We experimentally demonstrate third-harmonic generation (THG) in gases ionized by a femtosecond laser pulse superimposed on its second-harmonic (SH). The mechanism of THG has been investigated, and it demonstrates that a third-order nonlinear process dominates at low pump intensity. Asymmetric third-harmonic (TH) spectra are observed at different time delays in two color fields, which are attributed to the process of the four-wave mixing (FWM) of the broad spectrum of pump pulses. A joint measurement on the terahertz (THz) and the TH is performed. It reveals that the optimized phase for the THG jumps from 0 to 0.5π as the pump intensity increases, which is different from the THz being a constant, and indicates that the THG arises from the nonlinearity of the third-order bound electrons to the tunnel-ionization current.

Multiphoton microscopy is the enabling tool for biomedical research, but the aberrations of biological tissues have limited its imaging performance. Adaptive optics (AO) has been developed to partially overcome aberration to restore imaging performance. For indirect AO, algorithm is the key to its successful implementation. Here, based on the fact that indirect AO has an analogy to the black-box optimization problem, we successfully apply the covariance matrix adaptation evolution strategy (CMA-ES) used in the latter, to indirect AO in multiphoton microscopy (MPM). Compared with the traditional genetic algorithm (GA), our algorithm has a greater improvement in convergence speed and convergence accuracy, which provides the possibility of realizing real-time dynamic aberration correction for deep in vivo biological tissues.

In this Letter, the optical amplification characteristics of the home-made Bi/P co-doped silica fiber were systematically explored in the range of 1270–1360 nm. The maximum gain of 24.6 dB was obtained in the single-pass amplification device, and then improved to 38.3 dB in the double-pass amplification device for -30 dBm signal power. In addition, we simultaneously investigated the laser performance of the fiber with the linear cavity. A slope efficiency of 16.4% at ∼1313 nm was obtained with a maximum output power of about 133 mW under the input pump power of 869 mW at 1240 nm. As far as we know, it is the first laser reported based on the bismuth-doped fiber in China.

In this paper, we propose and experimentally demonstrate a joint shaping technique to improve the performance of a low-resolution transmission system for the first time, to the best of our knowledge. The joint shaping technique combines probabilistic shaping (PS) and error feedback noise shaping (EFNS). In the 40-Gbaud intensity-modulation direct-detection (IM/DD) experimental transmission system, a bit-error-rate (BER) of 3.8×10-3 can be achieved easily with the joint shaping at the physical number of bits (PNOB) of 3. In the 30-Gbaud dual polarization (DP) coherent experimental transmission system, a BER below 1×10-3 is easily obtained with a 3-bit quantizer by using joint shaping. The optimization of the shaping degree is also analyzed.

Polarized hyperspectral imaging, which has been widely studied worldwide, can obtain four-dimensional data including polarization, spectral, and spatial domains. To simplify data acquisition, compressive sensing theory is utilized in each domain. The polarization information represented by the four Stokes parameters currently requires at least two compressions. This work achieves full-Stokes single compression by introducing deep learning reconstruction. The four Stokes parameters are modulated by a quarter-wave plate (QWP) and a liquid crystal tunable filter (LCTF) and then compressed into a single light intensity detected by a complementary metal oxide semiconductor (CMOS). Data processing involves model training and polarization reconstruction. The reconstruction model is trained by feeding the known Stokes parameters and their single compressions into a deep learning framework. Unknown Stokes parameters can be reconstructed from a single compression using the trained model. Benefiting from the acquisition simplicity and reconstruction efficiency, this work well facilitates the development and application of polarized hyperspectral imaging.

Multi-channel detection is an effective way to improve data throughput of spectral-domain optical coherence tomography (SDOCT). However, current multi-channel OCT requires multiple detectors, which increases the complexity and cost of the system. We propose a novel multi-channel detection design based on a single spectrometer. Each camera pixel receives interferometric spectral signals from all the channels but with a spectral shift between two channels. This design effectively broadens the spectral bandwidth of each pixel, which reduces relative intensity noise (RIN) by √M times with M being the number of channels. We theoretically analyzed the noise of the proposed design under two cases: shot-noise limited and electrical noise or RIN limited. We show both theoretically and experimentally that this design can effectively improve the sensitivity, especially for electrical noise or RIN-dominated systems.

We proposed and experimentally demonstrated an all-fiber sensor for measuring bend with high sensitivity based on a ring core fiber (RCF) modal interferometer. The sensor was fabricated by splicing a segment of RCF between two pieces of multimode fiber (MMF) and single-mode fiber (SMF) at the ends of the MMF as lead-in and lead-out. Due to the first segment of the MMF, the transmitted light is coupled into the ring core, silica center, and cladding of the RCF, exciting multiple modes in the RCF. By the modal interferences in the structure, bending sensing can be realized by interrogating the intensity of the interference dip. Experimental results show a high bending sensitivity of -25.63 dB/m-1 in the range of 1.0954 m-1 to 1.4696 m-1. In addition, the advantages of the bend sensor, such as small size, low temperature sensitivity, and simple fabrication process, can be used for curvature measurement in building health monitoring.

The microring resonator based on lithium niobate on insulator (LNOI) is a promising platform for broadband nonlinearity process because of its strong second-order nonlinear coefficients, the capability of dispersion engineering, etc. It is important to control the energy transmitted into the resonator at different wavelengths, as this becomes difficult for two bands across an octave. In this Letter, we study the effect of different pulley bus-resonator configurations on phase mismatching and mode field overlap. We achieve the control of energy transmission coefficients at different wavebands simultaneously and provide a general design methodology for coupled structures for broadband applications. This paper can contribute to quantum and classical optical broadband applications based on LNOI microring resonators.

A high-energy 100-Hz optical parametric oscillator (OPO) based on a confocal unstable resonator with a Gaussian reflectivity mirror was demonstrated. A KTA-based OPO with a good beam quality was obtained when the magnification factor was 1.5, corresponding to the maximum signal (1.53 µm) energy of 56 mJ and idler (3.47 µm) energy of 20 mJ, respectively. The beam quality factors (M2) were measured to be M2x = 5.7, M2y = 5.9 for signal and M2x = 8.4, M2y = 8.1 for idler accordingly. The experimental results indicated that the beam quality positively changed with the increase of magnification factors, accompanied by an acceptable loss of pulse energy.

High-repetition rate femtosecond lasers are shown to drive heat accumulation processes that are attractive for femtosecond laser-induced subwavelength periodic surface structures on silicon. Femtosecond laser micromachining is no longer a nonthermal process, as long as the repetition rate reaches up to 100 kHz due to heat accumulation. Moreover, a higher repetition rate generates much better defined ripple structures on the silicon surface, based on the fact that accumulated heat raises lattice temperature to the melting point of silicon (1687 K), with more intense surface plasmons excited simultaneously. Comparison of the surface morphology on repetition rate and on the overlapping rate confirms that repetition rate and pulse overlapping rate are two competing factors that are responsible for the period of ripple structures. Ripple period drifts longer because of a higher repetition rate due to increasing electron density; however, the period of laser structured surface is significantly reduced with the pulse overlapping rate. The Maxwell–Garnett effect is confirmed to account for the ripple period-decreasing trend with the pulse overlapping rate.

A monolithic visible supercontinuum (SC) source with a record average output power of 204 W and a spectrum ranging from 580 nm to beyond 2400 nm is achieved in a piece of standard telecom graded-index multimode fiber (GRIN MMF) by designing the pumping system. The influence of the GRIN MMF length on the geometrical parameter instability (GPI) effect is analyzed for the first time, to the best of our knowledge, by comparing the SC spectral region dominated by the GPI effect under different fiber lengths. Our work could pave the way for robust, cost-effective, and high-power visible SC sources.

The high peak power of picosecond pulses produced by a self-mode-locked semiconductor disk laser can effectively improve the efficiency of nonlinear frequency conversion. This paper presents the intracavity frequency tripling in a self-mode-locked semiconductor disk laser, and a picosecond pulse train at 327 nm wavelength is achieved. The pulse repetition rate is 0.49 GHz, and the pulse width is 5.0 ps. The obtained maximum ultraviolet output power under mode locking is 30.5 mW, and the corresponding conversion efficiency is obviously larger than that of continuous-wave operation. These ultraviolet picosecond pulses have high spatial and temporal resolution and can be applied in some emerging fields.

SrMoO3 (SMO) thin films are deposited on LaAlO3 substrates by magnetron sputtering. The effects of ambient temperature on the structural, electrical, and optical properties of the films are investigated. As the temperature increases from 23°C to 800°C, the SMO film exhibits high crystallinity and low electrical resistivity, and the real part of dielectric functions becomes less negative in the visible and near-IR wavelength range, and the epsilon near zero (ENZ) wavelength increases from 460 nm to 890 nm. The optical loss of the SMO film is significantly lower than that of Au, and its plasmonic performance is comparable to or even higher than TiN in the temperature range of 23°C to 600°C. These studies are critical for the design of high-temperature SMO-based plasmonic devices.

We implement Monte Carlo-based parallel ray tracing to achieve quick irradiance evaluation for freeform lenses with non-uniform rational B-splines (NURBS) surfaces. We employ the inverse transform sampling method to sample rays uniformly from the Lambertian light source and adopt the analytical form of the B-spline basis function to achieve fast surface interpolation. When performing parallel calculations for the intersections between the rays and the NURBS surfaces, we propose a parameter transformation method to avoid the parameters escaping from the defined range in the iteration process. Simulation results of two complex picture-generating freeform lenses show that our method is fast and effective.

Bismuth (Bi)-doped near-infared (NIR) glass that can cover the entire optical communication window (850, 1310, and 1550 nm) has become the subject of extensive research for developing photonic devices, particularly, tunable fiber lasers and ultrabroadband optical amplifiers. However, the realization of highly efficient NIR luminescence from Bi-doped glass is still full of challenges. Notably, due to the co-existence of multiple Bi NIR centers in the glass, the origin of newly generated Bi NIR emission peaks at ∼930 and ∼1520 nm is still controversial. Here, we report a new Bi-doped nitridated germanate glass with tunable ultrabroadband NIR emission (850–1700 nm) and high external quantum efficiency (EQE) of ∼50%. A series of studies, including spectral analysis, nuclear magnetic resonance (NMR), and others, provide powerful evidence for the mechanism of luminescence enhancement and tunability, and make reasonable inferences about the origin of the new emission bands at ∼930 and ∼1520 nm. We believe that the results discussed above would enrich our understanding about multiple Bi NIR emission behaviors and contribute to the design and fabrication of highly efficient Bi-doped ultrabroadband wavelength-tunable optical glass fiber amplifiers and lasers in the future.

Dysprosium-doped orthorhombic yttrium aluminate (Dy:YAlO3 or Dy:YAP) single crystals were grown by the Czochralski method with a size of Φ43 mm×150 mm. Based on the measurements of spectra and theoretical analysis, the white-light emission was investigated with different doping concentrations. The optimal white emission was achieved at Dy3+ doping concentration of 1.0% under 450 nm excitation. Combining with residual pumping light, the white-light output was successfully obtained with Commission Internationale de l´Eclairage (CIE) coordinates x=0.3797, y=0.3685, the color temperature of 4000 K, and the largest fluorescence quantum yield of 46.9%. With the development of the GaN laser diode, the Dy:YAP single crystal has proven applicable in white-light-emitting diodes.

Rare-earth-doped upconversion (UC) materials are ideal candidates for solar photovoltaic conversion and NIR response devices due to their unique spectral conversion properties. However, their low efficiency remains a tremendous challenge for practical applications. Here, we constructed an efficient NIR light-responsive device by coating a Si-photoresistor with a transparent gel consisting of UC powders and an organic polymer matrix. We show that reasonable introduction of alkali metal ions (Na+, K+, and Cs+) into the lattice of UC crystals results in the improvement of photoelectricity conversion efficiency, due to the high crystallinity and surface reconstruction caused by alkali metal ion doping.

Ga2O3-based avalanche photodetectors (APDs) have gained increasing attention because of their excellent photoelectric conversion capability in the UV solar-blind region. Integrating high-quality epitaxial Ga2O3 with p-type semiconductor remains an open challenge associated with the integration difficulty on alleviating its defects and dislocations. Herein, we construct an APD consisting of epitaxial β-Ga2O3/La0.8Ca0.2MnO3 heterostructure. The pn junction APDs exhibit a high responsivity of 568 A/W as well as an enhanced avalanche gain of up to 3.0×105 at a reverse bias voltage of 37.9 V. The integration capability demonstrated in this work provides exciting opportunities for further development of high-performance Ga2O3-based electronics and optoelectronics.

In this paper, the influence of the delay time between the pre-pulse and the main pulse on the double-pass amplified 46.9 nm laser was studied for the first time, to the best of our knowledge, by using a high-precision polished SiC slice as a rear mirror. The temporal and spatial characteristics of the output laser were measured separately to investigate the effect of the delay time on the laser characteristics. The energy of the double-pass amplified laser was between 510 µJ and 890 µJ. In addition, a theoretical model of double-pass amplification was established to analyze the effect of the delay time on the double-pass amplified 46.9 nm laser.