that allows highly controllable, reliable, and strong coupling between the grating and the optical mode. We implement such a grating structure in a low-loss amorphous silicon material alongside GaAs lasers with InAs/GaAs QD gain layers. The resulting DFB laser arrays emit at pre-designed 0.8 THz local area network wavelength division multiplexing frequency intervals in the 1300 nm band with record performance parameters, including sidemode suppression ratios as high as 52.7 dB, continuous-wave output power of 26.6 mW (room temperature) and 6 mW (at 55°C), and ultralow relative intensity noise (RIN) of -165 dB/Hz (2.5–20 GHz). The devices are also capable of isolator-free operating under very high external reflection levels of up to -12.3 dB while maintaining high spectral purity and ultralow RIN qualities. These results validate the novel laterally coupled dielectric grating as a technologically superior and potentially cost-effective approach for fabricating DFB and DBR lasers free of their semiconductor material constraints, which are thus universally applicable across different material platforms and wavelength bands.

Photonics Research
Apr. 29, 2022, Vol. 10 Issue 5 05001271 (2022)
Four-wave mixing in 1.3 μm epitaxial quantum dot lasers directly grown on silicon
Jianan Duan, Bozhang Dong, Weng W. Chow, Heming Huang, Shihao Ding, Songtao Liu, Justin C. Norman, John E. Bowers, and Frédéric Grillot

This work compares the four-wave mixing (FWM) effect in epitaxial quantum dot (QD) lasers grown on silicon with quantum well (QW) lasers. A comparison of theory and experiment results shows that the measured FWM coefficient is in good agreement with theoretical predictions. The gain in signal power is higher for p-doped QD lasers than for undoped lasers, despite the same FWM coefficient. Owing to the near-zero linewidth enhancement factor, QD lasers exhibit FWM coefficients and conversion efficiency that are more than one order of magnitude higher than those of QW lasers. Thus, this leads to self-mode locking in QD lasers. These findings are useful for developing on-chip sources for photonic integrated circuits on silicon.

Photonics Research
Editors' PickApr. 21, 2022, Vol. 10 Issue 5 05001264 (2022)
Mode crossing induced soliton frequency comb generation in high-Q yttria-stabilized zirconia crystalline optical microresonators
Guoping Lin, and Tang Sun

We demonstrate for the first time, to the best of our knowledge, the fabrication of high-Q crystalline optical microresonators from cubic yttria-stabilized zirconia (YSZ). Intrinsic Q factors up to 80 million are obtained, indicating an upper bound absorption coefficient of 0.001 cm-1 for YSZ crystals at the telecom wavelength. Through laser-scanned spectroscopy on a few-mode YSZ microresonator with a radius of 300 μm, we find that the mode crossing effect in the case of weak coupling can induce a repelling disruption of free spectral range values between transverse mode families. Generation of soliton and soliton crystals in such a YSZ microcomb platform operated in the normal dispersion regime is observed. A breathing comb behavior is also reported. Our finding has enriched comb generation platforms with potential multidisciplinary capabilities linked to properties of YSZ crystals in superconducting films and sensors.

Photonics Research
Feb. 22, 2022, Vol. 10 Issue 3 03000731 (2022)
Revealing the dynamics of intensity fluctuation transfer in a random Raman fiber laser
Jun Ye, Xiaoya Ma, Yang Zhang, Jiangming Xu, Hanwei Zhang, Tianfu Yao, Jinyong Leng, and Pu Zhou

Temporal intensity fluctuation is one of the inherent features of fiber lasers. When utilizing the fiber lasers to pump a random Raman fiber laser (RRFL), the intensity fluctuation transfer from the pump to the random lasing could affect the output performance significantly. In this paper, we comprehensively compared the spectral, temporal, and power characteristics of an RRFL pumped by two different fiber lasers—a temporally unstable fiber oscillator and a temporally stable amplified spontaneous emission (ASE) source. Owing to less impact of the intensity fluctuation transfer, the ASE source-pumped RRFL shows ∼45.3% higher maximum output power, higher spectral purity (>99.9%) and optical signal-to-noise ratio (>40 dB), weaker spectral broadening, and more stable temporal behavior compared to the fiber oscillator-pumped RRFL. Furthermore, based on the temporal-spatial-coupled Raman equations and the generalized nonlinear Schrödinger equations, we numerically revealed the impact of the pump intensity fluctuations on the output characteristics of RRFLs, and found that the temporal walk-off effect played an important role in the dynamics of intensity fluctuation transfer. This work may provide a reference for designing and implementing high-performance RRFLs and promote their practicability in sensing, telecommunications, and high-power applications.

Photonics Research
Feb. 09, 2022, Vol. 10 Issue 3 03000618 (2022)
Spatial coherence of electrically pumped random terahertz lasers
Eva A. A. Pogna, Alessandra Di Gaspare, Kimberly Reichel, Chiara Liberatore, Harvey E. Beere, David A. Ritchie, and Miriam S. Vitiello

Light sources with high radiance and tailored coherence properties are highly desirable for imaging applications in the mid-infrared and terahertz (THz) spectral regions, which host a large variety of molecular absorptions and distinctive fingerprints to be exploited for sensing and tomography. Here, we characterize the spatial coherence of random multimode THz quantum cascade lasers (QCLs) emitting > mW optical power per mode and showing low divergence (10°–30°), performing a modified Young’s double-slit experiment. Partial spatial coherence values ranging between 0.16 and 0.34 are retrieved, depending on the specific degree of disorder. These values are significantly lower than those (0.82) of conventional Fabry–Perot THz QCLs exploiting an identical active region quantum design. We then incorporate the devised low spatial coherence random lasers into a confocal imaging system with micrometer spatial resolution and demonstrate notable imaging performances, at THz frequencies, against spatial cross talk and speckles.

Photonics Research
Feb. 01, 2022, Vol. 10 Issue 2 02000524 (2022)
All-fiber spatiotemporal mode-locking lasers with large modal dispersion
Huaiwei Zhang, Yunhong Zhang, Jiying Peng, Xinyang Su, Xiaosheng Xiao, Dongjian Xu, Junhao Chen, Tianran Sun, Kai Zheng, Jianquan Yao, and Yi Zheng

It is a challenging problem to balance the modal walk-off (modal dispersion) between multiple transverse modes and chromatic dispersion in long step-index multimode fibers (MMFs). By properly designing the oscillator, we have overcome the difficulty and successfully obtained an all-fiber spatiotemporal mode-locked laser based on step-index MMFs with large modal dispersion for the first time, to our knowledge. Various proofs of spatiotemporal mode-locking (STML) such as spatial, spectral, and temporal properties, are measured and characterized. This laser works at a fundamental frequency of 28.7 MHz, and achieves a pulse laser with single pulse energy of 8 nJ, pulse width of 20.1 ps, and signal-to-noise ratio of ∼70 dB. In addition, we observe a dynamic evolution of the transverse mode energy during the STML establishment process that has never been reported before.

Photonics Research
Feb. 01, 2022, Vol. 10 Issue 2 02000483 (2022)
Sub-terahertz-repetition-rate frequency comb generated by filter-induced instabilities in passive driven fiber resonators
Pan Wang, Jiangyong He, Xiaosheng Xiao, Zhi Wang, and Yange Liu

Ultrahigh-repetition-rate frequency comb generation exhibits great potential in applications of optical waveform synthesis, direct comb spectroscopy, and high capacity telecommunications. Here we present the theoretical investigations of a filter-induced instability mechanism in passive driven fiber resonators with a wide range of cavity dispersion regimes. In this novel concept of modulation instability, coherent frequency combs are demonstrated numerically with rates up to sub-terahertz level. Floquet stability analysis based on the Ikeda map is utilized to understand the physical origin of the filter-induced instability. Comparison with the well-known Benjamin–Feir instability and parametric instability is performed, revealing the intrinsic distinction in the family of modulation instabilities. Our investigations might benefit the development of ultrahigh-repetition-rate frequency comb generation, providing an alternative method for the microresonators.

Photonics Research
Jan. 24, 2022, Vol. 10 Issue 2 02000465 (2022)
Principle and numerical demonstration of high power all-fiber coherent beam combination based on self-imaging effect in a square core fiber
Yuefang Yan, Yu Liu, Haoyu Zhang, Yue Li, Yuwei Li, Xi Feng, Donglin Yan, Jianjun Wang, Honghuan Lin, Feng Jing, Wenhui Huang, and Rumao Tao

The self-imaging effect in a square core fiber has been investigated, and an integrated all-fiber combiner has been proposed based on a large mode area double clad fiber, which can be employed to construct high power coherent beam combining sources in the all-fiber format. The influence of various parameters on beam quality (M2) and efficiency of the all-fiber coherent beam combiner has been studied numerically, which reveals that the near diffraction-limited laser beam can be achieved. A principle demonstration of the self-imaging effect has been carried out experimentally in a square core fiber, which proves the feasibility of beam combining with the square fiber, and that it is a promising way to develop high power coherent beam combination sources.

Photonics Research
Jan. 24, 2022, Vol. 10 Issue 2 02000444 (2022)
Near-optimal intense and powerful terahertz source by optical rectification in lithium niobate crystal
L. Guiramand, J. E. Nkeck, X. Ropagnol, T. Ozaki, and F. Blanchard

Using a 400 μJ ytterbium laser combined with a novel pulse compression technique, we demonstrate a state-of-the-art terahertz (THz) source from the tilted-pulse front pumping scheme in lithium niobate at room temperature with record efficiency of 1.3% capable of generating 74 mW of average power and 400 kV/cm at focus. Key points of this demonstration include the use of a pump pulse duration of 280 fs in combination with a stair-step echelon mirror and an off-axis ellipsoidal mirror. This source has unmatched characteristics of generating intense and powerful THz pulses at the same time and remains highly scalable as compared to existing Ti:sapphire-based THz sources pumped in the millijoule range.

Photonics Research
Jan. 11, 2022, Vol. 10 Issue 2 02000340 (2022)
Continuous-wave single-mode quantum cascade laser at 5.1 THz based on graded sampled grating design
Weijiang Li, Yu Ma, Yunfei Xu, Junqi Liu, Lijun Wang, Ning Zhuo, Quanyong Lu, Jinchuan Zhang, Shenqiang Zhai, Shuman Liu, and Fengqi Liu

We report on the terahertz (THz) quantum cascade lasers in continuous-wave (CW) operation with an emitting frequency above 5 THz. Excellent performance with a smaller leakage current and higher population inversion efficiency is obtained by one-well bridged bound-to-continuum hybrid quantum design at 5 THz. By designing and fabricating a graded metallic sampled distributed feedback grating in the waveguide, the first single-mode THz quantum cascade laser at 5.13 THz in CW operation mode is achieved. The maximum single-mode optical power of ∼48 mW is achieved at 15 K with a side-mode suppression ratio above 24 dB. This will draw great interest in the spectroscopy applications above the 5 THz range for THz quantum cascade lasers.

Photonics Research
Nov. 04, 2022, Vol. 10 Issue 12 2686 (2022)
Multi-gigahertz mode-locked femtosecond Yb:KLuW waveguide lasers
Ji Eun Bae, Xavier Mateos, Magdalena Aguiló, Francesc Díaz, Javier García Ajates, Carolina Romero, Javier Rodríguez Vázquez de Aldana, and Fabian Rotermund

We demonstrate multi-gigahertz continuous-wave mode-locking of a Yb:KLuW waveguide laser. A femtosecond-laser-inscribed Yb:KLuW channel waveguide in an extended laser cavity delivers a fundamentally mode-locked laser near 1030 nm. A tunable few-centimeter-long cavity containing a single-walled carbon nanotube saturable absorber as mode-locker generates self-starting femtosecond pulses with average output powers exceeding 210 mW at repetition rates of 2.27, 2.69, and 3.55 GHz. The laser cavity, which includes a wedged waveguide, is extended by using a lens pair that controls the laser fluence on the saturable absorber for reliable mode-locked operation without instability. The presented laser performance, mode-locked up to 3.55 GHz, highly suggests the potential of crystalline Yb:KLuW waveguides for realizing high-power ultrafast lasers with higher GHz repetition rates in a quasi-monolithic cavity.

Photonics Research
Oct. 28, 2022, Vol. 10 Issue 11 2584 (2022)
On the initiation of fiber fuse damage in high-power ytterbium-doped fiber lasers
Jiading Tian, Zehui Wang, Qirong Xiao, Dan Li, Ping Yan, and Mali Gong

Fiber fuse effect can occur spontaneously and propagate along optical fibers to cause widespread damage; it threatens all applications involving optical fibers. This paper presents two results. First, it establishes that the initiation of fiber fuse (IFF) in silica fibers is caused by virtual-defect-induced absorption. Critical temperatures and critical optical powers for IFF are simulated for the first time using a 3D solid-state heat transfer model with heat source generated by the virtual-defect-induced absorption. In this method, formation energies of the virtual defects can be uniquely determined, which offers critical information on the chemical reasons for fiber fuse. Second, this paper offers a method to evaluate operating temperatures of fiber lasers. General analytical solutions of the operating temperatures along gain fibers are deduced. Results of 976-nm laser-diode-pumped and 1018-nm tandem-pumped ytterbium-doped fiber (YDF) amplifiers using 10/130-μm YDFs are calculated. Potential limits caused by fiber fuse are discussed.

Photonics Research
Oct. 17, 2022, Vol. 10 Issue 11 2513 (2022)
High-power wavelength-tunable and power-ratio-controllable dual-wavelength operation at 1319 nm and 1338 nm in a Q-switched Nd:YAG laser
Qi Bian, Yong Bo, Jun-Wei Zuo, Lei Yuan, Hong-Wei Gao, and Qin-Jun Peng

We report the first demonstration on three types of 1.3 μm spectral region in a Q-switched Nd:YAG laser. In order to dissipate the heat deposition effectively to obtain good beam quality, the Nd:YAG rod crystal with 1° cut-angle on end faces is side-pumped by the quasi-continuous-wave pulsed laser diode. A Suprasil etalon is well designed as the intracavity mode-selector to obtain wavelength-tunable single line or power-ratio-controllable dual line operation at 1319 nm and 1338 nm. With the pump pulse width of 200 μs and pump power of 410 W, the acousto-optic Q-switched laser delivered a pulse width of 117 ns at 400 Hz repetition rate, and the M2 factor was measured to be about 1.87. 1319 nm together with 1338 nm single-wavelength laser achieved an average output power of 47.6 W and 39.9 W with a linewidth of 0.48 nm and 0.32 nm, and a tunable range of 111.2 pm and 108.6 pm, respectively. Among dual-wavelength oscillation, both lines can be tuned at almost equal intensity level with 45.7 W total output power, which is input into an LBO crystal to generate red light of 11.4 W for 659 nm, 6.7 W for 664 nm, and 7.5 W for 669 nm. The 1.3 μm wavelength-selectable operation realized by using the same laser configuration may enhance the application in the fields of tunable lasers and THz frequency generation.

Photonics Research
Sep. 14, 2022, Vol. 10 Issue 10 2287 (2022)
Dynamic performance and reflection sensitivity of quantum dot distributed feedback lasers with large optical mismatch
Bozhang Dong, Jianan Duan, Heming Huang, Justin C. Norman, Kenichi Nishi, Keizo Takemasa, Mitsuru Sugawara, John E. Bowers, and Frédéric Grillot

This work reports on a high-efficiency InAs/GaAs distributed feedback quantum dot laser. The large optical wavelength detuning at room temperature between the lasing peak and the gain peak causes the static, dynamic, and nonlinear intrinsic properties to all improve with temperature, including the lasing efficiency, the modulation dynamics, the linewidth enhancement factor, and consequently the reflection insensitivity. Results reported show an optimum operating temperature at 75°C, highlighting the potential of the large optical mismatch assisted single-frequency laser for the development of uncooled and isolator-free high-speed photonic integrated circuits.

Photonics Research
Jul. 29, 2021, Vol. 9 Issue 8 08001550 (2021)
Nonlinear Fourier transform enabled eigenvalue spectrum investigation for fiber laser radiation
Yutian Wang, Songnian Fu, Jian Kong, Andrey Komarov, Mariusz Klimczak, Ryszard Buczyński, Xiahui Tang, Ming Tang, Yuwen Qin, and Luming Zhao

Fiber lasers are a paradigm of dissipative systems, which distinguish themselves from a Hamilton system where energy is conservative. Consequently, pulses generated in a fiber laser are always accompanied by the continuous wave (CW). Under certain hypothesis, pulses generated in a fiber laser can be considered as a soliton, a product of a Hamilton system. Therefore, all the descriptions of solitons of a fiber laser are approximate. Coexistence of solitons and the CW from a fiber laser prevents unveiling of real nonlinear dynamics in fiber lasers, such as soliton interactions. Pulse behavior in a fiber laser can be represented by the state of single pulse, the state of period doubling of single pulse, the states of two pulses either tightly bound or loosely distributed, the states of three pulses, and various combinations of the above-mentioned states. Recently, soliton distillation was proposed and numerically demonstrated based on the nonlinear Fourier transform (NFT) [J. Lightwave Technol.39, 2542 (2021)JLTEDG0733-872410.1109/JLT.2021.3051036]. Solitons can be separated from the coherent CW background. Therefore, it is feasible to isolate solitons from CW background in a fiber laser. Here, we applied the NFT to various pulses generated in a fiber laser, including single pulse, single pulse in period doubling, different double pulses, and multiple pulses. Furthermore, with the approach of soliton distillation, the corresponding pure solitons of those pulses are reconstructed. Simulation results suggest that the NFT can be used to identify soliton dynamics excluding CW influence in a fiber laser, which paves a new way for uncovering real soliton interaction in nonlinear systems.

Photonics Research
Jul. 29, 2021, Vol. 9 Issue 8 08001531 (2021)
Low-latency deep-reinforcement learning algorithm for ultrafast fiber lasers
Qiuquan Yan, Qinghui Deng, Jun Zhang, Ying Zhu, Ke Yin, Teng Li, Dan Wu, and Tian Jiang

The application of machine learning to the field of ultrafast photonics is becoming more and more extensive. In this paper, for the automatic mode-locked operation in a saturable absorber-based ultrafast fiber laser (UFL), a deep-reinforcement learning algorithm with low latency is proposed and implemented. The algorithm contains two actor neural networks providing strategies to modify the intracavity lasing polarization state and two critic neural networks evaluating the effect of the actor networks. With this algorithm, a stable fundamental mode-locked (FML) state of the UFL is demonstrated. To guarantee its effectiveness and robustness, two experiments are put forward. As for effectiveness, one experiment verifies the performance of the trained network model by applying it to recover the mode-locked state with environmental vibrations, which mimics the condition that the UFL loses the mode-locked state quickly. As for robustness, the other experiment, at first, builds a database with UFL at different temperatures. It then trains the model and tests its performance. The results show that the average mode-locked recovery time of the trained network model is 1.948 s. As far as we know, it is 62.8% of the fastest average mode-locked recovery time in the existing work. At different temperatures, the trained network model can also recover the mode-locked state of the UFL in a short time. Remote algorithm training and automatic mode-locked control are proved in this work, laying the foundation for long-distance maintenance and centralized control of UFLs.

Photonics Research
Jul. 20, 2021, Vol. 9 Issue 8 08001493 (2021)
Multigigawatt 50 fs Yb:CALGO regenerative amplifier system with 11 W average power and mid-infrared generation
Weizhe Wang, Han Wu, Cheng Liu, Biao Sun, and Houkun Liang

Lasers with high average and high peak power as well as ultrashort pulse width have been all along demanded by nonlinear optics studies, strong-field experiments, electron dynamics investigations, and ultrafast spectroscopy. While the routinely used titanium-doped sapphire (Ti:sapphire) laser faces a bottleneck in the average power upscaling, ytterbium (Yb)-doped lasers have remarkable advantages in achieving high average power. However, there is still a substantial gap of pulse width and peak power between the Ti:sapphire and Yb-doped lasers. Here we demonstrate a high-power Yb:CaAlGdO4 (Yb:CALGO) regenerative amplifier system, delivering 1040 nm pulses with 11 W average power, 50 fs pulse width, and 3.7 GW peak power at a repetition rate of 43 kHz, which to some extent bridges the gap between the Ti:sapphire and Yb lasers. An ultrabroadband Yb-doped fiber oscillator, specially designed spectral shapers, and Yb:CALGO gain medium with broad emission bandwidth, together with a double-end pumping scheme enable an amplified bandwidth of 19 nm and 95 fs output pulse width. To the best of our knowledge, this is the first demonstration of sub-100 fs regenerative amplifier based on Yb-doped bulk medium without nonlinear spectral broadening. The amplified pulse is further compressed to 50 fs via cascaded-quadratic compression with a simple setup, producing 3.7 GW peak power, which boosts the record of peak power from Yb:CALGO regenerative amplifiers by 1 order. As a proof of concept, pumped by the high-power, 50 fs pulses, 7.5–11.5 µm mid-infrared (MIR) generation via intrapulse difference-frequency generation is performed, without the necessity of nonlinear fiber compressors. It leads to a simple and robust apparatus, and it would find good usefulness in MIR spectroscopic applications.

Photonics Research
Jul. 08, 2021, Vol. 9 Issue 8 08001439 (2021)
Vector harmonic mode-locking by acoustic resonance
Sergey Sergeyev, Stanislav Kolpakov, and Yury Loika

For an Er-doped fiber laser, for the first time, to the best of our knowledge, we demonstrate both experimentally and theoretically a novel mechanism of harmonic mode-locking based on the electrostriction effect leading to excitation of the torsional acoustic modes in the transverse section of the laser. The exited torsional acoustic modes modulate the fiber birefringence that results in synchronization of oscillations at the harmonic modes and the linewidth narrowing with the increased signal-to-noise ratio of 30 dB. By adjusting the in-cavity birefringence based on tuning the polarization controller, we enable the selection of the harmonic mode to be stabilized.

Photonics Research
Jul. 08, 2021, Vol. 9 Issue 8 08001432 (2021)
Intermittent dynamical state switching in discrete-mode semiconductor lasers subject to optical feedback
Zhuqiang Zhong, Da Chang, Wei Jin, Min Won Lee, Anbang Wang, Shan Jiang, Jiaxiang He, Jianming Tang, and Yanhua Hong

Intermittent dynamics switching on the route to chaos in a discrete-mode laser with long time-delayed feedback is experimentally and numerically studied by analyzing the time series, power spectra, and phase portraits. The results show two types of dynamics switching: one or multiple times regular intermittent dynamics switching between stable state and square-wave envelope period-one oscillation within one feedback round time, and the irregular intermittent dynamics switching between stable state and quasi-periodic or multi-states or chaos with higher feedback ratio and bias currents. The relationship between the duty cycle of period-one oscillation and the feedback ratio has been analyzed. The map of the dynamics distribution in the parameter space of feedback ratio and bias current is plotted for a better understanding of dynamics evolution in long external cavity discrete-mode lasers.

Photonics Research
Jun. 28, 2021, Vol. 9 Issue 7 07001336 (2021)
Modeling of a SiGeSn quantum well laser
Bahareh Marzban, Daniela Stange, Denis Rainko, Zoran Ikonic, Dan Buca, and Jeremy Witzens

We present comprehensive modeling of a SiGeSn multi-quantum well laser that has been previously experimentally shown to feature an order of magnitude reduction in the optical pump threshold compared to bulk lasers. We combine experimental material data obtained over the last few years with k·p theory to adapt transport, optical gain, and optical loss models to this material system (drift-diffusion, thermionic emission, gain calculations, free carrier absorption, and intervalence band absorption). Good consistency is obtained with experimental data, and the main mechanisms limiting the laser performance are discussed. In particular, modeling results indicate a low non-radiative lifetime, in the 100 ps range for the investigated material stack, and lower than expected Γ-L energy separation and/or carrier confinement to play a dominant role in the device properties. Moreover, they further indicate that this laser emits in transverse magnetic polarization at higher temperatures due to lower intervalence band absorption losses. To the best of our knowledge, this is the first comprehensive modeling of experimentally realized SiGeSn lasers, taking the wealth of experimental material data accumulated over the past years into account. The methods described in this paper pave the way to predictive modeling of new (Si)GeSn laser device concepts.

Photonics Research
Jun. 15, 2021, Vol. 9 Issue 7 07001234 (2021)
Superior performance of a 2 kHz pulse Nd:YAG laser based on a gradient-doped crystal
Meng’en Wei, Tingqing Cheng, Renqin Dou, Qingli Zhang, and Haihe Jiang

Herein, we report a homemade new Nd:YAG crystal rod that contains a gradient dopant of 0.39–0.80 at.% Nd3+ from end to end, achieving superior performance of a 2 kHz Nd:YAG pulse laser at 1064 nm. The optical-to-optical conversion efficiency reached 53.8%, and the maximum output power of the laser was 24.2 W, enhanced by 35.9% compared with a uniform crystal rod with the same total concentration of Nd3+. Significantly, our experiments revealed that the gradient concentration crystal produced a relatively even pumping distribution along the rod axis, greatly reducing the temperature gradient as well as having a smaller thermal effect. The pump and thermal distribution smoothing obviously improved the features of laser oscillation and output.

Photonics Research
Jun. 08, 2021, Vol. 9 Issue 7 07001191 (2021)
Experimental demonstration of pyramidal neuron-like dynamics dominated by dendritic action potentials based on a VCSEL for all-optical XOR classification task
Yahui Zhang, Shuiying Xiang, Xingyu Cao, Shihao Zhao, Xingxing Guo, Aijun Wen, and Yue Hao

We experimentally and numerically demonstrate an approach to optically reproduce a pyramidal neuron-like dynamics dominated by dendritic Ca2+ action potentials (dCaAPs) based on a vertical-cavity surface-emitting laser (VCSEL) for the first time. The biological pyramidal neural dynamics dominated by dCaAPs indicates that the dendritic electrode evoked somatic spikes with current near threshold but failed to evoke (or evoked less) somatic spikes for higher current intensity. The emulating neuron-like dynamics is performed optically based on the injection locking, spiking dynamics, and damped oscillations in the optically injected VCSEL. In addition, the exclusive OR (XOR) classification task is examined in the VCSEL neuron equipped with the pyramidal neuron-like dynamics dominated by dCaAPs. Furthermore, a single spike or multiple periodic spikes are suggested to express the result of the XOR classification task for enhancing the processing rate or accuracy. The experimental and numerical results show that the XOR classification task is achieved successfully in the VCSEL neuron enabled to mimic the pyramidal neuron-like dynamics dominated by dCaAPs. This work reveals valuable pyramidal neuron-like dynamics in a VCSEL and offers a novel approach to solve XOR classification task with a fast and simple all-optical spiking neural network, and hence shows great potentials for future photonic spiking neural networks and photonic neuromorphic computing.

Photonics Research
May. 25, 2021, Vol. 9 Issue 6 06001055 (2021)
Arbitrary cylindrical vector beam generation enabled by polarization-selective Gouy phase shifter
Junliang Jia, Kepeng Zhang, Guangwei Hu, Maping Hu, Tong Tong, Quanquan Mu, Hong Gao, Fuli Li, Cheng-Wei Qiu, and Pei Zhang

Cylindrical vector beams (CVBs), which possess polarization distribution of rotational symmetry on the transverse plane, can be developed in many optical technologies. Conventional methods to generate CVBs contain redundant interferometers or need to switch among diverse elements, thus being inconvenient in applications containing multiple CVBs. Here we provide a passive polarization-selective device to substitute interferometers and simplify generation setup. It is accomplished by reversing topological charges of orbital angular momentum based on a polarization-selective Gouy phase. In the process, tunable input light is the only condition to generate a CVB with arbitrary topological charges. To cover both azimuthal and radial parameters of CVBs, we express the mapping between scalar Laguerre–Gaussian light on a basic Poincaré sphere and CVB on a high-order Poincaré sphere. The proposed device simplifies the generation of CVBs enormously and thus has potential in integrated devices for both quantum and classic optical experiments.

Photonics Research
May. 25, 2021, Vol. 9 Issue 6 06001048 (2021)
1.7-μm dissipative soliton Tm-doped fiber laser
Ji-Xiang Chen, Xiang-Yue Li, Ti-Jian Li, Ze-Yu Zhan, Meng Liu, Can Li, Ai-Ping Luo, Pu Zhou, Kenneth K.-Y. Wong, Wen-Cheng Xu, and Zhi-Chao Luo

We report on the dissipative soliton generation in a 1.7-μm net-normal dispersion Tm-doped fiber laser by nonlinear polarization rotation technique. An intra-cavity bandpass filter was employed to suppress the long-wavelength emission, while the cavity dispersion was compensated by a segment of ultrahigh numerical aperture (UHNA4) fiber. The dissipative soliton with a central wavelength of 1746 nm was obtained, covering a spectral range from 1737 nm to 1754 nm. The de-chirped duration and energy of the dissipative soliton were 370 fs and 0.2 nJ, respectively. In addition, the dynamics of multiple dissipative solitons were also investigated. Through optimization of the cavity dispersion, the 50 nm broadband dissipative soliton with de-chirped pulse duration of 230 fs could be achieved. The development of dissipative soliton seed laser represents the first step in achieving the chirped pulse amplification system at the 1.7-μm wave band, which would find potential applications in fields such as biomedical imaging and material processing.

Photonics Research
Apr. 30, 2021, Vol. 9 Issue 5 05000873 (2021)
Broadband mid-infrared supercontinuum generation in dispersion-engineered As2S3-silica nanospike waveguides pumped by 2.8 μm femtosecond laser
Pan Wang, Jiapeng Huang, Shangran Xie, Johann Troles, and Philip St.J. Russell

Broadband mid-infrared (IR) supercontinuum laser sources are essential for spectroscopy in the molecular fingerprint region. Here, we report generation of octave-spanning and coherent mid-IR supercontinua in As2S3-silica nanospike hybrid waveguides pumped by a custom-built 2.8 μm femtosecond fiber laser. The waveguides are formed by pressure-assisted melt-filling of molten As2S3 into silica capillaries, allowing the dispersion and nonlinearity to be precisely tailored. Continuous coherent spectra spanning from 1.1 μm to 4.8 μm (30 dB level) are observed when the waveguide is designed so that 2.8 μm lies in the anomalous dispersion regime. Moreover, linearly tapered millimeter-scale As2S3-silica waveguides are fabricated and investigated for the first time, to the best of our knowledge, showing much broader supercontinua than uniform waveguides, with improved spectral coherence. The waveguides are demonstrated to be long-term stable and water-resistant due to the shielding of the As2S3 by the fused silica sheath. They offer an alternative route to generating broadband mid-IR supercontinua, with applications in frequency metrology and molecular spectroscopy, especially in humid and aqueous environments.

Photonics Research
Mar. 31, 2021, Vol. 9 Issue 4 04000630 (2021)
On the upper limit of laser intensity attainable in nonideal vacuum
Yitong Wu, Liangliang Ji, and Ruxin Li

The upper limit of the laser field strength in a perfect vacuum is usually considered as the Schwinger field, corresponding to ~1029 W/cm2. We investigate such limitations under realistic nonideal vacuum conditions and find that intensity suppression appears starting from 1025 W/cm2, showing an upper threshold at 1026 W/cm2 level if the residual electron density in chamber surpasses 109 cm-3. This is because the presence of residual electrons triggers the avalanche of quantum electrodynamics cascade that creates copious electron and positron pairs. The leptons are further trapped within the driving laser field due to radiation reaction, which significantly depletes the laser energy. The relationship between the attainable intensity and the vacuity is given according to particle-in-cell simulations and theoretical analysis. These results answer a critical problem on the achievable light intensity based on present vacuum conditions and provide a guideline for future hundreds of petawatt class laser development.

Photonics Research
Mar. 24, 2021, Vol. 9 Issue 4 04000541 (2021)
Observation of transition between multimode Q-switching and spatiotemporal mode locking
Kewei Liu, Xiaosheng Xiao, and Changxi Yang

We report experimental observation of multimode Q-switching and spatiotemporal mode locking in a multimode fiber laser. A typical steady Q-switching state is achieved with a 1.88 μs pulse duration, a 70.14 kHz repetition rate, and a 215.8 mW output power, corresponding to the single pulse energy of 3.08 μJ. We find weak spatial filtering is essential to obtain stable Q-switched pulses, in contrast to the relatively stronger spatial filtering for spatiotemporal mode locking. Furthermore, a reversible transition process, as well as a critical bistable state, between multimode Q-switching and spatiotemporal mode locking, is achieved with specific spatial coupling and waveplates sets. We believe the results will not only contribute to understanding the complicated nonlinear dynamics in multimode, fiber-based platforms, but also benefit the development of promising high-pulse energy lasers.

Photonics Research
Mar. 24, 2021, Vol. 9 Issue 4 04000530 (2021)
Pulse combination and compression in hollow-core fiber for few-cycle intense mid-infrared laser generation
Junyu Qian, Pengfei Wang, Yujie Peng, Yanyan Li, Beijie Shao, Hongpeng Su, Xinlin Lv, Ding Wang, Yuxin Leng, and Ruxin Li

The generation of high-peak-power, few-cycle mid-infrared (MIR) pulses using coherent beam combination and nonlinear pulse compression techniques simultaneously is demonstrated. The two pulses, with identical pulse energy of 2.8 mJ and pulse duration of 160 fs, are coherently combined at the input end of a krypton-filled hollow-core fiber (HCF), and then the bandwidth of the combined pulse is broadened to near an optical octave due to strong phase modulations, and the temporal width is compressed into a few-cycle regime. Finally, a 2.7 mJ, 22.9 fs, 20 Hz laser at 4 μm can be obtained, and the pulse peak power is greatly enhanced compared with that of conventional single-channel optical parametric chirped pulse-amplification systems. Furthermore, the peak power generated from this system has the prospect of further scaling up through use of more channels of coherent combination, which can pave a way to generate higher peak power ultra-intense MIR pulses for strong-field physics.

Photonics Research
Mar. 19, 2021, Vol. 9 Issue 4 04000477 (2021)
Direct generation of watt-level yellow Dy3+-doped fiber laser
Jinhai Zou, Tianran Li, Yanbo Dou, Jin Li, Nan Chen, Yikun Bu, and Zhengqian Luo

Yellow lasers (~565–590 nm) are of tremendous interest in biomedicine, astronomy, spectroscopy, and display technology. So far, yellow lasers still have relied heavily on nonlinear frequency conversion of near-infrared lasers, precluding compact and low-cost yellow laser systems. Here, we address the challenge through demonstrating, for the first time, to the best of our knowledge, watt-level high-power yellow laser generation directly from a compact fiber laser. The yellow fiber laser simply consists of a Dy3+-doped ZBLAN fiber as gain medium, a fiber end-facet mirror with high reflectivity at yellow and a 450-nm diode laser as the pump source. We comprehensively investigated the dependence of the yellow laser performance on the output coupler reflectivity and the gain fiber length and demonstrated that the yellow fiber laser with an output coupler reflectivity of 4% and a gain fiber length of ~1.8 m yields a maximum efficiency of 33.6%. A maximum output power of 1.12 W at 575 nm was achieved at a pump power of 4.20 W. This work demonstrated the power scaling of yellow Dy3+-doped ZBLAN fiber lasers, showing their promise for applications in ophthalmology, astronomical exploration, and high-resolution spectroscopy.

Photonics Research
Mar. 16, 2021, Vol. 9 Issue 4 04000446 (2021)
Effects of background spectral noise in the phase-modulated single-frequency seed laser on high-power narrow-linewidth fiber amplifiers
Wei Liu, Jiaxin Song, Pengfei Ma, Hu Xiao, and Pu Zhou

In this work, we analyze the effects of the background spectral noise in phase-modulated single-frequency seed lasers on the spectral purity of high-power narrow-linewidth fiber amplifiers. Through demonstrating the spectral evolution of the phase-modulated single-frequency part and the background spectral noise in a narrow-linewidth fiber amplifier, the mechanism for the spectral wing broadening effect is clarified and design strategies to maintain high spectral purity are given. Specifically, the background spectral noise in phase-modulated single-frequency seed lasers could lead to obvious spectral wing broadening and degeneration of spectral purity in narrow-linewidth fiber amplifiers through the four-wave-mixing effect. Notably, the spectral wing broadening effect could be suppressed by filtering out the background spectral noise in the seed laser or applying a counter-pumped configuration in the fiber amplifier. We have also conducted contrast experiments, which have verified the validity of the theoretical model and the design strategies for high-spectral-purity operation.

Photonics Research
Mar. 02, 2021, Vol. 9 Issue 4 04000424 (2021)
Structured laser beams: toward 2-μm femtosecond laser vortices
Yongguang Zhao, Li Wang, Weidong Chen, Pavel Loiko, Xavier Mateos, Xiaodong Xu, Ying Liu, Deyuan Shen, Zhengping Wang, Xinguang Xu, Uwe Griebner, and Valentin Petrov

Structured ultrashort-pulse laser beams, and in particular eigenmodes of the paraxial Helmholtz equation, are currently extensively studied for novel potential applications in various fields, e.g., laser plasma acceleration, attosecond science, and fine micromachining. To extend these prospects further, in the present work we push forward the advancement of such femtosecond structured laser sources into the 2-μm spectral region. Ultrashort-pulse Hermite– and Laguerre–Gaussian laser modes both with a pulse duration around 100 fs are successfully produced from a compact solid-state laser in combination with a simple single-cylindrical-lens converter. The negligible beam astigmatism, the broad optical spectra, and the almost chirp-free pulses emphasize the high reliability of this laser source. This work, as a proof of principle study, paves the way toward few-cycle pulse generation of optical vortices at 2 μm. The presented light source can enable new research in the fields of interaction with organic materials, next generation optical detection, and optical vortex infrared supercontinuum.

Photonics Research
Feb. 22, 2021, Vol. 9 Issue 3 03000357 (2021)
Real-time collision dynamics of vector solitons in a fiber laser
Kangjun Zhao, Chenxin Gao, Xiaosheng Xiao, and Changxi Yang

Particle-like structures of solitons, as a result of the balance between dispersion and nonlinearity, enable remarkable elastic and inelastic soliton collisions in many fields. Despite the experimental observation of temporal vector-soliton collisions in birefringent fibers, collision dynamics of vector solitons in fiber lasers have not been revealed before, to the best of our knowledge. Here, the real-time spectral evolutions of vector solitons during collisions in a dual-comb fiber laser, which generates vector solitons with slightly different repetition rates, are captured by a time-stretch dispersive Fourier transform technique. We record the whole process of vector-soliton collisions, including the formation of weak pulses induced by cross-polarization coupling, opposite central wavelength shifts of both vector solitons, distinct intensity redistribution and dissipative energy, and gradual recovery to initial states. Furthermore, extreme collisions with strong four-wave mixing sidebands are observed by virtue of coherent coupling between the orthogonal polarization components of vector solitons. Numerical simulations match well with the experimental observations. The experimental and numerical evidences of vector-soliton collision dynamics could give insight into the understanding of nonlinear dynamics in fiber lasers and other physical systems, as well as the improvement of laser performance for application in dual-comb spectroscopy.

Photonics Research
Feb. 10, 2021, Vol. 9 Issue 3 03000289 (2021)
Vortex random fiber laser with controllable orbital angular momentum mode
Xiaoya Ma, Jun Ye, Yang Zhang, Jiangming Xu, Jian Wu, Tianfu Yao, Jinyong Leng, and Pu Zhou

In this paper, we propose and experimentally demonstrate a vortex random fiber laser (RFL) with a controllable orbital angular momentum (OAM) mode. The topological charge of the vortex RFL can range from -50 to 50 with nearly watt-level output power. A triangular toroidal interferometer is constructed to verify the spiral phase structure of the generated vortex random laser with a special coherence property. Vortex RFLs with fractional topological charge are also performed in this work. As the first demonstration of a vortex RFL with a controllable OAM mode (to the best of our knowledge), this work may not only offer a valuable reference on temporal modulation of a vortex beam and optical field control of an RFL but also provide a potential vortex laser source for applications in imaging, sensing, and communication.

Photonics Research
Feb. 01, 2021, Vol. 9 Issue 2 02000266 (2021)
Dichroic laser mirrors with mixture layers and sandwich-like-structure interfaces
Tingting Zeng, Meiping Zhu, Yingjie Chai, Jingping Li, and Jianda Shao

The requirements for dichroic laser mirrors continue to increase with the development of laser technology. The challenge of a dichroic laser mirror coating is to simultaneously obtain spectral performance with significantly different reflection or transmission properties as well as a high laser-induced damage threshold (LIDT) at two different wavelengths. Traditional dichroic laser mirrors composed of alternating high- and low-refractive-index pure materials often has difficulty achieving excellent spectral performance and high LIDTs at two wavelengths simultaneously. We propose to use a new design with mixture layers and sandwich-like-structure interfaces to meet the challenging requirements. An Al2O3-HfO2 mixture-based dichroic laser mirror, which can be used as a harmonic separator in a fusion-class laser or a pump/signal beam separator in a petawatt-class Ti-sapphire laser system, is experimentally demonstrated using e-beam deposition. The mixture-based dichroic mirror coating shows good spectral performance, fine mechanical property, low absorption, and high LIDT. For the s-polarized 7.7 ns pulses at a wavelength of 532 nm and the p-polarized 12 ns pulses at a wavelength of 1064 nm, the LIDTs are almost doubled. The excellent performance of this new design strategy with mixture layers and sandwich-like-structure interfaces suggests its wide applicability in high-performance laser coating.

Photonics Research
Editors' PickJan. 27, 2021, Vol. 9 Issue 2 02000229 (2021)
Buildup dynamics of multiple solitons in spatiotemporal mode-locked fiber lasers
Kewei Liu, Xiaosheng Xiao, Yihang Ding, Hongyan Peng, Dongdong Lv, and Changxi Yang

Spatiotemporal mode locking is a nonlinear process of multimode soliton self-organization. Here the real-time buildup dynamics of the multiple solitons in a spatiotemporal mode-locked multimode fiber laser are investigated, assisted by the time-stretch technique. We find that the buildup processes are transverse mode dependent, especially during the stages of relaxation oscillation and Q-switching prior to multiple soliton formation. Furthermore, we observe that the transverse modal composition of these generated pulses among the multiple solitons can be different from each other, indicating the spatiotemporal structure of the multiple soliton. A simplified theoretical model based on pulse energy evolution is put forward to interpret the role of 3D saturable absorber on spatiotemporal structures of spatiotemporal mode-locking multiple solitons. Our work has presented the spatiotemporal nonlinear dynamics in multimode fiber lasers, which are novel to those inside the single transverse mode fiber lasers.

Photonics Research
Sep. 08, 2021, Vol. 9 Issue 10 10001898 (2021)
Reconfigurable time-stretched swept laser source with up to 100 MHz sweep rate, 100 nm bandwidth, and 100 mm OCT imaging range
Dongmei Huang, Feng Li, Chao Shang, Zihao Cheng, and P. K. A. Wai

The sweep rate, sweep range, and coherence length of swept sources, respectively, determine the acquisition rate, axial resolution, and imaging range of optical coherence tomography (OCT). In this paper, we demonstrate a reconfigurable high-speed and broadband swept laser by time stretching of a flat spectrum femtosecond pulse train with over 100 nm bandwidth and a repetition rate of 100 MHz. By incorporating an optical modulator and utilizing appropriate dispersive modules, the reconfiguration of the swept source is demonstrated with sweep rates of 25 and 2.5 MHz. The 2.5 MHz swept source enables an imaging range of >110 mm with 6 dB sensitivity roll-off in OCT, which is the longest imaging range ever reported for megahertz OCT.

Photonics Research
Editors' PickJul. 23, 2020, Vol. 8 Issue 8 08001360 (2020)
Real-time observation of vortex mode switching in a narrow-linewidth mode-locked fiber laser
Jiafeng Lu, Fan Shi, Linghao Meng, Longkun Zhang, Linping Teng, Zhengqian Luo, Peiguang Yan, Fufei Pang, and Xianglong Zeng

Temporal and spatial resonant modes are always possessed in physical systems with energy oscillation. In ultrafast fiber lasers, enormous progress has been made toward controlling the interactions of many longitudinal modes, which results in temporally mode-locked pulses. Recently, optical vortex beams have been extensively investigated due to their quantized orbital angular momentum, spatially donut-like intensity, and spiral phase front. In this paper, we have demonstrated the first to our knowledge observation of optical vortex mode switching and their corresponding pulse evolution dynamics in a narrow-linewidth mode-locked fiber laser. The spatial mode switching is achieved by incorporating a dual-resonant acousto-optic mode converter in the vortex mode-locked fiber laser. The vortex mode-switching dynamics have four stages, including quiet-down, relaxation oscillation, quasi mode-locking, and energy recovery prior to the stable mode-locking of another vortex mode. The evolution dynamics of the wavelength shifting during the switching process are observed via the time-stretch dispersion Fourier transform method. The spatial mode competition through optical nonlinearity induces energy fluctuation on the time scale of ultrashort pulses, which plays an essential role in the mode-switching dynamic process. The results have great implications in the study of spatial mode-locking mechanisms and ultrashort laser applications.

Photonics Research
Jul. 01, 2020, Vol. 8 Issue 7 07001203 (2020)
Ultralow-quantum-defect Raman laser based on the boson peak in phosphosilicate fiber
Yang Zhang, Jiangming Xu, Jun Ye, Jiaxin Song, Tianfu Yao, and Pu Zhou

Quantum defects (QDs) have always been a key factor of the thermal effect in high-power fiber lasers. Much research on low-QD fiber lasers has been reported in the past decades, but most of it is based on active fibers. Besides, Raman fiber lasers based on the stimulated Raman scattering effect in passive fiber are also becoming an important kind of high-power fiber laser for their unique advantages, such as their significantly broader wavelength-tuning range and being free of photon darkening. In this paper, we demonstrate an ultralow-QD Raman fiber laser based on phosphosilicate fiber. There is a strong boson peak located at a frequency shift of 3.65 THz in the Raman gain spectrum of the phosphosilicate fiber we employed. By utilizing this boson peak to provide Raman gain and adopting an amplified spontaneous emission source at 1066 nm as the pump source, 1080 nm Stokes light is generated, corresponding to a QD of 1.3%. The spectral purity at 1080 nm can be up to 96.03%, and the output power is 12.5 W, corresponding to a conversion efficiency of 67.2%. Moreover, by increasing the pump wavelength to 1072 nm, the QD is reduced to 0.74%, and the output power at 1080 nm is 10.7 W, with a spectral purity of 82.82%. To the best of our knowledge, this is the lowest QD ever reported for Raman fiber lasers. This work proposes a promising way of achieving high-power, high-efficiency Raman fiber lasers.

Photonics Research
Jun. 16, 2020, Vol. 8 Issue 7 07001155 (2020)
Widely tunable ultra-narrow-linewidth dissipative soliton generation at the telecom band
Chang Kyun Ha, Ki Sang Lee, Dohyeon Kwon, and Myeong Soo Kang

Ultra-narrow-linewidth mode-locked lasers with wide wavelength tunability can be versatile light sources for a variety of newly emergent applications. However, it is very challenging to achieve the stable mode locking of substantially long, anomalously dispersive fiber laser cavities employing a narrowband spectral filter at the telecom band. Here, we show that a nearly dispersion-insensitive dissipative mode-locking regime can be accessed through a subtle counterbalance among significantly narrowband spectral filtering, sufficiently deep saturable absorption, and moderately strong in-fiber Kerr nonlinearity. This achieves ultra-narrow-linewidth (a few gigahertz) nearly transform-limited self-starting stable dissipative soliton generation at low repetition rates (a few megahertz) without cavity dispersion management over a broad tuning range of wavelengths covering the entire telecom C-band. This unique laser may have immediate application as an idealized pump source for high-efficiency nonlinear frequency conversion and nonclassical light generation in dispersion-engineered tightly light-confining microphotonic/nanophotonic systems.

Photonics Research
Jun. 04, 2020, Vol. 8 Issue 7 07001100 (2020)
MXene-Ti3C2Tx for watt-level high-efficiency pulse generation in a 2.8 μm mid-infrared fiber laser
Chen Wei, Liqiang Zhou, Dongsheng Wang, Hao Chi, Hua Huang, Han Zhang, and Yong Liu

We report a watt-level passively Q-switched 2.8 μm mid-infrared multi-mode fiber laser by employing multi-layered two-dimensional MXene-Ti3C2Tx as the saturable absorber (SA). The MXene-Ti3C2Tx is fabricated by selectively etching aluminum layers in Ti3AlC2. The non-saturable loss, modulation depth, and saturable intensity of the SA at 2866 nm were measured to be 25.0%, 33.2%, and 0.043 GW/cm2, respectively. The maximum average output power of the Ti3C2TxQ-switched fiber laser reached 1.09 W at 28.23% slope efficiency. The pulse repetition rate, shortest pulse width, pulse peak power, and single-pulse energy were 78.12 kHz, 1.04 μs, 13.4 W, and 13.93 μJ, respectively. This is the first demonstration of watt-level pulse generation in a mid-infrared fiber laser using low dimensional materials, to the best of our knowledge. These results indicate that the Ti3C2Tx is a reliable and superior broadband SA for high power mid-infrared pulsed laser generation.

Photonics Research
May. 29, 2020, Vol. 8 Issue 6 06000972 (2020)
Frequency comb swept laser with a high-Q microring filter
Dongmei Huang, Feng Li, Chao Shang, Zihao Cheng, S. T. Chu, and P. K. A. Wai

Frequency comb swept lasers are the enabling technology of circular interferometric imaging, which was proposed to break the bottleneck of data acquisition and processing in optical coherence tomography (OCT) at video rate. In this paper, we propose and demonstrate a highly coherent frequency comb swept laser by using a high-quality (high-Q) microring comb filter to discretize a Fourier-domain mode-locked (FDML) laser. The microring filter has a Q factor of ~2×106 and a linewidth of ~90 MHz. To demonstrate the improvement in performance, a Fabry–Pérot comb filter with a Q factor of 6.2×104 and a linewidth of 3.1 GHz is also used in the experiment for comparison. Both comb filters have free spectral ranges (FSRs) of ~50 GHz for consistence. Stable and clearly discretized temporal waveforms and frequency comb spectra with 50 GHz FSR are observed. Adoption of the high-Q microring filter narrows the instantaneous linewidth of the FDML laser down to 1.5 GHz. The OCT system with the frequency comb swept laser source with a microring filter demonstrates an ultralong imaging range, which has a 6, 10, and 15 dB sensitivity roll-off length of ~53, ~73, and over 100 mm, respectively.

Photonics Research
May. 19, 2020, Vol. 8 Issue 6 06000904 (2020)
Real-time dynamics of soliton triplets in fiber lasers
Yiyang Luo, Ran Xia, Perry Ping Shum, Wenjun Ni, Yusong Liu, Huy Quoc Lam, Qizhen Sun, Xiahui Tang, and Luming Zhao

The evolution of soliton molecules emphasizes the complex soliton dynamics akin to matter molecules. Beyond the simplest soliton molecule—a soliton pair constituted by two bound pulses—soliton molecules with more constituents have more degrees of freedom because of the temporal pulse separations and relative phases. Here we detailedly characterize the transient dynamics of soliton triplets in fiber lasers by using the dispersive Fourier transform measurement. A particular form of leading, central, and tailing pulses is constructed to shed new light on more intriguing scenarios and fuel the molecular analogy. Especially the vibrating dynamics of the central and tailing pulses are captured near the regime of equally spaced soliton triplets, which is reminiscent of the recurrent timing jitters within multi-pulse structures. Further insights enable access into a universal form of unequally spaced soliton triplets interpreted as 2+1 soliton molecules. Different binding strengths of intramolecular and intermolecular bonds are validated with respect to the diverse internal motions involved in this soliton triplet molecule. All these findings unveil the transient dynamics with more degrees of freedom as well as highlight the possible application for all-optical bit storage.

Photonics Research
May. 14, 2020, Vol. 8 Issue 6 06000884 (2020)
Build-up dynamics in bidirectional soliton fiber lasers
Igor Kudelin, Srikanth Sugavanam, and Maria Chernysheva

Bidirectional ultrafast fiber lasers present an attractive solution, enabling the generation of two mutually coherent ultrashort pulse trains in a simple and turnkey system. Still, the lack of a comprehensive numerical model describing steady-state bidirectional generation, and even less ultrafast soliton breakdowns and collisions, is obstructing the achievement of the performance compared with unidirectional lasers. In this paper, we have experimentally investigated real-time build-up dynamics of counter-propagating solitons in an ultrafast ring Er-doped fiber laser via the dispersive Fourier transform methodology. We parade that counter-propagating pulses experience independent build-up dynamics from modulation instability, undergoing breathing dynamics and diverging subordinate pulse structure formation and annihilation to a stable bidirectional pulse train. Yet, the interaction of pulses in the cavity presents the key underlying phenomenon driving formation evolution distinct from unidirectional pulse build-up. Our findings will provide physical foundations for bidirectional ultrafast fiber laser design to carry forward their application.

Photonics Research
May. 06, 2020, Vol. 8 Issue 6 06000776 (2020)
Deep-learning-assisted, two-stage phase control method for high-power mode-programmable orbital angular momentum beam generation
Tianyue Hou, Yi An, Qi Chang, Pengfei Ma, Jun Li, Liangjin Huang, Dong Zhi, Jian Wu, Rongtao Su, Yanxing Ma, and Pu Zhou

High-power mode-programmable orbital angular momentum (OAM) beams have received substantial attention in recent years. They are widely used in optical communication, nonlinear frequency conversion, and laser processing. To overcome the power limitation of a single beam, coherent beam combining (CBC) of laser arrays is used. However, in specific CBC systems used to generate structured light with a complex wavefront, eliminating phase noise and realizing flexible phase modulation proved to be difficult challenges. In this paper, we propose and demonstrate a two-stage phase control method that can generate OAM beams with different topological charges from a CBC system. During the phase control process, the phase errors are preliminarily compensated by a deep-learning (DL) network, and further eliminated by an optimization algorithm. Moreover, by modulating the expected relative phase vector and cost function, all-electronic flexible programmable switching of the OAM mode is realized. Results indicate that the proposed method combines the characteristics of DL for undesired convergent phase avoidance and the advantages of the optimization algorithm for accuracy improvement, thereby ensuring the high mode purity of the generated OAM beams. This work could provide a valuable reference for future implementation of high-power, fast switchable structured light generation and manipulation.

Photonics Research
Apr. 24, 2020, Vol. 8 Issue 5 05000715 (2020)
Narrow-linewidth thermally tuned multi-channel interference widely tunable semiconductor laser with thermal tuning power below 50 mW
Quanan Chen, Chun Jiang, Kuankuan Wang, Miao Zhang, Xiang Ma, Ye Liu, Qiaoyin Lu, and Weihua Guo

A thermally tuned multi-channel interference widely tunable semiconductor laser is designed and demonstrated, for the first time to our knowledge, that realizes a tuning range of more than 45 nm, side-mode suppression ratios up to 56 dB, and Lorentzian linewidth below 160 kHz. AlGaInAs multiple quantum wells (MQWs) were used to reduce linewidth, which have a lower linewidth enhancement factor compared with InGaAsP MQWs. To decrease the power consumption of micro-heaters, air gaps were fabricated below the arm phase sections. For a 75 μm long suspended thermal tuning waveguide, about 6.3 mW micro-heater tuning power is needed for a 2π round-trip phase change. Total micro-heater tuning power required is less than 50 mW across the whole tuning range, which is lower than that of the reported thermally tuned tunable semiconductor lasers.

Photonics Research
Apr. 23, 2020, Vol. 8 Issue 5 05000671 (2020)
Low threshold random lasing in dye-doped and strongly disordered chiral liquid crystals
Shaohua Gao, Jiayi Wang, Wenhua Li, Xuanyi Yu, Xinzheng Zhang, Xiao Song, Andrey Iljin, Irena Drevensek-Olenik, Romano A. Rupp, and Jingjun Xu

Random lasing was experimentally investigated in pyrromethene 597-doped strongly disordered chiral liquid crystals (CLCs) composed of the nematic liquid crystal SLC1717 and the chiral agent CB15. The concentration of the chiral agent tuned the bandgap, and disordered CLC microdomains were achieved by fast quenching of the mixture from the isotropic to the cholesteric phase. Random lasing and band edge lasing were observed synchronously, and their behavior changed with the spectral location of the bandgap. The emission band for band edge lasing shifted with the change of the bandgap, while the emission band for random lasing remained practically constant. The results show that the threshold for random lasing sharply decreases if the CLC selective reflection band overlaps with the fluorescence peak of the dye molecules and if the band edge coincides at the same time with the excitation wavelength.

Photonics Research
Apr. 20, 2020, Vol. 8 Issue 5 05000642 (2020)
Laser-induced rotary micromotor with high energy conversion efficiency
Yu Zhang, Siyu Lin, Zhihai Liu, Yaxun Zhang, Jianzhong Zhang, Jun Yang, and Libo Yuan

Light is a precious resource that nature has given to human beings. Converting green, recyclable light energy into the mechanical energy of a micromotor is undoubtedly an exciting challenge. However, the performance of current light-induced micromotor devices is unsatisfactory, as the light-to-work conversion efficiency is only 10?15–10?12. In this paper, we propose and demonstrate a laser-induced rotary micromotor operated by Δα-type photopheresis in pure liquid glycerol, whose energy conversion ratio reaches as high as 10?9, which is 3–6 orders of magnitude higher than that of previous light-induced micromotor devices. In addition, we operate the micromotor neither with a light field carrying angular momentum nor with a rotor with a special rotating symmetrical shape. We just employ an annular-core fiber to configure a conical-shaped light field and select a piece of graphite sheet (with an irregular shape) as the micro-rotor. The Δα-type photophoretic force introduced by the conical-shaped light field drives the rotation of the graphite sheet. We achieve a rotation rate up to 818.2 r/min, which can be controlled by tuning the incident laser power. This optical rotary micromotor is available for twisting macromolecules or generating vortex and shear force in a medium at the nanoscale.

Photonics Research
Mar. 25, 2020, Vol. 8 Issue 4 04000534 (2020)
Passively Q-switched and femtosecond mode-locked erbium-doped fiber laser based on a 2D palladium disulfide (PdS2) saturable absorber
Ping Kwong Cheng, Chun Yin Tang, Xin Yu Wang, Long-Hui Zeng, and Yuen Hong Tsang

Stable Q-switched and mode-locked erbium-doped fiber lasers (EDFLs) are first demonstrated by using the novel layered palladium disulfide (PdS2), a new member of group 10 transition metal dichalcogenides (TMDs)-based saturable absorbers (SAs). Self-started Q-switched operation at 1567 nm was achieved with a threshold pump power of 50.6 mW. The modulation ranges of pulse duration and repetition rate were characterized as 12.6–4.5 μs and 17.2–26.0 kHz, respectively. Meanwhile, a mode-locked EDFL was also obtained with a pump power threshold of 106.4 mW. The achieved pulse duration is 803 fs, corresponding to a center wavelength of 1565.8 nm and 4.48 nm 3 dB bandwidth. To the best of our knowledge, the achieved pulse duration of the mode-locked EDFL in this work is the narrowest compared with all other group 10 TMD SA-based lasers.

Photonics Research
Mar. 24, 2020, Vol. 8 Issue 4 04000511 (2020)
Femtosecond mid-IR optical vortex laser based on optical parametric chirped pulse amplification
Junyu Qian, Yujie Peng, Yanyan Li, Pengfei Wang, Beijie Shao, Zhe Liu, Yuxin Leng, and Ruxin Li

A femtosecond mid-infrared optical vortex laser can be used for high harmonic generation to extend cutoff energy to the kilo-electron-volt range with orbital angular momentum, as well as other secondary radiations. For these, we demonstrate a high-energy femtosecond 4 μm optical vortex laser based on optical parametric chirped pulse amplification (OPCPA) for the first time. The optical vortex seed is generated from a femtosecond 4 μm laser by a silicon spiral phase plate with the topological charge l of 1 before the stretcher. Through using a two-stage collinear OPCPA amplifier, the chirped vortex pulse is amplified to 12.4 mJ with 200 nm full width at half-maximum bandwidth. After compression, the vortex laser pulse with 9.53 mJ, 119 fs can be obtained. Furthermore, the vortex characteristics of the laser beam are investigated and evaluated. This demonstration can scale to generate a higher-peak-power vortex mid-IR laser and pave a new way for high field physics.

Photonics Research
Editors' PickFeb. 28, 2020, Vol. 8 Issue 3 03000421 (2020)
High-efficiency and high-power single-frequency fiber laser at 1.6 μm based on cascaded energy-transfer pumping
Xianchao Guan, Qilai Zhao, Wei Lin, Tianyi Tan, Changsheng Yang, Pengfei Ma, Zhongmin Yang, and Shanhui Xu

In this paper, a technique combining cascaded energy-transfer pumping (CEP) method and master-oscillator power-amplifier (MOPA) configuration is proposed for power scaling of 1.6-μm-band single-frequency fiber lasers (SFFLs), where the Er3+ ion has a limited gain. The CEP technique is fulfilled by coupling a primary signal light at 1.6 μm and a C-band auxiliary laser. The numerical model of the fiber amplifier with the CEP technique reveals that the energy transfer process involves the pump competition and the in-band particle transition between the signal and auxiliary lights. Moreover, for the signal emission, the population density in the upper level is enhanced, and the effective population inversion is achieved thanks to the CEP. A single-frequency MOPA laser at 1603 nm with an output power of 52.6 W and an improved slope efficiency of 30.4% is experimentally obtained through the CEP technique. Besides, a laser linewidth of 5.2 kHz and a signal-to-auxiliary laser ratio of 60.7 dB are obtained at the maximum output power. The proposed technique is anticipated to be promising for increasing the slope efficiency and power scaling for fiber lasers operating at L band.

Photonics Research
Feb. 28, 2020, Vol. 8 Issue 3 03000414 (2020)
High-power hybrid GaN-based green laser diodes with ITO cladding layer
Lei Hu, Xiaoyu Ren, Jianping Liu, Aiqin Tian, Lingrong Jiang, Siyi Huang, Wei Zhou, Liqun Zhang, and Hui Yang

Green laser diodes (LDs) still perform worst among the visible and near-infrared spectrum range, which is called the “green gap.” Poor performance of green LDs is mainly related to the p-type AlGaN cladding layer, which on one hand imposes large thermal budget on InGaN quantum wells (QWs) during epitaxial growth, and on the other hand has poor electrical property especially when low growth temperature has to be used. We demonstrate in this work that a hybrid LD structure with an indium tin oxide (ITO) p-cladding layer can achieve threshold current density as low as 1.6 kA/cm2, which is only one third of that of the conventional LD structure. The improvement is attributed to two benefits that are enabled by the ITO cladding layer. One is the reduced thermal budget imposed on QWs by reducing p-AlGaN layer thickness, and the other is the increasing hole concentration since a low Al content p-AlGaN cladding layer can be used in hybrid LD structures. Moreover, the slope efficiency is increased by 25% and the operation voltage is reduced by 0.6 V for hybrid green LDs. As a result, a 400 mW high-power green LD has been obtained. These results indicate that a hybrid LD structure can pave the way toward high-performance green LDs.

Photonics Research
Feb. 12, 2020, Vol. 8 Issue 3 03000279 (2020)
Enhancement of femtosecond laser-induced surface ablation via temporal overlapping double-pulse irradiation
Zhenyuan Lin, Lingfei Ji, and Minghui Hong

This paper reports the physical phenomenon of the temporal overlapping double femtosecond laser-induced ablation enhancement at different time delays. Detailed thermodynamic modeling demonstrates the ablation enhancement is highly dependent on the first pulse’s laser fluence. In the case of the first pulse laser fluence being higher than material’s ablation threshold, the ablation enhancement is attributed to optical absorption modification by the first pulse ablation. While the first pulse’s laser fluence is lower than the material’s ablation threshold, the first pulse-induced melting leads to much higher absorption of the second pulse. However, for the case of the first pulse’s laser fluence even lower than melting threshold, the ablation enhancement decreases obviously with time delay. The results of the temporal overlapping double femtosecond laser ablation of poly(ε-caprolactone) are in good agreement with the theoretical predictions.

Photonics Research
Feb. 12, 2020, Vol. 8 Issue 3 03000271 (2020)
“Periodic” soliton explosions in a dual-wavelength mode-locked Yb-doped fiber laser
Meng Liu, Ti-Jian Li, Ai-Ping Luo, Wen-Cheng Xu, and Zhi-Chao Luo

We report the “periodic” soliton explosions induced by intracavity soliton collisions in a dual-wavelength mode-locked Yb-doped fiber laser. Owing to the different group velocities of the two wavelengths, the mode-locked solitons centered at different wavelengths would periodically collide with each other. By using the dispersive Fourier transformation technique, it was found that each collision would induce soliton explosions, but none of them would be identical. Therefore, this phenomenon was termed as “periodic” soliton explosions. In addition, the dissipative rogue waves were detected in the dual-wavelength mode-locked state. The experimental results would be fruitful to the communities interested in soliton dynamics and dual-comb lasers.

Photonics Research
Feb. 06, 2020, Vol. 8 Issue 3 03000246 (2020)
High-energy all-fiber gain-switched thulium-doped fiber laser for volumetric photoacoustic imaging of lipids
Can Li, Jiawei Shi, Xiatian Wang, Boquan Wang, Xiaojing Gong, Liang Song, and Kenneth K. Y. Wong

We demonstrate a high-energy all-fiber short wavelength gain-switched thulium-doped fiber laser for volumetric photoacoustic (PA) imaging of lipids. The laser cavity is constructed by embedding a short piece of gain fiber between a pair of fiber Bragg gratings (FBGs). Through using three pairs of FBGs with operation wavelengths at 1700, 1725, and 1750 nm, three similar lasers are realized with a cavity length of around 25 cm. Under a maximum pump energy of 300 μJ at 1560 nm, laser pulse energies of 58.2, 66.8, and 75.3 μJ are, respectively, achieved with a minimum pulse width of 16.7 ns at a repetition rate of 10 kHz. Volumetric imaging of lipids is validated through scanning a fat beef slice with a PA microscopy system incorporated with the newly developed source, and a lateral resolution of 18.8 μm and an axial resolution of 172.9 μm are achieved. Moreover, the higher shooting speed of the developed source can potentially allow for increasing at twice the frame rate of current intravascular PA imaging.

Photonics Research
Jan. 27, 2020, Vol. 8 Issue 2 02000160 (2020)
First experimental demonstration of coherent beam combining of more than 100 beams
Hongxiang Chang, Qi Chang, Jiachao Xi, Tianyue Hou, Rongtao Su, Pengfei Ma, Jian Wu, Can Li, Man Jiang, Yanxing Ma, and Pu Zhou

Coherent beam combining of 107 beams has been demonstrated for the first time to the best of our knowledge. When the system was in closed loop, the pattern in far-field was stable and the fringe contrast was >96%. The impact of the dynamic tilt error, the piston error, and power inconsistency was theoretically analyzed. Meanwhile, the distribution law of dynamic tilt error was estimated and the correlation of the tilt dithering of different axis was analyzed statistically. The ratio of power in the central lobe was ~22.5%. The phase residue error in the closed loop was ~λ/22, which was evaluated by the root-mean-square error of the signal generated from the photoelectric detector.

Photonics Research
Nov. 30, 2020, Vol. 8 Issue 12 12001943 (2020)
Quantum dot mode-locked frequency comb with ultra-stable 25.5 GHz spacing between 20°C and 120°C
Shujie Pan, Jianou Huang, Zichuan Zhou, Zhixin Liu, Lalitha Ponnampalam, Zizhuo Liu, Mingchu Tang, Mu-Chieh Lo, Zizheng Cao, Kenichi Nishi, Keizo Takemasa, Mitsuru Sugawara, Richard Penty, Ian White, Alwyn Seeds, Huiyun Liu, and Siming Chen

Semiconductor mode-locked lasers (MLLs) are promising frequency comb sources for dense wavelength-division-multiplexing (DWDM) data communications. Practical data communication requires a frequency-stable comb source in a temperature-varying environment and a minimum tone spacing of 25 GHz to support high-speed DWDM transmissions. To the best of our knowledge, however, to date, there have been no demonstrations of comb sources that simultaneously offer a high repetition rate and stable mode spacing over an ultrawide temperature range. Here, we report a frequency comb source based on a quantum dot (QD) MLL that generates a frequency comb with stable mode spacing over an ultrabroad temperature range of 20–120°C. The two-section passively mode-locked InAs QD MLL comb source produces an ultra-stable fundamental repetition rate of 25.5 GHz (corresponding to a 25.5 GHz spacing between adjacent tones in the frequency domain) with a variation of 0.07 GHz in the tone spacing over the tested temperature range. By keeping the saturable absorber reversely biased at -2 V, stable mode-locking over the whole temperature range can be achieved by tuning the current of the gain section only, providing easy control of the device. At an elevated temperature of 100°C, the device shows a 6 dB comb bandwidth of 4.81 nm and 31 tones with >36 dB optical signal-to-noise ratio. The corresponding relative intensity noise, averaged between 0.5 GHz and 10 GHz, is -146 dBc/Hz. Our results show the viability of the InAs QD MLLs as ultra-stable, uncooled frequency comb sources for low-cost, large-bandwidth, and low-energy-consumption optical data communications.

Photonics Research
Spotlight on OpticsNov. 30, 2020, Vol. 8 Issue 12 12001937 (2020)
Generation of multi-channel chaotic signals with time delay signature concealment and ultrafast photonic decision making based on a globally-coupled semiconductor laser network
Yanan Han, Shuiying Xiang, Yang Wang, Yuanting Ma, Bo Wang, Aijun Wen, and Yue Hao

We propose and demonstrate experimentally and numerically a network of three globally coupled semiconductor lasers (SLs) that generate triple-channel chaotic signals with time delayed signature (TDS) concealment. The effects of the coupling strength and bias current on the concealment of the TDS are investigated. The generated chaotic signals are further applied to reinforcement learning, and a parallel scheme is proposed to solve the multiarmed bandit (MAB) problem. The influences of mutual correlation between signals from different channels, the sampling interval of signals, and the TDS concealment on the performance of decision making are analyzed. Comparisons between the proposed scheme and two existing schemes show that, with a simplified algorithm, the proposed scheme can perform as well as the previous schemes or even better. Moreover, we also consider the robustness of decision making performance against a dynamically changing environment and verify the scalability for MAB problems with different sizes. This proposed globally coupled SL network for a multi-channel chaotic source is simple in structure and easy to implement. The attempt to solve the MAB problem in parallel can provide potential values in the realm of the application of ultrafast photonics intelligence.

Photonics Research
Oct. 30, 2020, Vol. 8 Issue 11 11001792 (2020)
Fast and wide-band tuning single-mode microlaser based on fiber Fabry–Pérot microcavities
Xin-Xia Gao, Jin-Ming Cui, Zhi-Hao Hu, Chun-Hua Dong, Jian Wang, Yun-Feng Huang, Chuan-Feng Li, and Guang-Can Guo

A narrow-linewidth laser operating at the telecommunications band combined with both fast and wide-band tuning features will have promising applications. Here we demonstrate a single-mode (both transverse and longitudinal mode) continuous microlaser around 1535 nm based on a fiber Fabry–Pérot microcavity, which achieves wide-band tuning without mode hopping to the 1.3 THz range and fast tuning rate to 60 kHz and yields a frequency scan rate of 1.6×1017 Hz/s. Moreover, the linewidth of the laser is measured as narrow as 3.1 MHz. As the microlaser combines all these features into one fiber component, it can serve as the seed laser for versatile applications in optical communication, sensing, frequency-modulated continuous-wave radar, and high-resolution imaging.

Photonics Research
Sep. 29, 2020, Vol. 8 Issue 10 10001642 (2020)
Extraction of internal phase motions in femtosecond soliton molecules using an orbital-angular-momentum-resolved method
Yuwei Zhao, Jintao Fan, Youjian Song, Uwe Morgner, and Minglie Hu

Internal motions in femtosecond soliton molecules provide insight into universal collective dynamics in various nonlinear systems. Here we introduce an orbital-angular-momentum (OAM)-resolved method that maps the relative phase motion within a femtosecond soliton molecule into the rotational movement of the interferometric beam profile of two optical vortices. By this means, long-term relative phase evolutions of doublet and triplet soliton molecules generated in an all-polarization-maintaining mode-locked Er-fiber laser are revealed. This simple and practical OAM-resolved method represents a promising way to directly visualize the complex phase dynamics in a diversity of multisoliton structures.

Photonics Research
Editors' PickSep. 18, 2020, Vol. 8 Issue 10 10001580 (2020)
Breathing dissipative soliton explosions in a bidirectional ultrafast fiber laser
Yi Zhou, Yu-Xuan Ren, Jiawei Shi, and Kenneth K. Y. Wong

Soliton explosions, among the most exotic dynamics, have been extensively studied on parameter invariant stationary solitons. However, the explosion dynamics are still largely unexplored in breathing dissipative solitons as a dynamic solution to many nonlinear systems. Here, we report on the first observation of a breathing dissipative soliton explosion in a net-normal-dispersion bidirectional ultrafast fiber laser. The breathing soliton explosions could be stimulated by the soliton buildup process or alteration of polarization settings. Transient breathing soliton pairs with intensive repulsion that is sensitive to initial conditions can also be triggered by multiple soliton explosions in the soliton buildup process instead of being triggered by varying polarization settings. The high behavior similarity also exists in the breathing soliton buildup and explosion process owing to the common gain and loss modulation. In addition, dissipative rogue waves were detected in the breathing soliton explosion, and the collision of breathing soliton significantly enhanced the amplitude of rogue waves, which is characteristic of the breathing solitons in a bidirectional fiber laser. These results shed new insights into complex dissipative soliton dynamics.

Photonics Research
Sep. 15, 2020, Vol. 8 Issue 10 10001566 (2020)
Fe3O4 nanoparticles as a saturable absorber for a tunable Q-switched dysprosium laser around 3 μm
Jian Yang, Jiyi Hu, Hongyu Luo, Jianfeng Li, Jishu Liu, Xiaohui Li, and Yong Liu

We demonstrate for the first time to our knowledge the use of Fe3O4 nanoparticles for Q-switching a tunable mid-infrared (Mid-IR) Dy3+-doped ZBLAN fiber laser around 3 μm. The Q-switcher was fabricated by depositing the prepared Fe3O4 nanoparticles solution onto an Au mirror. Its nonlinear optical response was characterized using a mode locked Ho3+/Pr3+-codoped ZBLAN fiber laser at 2.87 μm, and showed a modulation depth of 11.9% as well as a saturation intensity of 1.44 MW/cm2. Inserting the device into a tunable Dy3+-doped ZBLAN fiber laser, stable Q-switched pulses within the tunable range of 2812.4–3031.6 nm were obtained. When tuning the wavelength to 2931.2 nm, a maximum Q-switching output power of 111.0 mW was achieved with a repetition rate of 123.0 kHz and a pulse width of 1.25 μs. The corresponding pulse energy was 0.90 μJ. This demonstration suggests that Fe3O4 nanoparticles are a promising broadband saturable absorption material for mid-infrared operation.

Photonics Research
Dec. 24, 2019, Vol. 8 Issue 1 01000070 (2020)
Optical rogue wave in random fiber laser
Jiangming Xu, Jian Wu, Jun Ye, Jiaxin Song, Baicheng Yao, Hanwei Zhang, Jinyong Leng, Weili Zhang, Pu Zhou, and Yunjiang Rao

The famous demonstration of optical rogue waves (RWs), a powerful tool to reveal the fundamental physics in different laser scenarios, opened a flourishing time for temporal statistics. Random fiber laser (RFL) has likewise attracted wide attention due to its great potential in multidisciplinary demonstrations and promising applications. However, owing to the distinctive cavity-free structure, it is a scientific challenge to achieve temporal localized RWs in RFLs, whose feedback arises from multiple scattering in disordered medium. Here, we report the exploration of RW in the highly skewed, transient intensity of an incoherently pumped RFL for the first time, to our knowledge, and unfold the involved kinetics successfully. The corresponding frequency domain measurements demonstrate that the RW event arises from a crucial sustained stimulated Brillouin scattering process with intrinsic stochastic nature. This investigation highlights a novel path to fully understanding the complex physics, such as photon propagation and localization, in disordered media.

Photonics Research
Dec. 06, 2019, Vol. 8 Issue 1 01000001 (2020)
Widely tunable passively Q-switched Er3+-doped ZrF4 fiber laser in the range of 3.4–3.7 μm based on a Fe2+:ZnSe crystal
Hongyu Luo, Jian Yang, Jianfeng Li, and Yong Liu

We report the first (to the best of our knowledge) tunable passively Q-switched Er3+-doped ZrF4 fiber laser around 3.5 μm. In this case, a Fe2+:ZnSe crystal is used as the saturable absorber, and a plane-ruled grating in a Littrow configuration acts as the tuning element. At the tuned wavelength of 3478.0 nm, stable Q-switching with a maximum average power of 583.7 mW was achieved with a slope efficiency of 15.2% relative to the launched 1981 nm pump power. Further power scaling is mainly limited by the available 1981 nm pump power. The corresponding pulse width, repetition rate, and pulse energy are 1.18 μs, 71.43 kHz, and 7.54 μJ, respectively. By rotating the grating, the Q-switching can be continuously tuned in the region of 3.4–3.7 μm. To the best of our knowledge, this is the first pulsed rare-earth-doped fiber laser tunable in the region beyond 3.4 μm.

Photonics Research
Aug. 28, 2019, Vol. 7 Issue 9 09001106 (2019)
Towards an optimum saturable absorber for the multi-gigahertz harmonic mode locking of fiber lasers
Jakub Bogusławski, Grzegorz Soboń, Rafał Zybała, and Jarosław Sotor

Ultra-high-pulse-repetition-rate lasers are essential for a number of applications, including, e.g., optical communication and ablation-cooled material processing. Despite several techniques to generate pulses with gigahertz-range repetition rate, incorporating mainly short-length resonators, more widespread applications are still limited by the lack of a robust, simple, and cost-effective solution. Here, we report for the first time, to the best of our knowledge, fully passive harmonic mode locking in an all-polarization-maintaining (PM) fiber laser. The design guarantees a fixed polarization state and stable operation, where the cavity harmonic number is controlled by the pump power only. Self-starting operation is provided by the antimony telluride (Sb2Te3) thin-film saturable absorber (SA), which facilitates multiple pulse operation. The SA acts by means of low modulation depth, low saturation fluence, and an inverse slope in the saturable absorption curve. The optimum features of the SA and limiting factors for high-repetition-rate pulse generation in this regime of operation are discussed. As a result, 2.2 ps pulses with 3 GHz repetition rate are generated at 1560 nm wavelength. The study reports a new approach towards an optimal SA for multi-gigahertz pulse generation in practical, all-PM fiber lasers.

Photonics Research
Aug. 28, 2019, Vol. 7 Issue 9 09001094 (2019)
Room temperature III–V nanolasers with distributed Bragg reflectors epitaxially grown on (001) silicon-on-insulators
Yu Han, Wai Kit Ng, Ying Xue, Kam Sing Wong, and Kei May Lau

Efficient, scalable, bufferless, and compact III–V lasers directly grown on (001)-oriented silicon-on-insulators (SOIs) are preferred light sources in Si-photonics. In this article, we present the design and operation of III–V telecom nanolaser arrays with integrated distributed Bragg reflectors (DBRs) epitaxially grown on industry-standard (001) SOI wafers. We simulated the mirror reflectance of different guided modes under various mirror architectures, and accordingly devised nanoscale DBR gratings to support high reflectivity around 1500 nm for the doughnut-shaped TE01 mode. Building from InP/InGaAs nanoridges grown on SOI, we fabricated subwavelength DBR mirrors at both ends of the nanoridge laser cavities and thus demonstrated room-temperature low-threshold InP/InGaAs nanolasers with a 0.28 μm2 cross-section and a 20 μm effective cavity length. The direct growth of these bufferless nanoscale III–V light emitters on Si-photonics standard (001) SOI wafers opens future options of fully integrated Si-based nanophotonic integrated circuits in the telecom wavelength regime.

Photonics Research
Aug. 27, 2019, Vol. 7 Issue 9 09001081 (2019)
30-W supercontinuum generation based on ZBLAN fiber in an all-fiber configuration
Linyong Yang, Ying Li, Bin Zhang, Tianyi Wu, Yijun Zhao, and Jing Hou

We report an all-fiberized 30-W supercontinuum (SC) generation in a piece of ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber. The pump source is a thulium-doped fiber amplifier (TDFA) with broadband output spectrum spanning the 1.9 to ~2.6 μm region. The used ZBLAN fiber has a core diameter of 10 μm, and was directly fusion-spliced to the pigtail of the TDFA without using a traditional mode field adapter (MFA) or a piece of transition fiber. Such a low-loss and robust fusion splice joint, together with a robust AlF3-fiber-based endcap, enables efficient and high-power SC generation in the ZBLAN fiber. An SC with an average power up to 30.0 W and a spectral coverage of 1.9–3.35 μm with 20-dB bandwidth of 1.92–3.20 μm was obtained. Moreover, an SC with a broader spectrum was achieved by raising the pump pulse peak power (via reducing the duty ratio of the pump laser pulse). An SC with an output power of 27.4 W and a spectral coverage of 1.9–3.63 μm (with 20-dB bandwidth of 1.92–3.47 μm) was obtained, as well as an SC with output power of 24.8 W and a spectral coverage of 1.9–3.70 μm (with 20-dB bandwidth of 1.93–3.56 μm). The power conversion efficiency was measured as >69%. To the best of the authors’ knowledge, this research demonstrates the record output power of SC lasers based on ZBLAN fibers, paving the way for broadband and efficient multi-tens-of-watts SC generation in soft-glass fibers.

Photonics Research
Aug. 23, 2019, Vol. 7 Issue 9 09001061 (2019)
Investigation of nonlinear optical properties of rhenium diselenide and its application as a femtosecond mode-locker
Jinho Lee, Kyungtaek Lee, Suhyoung Kwon, Bumsoo Shin, and Ju Han Lee

We investigated the nonlinear optical properties of ReSe2. First, we measured the nonlinear absorption coefficient and the nonlinear refractive index of a ReSe2 thin film using open-aperture (OA) and closed-aperture (CA) Z-scan techniques. ReSe2 was shown to possess both saturable absorption and self-defocusing properties. The nonlinear absorption coefficient of ReSe2 was measured to be (5.67±0.35)×103 cm/GW, and its nonlinear refractive index was (2.81±0.13)×10 2 cm2/GW at 1560 nm. Next, a fiberized saturable absorber (SA) based on ReSe2 was fabricated with a side-polished fiber platform and was tested as an ultrafast mode-locker capable of producing femtosecond pulses operating at a wavelength of 1560 nm. The estimated modulation depth and saturation power are ~3.9% and ~42 W, respectively, for the transverse electric mode, while they are ~2.4% and ~53 W for the transverse magnetic mode. Using the prepared SA, stable soliton pulses with a temporal width of ~862 fs were produced from an erbium-doped fiber ring cavity. To the best of the authors’ knowledge, this is the first demonstration of using a ReSe2-based SA for femtosecond mode-locked pulse generation.

Photonics Research
Aug. 09, 2019, Vol. 7 Issue 9 09000984 (2019)
Pump scheme optimization of an incoherently pumped high-power random fiber laser
Jun Ye, Jiangming Xu, Jiaxin Song, Yang Zhang, Hanwei Zhang, Hu Xiao, Jinyong Leng, and Pu Zhou

Optical signal-to-noise ratio (OSNR) is one of the most significant parameters for the performance characterization of random fiber lasers (RFLs) and their application potentiality in sensing and telecommunication. An effective way to improve the OSNR of RFLs is pump scheme optimization, for example, employing a temporally stable source as the pump. In this paper, the output performance of an incoherently pumped RFL dependence on the pump bandwidth has been investigated both in experiment and theory. It is found that a high-OSNR RFL can be achieved with broadband amplified spontaneous emission (ASE) source pumping, and a relatively broad pump bandwidth can also help suppress the spectral broadening while maintaining an ultra-high spectral purity. By optimizing the pump bandwidth to ~10 nm, maximum OSNR of ~39 dB (corresponding to a spectral purity of ~99.96%) with more than 99 W output power can be obtained. Moreover, for the pump bandwidth of 0.6–40 nm, the spectral purity can reach as high as >99% with the pump power ranging from ~85 to ~117 W. In addition, with the aid of theoretical simulation based on a modified power balance model, we find that the increment of pump bandwidth can decrease the effective Raman gain coefficient, further influencing the gain characteristics, nonlinear effects, and eventually the output performance. This work provides new insight into the influence of the pump characteristics on the output performance of incoherently pumped RFLs.

Photonics Research
Aug. 07, 2019, Vol. 7 Issue 9 09000977 (2019)
Generation of wavelength-tunable and coherent dual-wavelength solitons in the C + L band by controlling the intracavity loss
Tianyu Zhu, Zhaokun Wang, D. N. Wang, Fan Yang, and Liujiang Li

A wavelength-tunable and dual-wavelength mode-locking operation is achieved in an Er-doped fiber laser using a hybrid no-core fiber graded index multimode fiber as the saturable absorber. In the tuning operation, continuously wavelength-tunable pulses with a tuning range of 46.7 nm, stable 3-dB bandwidth of around 5 nm, and pulse duration of ~850 fs are obtained by increasing the intracavity loss of a variable optical attenuator. In the dual-wavelength operation, the two solitons at different wavelengths demonstrate the characteristics of mutual coherence. By increasing the intracavity loss, the spectral spacing can be tuned from 11 to 33.01 nm while maintaining the coherence of the solitons. Such coherent solitons have high potential for applications in dual-comb frequency and multicolor pulses in nonlinear microscopy.

Photonics Research
Jul. 18, 2019, Vol. 7 Issue 8 08000853 (2019)
Graphene-based saturable absorber and mode-locked laser behaviors under gamma-ray radiation
Dohyun Kim, Nam Hun Park, Hyunju Lee, Jaegoan Lee, Dong-Il Yeom, and Jungwon Kim

We investigate optical and electrical behaviors of a graphene saturable absorber (SA) and mode-locking performance of a graphene-SA-based mode-locked Er fiber laser in gamma-ray radiation. When irradiated up to 4.8 kGy at ~100 Gy/hr dose rate, the overall nonlinear transmittance in transverse electric mode was increased, while maintaining modulation depth to >10%. The corresponding polarization-dependent loss was reduced at a 1.2-dB/kGy rate. In the electrical properties, the charge carrier mobility was reduced, and the Dirac voltage shift was increased to positive under gamma-ray radiation. The radiation-induced optical and electrical changes turned out to be almost recovered after a few days. In addition, we confirmed that the graphene-SA-based laser showed stable CW mode-locking operation while the inserted graphene SA was irradiated for 2-kGy at a 45-Gy/hr dose rate, which corresponds to >40 years of operation in low Earth orbit satellites. To the best of our knowledge, this is the first evaluation of graphene SAs and graphene-SA-based mode-locked lasers in gamma-ray radiation, and the measured results confirm the high potential of graphene SAs and graphene-SA-based lasers in various outer-space environments as well as other radiation environments, including particle accelerators and radiation-based medical instruments.

Photonics Research
Jun. 07, 2019, Vol. 7 Issue 7 07000742 (2019)
GaN-based ultraviolet microdisk laser diode grown on Si
Jin Wang, Meixin Feng, Rui Zhou, Qian Sun, Jianxun Liu, Yingnan Huang, Yu Zhou, Hongwei Gao, Xinhe Zheng, Masao Ikeda, and Hui Yang

This work reports a demonstration of electrically injected GaN-based near-ultraviolet microdisk laser diodes with a lasing wavelength of 386.3?nm at room temperature. The crack-free laser structure was epitaxially grown on Si substrates using an Al-composed down-graded AlN/AlGaN multilayer buffer to mitigate the mismatches in the lattice constant and coefficient of thermal expansion, and processed into “sandwich-like” microdisk structures with a radius of 12?μm. Air-bridge electrodes were successfully fabricated to enable the device electrical characterization. The electrically pumped lasing of the as-fabricated microdisk laser diodes was evidenced by the rapid narrowing down of electroluminescence spectra and dramatic increase in the light output power, as the current exceeded the threshold of 248?mA.

Photonics Research
May. 20, 2019, Vol. 7 Issue 6 06000B32 (2019)
Fiber laser with simultaneous multi-wavelength Er/Yb passively Q-switched and single-wavelength Tm gain-switched operations
J. Alaniz-Baylon, M. Durán-Sánchez, R. I. álvarez-Tamayo, B. Posada-Ramírez, M. Bello-Jiménez, B. Ibarra-Escamilla, A. A. Castillo-Guzman, and E. A. Kuzin

We report the experimental investigation of an all-fiber multi-wavelength passively Q-switched Er/Yb laser with simultaneous gain-switched pulsed operation by using a thulium-doped fiber as a saturable absorber. Laser emission is obtained in three wavelength regions with central peaks at around 1546?nm, 1561?nm, and 1862?nm. Multi-wavelength emission with separation of approximately 1?nm is obtained around the wavelength regions of 1546?nm and 1561?nm. Stable laser pulses are generated in the pump power range from 3.6?W to 7.3?W.

Photonics Research
May. 08, 2019, Vol. 7 Issue 6 06000608 (2019)
Whispering-gallery mode hexagonal micro-/nanocavity lasers [Invited]
Yue-De Yang, Min Tang, Fu-Li Wang, Zhi-Xiong Xiao, Jin-Long Xiao, and Yong-Zhen Huang

Whispering-gallery-mode (WGM) hexagonal optical micro-/nanocavities can be utilized as high-quality (Q) resonators for realizing compact-size low-threshold lasers. In this paper, the progress in WGM hexagonal micro-/nanocavity lasers is reviewed comprehensively. High-Q WGMs in hexagonal cavities are divided into two kinds of resonances propagating along hexagonal and triangular periodic orbits, with distinct mode characteristics according to theoretical analyses and numerical simulations; however, WGMs in a wavelength-scale nanocavity cannot be well described by the ray model. Hexagonal micro-/nanocavity lasers can be constructed by both bottom-up and top-down processes, leading to a diversity of these lasers. The ZnO- or nitride-based semiconductor material generally has a wurtzite crystal structure and typically presents a natural hexagonal cross section. Bottom-up growth guarantees smooth surface faceting and hence reduces the scattering loss effectively. Laser emissions have been successfully demonstrated in hexagonal micro-/nanocavities synthesized with various materials and structures. Furthermore, slight deformation can be easily introduced and precisely controlled in top-down fabrication, which allows lasing-mode manipulation. WGM lasing with excellent single-transverse-mode property was realized in waveguide-coupled ideal and deformed hexagonal microcavity lasers.

Photonics Research
Apr. 30, 2019, Vol. 7 Issue 5 05000594 (2019)
Widely tunable single-mode lasers based on a hybrid square/rhombus-rectangular microcavity
You-Zeng Hao, Fu-Li Wang, Min Tang, Hai-Zhong Weng, Yue-De Yang, Jin-Long Xiao, and Yong-Zhen Huang

Hybrid square/rhombus-rectangular lasers (HSRRLs) consisting of a Fabry–Perot (FP) cavity and a square/rhombus microcavity (SRM) are proposed and demonstrated for realizing single-mode lasing with a wide wavelength tuning range. The SRM is a deformed square microcavity with a vertex extended to the FP cavity to control the coupled mode field pattern in the FP cavity. Single-mode operation with a side-mode suppression ratio (SMSR) over 45.3 dB is realized, and a wide wavelength tuning range of 21 nm with SMSR >35 dB is further demonstrated by adjusting the injection currents of the SRM and the FP cavity simultaneously. Furthermore, a 3-dB modulation bandwidth of 14.1 GHz and an open-eye diagram at 35 Gb/s are demonstrated for the HSRRL.

Photonics Research
Apr. 18, 2019, Vol. 7 Issue 5 05000543 (2019)
Soliton and bound-state soliton mode-locked fiber laser based on a MoS2/fluorine mica Langmuir–Blodgett film saturable absorber
Ruidong Lü, Yonggang Wang, Jiang Wang, Wei Ren, Lu Li, Sicong Liu, Zhendong Chen, Yongfang Li, Hongying Wang, and Fuxing Fu

In this article, we report on an experimentally generated soliton and bound-state soliton passively mode-locked erbium-doped fiber laser by incorporating a saturable absorber (SA) made of MoS2/fluorine mica (FM) that was fabricated with the Langmuir–Blodgett (LB) method. The FM substrate is 20 μm thick and easy to bend or cut, like a polymer. However, it has a higher damage threshold and a better thermal dissipation than polymers. In addition, the LB method can be used to fabricate a thin film with good uniformity. In this study, the modulation depth, saturable intensity, and unsaturated loss of the SA are measured as 5.9%, 57.69 MW/cm2, and 13.4%, respectively. Based on the SA, a soliton mode-locked laser is achieved. The pulse duration, repetition rate, and signal-to-noise ratio are 581 fs, 15.67 MHz, and 65 dB, respectively. By adjusting the polarization controller and pump power, we obtain a bound-state soliton mode-locked pulse. The temporal interval between the two solitons forming the bound-state pulse is 2.7 ps. The repetition rate of the bound-state pulses is proportional to the pump power. The maximum repetition rate is 517 MHz, corresponding to the 33rd harmonic of the fundamental repetition rate. The results indicate that the MoS2/FM LB film absorber is a promising photonic device in ultrafast fiber lasers.

Photonics Research
Apr. 11, 2019, Vol. 7 Issue 4 04000431 (2019)
Revelation of the birth and extinction dynamics of solitons in SWNT-mode-locked fiber lasers
Yudong Cui, and Xueming Liu

The dispersive Fourier transform (DFT) technique opens a fascinating pathway to explore ultrafast non-repetitive events and has been employed to study the build-up process of mode-locked lasers. However, the shutting process for the mode-locked fiber laser seems to be beyond the scope of researchers, and the starting dynamics under near-zero dispersion remains unclear. Here, the complete evolution dynamics (from birth to extinction) of the conventional soliton (CS), stretched pulse (SP), and dissipative soliton (DS) are investigated by using the DFT technique. CS, SP, and DS fiber lasers mode locked by single-walled carbon nanotubes (SWNTs) are implemented via engineering the intracavity dispersion map. The relaxation oscillation can always be observed before the formation of stable pulse operation due to the inherent advantage of SWNT, but it exhibits distinct evolution dynamics in the starting and shutting processes. The shutting processes are dependent on the dispersion condition and turn-off time, which is against common sense. Some critical phenomena are also observed, including transient complex spectrum broadening and frequency-shift interaction of SPs and picosecond pulses. These results will further deepen understanding of the mode-locked fiber laser from a real-time point of view and are helpful for laser design and applications.

Photonics Research
Apr. 11, 2019, Vol. 7 Issue 4 04000423 (2019)
Supersymmetric microring laser arrays
Bikashkali Midya, Han Zhao, Xingdu Qiao, Pei Miao, Wiktor Walasik, Zhifeng Zhang, Natalia M. Litchinitser, and Liang Feng

A coherent combination of emission power from an array of coupled semiconductor lasers operating on the same chip is of fundamental and technological importance. In general, the nonlinear competition among the array supermodes can entail incoherence and spectral broadening, leading to a spatiotemporally unstable and multimode emission pattern and thus poor beam quality. Here, by harnessing notions from supersymmetric (SUSY) quantum mechanics, we report that the strategic coupling between a class III-V semiconductor microring laser array with its dissipative superpartner can be used to limit the number of supermodes available for laser actions to one. We introduce a novel approach based on second-order SUSY transformation in order to drastically simplify the superpartner array engineering. Compared to a conventional laser array, which has a multimode spectrum, a SUSY laser array is observed to be capable of operating in a single (transverse) supermode. Enhancement of the peak output intensity of the SUSY laser array has been demonstrated with high efficiency and lower lasing threshold, compared with a single laser and a conventional laser array. Our experimental findings pave the way towards broad-area and high-power light generation in a scalable and stable fashion.

Photonics Research
Mar. 01, 2019, Vol. 7 Issue 3 03000363 (2019)
Tunable and switchable harmonic h-shaped pulse generation in a 3.03  km ultralong mode-locked thulium-doped fiber laser
Junqing Zhao, Lei Li, Luming Zhao, Dingyuan Tang, Deyuan Shen, and Lei Su

We experimentally demonstrated a type of tunable and switchable harmonic h-shaped pulse generation in a thulium-doped fiber (TDF) laser passively mode locked by using an ultralong nonlinear optical loop mirror. The total cavity length was ~3.03 km, the longest ever built for a TDF laser to our best knowledge, which resulted in an ultralarge anomalous dispersion over 200 ps2 around the emission wavelength. The produced h-shaped pulse can operate either in a fundamental or in a high-order harmonic mode-locking (HML) state depending on pump power and intra-cavity polarization state (PS). The pulse duration, no matter of the operation state, was tunable with pump power. However, pulse breaking and self-organizing occurred, resulting in high-order HML, when the pump power increased above a threshold. At a fixed pump power, the order of HML was switchable from one to another by manipulating the PS. Switching from the 8th up to the 48th order of HML was achieved with a fixed pump power of ~4.15 W. Our results revealed the detailed evolution and switching characteristics of the HML and individual pulse envelope with respect to both the pump power and PS. We have also discussed in detail the mechanisms of both the h-shaped pulse generation and the switching of its HML. This contribution would be helpful for further in-depth study on the underlying dynamics of long-duration particular-envelope pulses with ultralarge anomalous dispersion and ultralong roundtrip time.

Photonics Research
Feb. 27, 2019, Vol. 7 Issue 3 03000332 (2019)
Ternary ReS2(1-x)Se2x alloy saturable absorber for passively Q-switched and mode-locked erbium-doped all-fiber lasers
Chenxi Dou, Wen Wen, Junli Wang, Mengyuan Ma, Liming Xie, Ching-Hwa Ho, and Zhiyi Wei

We report Q-switched and mode-locked erbium-doped all-fiber lasers using ternary ReS2(1 x)Se2x as saturable absorbers (SAs). The modulation depth and saturable intensity of the film SA are 1.8% and 0.046 MW/cm2. In Q-switched mechanism output, the pulse was centered at 1531.1 nm with maximum pulse energy and minimum pulse width of 28.29 nJ and 1.07 μs, respectively. In mode-locked operation, the pulse was centered at 1561.15 nm with pulse width of 888 fs, repetition rate of 2.95 MHz, and maximum pulse energy of 0.275 nJ. To the best of our knowledge, this is the first report on the mode-locked Er3+-doped fiber laser using ternary transition metal dichalcogenides. This work suggests prospective 2D-material SAs can be widely used in versatile fields due to their attractive optoelectronic and tunable energy bandgap properties.

Photonics Research
Feb. 15, 2019, Vol. 7 Issue 3 03000283 (2019)
Highly stable femtosecond pulse generation from a MXene Ti3C2Tx (T = F, O, or OH) mode-locked fiber laser
Jie Li, Zilong Zhang, Lin Du, Lili Miao, Jun Yi, Bin Huang, Yanhong Zou, Chujun Zhao, and Shuangchun Wen

Ultrafast fiber lasers are in great demand for various applications, such as optical communication, spectroscopy, biomedical diagnosis, and industrial fabrication. Here, we report the highly stable femtosecond pulse generation from a MXene mode-locked fiber laser. We have prepared the high-quality Ti3C2Tx nanosheets via the etching method, and characterized their ultrafast dynamics and broadband nonlinear optical responses. The obvious intensity- and wavelength-dependent nonlinear responses have been observed and investigated. In addition, a highly stable femtosecond fiber laser with signal-to-noise ratio up to 70.7 dB and central wavelength of 1567.3 nm has been delivered. The study may provide some valuable design guidelines for the development of ultrafast, broadband nonlinear optical modulators, and open new avenues toward advanced photonic devices based on MXenes.

Photonics Research
Feb. 07, 2019, Vol. 7 Issue 3 03000260 (2019)
Ultrabroadband wavelength-swept source based on total mode-locking of an Yb:CaF2 laser
Maciej Kowalczyk, Tadeusz Martynkien, Paweł Mergo, Grzegorz Soboń, and Jarosław Sotor

We present an ultrabroadband, high-speed wavelength-swept source based on a self-modulated femtosecond oscillator. Photonic crystal fiber was pumped by a mode-locked Yb:CaF2 laser, resulting in a strong spectral broadening from 485 to 1800 nm. The pump laser cavity could be realigned in order to achieve total mode-locking of the longitudinal and transverse TEM00 and TEM01 electromagnetic modes. This led to spatial oscillations of the output beam, which induced modulation of the coupling efficiency to the fiber. Due to the fact that nonlinear spectral broadening was intensity dependent, this mechanism introduced wavelength sweeping at the fiber output. The sweeping rate could be adjusted between 7 and 21.5 MHz by changing the geometry of the pump cavity. By controlling the ratio of the transverse mode amplitudes, we were able to tune the sweeping bandwidth, eventually covering both the 1300 nm and 1700 nm bioimaging transparency windows. When compared with previously demonstrated wavelength-swept sources, our concept offers much broader tunability and higher speed. Moreover, it does not require an additional intensity modulator.

Photonics Research
Jan. 24, 2019, Vol. 7 Issue 2 02000182 (2019)
Effective suppression of stimulated Raman scattering in half 10  kW tandem pumping fiber lasers using chirped and tilted fiber Bragg gratings
Meng Wang, Zefeng Wang, Le Liu, Qihao Hu, Hu Xiao, and Xiaojun Xu

The average power of fiber lasers has been scaled deeply into the kW regime in the past years. However, stimulated Raman scattering (SRS) is still a major factor limiting further power scaling. Here, we have demonstrated for the first time, to the best of our knowledge, the suppression of SRS in a half 10 kW tandem pumping fiber amplifier using chirped and tilted fiber Bragg gratings (CTFBGs). With specially self-designed and manufactured CTFBGs inserted between the seed laser and the amplifier stage, a maximum SRS suppression ratio of >15 dB in spectrum is observed with no reduction in laser efficiency. With one CTFBG, the effective output power is improved to 3.9 kW with a beam quality M2 factor of ~1.7 from <3.5 kW with an M2 factor of >2; with two CTFBGs, the effective laser power reaches 4.2 kW with an increasing ratio of 20% and an M2 factor of ~1.8, and further power improvement is limited by the power and performance of the 1018 nm pump sources. This work provides an effective SRS suppression method for high-power all-fiber lasers, which is useful for further power scaling of these systems.

Photonics Research
Jan. 23, 2019, Vol. 7 Issue 2 02000167 (2019)
High power and energy generation in a Nd:YAG single-crystal fiber laser at 1834  nm
Yaqi Cai, Bin Xu, Yunshan Zhang, Qingyu Tian, Xiaodong Xu, Qingsong Song, Dongzhen Li, Jun Xu, and Ivan Buchvarov

We report on a diode-end-pumped high-power and high-energy Nd:YAG single-crystal fiber laser at 1834 nm. Two 808 nm diodes injecting about 58 W pump power into the Nd:YAG fiber have generated 3.28 W continuous-wave and 1.66 W Cr:ZnSe-based passively Q-switched lasers. Slope efficiencies with respect to pump powers are 8.7% for the continuous-wave laser and 4.9% for the Q-switched laser. The extracted maximum pulse energy is about 266.9 μJ, and the corresponding maximum pulse peak power is 2.54 kW. These performances greatly surpass previous results regarding this specific laser emission because the laser gain medium in the form of fiber can significantly mitigate thermally induced power saturation thanks to its significantly reduced thermal lensing effect. Single-crystal fiber lasers show great potential for high average power, pulse energy, and peak power.

Photonics Research
Jan. 18, 2019, Vol. 7 Issue 2 02000162 (2019)
Bound states of solitons in a harmonic graphene-mode-locked fiber laser
Bo Fu, Jin Li, Zhang Cao, and Daniel Popa

We report bound states of solitons from a harmonic mode-locked fiber laser based on a solution-processed graphene saturable absorber. Stable soliton pairs, 26.2 ps apart, are generated with 720 fs duration. By simply increasing the pump power, the laser can also generate harmonic mode-locking with harmonics up to the 26th order (409.6 MHz repetition rate). This is a simple, low-cost, all-fiber, versatile multifunction ultrafast laser that could be used for many applications.

Photonics Research
Jan. 08, 2019, Vol. 7 Issue 2 02000116 (2019)
Polarization evolution dynamics of dissipative soliton fiber lasers
Lei Gao, Yulong Cao, Stefan Wabnitz, Hongqing Ran, Lingdi Kong, Yujia Li, Wei Huang, Ligang Huang, Danqi Feng, and Tao Zhu

Dissipative solitons emerge as stable pulse solutions of nonintegrable and nonconservative nonlinear physical systems, owing to a balance of nonlinearity, dispersion, and loss/gain. A considerable research effort has been dedicated to characterizing amplitude and phase evolutions in the spatiotemporal dynamics of dissipative solitons emerging from fiber lasers. Yet, the picture of the buildup process of dissipative solitons in fiber lasers is incomplete in the absence of corresponding information about the polarization evolution. Here, we characterize probabilistic polarization distributions in the buildup of dissipative solitons in a net-normal dispersion fiber laser system, mode-locked by single-wall carbon nanotubes. The output optical spectra under different pump powers are filtered by a tunable filter, and are detected by a polarization state analyzer. The laser system operates from random amplified spontaneous emission into a stable dissipative soliton state as the cavity gain is progressively increased. Correspondingly, the state of polarization of each spectral wavelength converges towards a fixed point. To reveal the invariant polarization relationship among the various wavelength components of the laser output field, the phase diagram of the ellipticity angle and the spherical orientation angle is introduced. We find that, within the central spectral region of the dissipative soliton, the state of polarization evolves with frequency by tracing a uniform arc on the Poincaré sphere, whereas in the edges of the dissipative soliton spectrum, the state of polarization abruptly changes its path. Increasing cavity gain leads to spectral broadening, accompanied by a random scattering of the state of polarization of newly generated frequencies. Further increases of pump power result in dissipative soliton explosions, accompanied by the emergence of a new type of optical polarization rogue waves. These experimental results provide a deeper insight into the transient dynamics of dissipative soliton fiber lasers.

Photonics Research
Nov. 01, 2019, Vol. 7 Issue 11 11001331 (2019)
Interference-enhanced optical magnetism in surface high-index resonators: a pathway toward high-performance ultracompact linear and nonlinear meta-optics
Lei Kang, Huaguang Bao, and Douglas H. Werner

Artificial magnetism in optical frequencies is one of the most intriguing phenomena associated with metamaterials. The Mie resonance of high-index resonators provides an alternative approach to achieving optical magnetism with simple structures. Given the generally moderate refractive index exhibited by available materials at optical frequencies, Mie resonances usually suffer from coupling between the multipole modes, and the corresponding response of the Mie metasurfaces can be analyzed based on the concept of “meta-optics.” Here, we show that the optical magnetism in high-index resonators can be significantly enhanced by adding a highly reflective back mirror to the system. To highlight the transformative ability of this approach for improving meta-optics in the linear and nonlinear regimes, two proof-of-concept demonstrations are presented. Theoretical modeling reveals that low-pump power ultrafast nonlinear optics can be realized in periodic Si nanodisk arrays backed with a gold film, a system supporting guided resonance modes. Moreover, based on the enhanced magnetism of individual high-index resonators, a pair of silicon cuboids is demonstrated as a magnetic antenna for directional excitation of surface plasmon waves. The interference-enhanced magnetism of high-index resonators provides a disruptive technology for enabling meta-optics comprising ultracompact, high-speed, and power-efficient photonic devices.

Photonics Research
On the CoverOct. 30, 2019, Vol. 7 Issue 11 11001296 (2019)
Pattern formation in 2-μm Tm Mamyshev oscillators associated with the dissipative Faraday instability
Pan Wang, Shunyu Yao, Philippe Grelu, Xiaosheng Xiao, and Changxi Yang

We investigate numerically the pattern formation in 2-μm thulium-doped Mamyshev fiber oscillators, associated with the dissipative Faraday instability. The dispersion-managed fiber ring oscillator is designed with commercial fibers, allowing the dynamics for a wide range of average dispersion regimes to be studied, from normal to near-zero cavity dispersion where the Benjamin–Feir instability remains inhibited. For the first time in the 2-μm spectral window, the formation of highly coherent periodic patterns is demonstrated numerically with rates up to ~100 GHz. In addition, irregular patterns are also investigated, revealing the generation of rogue waves via nonlinear collision processes. Our investigations have potential applications for the generation of multigigahertz frequency combs. They also shed new light on the dissipative Faraday instability mechanisms in the area of nonlinear optical cavity dynamics.

Photonics Research
Oct. 29, 2019, Vol. 7 Issue 11 11001287 (2019)
14 μm quantum cascade lasers based on diagonal transition and nonresonant extraction
Shouzhu Niu, Junqi Liu, Fengmin Cheng, Huan Wang, Jinchuan Zhang, Ning Zhuo, Shenqiang Zhai, Lijun Wang, Shuman Liu, Fengqi Liu, Zhanguo Wang, Xiaohua Wang, and Zhipeng Wei

We report InP-based room-temperature high-average-power quantum cascade lasers emitting at 14 μm. Using a novel active region design, a diagonal bound-to-bound lasing transition is guaranteed by efficient electron injection into the upper laser level and fast nonresonant electron extraction through a miniband from the lower laser level. For a 4 mm long and 40 μm wide double channel ridge waveguide laser with 55 stages of the active region, the threshold current density is only 3.13 kA/cm2 at room temperature. At 293 K, and the maximum single-facet peak power and average power are up to 830 mW and 75 mW, respectively. The laser exhibits a characteristic temperature T0 of 395 K over a temperature range from 293 to 353 K.

Photonics Research
Oct. 24, 2019, Vol. 7 Issue 11 11001244 (2019)
Dynamic and nonlinear properties of epitaxial quantum dot lasers on silicon for isolator-free integration
Jianan Duan, Heming Huang, Bozhang Dong, Justin C. Norman, Zeyu Zhang, John E. Bowers, and Frédéric Grillot

This work investigates the dynamic and nonlinear properties of quantum dot (QD) lasers directly grown on silicon with a view to isolator-free applications. Among them, the chirp parameter, also named the αH factor, is featured through a thermally insensitive method analyzing the residual side-mode dynamics under optical injection locking. The αH at threshold is found as low as 0.32. Then, the nonlinear gain is investigated from the gain compression factor viewpoint. The latter is found higher for epitaxial QD lasers on silicon than that in heterogeneously integrated quantum well (QW) devices on silicon. Despite that, the power dependence of the αH does not lead to a large increase of the chirp coefficient above the laser’s threshold at higher bias. This effect is confirmed from an analytical model and attributed to the strong lasing emission of the ground-state transition, which transforms into a critical feedback level as high as 6.5 dB, which is ~19 dB higher than a comparable QW laser. Finally, the intensity noise analysis confirms that QD lasers are overdamped oscillators with damping frequencies as large as 33 GHz. Altogether, these features contribute to fundamentally enhancing the reflection insensitivity of the epitaxial QD lasers. This last feature is unveiled by the 10 Gbit/s error-free high-speed transmission experiments. Overall, we believe that this work is of paramount importance for future isolator-free photonics technologies and cost-efficient high-speed transmission systems.

Photonics Research
Oct. 17, 2019, Vol. 7 Issue 11 11001222 (2019)
Graded index fiber as an all-fiber saturable absorber for large energy conventional soliton and dissipative soliton generation
Zhaokun Wang, Jikai Chen, Tianyu Zhu, D. N. Wang, and Feng Gao

In this work, we investigate the possibility of achieving nanojoule level pulse energy in an all-fiber Er-doped oscillator by using a graded index multimode fiber (GIMF) as the saturable absorber (SA). This GIMF-based SA demonstrates the desirable characteristics of high-power tolerance, large modulation depth of 29.6%, and small saturation fluence of ~7.19×10 3 μJ/cm2, which contribute to the high-energy soliton generation. In the experiments, the oscillator generates stable ultrafast pulse trains with high pulse energy/average output power up to 13.65 nJ/212.4 mW in the anomalous regime and 6.25 nJ/72.5mW in the normal regime, which are among the highest energy/average output power values achieved by all-fiber Er lasers. The results obtained demonstrate that the GIMF-based SA can be used as an effective photonic device for high-energy wave-breaking free pulse generation.

Photonics Research
Oct. 10, 2019, Vol. 7 Issue 11 11001214 (2019)
Direct generation of an ultrafast vortex beam in a CVD-graphene-based passively mode-locked Pr:LiYF4 visible laser
Nan Li, Junjie Huang, Bin Xu, Yaqi Cai, Jie Lu, Linjie Zhan, Zhengqian Luo, Huiying Xu, Zhiping Cai, and Weiwei Cai

We report on the direct generation of passively mode-locked vortex lasers in the visible spectral region, for the first time to the best of our knowledge, using a Pr:LiYF4 (Pr:YLF) crystal as the gain medium. A stable mode-locked TEM00 mode has been achieved with a maximum average output power of 75 mW using a graphene saturable absorber mirror. The mode-locked pulse width is measured to be as short as about 73.4 ps at a repetition rate of about 140 MHz, and the laser wavelength is at about 721 nm with spectral width of about 0.5 nm. By slightly misaligning the laser resonator, a first-order Laguerre-Gaussian mode (LG0,1) has also been obtained with output power reduced to about 22 mW. The achieved LG0,1 mode has been verified via a home made improved Fizeau interferometer. This work provides a simple and universal method for direct generation of an ultrafast vortex laser, which can be readily extended to other spectral regions by using different laser gain mediums.

Photonics Research
Oct. 04, 2019, Vol. 7 Issue 11 11001209 (2019)
Solid-state Mamyshev oscillator
Mingming Nie, Jiarong Wang, and Shu-Wei Huang

We present the first design and analysis of a solid-state Mamyshev oscillator. We utilize the phase-mismatched cascaded quadratic nonlinear process in a periodically poled lithium niobate waveguide to generate substantial spectral broadening for Mamyshev mode locking. The extensive spectral broadening bridges the two narrowband gain media in the two arms of the same cavity, leading to a broadband mode locking not attainable with either gain medium alone. Two pulses are coupled out of the cavity, and each of the output pulses carries a pulse energy of 25.3 nJ at a repetition rate of 100 MHz. The 10 dB bandwidth of 2.1 THz supports a transform-limited pulse duration of 322 fs, more than 5 times shorter than what can be achieved with either gain medium alone. Finally, effects of group velocity mismatch, group velocity dispersion, and nonlinear saturation on the performance of Mamyshev mode locking are numerically discussed in detail.

Photonics Research
Oct. 01, 2019, Vol. 7 Issue 10 10001175 (2019)
High-Q, low-mode-volume microsphere-integrated Fabry–Perot cavity for optofluidic lasing applications
Xiaoqin Wu, Yipei Wang, Qiushu Chen, Yu-Cheng Chen, Xuzhou Li, Limin Tong, and Xudong Fan

We develop a hybrid optofluidic microcavity by placing a microsphere with a diameter ranging from 1 to 4 μm in liquid-filled plano-plano Fabry–Perot (FP) cavities, which can provide an extremely low effective mode volume down to 0.3–5.1 μm3 while maintaining a high Q-factor up to 1×104–5×104 and a finesse of ~2000. Compared to the pure plano-plano FP cavities that are known to suffer from the lack of mode confinement, diffraction, and geometrical walk-off losses as well as being highly susceptible to mirror misalignment, our microsphere-integrated FP (MIFP) cavities show strong optical confinement in the lateral direction with a tight mode radius of only 0.4–0.9 μm and high tolerance to mirror misalignment as large as 2°. With the microsphere serving as a waveguide, the MIFP is advantageous over a fiber-sandwiched FP cavity due to the open-cavity design for analytes/liquids to interact strongly with the resonant mode, the ease of assembly, and the possibility to replace the microsphere. In this work, the main characteristics of the MIFP, including Q-factor, finesse, effective mode radius and volume, and their dependence on the surrounding medium’s refractive index, mirror spacing, microsphere position inside the FP cavity, and mirror misalignment, are systematically investigated using a finite-element method. Then, by inserting dye-doped polystyrene microspheres of various sizes into the FP cavity filled with water, we experimentally realize single-mode MIFP optofluidic lasers that have a lasing threshold as low as a few microjoules per square millimeter and a lasing spot radius of only ~0.5 μm. Our results suggest that the MIFP cavities provide a promising technology platform for novel photonic devices and biological/chemical detection with ultra-small detection volumes.

Photonics Research
Dec. 14, 2018, Vol. 7 Issue 1 01000050 (2019)
High-order mode lasing in all-FMF laser cavities
Teng Wang, Ao Yang, Fan Shi, Yiping Huang, Jianxiang Wen, and Xianglong Zeng

We experimentally demonstrate two kinds of all few-mode fiber (FMF) ring lasers with high-order mode (HOM) oscillation in the laser cavity. One kind is a switchable-wavelength all-FMF HOM laser with an output of tunable optical vortex beams (OVBs); the other is a Q-switched all-FMF HOM laser with an output of pulsed cylindrical vector beams (CVBs). The lasers are composed of all-FMF components and few-mode erbium-doped fiber. A Sagnac interferometer made of a 3 dB FMF coupler functions as the wavelength selector, and switchable multiwavelength tunable OVBs are experimentally realized. Carbon nanotube-based saturable absorbers and the nonlinear polarization rotation technique are used to achieve Q-switched CVB lasers. This is the first report, to our knowledge, on the generation of switchable-wavelength and Q-switched HOM beams in all-FMF laser cavities.

Photonics Research
Dec. 13, 2018, Vol. 7 Issue 1 01000042 (2019)
Ultracompact fiber laser based on a highly integrated optical device
Xiaoxiang Han, and Xueming Liu

The ultrafast fiber laser has attracted a great deal of research interest due to its low cost, high efficiency, and simple maintenance. Optical passive devices are vital parts of a fiber laser. In order to obtain a fiber laser with high quality, optical passive devices with high performance are required. Here, we demonstrate a highly integrated optical device with the combination of a saturable absorber (SA), coupler, isolator, wavelength division multiplexer, and erbium-doped fiber. The built-in SA has a modulation depth of 7% and can withstand high pump power due to the unique structure of the proposed device. The proposed device is applied to an ultracompact fiber laser, which greatly simplifies the laser structure and reduces the size of the proposed laser. The central wavelength, pulse duration, repetition rate, and signal-to-noise ratio of the output soliton are 1560 nm, 1.06 ps, 25.8 MHz, and 50 dB, respectively. The proposed device has great potential for application in high-power and high-frequency fiber lasers. The proposed ultracompact fiber laser has important applications in optical communication, optical sensing, optical frequency combs, and micromachining.

Photonics Research
Dec. 11, 2018, Vol. 7 Issue 1 01000036 (2019)
Wideband tunable passively Q-switched fiber laser at 2.8  μm using a broadband carbon nanotube saturable absorber
Yanjia Lü, Chen Wei, Han Zhang, Zhe Kang, Guanshi Qin, and Yong Liu

We propose and demonstrate a widely tunable passively Q-switched Ho3+/Pr3+-codoped ZrF4-BaF2-LaF3-AlF3-NaF fiber laser operating in the 2.8 μm mid-infrared (MIR) waveband based on a single-walled carbon nanotube (SWCNT) saturable absorber (SA). The SWCNTs have diameters ranging from 1.4 to 1.7 nm. The modulation depth and saturation intensity of the SWCNT SA measured at 2850 nm are 16.5% and 1.66 MW/cm2, respectively. Stable Q-switched pulses with the shortest pulse duration of 1.46 μs and the maximum pulse energy of 0.43 μJ are achieved at a launched pump power of 445.6 mW. The combined use of a broadband SWCNT SA and a plane ruled grating ensures a broad continuously tuning range of 55.0 nm from 2837.6 to 2892.6 nm. The output powers, emission spectra, repetition rates, and pulse durations at different tuning wavelengths are also characterized and analyzed. Our results indicate that SWCNTs can be excellent broadband SAs in the 3 μm MIR region. To the author’s knowledge, this is the first demonstration of a widely tunable carbon-nanotube-enabled passively Q-switched fiber laser operating in the 2.8 μm MIR waveband.

Photonics Research
Dec. 07, 2018, Vol. 7 Issue 1 01000014 (2019)
Modulation properties of solitary and optically injected phased-array semiconductor lasers
Nianqiang Li, H. Susanto, B. R. Cemlyn, I. D. Henning, and M. J. Adams

We study modulation properties of two-element phased-array semiconductor lasers that can be described by coupled mode theory. We consider four different waveguide structures and modulate the array either in phase or out of phase within the phase-locked regions, guided by stability diagrams obtained from direct numerical simulations. Specifically, we find that out-of-phase modulation allows for bandwidth enhancement if the waveguide structure is properly chosen; for example, for a combination of index antiguiding and gain-guiding, the achievable modulation bandwidth in the case of out-of-phase modulation could be much higher than the one when they are modulated in phase. Proper array design of the coupling, controllable in terms of the laser separation and the frequency offset between the two lasers, is shown to be beneficial to slightly improve the bandwidth but not the resonance frequency, while the inclusion of the frequency offset leads to the appearance of double peak response curves. For comparison, we explore the case of modulating only one element of the phased array and find that double peak response curves are found. To improve the resonance frequency and the modulation bandwidth, we introduce simultaneous external injection into the phased array and modulate the phased array or its master light within the injection locking region. We observe a significant improvement of the modulation properties, and in some cases, by modulating the amplitude of the master light before injection, the resulting 3 dB bandwidths could be enhanced up to 160 GHz. Such a record bandwidth for phased-array modulation could pave the way for various applications, notably optical communications that require high-speed integrated photonic devices.

Photonics Research
Aug. 23, 2018, Vol. 6 Issue 9 09000908 (2018)
Q-switched and mode-locked Er-doped fiber laser using PtSe2 as a saturable absorber
Kang Zhang, Ming Feng, Yangyang Ren, Fang Liu, Xingshuo Chen, Jie Yang, Xiao-Qing Yan, Feng Song, and Jianguo Tian

We report a passively Q-switched and mode-locked erbium-doped fiber laser (EDFL) based on PtSe2, a new two-dimensional material, as a saturable absorber (SA). Self-started Q-switching at 1560 nm in the EDFL was achieved at a threshold pump power of 65 mW, and at the maximum pump power of 450 mW, the maximum single Q-switched pulse energy is 143.2 nJ. Due to the polarization-dependent characteristics of the PtSe2-based SA, the laser can be switched from the Q-switched state to the mode-locked state by adjusting the polarization state. A mode-locked pulse train with a repetition rate of 23.3 MHz and a pulse width of 1.02 ps can be generated when the pump power increases to about 80 mW, and the stable mode-locked state is maintained until the pump power reaches its maximum 450 mW. The maximum single mode-locked pulse energy is 0.53 nJ. This is the first time to our knowledge that successful generation of stable Q-switched and mode-locked pulses in an Er-doped fiber laser has been obtained by using PtSe2 as a saturable absorber.

Photonics Research
Aug. 23, 2018, Vol. 6 Issue 9 09000893 (2018)
Widely tunable 2.3  μm III-V-on-silicon Vernier lasers for broadband spectroscopic sensing
Ruijun Wang, Stephan Sprengel, Anton Vasiliev, Gerhard Boehm, Joris Van Campenhout, Guy Lepage, Peter Verheyen, Roel Baets, Markus-Christian Amann, and Gunther Roelkens

Heterogeneously integrating III-V materials on silicon photonic integrated circuits has emerged as a promising approach to make advanced laser sources for optical communication and sensing applications. Tunable semiconductor lasers operating in the 2–2.5 μm range are of great interest for industrial and medical applications since many gases (e.g., CO2, CO, CH4) and biomolecules (such as blood glucose) have strong absorption features in this wavelength region. The development of integrated tunable laser sources in this wavelength range enables low-cost and miniature spectroscopic sensors. Here we report heterogeneously integrated widely tunable III-V-on-silicon Vernier lasers using two silicon microring resonators as the wavelength tuning components. The laser has a wavelength tuning range of more than 40 nm near 2.35 μm. By combining two lasers with different distributed Bragg reflectors, a tuning range of more than 70 nm is achieved. Over the whole tuning range, the side-mode suppression ratio is higher than 35 dB. As a proof-of-principle, this III-V-on-silicon Vernier laser is used to measure the absorption lines of CO. The measurement results match very well with the high-resolution transmission molecular absorption (HITRAN) database and indicate that this laser is suitable for broadband spectroscopy.

Photonics Research
On the CoverAug. 09, 2018, Vol. 6 Issue 9 09000858 (2018)
Polarization-multiplexed, dual-comb all-fiber mode-locked laser
Xin Zhao, Ting Li, Ya Liu, Qian Li, and Zheng Zheng

Mode-locked fiber lasers that can simultaneously generate two asynchronous ultrashort pulse trains could play an attractive role as the alternative light sources for low-complexity dual-comb metrology applications. Although a few multiplexing schemes to realize such lasers have been proposed and demonstrated, here we investigate the lasing characteristics of a passively mode-locked fiber laser with a finite amount of intracavity birefringence. By introducing a section of polarization-maintaining (PM) fiber into the otherwise-non-PM-single-mode cavity, dual asynchronous pulses with nearly orthogonal states of polarization are generated. With a repetition rate difference of hundreds of hertz, the pulses have well-overlapped spectra and show typical features of polarization-locked vector solitons. It is demonstrated that under an anomalous or net normal dispersion regime, either dual vector solitons or dual dissipative vector solitons can be generated, respectively. Such polarization-multiplexed single single-cavity dual-comb lasers could find further uses in various applications in need of simple dual-comb system solutions.

Photonics Research
Aug. 08, 2018, Vol. 6 Issue 9 09000853 (2018)
Self-Q-switched and wavelength-tunable tungsten disulfide-based passively Q-switched Er:Y2O3 ceramic lasers
Xiaofeng Guan, Jiawei Wang, Yuzhao Zhang, Bin Xu, Zhengqian Luo, Huiying Xu, Zhiping Cai, Xiaodong Xu, Jian Zhang, and Jun Xu

We report on diode-pumped Er:Y2O3 ceramic lasers at about 2.7 μm in the tunable continuous-wave, self-Q-switching and tungsten disulfide (WS2)-based passively Q-switching regimes. For stable self-Q-switched operation, the maximum output power reaches 106.6 mW under an absorbed power of 2.71 W. The shortest pulse width is measured to be about 1.39 μs at a repetition rate of 26.7 kHz at maximum output. Using a spin-coated WS2 as a saturable absorber, a passively Q-switched Er:Y2O3 ceramic laser is also realized with a maximum average output power of 233.5 mW (for the first time, to the best of our knowledge). The shortest pulse width decreases to 0.72 μs at a corresponding repetition rate of 29.4 kHz, which leads to a pulse energy of 7.92 μJ and a peak power of 11.0 W. By inserting an undoped YAG thin plate as a Fabry–Perot etalon, for the passive Q switching, wavelength tunings are also demonstrated at around 2710, 2717, 2727, and 2740 nm.

Photonics Research
Aug. 02, 2018, Vol. 6 Issue 9 09000830 (2018)
Generation of 84-fs pulses from a mode-locked Tm:CNNGG disordered garnet crystal laser
Zhongben Pan, Yicheng Wang, Yongguang Zhao, Hualei Yuan, Xiaojun Dai, Huaqiang Cai, Ji Eun Bae, Sun Young Choi, Fabian Rotermund, Xavier Mateos, Josep Maria Serres, Pavel Loiko, Uwe Griebner, and Valentin Petrov

A mode-locked laser based on a Tm:CNNGG disordered crystal as an active medium and a single-walled carbon nanotube saturable absorber is demonstrated, operating at a central wavelength of 2018 nm. Transform-limited 84 fs pulses are generated with an average output power of 22 mW at a repetition rate of ~90 MHz. A maximum output power of 98 mW is obtained at a slightly longer pulse duration of 114 fs.

Photonics Research
Jul. 13, 2018, Vol. 6 Issue 8 08000800 (2018)
Bismuth nanosheets as a Q-switcher for a mid-infrared erbium-doped SrF2 laser
Jingjing Liu, Hao Huang, Feng Zhang, Zhen Zhang, Jie Liu, Han Zhang, and Liangbi Su

Bismuth nanosheets (Bi-NSs) were successfully prepared and employed as saturable absorbers to generate a diode-pumped dual-wavelength Er3+:SrF2 laser in the mid-infrared region. Q-switched pulses with a maximum output power of 0.226 W were obtained at an absorbed pump power of 1.97 W. A repetition rate of 56.20 kHz and a minimum pulse duration of 980 ns were achieved. To the best of our knowledge, we present the first application of Bi-NSs in a mid-infrared all-solid-state laser. The results prove that Bi-NSs may be applied as an optical modulator in mid-infrared photonic devices or as a mode-locker and Q-switcher.

Photonics Research
Jul. 05, 2018, Vol. 6 Issue 8 08000762 (2018)
Multi-wavelength sampled Bragg grating quantum cascade laser arrays
Xue-Feng Jia, Li-Jun Wang, Ning Zhuo, Jin-Chuan Zhang, Shen-Qiang Zhai, Jun-Qi Liu, Shu-Man Liu, Feng-Qi Liu, and Zhanguo Wang

A multi-wavelength sampled Bragg grating (SBG) quantum cascade laser array operating between 7.32 and 7.85 μm is reported. The sampling grating structure, which can be analyzed as a conventional grating multiplied by a sampling function, is fabricated by holographic exposure combined with optical photolithography. The sampling grating period was varied from 8 to 32 μm, and different sampling order ( 1st, 2nd, and 3rd order) modes were achieved. We propose that higher-order modes with optimized duty cycles can be used to take full advantage of the gain curve and improve the wavelength coverage of the SBG array, which will be beneficial to many applications.

Photonics Research
Jun. 19, 2018, Vol. 6 Issue 7 07000721 (2018)
Active/passive Q-switching operation of 2  μm Tm,Ho:YAP laser with an acousto-optical Q-switch/MoS2 saturable absorber mirror
Linjun Li, Xining Yang, Long Zhou, Wenqiang Xie, Yunlong Wang, Yingjie Shen, Yuqiang Yang, Wenlong Yang, Wei Wang, Zhiwei Lv, Xiaoming Duan, and Minghua Chen

The active/passive Q-switching operation of a 2 μm a-cut Tm,Ho:YAP laser was experimentally demonstrated with an acousto-optical Q–switch/MoS2 saturable absorber mirror. The active Q-switch laser was operated for the first time, to the best of our knowledge, with an average output power of 12.3 W and a maximum pulse energy of 10.3 mJ. The passive Q-switch laser was also the first acquired with an average output power of 3.3 W and per pulse energy of 23.31 μJ, and the beam quality factors of Mx2=1.06 and My2=1.06 were measured at the average output power of 2 W.

Photonics Research
May. 23, 2018, Vol. 6 Issue 6 06000614 (2018)
Passively Q-switched erbium doped fiber laser using a gold nanostars based saturable absorber
Zhe Kang, Mingyi Liu, Zhenwei Li, Siqing Li, Zhixu Jia, Chengzhi Liu, Weiping Qin, and Guanshi Qin

In this paper, we propose and demonstrate an all-fiber passively Q-switched erbium doped fiber laser (EDFL) by using gold nanostars (GNSs) as a saturable absorber (SA) for the first time, to the best of our knowledge. In comparison with other gold nanomorphologies, GNSs have multiple localized surface plasmon resonances, which means that they can be used to construct wideband ultrafast pulse lasers. By inserting the GNS SA into an EDFL cavity pumped by a 980 nm laser diode, a stable passively Q-switched laser at 1564.5 nm was achieved for a threshold pump power of 40 mW. By gradually increasing the pump power from 40 to 120 mW, the pulse duration decreases from 12.8 to 5.3 μs and the repetition rate increases from 10 to 17 kHz. Our results indicate that the GNSs are a promising SA for constructing pulse lasers.

Photonics Research
May. 07, 2018, Vol. 6 Issue 6 06000549 (2018)
Integrated heterogeneous silicon/III–V mode-locked lasers
Michael L. Davenport, Songtao Liu, and John E. Bowers

The mode-locked laser diode has emerged as a promising candidate as a signal source for photonic radar systems, wireless data transmission, and frequency comb spectroscopy. They have the advantages of small size, low cost, high reliability, and low power consumption, thanks to semiconductor technology. Mode-locked lasers based on silicon photonics advance these qualities by the use of highly advanced silicon manufacturing technology. This paper will begin by giving an overview of mode-locked laser diode literature, and then focus on mode-locked lasers on silicon. The dependence of mode-locked laser performance on design details is presented.

Photonics Research
Apr. 20, 2018, Vol. 6 Issue 5 05000468 (2018)
Band-gap-tailored random laser
Hongbo Lu, Jian Xing, Cheng Wei, Jiangying Xia, Junqing Sha, Yunsheng Ding, Guobing Zhang, Kang Xie, Longzhen Qiu, and Zhijia Hu

A band-gap-tailored random laser with a wide tunable range and low threshold through infrared radiation is demonstrated. When fluorescent dyes are doped into the liquid crystal and heavily doped chiral agent system, we demonstrate a wavelength tuning random laser instead of a side-band laser, which is caused by the combined effect of multi-scattering of liquid crystal (LC) and band-gap control. Through rotating the infrared absorbing material on the side of the LC cell, an adjustable range for random lasing of 80 nm by infrared light irradiation was observed.

Photonics Research
Apr. 18, 2018, Vol. 6 Issue 5 05000390 (2018)
Evanescent-wave pumped single-mode microcavity laser from fiber of 125  μm diameter
Yuchen Wang, Shu Hu, Xiao Yang, Ruizhi Wang, Heng Li, and Chuanxiang Sheng

A microcavity laser based on evanescent-wave-coupled gain is formed using a silica fiber with a diameter of 125 μm in a rhodamine 6G ethanol solution. When the fiber is sticking to the cuvette wall by capillary force, using the excitation of a 532 nm nanosecond pulsed laser, single-mode laser emission is observed. While increasing the distance between the fiber and the cuvette wall, the typical multi-peak whispering-gallery-mode (WGM) laser emission can also be demonstrated. On the other hand, while increasing the refractive index of the solution by mixing ethanol and ethylene glycol with different ratios as a solvent, the single-mode emission would evolve to multi-peak WGM laser emission controllably.

Photonics Research
Mar. 27, 2018, Vol. 6 Issue 4 04000332 (2018)
Noise-sidebands-free and ultra-low-RIN 1.5  μm single-frequency fiber laser towards coherent optical detection
Qilai Zhao, Zhitao Zhang, Bo Wu, Tianyi Tan, Changsheng Yang, Jiulin Gan, Huihui Cheng, Zhouming Feng, Mingying Peng, Zhongmin Yang, and Shanhui Xu

A noise-sidebands-free and ultra-low relative intensity noise (RIN) 1.5 μm single-frequency fiber laser is demonstrated for the first time to our best knowledge. Utilizing a self-injection locking framework and a booster optical amplifier, the noise sidebands with relative amplitudes as high as 20 dB are completely suppressed. The RIN is remarkably reduced by more than 64 dB at the relaxation oscillation peak to retain below 150 dB/Hz in a frequency range from 75 kHz to 50 MHz, while the quantum noise limit is 152.9 dB/Hz. Furthermore, a laser linewidth narrower than 600 Hz, a polarization-extinction ratio of more than 23 dB, and an optical signal-to-noise ratio of more than 73 dB are acquired simultaneously. This noise-sidebands-free and ultra-low-RIN single-frequency fiber laser is highly competitive in advanced coherent light detection fields including coherent Doppler wind lidar, high-speed coherent optical communication, and precise absolute distance coherent measurement.

Photonics Research
Mar. 27, 2018, Vol. 6 Issue 4 04000326 (2018)
1.3  μm InAs/GaAs quantum dot lasers on silicon with GaInP upper cladding layers
Jun Wang, Haiyang Hu, Haiying Yin, Yiming Bai, Jian Li, Xin Wei, Yuanyuan Liu, Yongqing Huang, Xiaomin Ren, and Huiyun Liu

We report on the first electrically pumped continuous-wave (CW) InAs/GaAs quantum dot (QD) laser grown on Si with a GaInP upper cladding layer. A QD laser structure with a Ga0.51In0.49P upper cladding layer and an Al0.53Ga0.47As lower cladding layer was directly grown on Si by metal–organic chemical vapor deposition. It demonstrates the postgrowth annealing effect on the QDs was relieved enough with the GaInP upper cladding layer grown at a low temperature of 550°C. Broad-stripe edge-emitting lasers with 2-mm cavity length and 15-μm stripe width were fabricated and characterized. Under CW operation, room-temperature lasing at ~1.3 μm has been achieved with a threshold density of 737 A/cm2 and a single-facet output power of 21.8 mW.

Photonics Research
Mar. 27, 2018, Vol. 6 Issue 4 04000321 (2018)
High-quality 2-μm Q-switched pulsed solid-state lasers using spin-coating-coreduction approach synthesized Bi2Te3 topological insulators
Junpeng Qiao, Shengzhi Zhao, Kejian Yang, Wei-Heng Song, Wenchao Qiao, Chung-Lung Wu, Jia Zhao, Guiqiu Li, Dechun Li, Tao Li, Hong Liu, and Chao-Kuei Lee

In this paper, the fabrication process and characterization of Bi2Te3 topological insulators (TIs) synthesized by the spin-coating-coreduction approach (SCCA) is reported. With this approach, high-uniformity nano-crystalline TI saturable absorbers (TISAs) with large-area uniformity and controllable thickness are prepared. By employing these prepared TIs with different thicknesses as SAs in 2-μm solid-state Q-switched lasers, thickness-dependent output powers and pulse durations of the laser pulses are obtained, and the result also exhibits stability and reliability. The shortest pulse duration is as short as 233 ns, and the corresponding clock amplitude jitter is around 2.1%, which is the shortest pulse duration in TISA-based Q-switched 2-μm lasers to the best of our knowledge. Moreover, in comparison with the TISA synthesized by the ultrasound-assisted liquid phase exfoliation (UALPE) method, the experimental results show that lasers with SCCA synthesized TISAs have higher output powers, shorter pulse durations, and higher pulse peak powers. Our work suggests that the SCCA synthesized TISAs could be used as potential SAs in pulsed lasers.

Photonics Research
Mar. 27, 2018, Vol. 6 Issue 4 04000314 (2018)
Relative intensity noise in high-speed hybrid square-rectangular lasers
Fu-Li Wang, Xiu-Wen Ma, Yong-Zhen Huang, Yue-De Yang, Jun-Yuan Han, and Jin-Long Xiao

Relative intensity noise (RIN) and high-speed modulation characteristics are investigated for an AlGaInAs/InP hybrid square-rectangular laser (HSRL) with square side length, rectangular length, and width of 15,300, and 2 μm, respectively. Single-mode operation with side-mode suppression larger than 40 dB has been realized for the HSRL over wide variation of the injection currents. In addition, the HSRL exhibits a 3 dB modulation bandwidth of 15.5 GHz, and an RIN nearly approaches standard quantum shot-noise limit 2hv/P= 164 dB/Hz at high bias currents due to the strong mode selection of the square microcavity. With the increase of the DC bias current of the Fabry–Perot section, significantly enhanced modulation bandwidth and decreased RIN are observed. Furthermore, intrinsic parameters such as resonance frequency, damping factor, and modified Schawlow–Townes linewidth are extracted from the noise spectra.

Photonics Research
Feb. 27, 2018, Vol. 6 Issue 3 03000193 (2018)
Diode-pumped power scalable Kerr-lens mode-locked Yb:CYA laser
Wenlong Tian, Yingnan Peng, Ziyue Zhang, Zijiao Yu, Jiangfeng Zhu, Xiaodong Xu, and Zhiyi Wei

Stable 68 fs pulses with the average power of 1.5 W is directly generated from a multimode diode-pumped Kerr-lens mode-locked Yb:CYA laser by separating the gain medium and Kerr medium. The repetition rate is about 50 MHz, resulting in a single pulse energy of 30 nJ and a peak power of 0.44 MW. To the best of our knowledge, this is the highest single pulse energy ever produced from a mode-locked Yb:CYA oscillator. Our experimental results show that Yb:CYA crystal is an excellent candidate for multiwatt, sub-100 fs pulse generation in diode-pumped all-solid-state lasers. It is believed that the output power can be scalable to multi-W while the pulse duration is maintained with this simple method.

Photonics Research
On the CoverJan. 25, 2018, Vol. 6 Issue 2 02000127 (2018)
30  W monolithic 2–3  μm supercontinuum laser
Ke Yin, Bin Zhang, Linyong Yang, and Jing Hou

A high-power all-fiber supercontinuum (SC) laser source based on germania-core fiber (GCF) was presented. The lesser absorption loss of GCF than silica fiber beyond 2.0 μm makes GCF more suitable for extending the SC spectrum to the long wavelength side. In this work, the GCF-based SC laser had a maximum power of 30.1 W, together with a 10 dB spectral bandwidth of >1000 nm spanning from 1.95 to 3.0 μm. To the best of our knowledge, this is the highest output power level ever reported for a GCF-based SC laser as well as a 2–3 μm SC laser.

Photonics Research
Jan. 25, 2018, Vol. 6 Issue 2 02000123 (2018)
Passively mode-locked Er-doped fiber laser based on SnS2 nanosheets as a saturable absorber
Kangdi Niu, Ruyi Sun, Qingyun Chen, Baoyuan Man, and Huanian Zhang

In this paper, tin disulfide (SnS2), a two-dimensional (2D) n-type direct bandgap layered metal dichalcogenide with a gap value of 2.24 eV, was employed as a saturable absorber. Its appearance and nonlinear saturable absorption characteristics were also investigated experimentally. SnS2-PVA (polyvinyl alcohol) film was successfully prepared and employed as a mode-locker for achieving a mode-locked Er-doped fiber laser with a pulse width of 623 fs at a pulse repetition rate of 29.33 MHz. The results prove that SnS2 nanosheets will have wide potential ultrafast photonic applications due to their suitable bandgap value and excellent nonlinear saturable absorption characteristics.

Photonics Research
Jan. 24, 2018, Vol. 6 Issue 2 02000072 (2018)
Control of the phase of the magnetization precession excited by circularly polarized femtosecond-laser pulses
Alexander I. Chernov, Mikhail A. Kozhaev, Anastasiia Khramova, Alexander N. Shaposhnikov, Anatoly R. Prokopov, Vladimir N. Berzhansky, Anatoly K. Zvezdin, and Vladimir I. Belotelov

The inverse Faraday effect induced in magnetic films by ultrashort laser pulses allows excitation and control of spins at gigahertz and sub-terahertz frequencies. The frequency of the optically excited magnetization precession is easily tunable by the external magnetic field. On the other hand, the initial phase of the precession marginally depends on the magnetic field. Here we demonstrate an approach for the control of the precession phase by variation of the pump beam direction. In particular, we consider the case when the magnetization precession is excited by obliquely incident pump pulses in a magnetic dielectric film placed in the in-plane magnetic field. Theoretical consideration predicts that the initial phase should appear for a non-zero in-plane component of the pump wavevector orthogonal to the external magnetic field. Experimental studies confirm this conclusion and reveal that the phase grows with increase of the in-plane wavevector component. Variation of phase by 15 deg is demonstrated. Potentially, the phase could be changed even more pronouncedly by more than 90 deg. This work provides a simple way for additional manipulation with optically excited magnetization dynamics, which is of importance for different spintronic applications.

Photonics Research
Oct. 22, 2018, Vol. 6 Issue 11 11001079 (2018)
2.8  μm all-fiber Q-switched and mode-locked lasers with black phosphorus
Zhipeng Qin, Guoqiang Xie, Jingui Ma, Peng Yuan, and Liejia Qian

In past years, rare-earth-doped fluoride fiber lasers (FFLs) have developed rapidly in the mid-infrared (mid-IR) region. However, due to the lack of fiber optic devices and challenge of fluoride fiber splicing, most mid-IR FFLs have been demonstrated with free-space optic elements, limiting the advantages of all-fiber lasers for flexible delivery, stability, and compactness. Here, we report, to the best of our knowledge, the first pulsed all-fiber FFL in the mid-IR region. By taking advantage of the integration of black phosphorus flake, stable Q-switched and mode-locked pulses were obtained at 2.8 μm wavelength. We believe that this all-fiber design will promote the application of pulsed FFL in the mid-IR region.

Photonics Research
Oct. 18, 2018, Vol. 6 Issue 11 11001074 (2018)
All-fiber passively mode-locked laser using nonlinear multimode interference of step-index multimode fiber
Tao Chen, Qiaoli Zhang, Yaping Zhang, Xin Li, Haikun Zhang, and Wei Xia

We experimentally demonstrate for the first time, to the best of our knowledge, an all-fiber passively mode-locked laser operation based on the nonlinear multimode interference of step-index multimode fiber. Such a structure couples the light in and out of the multimode fiber via single-mode fibers, and its physical mechanisms for saturable absorption have been analyzed theoretically based on the third-order nonlinear Kerr effect of multimode fiber. Using the nonlinear multimode interference structure with 48.8 mm length step-index multimode fiber, the modulation depth has been measured to be ~5%. The passively mode-locked laser output pulses have a central wavelength of 1596.66 nm, bandwidth of 2.18 nm, pulsewidth of ~625 fs, and fundamental repetition rate of 8.726 MHz. Furthermore, the influence of total cavity dispersion on the optical spectrum, pulse width, and output power is investigated systematically by adding different lengths of single-mode fiber and dispersion compensation fiber in the laser cavity.

Photonics Research
Oct. 16, 2018, Vol. 6 Issue 11 11001033 (2018)
PbS quantum dots as a saturable absorber for ultrafast laser
Ling Yun, Yang Qiu, Conghao Yang, Jie Xing, Kehan Yu, Xiangxing Xu, and Wei Wei

Low-dimensional nanomaterials, owing to their unique and versatile properties, are very attractive for enormous electronic and optoelectronic applications. PbS quantum dots (QDs), characterized by a large Bohr radius and size-tunable bandgap, are especially interesting for photonic applications in the near-infrared region. Here, oleic acid capped colloidal PbS QDs as a saturable absorber are investigated for ultrashort-pulse generation. The PbS QDs exhibit outstanding nonlinear saturable absorption properties at 1550 nm: a modulation depth up to 44.5% and a thermal damage threshold larger than 30 mJ/cm2. By incorporating PbS QDs into a fiber laser, a transform-limited soliton pulse as short as 559 fs with a bandwidth of 4.78 nm is realized at 1563 nm. Numerous applications may benefit from the nonlinear saturable absorption properties of PbS QDs, such as near-infrared pulsed lasers and modulators.

Photonics Research
Oct. 16, 2018, Vol. 6 Issue 11 11001028 (2018)
Facile active control of a pulsed erbium-doped fiber laser using modulation depth tunable carbon nanotubes
Xintong Xu, Shuangchen Ruan, Jianpang Zhai, Ling Li, Jihong Pei, and Zikang Tang

Short pulsed fiber lasers have been widely made using single-walled carbon nanotubes as a saturable absorber (SA). However, most of the currently used devices can only operate in one determined operation state with an unchangeable modulation SA depth in the cavity, which significantly limits their application in photonic devices. In this paper, well-aligned carbon nanotube arrays are synthesized using zeolite AlPO4-5 as a template, which features anisotropic optical absorption. The linear optical absorption of the as-synthesized carbon nanotube arrays can easily be tuned by adjusting a polarization controller, thus providing a tunable modulation depth for the carbon nanotube SA. By exploiting this SA in an erbium-doped fiber laser cavity, both Q-switched and mode-locked pulsed lasers are achieved by simply adjusting a polarization controller under a fixed pump power of 330 mW. In addition, the repetition rate of the Q-switching and pulse duration of the mode-locking can be tuned according to the variation of modulation depth. Moreover, soliton molecules can be obtained when the modulation depth of the SA is 4.5%.

Photonics Research
Oct. 12, 2018, Vol. 6 Issue 11 11000996 (2018)
Passively Q-switched femtosecond-laser-written thulium waveguide laser based on evanescent field interaction with carbon nanotubes
Esrom Kifle, Pavel Loiko, Javier Rodríguez Vázquez de Aldana, Carolina Romero, Airán Ródenas, Sun Yung Choi, Ji Eun Bae, Fabian Rotermund, Viktor Zakharov, Andrey Veniaminov, Magdalena Aguiló, Francesc Díaz, Uwe Griebner, Valentin Petrov, and Xavier Mateos

Surface channel waveguides (WGs) were fabricated in a monoclinic Tm3+:KLu(WO4)2 crystal by femtosecond direct laser writing (fs-DLW). The WGs consisted of a half-ring cladding with diameters of 50 and 60 μm located just beneath the crystal surface. They were characterized by confocal laser microscopy and μ-Raman spectroscopy, indicating a reduced crystallinity and stress-induced birefringence of the WG cladding. In continuous-wave (CW) mode, under Ti:sapphire laser pumping at 802 nm, the maximum output power reached 171.1 mW at 1847.4 nm, corresponding to a slope efficiency η of 37.8% for the 60 μm diameter WG. The WG propagation loss was 0.7±0.3 dB/cm. The top surface of the WGs was spin-coated by a polymethyl methacrylate film containing randomly oriented (spaghetti-like) arc-discharge single-walled carbon nanotubes serving as a saturable absorber based on evanescent field coupling. Stable passively Q-switched (PQS) operation was achieved. The PQS 60 μm diameter WG laser generated a record output power of 150 mW at 1846.8 nm with η=34.6%. The conversion efficiency with respect to the CW mode was 87.6%. The best pulse characteristics (energy/duration) were 105.6 nJ/98 ns at a repetition rate of 1.42 MHz.

Photonics Research
Sep. 26, 2018, Vol. 6 Issue 10 10000971 (2018)
Integration of nanoscale light emitters: an efficient ultraviolet and blue random lasing from NaYF4:Yb/Tm hexagonal nanocrystals
Ya-Pei Peng, Wei Lu, Pengpeng Ren, Yiqun Ni, Yunfeng Wang, Long Zhang, Yu-Jia Zeng, Wenfei Zhang, and Shuangchen Ruan

Near infrared light-controlled release of payloads from ultraviolet-sensitive (UV-sensitive) polymer hydrogels or nanocarriers is one of the most promising strategies for biotherapy. Here, we propose the concept of light activation of NaYF4:20%Yb,2%Tm nanocrystals (NCs). NaYF4:20%Yb,2%Tm NCs are synthesized by a solvothermal method. Effective upconversion luminescence from NaYF4:20%Yb,2%Tm NCs excited by a continuous wave (CW) 980 nm laser is obtained. The NaYF4:20%Yb,2%Tm NCs are then used as a laser gain medium and sandwiched between Al and quartz reflectors to form laser microcavities. UV and blue upconverted random lasing is obtained from the laser microcavities. Hence, we verify explicitly that the NaYF4:Yb,Tm NCs support UV and blue upconversion random lasing via a 980 nm nanosecond laser excitation. Our work provides what we believe is a new concept for precision and localized cancer therapy by external light excitation.

Photonics Research
Sep. 07, 2018, Vol. 6 Issue 10 10000943 (2018)
Generation of high-energy narrowband 2.05  μm pulses for seeding a Ho:YLF laser
Yanchun Yin, Xiaoming Ren, Yang Wang, Fengjiang Zhuang, Jie Li, and Zenghu Chang

We experimentally demonstrate efficient generation of high-energy (82 μJ) narrowband 2.05 μm pulses pumped with 1 mJ broadband Ti:sapphire laser pulses, utilizing dual-chirped optical parametric amplification (DC-OPA) in a BBO crystal. The narrowband 2.05 μm pulses will be primarily used for seeding an Ho:YLF laser, which solves the synchronization issue when Ti:sapphire and Ho:YLF lasers are needed for developing midinfrared lasers. The narrowband 2.05 μm pulse from the unique DC-OPA design can seed the Ho:YLF laser much more efficiently than the broadband 2.05 μm pulse from traditional OPA technology.

Photonics Research
Dec. 05, 2017, Vol. 6 Issue 1 01000001 (2018)
Nonlinearity-tailored fiber laser technology for low-noise, ultra-wideband tunable femtosecond light generation
Xiaomin Liu, Jesper Laegsgaard, Roman Iegorov, Ask S. Svane, F. ?mer Ilday, Haohua Tu, Stephen A. Boppart, and Dmitry Turchinovich

The emission wavelength of a laser is physically predetermined by the gain medium used. Consequently, arbitrary wavelength generation is a fundamental challenge in the science of light. Present solutions include optical parametric generation, requiring complex optical setups and spectrally sliced supercontinuum, taking advantage of a simpler fiber technology: a fixed-wavelength pump laser pulse is converted into a spectrally very broadband output, from which the required resulting wavelength is then optically filtered. Unfortunately, this process is associated with an inherently poor noise figure, which often precludes many realistic applications of such supercontinuum sources. Here, we show that by adding only one passive optical element—a tapered photonic crystal fiber—to a fixed-wavelength femtosecond laser, one can in a very simple manner resonantly convert the laser emission wavelength into an ultra-wide and continuous range of desired wavelengths, with very low inherent noise, and without mechanical realignment of the laser. This is achieved by exploiting the double interplay of nonlinearity and chirp in the laser source and chirp and phase matching in the tapered fiber. As a first demonstration of this simple and inexpensive technology, we present a femtosecond fiber laser continuously tunable across the entire red–green–blue spectral range.

Photonics Research
Oct. 26, 2017, Vol. 5 Issue 6 06000750 (2017)
Bismuth-doped glass microsphere lasers
Zaijin Fang, Síle Nic Chormaic, Shanyu Wang, Xin Wang, Jibo Yu, Yuxuan Jiang, Jianrong Qiu, and Pengfei Wang

In this work, a hybrid structure consisting of a multicomponent germanate glass microsphere containing bismuth as a gain medium is proposed and presented. The bismuth-doped germanate glass microspheres were fabricated from a glass fiber tip with no precipitation of the bismuth metal. Coupling with a fiber taper, the bismuth-doped microsphere single-mode laser was observed to lase at around 1305.8 nm using 808 nm excitation. The low threshold of absorbed pump power at 215 μW makes this microlaser appealing for various applications, including tunable lasers for a range of purposes in telecommunication, biomedical, and optical information processing.

Photonics Research
Oct. 26, 2017, Vol. 5 Issue 6 06000740 (2017)
Generating laser transverse modes analogous to quantum Green’s functions of two-dimensional harmonic oscillators
J. C. Tung, Y. H. Hsieh, T. Omatsu, K. F. Huang, and Y. F. Chen

We theoretically analyzed the relationship between quantum Green’s functions of two-dimensional harmonic oscillators and radial-order Laguerre–Gaussian laser modes of spherical resonators. By using a nearly hemispherical resonator and a tight focusing in the end-pumped solid-state laser, we successfully generated various laser transverse modes analogous to quantum Green’s functions. We further experimentally and numerically verified that the transverse order associated with quantum Green’s functions is noticeably raised with increasing the pump power induced by the thermal effect. More importantly, the high lasing efficiency and the salient structure enable the present laser source to be used in exploring the light–matter interaction.

Photonics Research
Oct. 29, 2017, Vol. 5 Issue 6 06000733 (2017)
Intracavity biosensor based on the Nd:YAG waveguide laser: tumor cells and dextrose solutions
Guanhua Li, Huiyuan Li, Rumei Gong, Yang Tan, Javier Rodríguez Vázquez de Aldana, Yuping Sun, and Feng Chen

This work demonstrates the Nd:YAG waveguide laser as an efficient platform for the bio-sensing of dextrose solutions and tumor cells. The waveguide was fabricated in an Nd:YAG crystal with the cooperation of ultrafast laser writing and ion irradiation. The laser oscillation in the Nd:YAG waveguide is ultrasensitive to the external environment of the waveguide. Even a weak disturbance leads to a large variation of the output power of the laser. According to this feature, an Nd:YAG waveguide coated with graphene and WSe2 layers is used as substrate for the microfluidic channel. When the microflow crosses the Nd:YAG waveguide, the laser oscillation in the waveguide is disturbed and induces fluctuation of the output laser. According to the fluctuation, the microflow is detected with a sensitivity of 10 mW/RIU.

Photonics Research
Oct. 26, 2017, Vol. 5 Issue 6 06000728 (2017)
Variable-curvature microresonators for dual-wavelength lasing
Min Tang, Yong-Zhen Huang, Yue-De Yang, Hai-Zhong Weng, and Zhi-Xiong Xiao

Stable dual-mode semiconductor lasers can be applied for the photonic generation of microwave and terahertz waves. In this paper, the mode characteristics of a variable curvature microresonator are investigated by a two-dimensional finite element method for realizing stable dual-mode lasing. The microresonator features a smooth boundary and the same symmetry as a square resonator. A small variable-curvature microresonator with a radius of 4 μm can support the fundamental four-bounce mode and the circular-like mode simultaneously, with quality factors up to the order of 104 and 105, respectively. The dual modes in the phase space of the Poincaré surface of sections distribute far from each other and can maintain enough stability for dual-mode lasing. Furthermore, the refractive index and waveguide can modulate the dual-mode wavelength difference and quality factors efficiently thanks to the spatially separated fields of these two modes.

Photonics Research
Oct. 17, 2017, Vol. 5 Issue 6 06000695 (2017)
Graphene/phosphorene nano-heterojunction: facile synthesis, nonlinear optics, and ultrafast photonics applications with enhanced performance
Shunxiang Liu, Zhongjun Li, Yanqi Ge, Huide Wang, Rui Yue, Xiantao Jiang, Jianqing Li, Qiao Wen, and Han Zhang

Owing to its thickness-modulated direct energy band gap, relatively strong light–matter interaction, and unique nonlinear optical response at a long wavelength, few-layer black phosphorus, or phosphorene, becomes very attractive in ultrafast photonics applications. Herein, we synthesized a graphene/phosphorene nano-heterojunction using a liquid phase-stripping method. Tiny lattice distortions in graphene and phosphorene suggest the formation of a nano-heterojunction between graphene and phosphorene nanosheets. In addition, we systematically investigate their nonlinear optical responses at different wavelength regimes. Our experiments indicate that the combined advantages of ultrafast relaxation, broadband response in graphene, and the strong light–matter interaction in phosphorene can be combined together by nano-heterojunction. We have further fabricated two-dimensional (2D) nano-heterojunction based optical saturable absorbers and integrated them into an erbium-doped fiber laser to demonstrate the generation of a stable ultrashort pulse down to 148 fs. Our results indicate that a graphene/phosphorene nano-heterojunction can operate as a promising saturable absorber for ultrafast laser systems with ultrahigh pulse energy and ultranarrow pulse duration. We believe this work opens up a new approach to designing 2D heterointerfaces for applications in ultrafast photonics and other research. The fabrication of a 2D nano-heterojunction assembled from stacking different 2D materials, via this facile and scalable growth approach, paves the way for the formation and tuning of new 2D materials with desirable photonic properties and applications.

Photonics Research
Oct. 01, 2017, Vol. 5 Issue 6 06000662 (2017)
Passively spatiotemporal gain-modulation-induced stable pulsing operation of a random fiber laser
Jiangming Xu, Jun Ye, Wei Liu, Jian Wu, Hanwei Zhang, Jinyong Leng, and Pu Zhou

Unlike a traditional fiber laser with a defined resonant cavity, a random fiber laser (RFL), whose operation is based on distributed feedback and gain via Rayleigh scattering (RS) and stimulated Raman scattering in a long passive fiber, has fundamental scientific challenges in pulsing operation for its remarkable cavity-free feature. For the time being, stable pulsed RFL utilizing a passive method has not been reported. Here, we propose and experimentally realize the passive spatiotemporal gain-modulation-induced stable pulsing operation of counterpumped RFL. Thanks to the good temporal stability of an employed pumping amplified spontaneous emission source and the superiority of this pulse generation scheme, a stable and regular pulse train can be obtained. Furthermore, the pump hysteresis and bistability phenomena with the generation of high-order Stokes light is presented, and the dynamics of pulsing operation is discussed after the theoretical investigation of the counterpumped RFL. This work extends our comprehension of temporal property of RFL and paves an effective novel avenue for the exploration of pulsed RFL with structural simplicity, low cost, and stable output.

Photonics Research
Sep. 14, 2017, Vol. 5 Issue 6 06000598 (2017)
Watt-level broadly wavelength tunable mode-locked solid-state laser in the 2 μm water absorption region
Wei Zhou, Xiaodong Xu, Rui Xu, Xuliang Fan, Yongguang Zhao, Lei Li, Dingyuan Tang, and Deyuan Shen

We report on broadly wavelength-tunable passive mode-locking with high power operating at the 2 μm water absorption band in a Tm:CYA crystal laser. With a simple quartz plate, stable mode-locking wavelengths can be tuned from 1874 to 1973 nm, with a tunable wavelength range up to ~100 nm and maximum output power up to 1.35 W. The bandwidth is narrow as ~6 GHz, corresponding to a high coherence. To our knowledge, this is the first demonstration of wavelength-tunable mode-locking with watt-level in the 2 μm water absorption band. The high temporal coherent laser can be further applied in spectroscopy, the efficient excitation of molecules, sensing, and quantum optics.

Photonics Research
Sep. 14, 2017, Vol. 5 Issue 6 06000583 (2017)
High-peak-power large-angular-momentum beams generated from passively Q-switched geometric modes with astigmatic transformation
Y. F. Chen, C. C. Chang, C. Y. Lee, C. L. Sung, J. C. Tung, K. W. Su, H. C. Liang, W. D. Chen, and G. Zhang

The criterion of achieving efficient passive Q-switching is analyzed to design an off-axis pumped Nd:YVO4/Cr4+:YAG laser with a degenerate cavity. Experimental results reveal that pure high-order HG0,m or HGm,0 eigenmodes with the order m between 0 and 14 can be generated, depending on the off-axis displacement along the y axis or the x axis. On the other hand, lasing modes naturally turn into planar geometric modes when the off-axis displacement is larger than the value for exciting the HG0,m or HGm,0 eigenmodes with m>14. The overall peak powers for high-order eigenmodes or geometric modes can exceed 140 W. Furthermore, the high-order eigenmodes and geometric modes are employed to generate vortex beams with large orbital angular momentum by using an external cylindrical mode converter. Theoretical analyses are performed to confirm experimental results and to manifest the phase structures of the generated vortex beams.

Photonics Research
Sep. 17, 2017, Vol. 5 Issue 6 06000561 (2017)
Nanoparticle mediated microcavity random laser
Zhaoji Yang, Weili Zhang, Rui Ma, Xiang Dong, Sofie Lindskov Hansen, Xiaofeng Li, and Yunjiang Rao

This paper reports a coherent random microcavity laser that consists of a disordered cladding (scattering) layer and a light-amplification core filled with dye solution. Cold cavity analysis indicates that the random resonance modes supported by the proposed cavity can be effectively excited. With introducing the gain material, random lasing by specific modes is observed to show typical features of coherent random lasers, such as spatially incoherent emission of random modes. By inserting a metal nanoparticle into the gain region, emission wavelength/intensity of the random lasers can be considerably tuned by changing the position of the inserted nanoparticle, opening up new avenues for controlling output of random lasers and sensing applications (e.g., small particle identification, location, etc.).

Photonics Research
Sep. 08, 2017, Vol. 5 Issue 6 06000557 (2017)
Few-mode VCSEL chip for 100-Gb/s transmission over 100 m multimode fiber
Hsuan-Yun Kao, Yu-Chieh Chi, Cheng-Ting Tsai, Shan-Fong Leong, Chun-Yen Peng, Huai-Yung Wang, Jian Jang Huang, Jau-Ji Jou, Tien-Tsorng Shih, Hao-Chung Kuo, Wood-Hi Cheng, Chao-Hsin Wu, and Gong-Ru Lin

A few-mode (FM) vertical cavity surface emitting laser (VCSEL) chip with heavily zinc-diffused contact layer and oxide-confined cross-section is demonstrated for carrying pre-leveled 16-quadrature amplitude modulation orthogonal frequency division multiplexing (QAM-OFDM) data in OM4 multi-mode fiber (MMF) over 100 m for intra-data-center applications. The FM VCSEL chip, which has an oxide-confined emission aperture of 5 μm, demonstrates high external quantum efficiency, provides an optical power of 2.2 mW at 38 times threshold condition, and exhibits 3 dB direct-modulation bandwidth beyond 22 GHz at a cost of slight heat accumulation. At a DC bias point of 5 mA (22.6Ith) the FM VCSEL chip, with sufficiently normalized modulation output, supports Baud and data rates of 25 and 100 Gb/s, respectively, with forward error correction (FEC) certifying receiving quality after back-to-back transmission. After passing through 100 m OM4 MMF with a receiving power penalty of 4 dB, the FM VCSEL chip demonstrates FEC-certified transmission of the pre-leveled 16-QAM OFDM data at 92 Gb/s.

Photonics Research
Aug. 14, 2017, Vol. 5 Issue 5 05000507 (2017)
Enhancing the expansion of a plasma shockwave by crater-induced laser refocusing in femtosecond laser ablation of fused silica
Qingsong Wang, Lan Jiang, Jingya Sun, Changji Pan, Weina Han, Guoyan Wang, Hao Zhang, Costas P. Grigoropoulos, and Yongfeng Lu

The dynamics of plasma and shockwave expansion during two femtosecond laser pulse ablation of fused silica are studied using a time-resolved shadowgraph imaging technique. The experimental results reveal that during the second pulse irradiation on the crater induced by the first pulse, the expansion of the plasma and shockwave is enhanced in the longitudinal direction. The plasma model and Fresnel diffraction theory are combined to calculate the laser intensity distribution by considering the change in surface morphology and transient material properties. The theoretical results show that after the free electron density induced by the rising edge of the pulse reaches the critical density, the originally transparent surface is transformed into a transient high-reflectivity surface (metallic state). Thus, the crater with a concave-lens-like morphology can tremendously reflect and refocus the latter part of the laser pulse, leading to a strong laser field with an intensity even higher than the incident intensity. This strong refocused laser pulse results in a stronger laser-induced air breakdown and enhances the subsequent expansion of the plasma and shockwave. In addition, similar shadowgraphs are also recorded in the single-pulse ablation of a concave microlens, providing experimental evidence for the enhancement mechanism.

Photonics Research
Aug. 23, 2017, Vol. 5 Issue 5 05000488 (2017)
High-power passively Q-switched 2 μm all-solid-state laser based on a Bi2Te3 saturable absorber
X. Liu, K. Yang, S. Zhao, T. Li, W. Qiao, H. Zhang, B. Zhang, J. He, J. Bian, L. Zheng, L. Su, and J. Xu

By using the ultrasound-assisted liquid phase exfoliation method, Bi2Te3 nanosheets are synthesized and deposited onto a quartz plate to form a kind of saturable absorber (SA), in which nonlinear absorption properties around 2 μm are analyzed with a home-made mode-locked laser. With the as-prepared Bi2Te3 SA employed, a stable passively Q-switched all-solid-state 2 μm laser is successfully realized. Q-switched pulses with a maximum average output power of 2.03 W are generated under an output coupling of 5%, corresponding to the maximum single-pulse energy of 18.4 μJ and peak power of 23 W. The delivered shortest pulse duration and maximum repetition rate are 620 ns and 118 kHz under an output coupling of 2%, respectively. It is the first presentation of such Bi2Te3 SA employed in a solid-state Q-switched crystalline laser at 2 μm, to the best of our knowledge. In comparison with other 2D materials suitable for pulsed 2 μm lasers, the saturable absorption performance of Bi2Te3 SA is proved to be promising in generating high power and high-repetition-rate 2 μm laser pulses.

Photonics Research
Aug. 08, 2017, Vol. 5 Issue 5 05000461 (2017)
Q-switching of waveguide lasers based on graphene/WS2 van der Waals heterostructure
Ziqi Li, Chen Cheng, Ningning Dong, Carolina Romero, Qingming Lu, Jun Wang, Javier Rodríguez Vázquez de Aldana, Yang Tan, and Feng Chen

We report on the operation of passively Q-switched waveguide lasers at 1 μm wavelength based on a graphene/WS2 heterostructure as a saturable absorber (SA). The gain medium is a crystalline Nd:YVO4 cladding waveguide produced by femtosecond laser writing. The nanosecond waveguide laser operation at 1064 nm has been realized with the maximum average output power of 275 mW and slope efficiency of 37%. In comparison with the systems based on single WS2 or graphene SA, the lasing Q-switched by a graphene/WS2 heterostructure SA possesses advantages of a higher pulse energy and enhanced slope efficiency, indicating the promising applications of van der Waals heterostructures for ultrafast photonic devices.

Photonics Research
Jul. 10, 2017, Vol. 5 Issue 5 05000406 (2017)
Near-diffraction-limited linearly polarized narrow-linewidth random fiber laser with record kilowatt output
Jiangming Xu, Long Huang, Man Jiang, Jun Ye, Pengfei Ma, Jinyong Leng, Jian Wu, Hanwei Zhang, and Pu Zhou

In this paper, we propose and experimentally investigate a linearly polarized narrow-linewidth random fiber laser (RFL) operating at 1080 nm and boost the output power to kilowatt level with near-diffraction-limited beam quality using a master oscillation power amplifier. The RFL based on a half-opened cavity, which is composed of a linearly polarized narrow-linewidth fiber Bragg grating and a 500 m piece of polarization-maintained Ge-doped fiber, generates a 0.71 W seed laser with an 88 pm full width at half-maximum (FWHM) linewidth and a 22.5 dB polarization extinction ratio (PER) for power scaling. A two-stage fiber amplifier enhances the seed laser to the maximal 1.01 kW with a PER value of 17 dB and a beam quality of Mx2=1.15 and My2=1.13. No stimulated Brillouin scattering effect is observed at the ultimate power level, and the FWHM linewidth of the amplified random laser broadens linearly as a function of the output power with a coefficient of about 0.1237 pm/W. To the best of our knowledge, this is the first demonstration of a linearly polarized narrow-linewidth RFL with even kilowatt-level near-diffraction-limited output, and further performance scaling is ongoing.

Photonics Research
Jul. 10, 2017, Vol. 5 Issue 4 04000350 (2017)
High power and beam quality continuous-wave Nd:GdVO4 laser in-band diode-pumped at 912  nm
Mohammad Nadimi, Tanant Waritanant, and Arkady Major

We report on the performance of a continuous-wave Nd:GdVO4 laser in-band diode-pumped at 912 nm with high output power and excellent beam quality. The laser produced an output power of 19.8 W at 1063 nm with an optical efficiency of 59.3% and slope efficiency of 62.7%. The laser threshold was ~2.04 W of the absorbed pump power, and laser output beam quality was ≤1.2 in the horizontal and vertical directions. The strength of thermal lensing at full output power (33.4 W of absorbed power) was measured to be an average of 8.6 diopters. It is shown that thermal lensing is reduced by a factor of 2 with respect to the Nd:YVO4 lasers, thus opening a way for further output-power scaling.

Photonics Research
Jul. 05, 2017, Vol. 5 Issue 4 04000346 (2017)
Stable single-mode operation of a distributed feedback quantum cascade laser integrated with a distributed Bragg reflector
Feng-Min Cheng, Zhi-Wei Jia, Jin-Chuan Zhang, Ning Zhuo, Shen-Qiang Zhai, Li-Jun Wang, Jun-Qi Liu, Shu-Man Liu, Feng-Qi Liu, and Zhan-Guo Wang

We report an index-coupled distributed feedback quantum cascade laser by employing an equivalent phase shift (EPS) of quarter-wave integrated with a distributed Bragg reflector (DBR) at λ~5.03 μm. The EPS is fabricated through extending one sampling period by 50% in the center of a sampled Bragg grating. The key EPS and DBR pattern are fabricated by conventional holographic exposure combined with the optical photolithography technology, which leads to improved flexibility, repeatability, and cost-effectiveness. Stable single-mode emission can be obtained by changing the injection current or heat sink temperature even under the condition of large driving pulse width.

Photonics Research
Jun. 29, 2017, Vol. 5 Issue 4 04000320 (2017)
Effects of gain nonlinearities in an optically injected gain lever semiconductor laser
J. M. Sarraute, K. Schires, S. LaRochelle, and F. Grillot

The effects of gain compression on the modulation dynamics of an optically injected gain lever semiconductor laser are studied. Calculations reveal that the gain compression is not necessarily a drawback affecting the laser dynamics. With a practical injection strength, a high gain lever effect and a moderate compression value allow us to theoretically predict a modulation bandwidth four times higher than the free-running one without a gain lever, which is of paramount importance for the development of directly modulated broadband optical sources compatible with short-reach communication links.

Photonics Research
On the CoverJun. 29, 2017, Vol. 5 Issue 4 04000315 (2017)
Exploiting a metal wire grating in total internal reflection geometry to achieve achromatic polarization conversion
Xudong Liu, Xuequan Chen, Edward P. J. Parrott, and Emma Pickwell-MacPherson

We demonstrate how a metal wire grating can work as a 45° polarization converter, a quarter-wave retarder, and a half-wave retarder over a broadband terahertz range when set up in total internal reflection geometry. Classical electromagnetic theory is applied to understand the mechanism, and equations to calculate the polarization state of reflected light are derived. We use a metal grating with a period of 20 μm and width of 10 μm on a fused silica surface: linearly polarized terahertz light incident from fused silica with a supercritical incident angle of 52° is totally reflected by the metal grating and air. The polarization of the terahertz light is rotated by 45°, 90°, and circularly polarized by simply rotating the wire grating. The performance is achromatic over the measured range of 0.1–0.7 THz and comparable to commercial visible light wave retarders.

Photonics Research
Jun. 14, 2017, Vol. 5 Issue 4 04000299 (2017)
Fiber laser with random-access pulse train profiling for a photoinjector driver
Ekaterina I. Gacheva, Anatoly K. Poteomkin, Sergey Yu. Mironov, Viktor V. Zelenogorskii, Efim A. Khazanov, Konstantin B. Yushkov, Alexander I. Chizhikov, and Vladimir Ya. Molchanov

We report on the design and performance of a fiber laser system with adaptive acousto-optic macropulse control for a novel photocathode laser driver with 3D ellipsoidal pulse shaping. The laser system incorporates a three-stage fiber amplifier with an integrated acousto-optical modulator. A digital electronic control system with feedback combines the functions of the arbitrary micropulse selection and modulation resulting in macropulse envelope profiling. As a benefit, a narrow temporal transparency window of the modulator, comparable to a laser pulse repetition period, effectively improves temporal contrast. In experiments, we demonstrated rectangular laser pulse train profiling at the output of a three-cascade Yb-doped fiber amplifier.

Photonics Research
Jun. 14, 2017, Vol. 5 Issue 4 04000293 (2017)
Multipoint fiber-optic laser–ultrasound generation along a fiber based on the core-offset splicing of fibers
Xiaolong Dong, Shimin Gao, Jiajun Tian, and Yong Yao

Fiber-optic laser–ultrasound generation is being used in an increasing number of applications, including medical diagnosis, material characterization, and structural health monitoring. However, most currently used fiber-optic ultrasonic transducers allow effective ultrasound generation at only a single location, namely, at the fiber tip, although there have been a few limited proposals for achieving multipoint ultrasound generation along the length of a fiber. Here we present a novel fiber-optic ultrasound transducer that uses the core-offset splicing of fibers to effectively generate ultrasound at multiple locations along the fiber. The proposed laser–ultrasonic transducer can produce a balanced-strength signal between ultrasonic generation points by reasonably controlling the offsets of the fibers. The proposed transducer has other outstanding characteristics, including simple fabrication and low cost.

Photonics Research
Jun. 14, 2017, Vol. 5 Issue 4 04000287 (2017)
Fiber laser for on-demand mode generation in 1550  nm band
Chenghui Tian, Song Yu, Shanyong Cai, Mingying Lan, and Wanyi Gu

Transverse mode characteristics of a laser are related to a variety of interesting applications. An on-demand mode solid laser in the 1064 nm band was proposed previously. In this paper, we provide a fiber laser for on-demand modes in the 1550 nm band to prescribe the pure and high-quality emission of a higher-order transverse laser mode, based on a simple construction with one spatial light modulator (SLM) and a single-mode erbium-doped fiber (SM-EDF). The SLM is designated to generate the desired higher-order mode and separate the higher-order mode and the fundamental mode. The fundamental mode oscillates in the fiber ring laser, and therefore the SM-EDF can be pumped with a single-mode 980 nm laser, no matter what higher-order mode is prescribed. In this proof-of-principle experiment, high-quality higher-order modes are observed from LP01 to LP105. Stable emission and real-time switching between modes can be easily realized by altering the phase on the SLM. In addition, the propagation of the LP01, LP11, LP21, and LP02 modes from the fiber laser is also demonstrated in a four-mode few-mode fiber.

Photonics Research
May. 30, 2017, Vol. 5 Issue 3 03000256 (2017)
Methods to improve the performance of the swept source at 1.0  μm based on a polygon scanner
Jing Cao, Pinghe Wang, Yan Zhang, Guohua Shi, Bo Wu, Shangjian Zhang, and Yong Liu

In this work, we investigate the methods to improve the performance of the swept source at 1.0 μm based on a polygon scanner, including in-cavity parameters and booster structures out of the cavity. The three in-cavity parameters are the cavity length, the rotating speed of the polygon scanner, and the in-cavity energy. With the decrease of cavity length, the spectrum bandwidth becomes wider and the duty cycle becomes higher. With the increase of the rotating speed of the polygon, the spectrum bandwidth becomes narrower, and the duty cycle becomes lower but the repetition rate becomes higher. With more energy in-cavity, the spectrum bandwidth becomes wider and the duty cycle becomes higher. The booster structures include the buffered structure, secondary amplifier, and dual-semiconductor optical amplifier configuration, which are used to increase the sweep frequency to 86 kHz, the output power to 18 mW, and the tuning bandwidth to 131 nm, respectively.

Photonics Research
May. 23, 2017, Vol. 5 Issue 3 03000245 (2017)
Wavelength-swept fiber laser based on bidirectional used linear chirped fiber Bragg grating
Lin Wang, Minggui Wan, Zhenkun Shen, Xudong Wang, Yuan Cao, Xinhuan Feng, and Bai-ou Guan

A wavelength-swept fiber laser is proposed and successfully demonstrated based on a bidirectional used linear chirped fiber Bragg grating (LC-FBG). The wavelength-swept operation principle is based on intracavity pulse stretching and compression. The LC-FBG can introduce equivalent positive and negative dispersion simultaneously, which enables a perfect dispersion matching to obtain wide-bandwidth mode-locking. Experimental results demonstrate a wavelength-swept fiber laser that exhibits a sweep rate of about 5.4 MHz over a 2.1 nm range at a center wavelength of 1550 nm. It has the advantages of simple configuration and perfect dispersion matching in the laser cavity.

Photonics Research
Apr. 18, 2017, Vol. 5 Issue 3 03000219 (2017)
Demonstration of an ultra-low-threshold phonon laser with coupled microtoroid resonators in vacuum
Guanzhong Wang, Mingming Zhao, Yingchun Qin, Zhangqi Yin, Xiaoshun Jiang, and Min Xiao

We demonstrate an ultra-low-threshold phonon laser using a coupled-microtoroid-cavity system by introducing a novel coupling approach. The scheme exhibits both high optical quality factors and high mechanical quality factors. We have experimentally obtained the mechanical quality factor up to 18,000 in vacuum for a radial-breathing mode of 59.2 MHz. The measured phonon lasing threshold is as low as 1.2 μW, which is ~5 times lower than the previous result.

Photonics Research
On the CoverFeb. 09, 2017, Vol. 5 Issue 2 02000073 (2017)
Self-starting passively mode-locked all fiber laser based on carbon nanotubes with radially polarized emission
Yong Zhou, Jian Lin, Xiaoqiang Zhang, Lixin Xu, Chun Gu, Biao Sun, Anting Wang, and Qiwen Zhan

We demonstrate an all fiber passively mode-locked laser emitting a radially polarized beam by using a few-mode fiber Bragg grating to achieve mode selection and spectrum filtering. An offset splicing of single-mode fiber with four-mode fiber is utilized as a mode coupler in the laser cavity. Carbon nanotubes are introduced into the laser cavity as the saturable absorber to achieve self-start mode locking. The laser operates at 1547.5 nm with a narrow spectrum width of 0.3 nm at 30 dB. The emitted mode-locked pulses have a duration of 22.73 ps and repetition of 10.61 MHz. A radially polarized beam has been obtained with high mode purity by adjusting the polarization in the laser cavity.

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 6 06000327 (2016)
Midinfrared optical frequency comb based on difference frequency generation using high repetition rate Er-doped fiber laser with single wall carbon nanotube film
M. Tsuzuki, L. Jin, M. Yamanaka, V. Sonnenchein, H. Tomita, A. Sato, T. Ohara, Y. Sakakibara, E. Omoda, H. Kataura, T. Iguchi, and N. Nishizawa

We demonstrated stable midinfrared (MIR) optical frequency comb at the 3.0 μm region with difference frequency generation pumped by a high power, Er-doped, ultrashort pulse fiber laser system. A soliton mode-locked 161 MHz high repetition rate fiber laser using a single wall carbon nanotube was fabricated. The output pulse was amplified in an Er-doped single mode fiber amplifier, and a 1.1–2.2 μm wideband supercontinuum (SC) with an average power of 205 mW was generated in highly nonlinear fiber. The spectrogram of the generated SC was examined both experimentally and numerically. The generated SC was focused into a nonlinear crystal, and stable generation of MIR comb around the 3 μm wavelength region was realized.(AMED).

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 6 06000313 (2016)
Power-scaled dissipative soliton using double-claddingpumped Yb-doped all-fiber amplifier
Mohamed A. Abdelalim, Hussein E. Kotb, Hanan Anis, and Serguei Tchouragoulov

We report on an all-fiber oscillator followed by an all-fiber amplifier to produce as short as 382 fs laser pulses with up to 0.9Waverage power. The oscillator is an all-normal-dispersion all-fiber dissipative soliton laser operating at 1030 nm, and operating in dissipative soliton mode. The amplifier stage is mainly based on a double-cladding 20 μm radius ytterbium-doped fiber pumped by an up to 2.5WCWlaser source. The optical-to-optical conversion amplifier efficiency is around 40%. To our knowledge, this is the first report of an all-fiber mode-locked fiber laser oscillator amplified by an all-fiber amplifier.

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 6 06000277 (2016)
High-peak-power passivelyQ-switched Nd:YAG/Cr4+:YAG composite laser with multiple-beam output
T. Dascalu, G. Croitoru, O. Grigore, and N. Pavel

We report on the design, realization, and output performance of a diode-pumped high-peak-power passively Q-switched Nd:YAG∕Cr4:YAG composite medium onolithic laser with four-beam output. The energy of a laser pulse was higher than 3 mJ with duration of 0.9 ns. The proposed system has the ability to choose independently the focus of each beam. Such a laser device can be used for multipoint ignition of an automobile gasoline engine, but could also be of interest for ignition in space propulsion or in turbulent conditions specific to aeronautics.

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 6 06000267 (2016)
Subnanosecond KTiOPO4 optical parametric oscillator intracavity pumped by a Kerr-lens, mode-locked YVO4/Nd:YVO4 laser coupled with an acousto-optic modulator
Hongwei Chu, Jia Zhao, Kejian Yang, Shengzhi Zhao, Tao Li, Dechun Li, Guiqiu Li, and Wenchao Qiao

A Kerr-lens, mode-locked YVO4∕Nd:YVO4 laser coupled with an acousto-optic modulator (AOM) Q-switching near 1064 nm was employed to pump an intracavity KTiOPO4 (KTP) optical parametric oscillator. A subnanosecond signal wave near 1572 nm with low repetition rate was realized. At an AOM repetition rate of 8 kHz, the maximum output power was 165 mW. The highest average pulse energy, the shortest duration, and the highest peak power of a mode-locking signal pulse were estimated to be ~10.3 μJ, ~120 ps, and ~82 kW, respectively.

Photonics Research
Jan. 01, 1900, Vol. 3 Issue 5 05000260 (2015)
Compact Q-switched 2 μm Tm:GdVO4 laser with MoS2 absorber
Pingguang Ge, Jie Liu, Shouzhen Jiang, Yuanyuan Xu, and Baoyuan Man

A molybdenum disulfide (MoS2) saturable absorber was fabricated by thermally decomposing the ammonium thiomolybdate. By using the MoS2 absorber, a compact diode-pumped passively Q-switched Tm:GdVO4 laser has been demonstrated. A stable Q-switched laser with repetition rates from 25.58 to 48.09 kHz was achieved. Maximum average output power was 100 mW with the shortest pulse duration of 0.8 μs. Maximum pulse energy is 2.08 μJ at center of 1902 nm.

Photonics Research
Aug. 31, 2015, Vol. 3 Issue 5 05000256 (2015)
73 nJ, 109 fs Yb-doped fiber laser at 19 MHz repetition rate in amplifier similariton regime
Yizhou Liu, Yifan Cui, Jian Zhang, Aimin Wang, and Zhigang Zhang

We report femtosecond pulse generation in an amplifier similariton oscillator and a prechirped fiber amplifier system. The final output power is 1.4 W, and the fundamental repetition rate is 19.1 MHz after a single state fiber amplifier. The pulsewidth is 109 fs.

Photonics Research
Aug. 25, 2015, Vol. 3 Issue 5 05000248 (2015)
Propagation of high-power fiber laser with high-order-mode content
Rumao Tao, Long Huang, Pu Zhou, Lei Si, and Zejin Liu

Propagation properties of high-power fiber laser with high-order-mode (HOM) content are studied numerically for the first time to the best of our knowledge. The effect of HOM on the propagation property is evaluated by the power in the bucket (PIB) metric. It is shown that PIB is mainly dependent on HOM content rather than the relative phase between the fundamental mode and HOM. The PIB in vacuum is more than 80% when the power fraction of the HOM is controlled to be less than 50% at 5 km. The relative phase has an impact on the peak intensity position and concentration of the far-field intensity distribution. If an adaptive optics system is used to correct the peak intensity deviation, the results indicate that there exists a maximal value of PIB as relative phase increases. Such effect is weakened when propagating in turbulence. Compared to the laser beams without HOM, laser beams with HOM content are less influenced by the turbulence and can reduce average intensity fluctuation. The results may be useful in the design of a high-power fiber laser system.

Photonics Research
May. 11, 2015, Vol. 3 Issue 4 04000192 (2015)
Modification of wettability of stainless steel by picosecond laser surface microstructuring
Zongjie Bao, Chengwei Wang, Yang Zhang, and Quan-Zhong Zhao

We report on the modification of the wettability of stainless steel by picosecond laser surface microstructuring in this paper. Compared with traditional methods, picosecond laser-induced surface modification provides a fast and facile method for surface modification without chemical damage and environmental pollution. As a result of treatment by 100 ps laser pulses, microstructures are fabricated on the stainless steel sample surface, contributing to the increase of the contact angle from 88° to 105°, which realizes a transformation from hydrophilicity to hydrophobicity. The morphological features of fabricated microstructures are characterized by scanning electron microscopy and optical microscopy.

Photonics Research
Jun. 06, 2015, Vol. 3 Issue 4 04000180 (2015)
Optimization of spectral distortion in a ytterbium-doped mode-locked fiber laser system
Fuqiang Lian, Zhongwei Fan, Zhenao Bai, Xiaohui Li, and Qi Jie Wang

A method for optimizing the spectral distortion of an ultrafast pulse in a polarization-maintaining picosecond linear-cavity fiber laser with a one-stage fiber amplifier is proposed and demonstrated. The mechanism of control of the spectral distortion in the fiber system has been investigated. The experimental and theoretical results illustrate that the filtering effect of a fiber Bragg grating can effectively decrease the spectral oscillatory distortion accumulated by self-phase modulation. Injected into a Nd:YAG regenerative amplifier, the ultrafast pulse could produce high pulse energy of 1.2 mJ at a repetition rate of 1 kHz.

Photonics Research
May. 21, 2015, Vol. 3 Issue 4 04000129 (2015)
Graphene oxide paper as a saturable absorber for Er- and Tm-doped fiber lasers
Jakub Boguslawski, Jaroslaw Sotor, Grzegorz Sobon, Rafal Kozinski, Krzysztof Librant, Magdalena Aksienionek, Ludwika Lipinska, and Krzysztof M. Abramski

In this work pulse generation in both the 1.5 and 2 μm spectral ranges using a graphene oxide (GO)-paper-based saturable absorber in Er- and Tm-doped fiber lasers is presented. The article describes the fabrication method of GO paper and its characterization. The performance of both lasers is discussed in detail. Stable, mode-locked operation provides 613 fs and 1.36 ps soliton pulses centered at 1565.9 and 1961.6 nm in Er- and Tm-doped fiber lasers, respectively. Furthermore, scaling of spectral width, and hence the pulse duration, by increasing the number of GO paper layers in the Er-doped laser is described. The versatility and simplicity ofGOpaper fabrication combined with the possibility of scaling the optical spectrum full width at half-maximum are essential features that make it a good candidate for ultrafast low-power mode-locked lasers operating in different spectral regions.

Photonics Research
Jan. 01, 1900, Vol. 3 Issue 4 04000119 (2015)
Theoretical analysis of a quasi-Bessel beam for laser ablation
Pinghui Wu, Chenghua Sui, and and Wenhua Huang

A quasi-Bessel beam (QBB) is suitable for laser ablation because it possesses a micrometer-sized focal spot and long depth of focus simultaneously. In this paper, the characterizations of QBBs formed by the ideal axicon and oblate-tip axicon are described. Strong on-axis intensity oscillations occur due to interference between the QBB and the refracted beam by the oblate tip. Using the axicon for laser ablation was theoretically investigated. Simple analytical formulas can be used to predict the required laser parameters, including the laser pulse energy, the generated fluence distributions, and the beam diameters.

Photonics Research
Jan. 01, 1900, Vol. 2 Issue 3 03000082 (2014)
Coherent beam combination of fiber lasers with a strongly confined tapered self-imaging waveguide: theoretical modeling and simulation
Rumao Tao, Xiaolin Wang, Hu Xiao, Pu Zhou, and Lei Si

Coherent beam combination (CBC) of fiber lasers based on self-imaging properties of a strongly confined tapered waveguide (SCTW) is studied systematically. Analytical formulas are derived for the positions, amplitudes, and phases of the N self-images at the output of a SCTW, which are important for quantitative analysis of waveguide-based CBC. The formulas are verified with numerical examples by a finite difference beam propagation method (FDBPM) and the errors of the analytical expressions are studied. This study shows that the analytical formulas agree well with the FDBPM simulation results when the taper angle is less than 1.4° and the phase distortion is less than λ/10. The relative errors increase as the taper angle increases. Based on the theoretical model and the FDBPM, we simulated the CBC of fiber laser array and compared the CBC based on the tapered waveguide with that based on the nontapered one. The effects of input beam number, aperture fill factor, and taper angle on the combination performance have been studied. The study reveals that a beam which has near-diffraction limited beam quality (M2≤1.41) and a single beam without side lobe in the far field can be achieved with tapered-waveguide-based CBC. It is shown that beam quality depends on input beam number, aperture fill factor, and taper angle. There exists a best fill factor which will increase as input beam number increases. The tolerance of the system on the fill factor and the taper angle is studied, which is 0.45t0.67 and θ0.8°, respectively. The results may be useful for practical, high-power fiber laser systems.

Photonics Research
Oct. 07, 2013, Vol. 1 Issue 4 04000186 (2013)
Engineered quasi-phase-matching for laser techniques [Invited]
X. P. Hu, P. Xu, and S. N. Zhu

The quasi-phase-matching (QPM) technique has drawn increasing attention due to its promising applications in areas such as nonlinear frequency conversion for generating new laser light sources. In this paper, we will briefly review the main achievements in this field.Wegive a brief introduction of the invention ofQPMtheory, followed by the QPM-material fabrication techniques. When combing QPM with the solid-state laser techniques, various laser light sources, such as single-wavelength visible lasers and ultraviolet lasers, red–green–blue three-fundamentalcolor lasers, optical parametric oscillators in different temporal scales, and passive mode-locking lasers based on cascaded second-order nonlinearity, have been presented. The QPM technique has been extended to quantum optics recently, and prospects for the studies are bright.

Photonics Research
Jan. 01, 1900, Vol. 1 Issue 4 04000171 (2013)
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