In this paper, we propose and demonstrate a dual-beam delay-encoded Doppler spectral domain optical coherence tomography (OCT) system for in vivo measurement of absolute retinal blood velocity and flow with arbitrary orientation. The incident beam is split by a beam displacer into two probe beams of the single-spectrometer spectral domain OCT system with orthogonal polarization states and an optical path length delay. We validate our approach with a phantom and in vivo experiments of human retinal blood flow, respectively.

Laser speckle imaging is a common technique to monitor blood flow. The fluctuations in speckle intensity can be related to the blood flow by calculating the speckle contrast, the ratio between the standard deviation of speckle fluctuations and the average intensity. However, this simple statistic calculation is easily affected by motion artifacts. In this study, we applied sample entropy analysis instead of calculating standard deviations of the speckle fluctuations. Similar to the traditional method, sample entropy-based speckle contrast increases linearly with flow rate but was shown to be more immune to sudden movements during an upper arm occlusion test.

A holographic visualization of volume data based on adjustable ray to optical-wave conversion is presented. Computer-generated holograms are generated by emitting multiple rays to sample the volumetric field. Equal interval sampling, object light wave adjustment, and information composition are sequentially performed during the march of rays. The program is accelerated in parallel to reduce the total time, and the reconstructions are dynamically adjusted to express different parts of an object. Optical experiments verify that the proposed method can holographically reconstruct the surface and interior information of objects.

Micromachining based on femtosecond lasers usually requires accurate control of the sample movement, which may be very complex and costly. Therefore, the exploration of micromachining without sample movement is valuable. Herein, we have illustrated the manipulation of optical fields by controlling the polarization or phase to vary periodically and then realized certain focal traces by real-time loading of the computer-generated holograms (CGHs) on the spatial light modulator. The focal trace is composed of many discrete focal spots, which are generated experimentally by using the real-time dynamically controlled CGHs. With the designed focal traces, various microstructures such as an ellipse, a Chinese character “Nan”, and an irregular quadrilateral grid structure are fabricated in the z-cut LiNbO3 wafers, showing good qualities in terms of continuity and homogeneity. Our method proposes a movement free solution for micromachining samples and completely abandons the high precision stage and complex movement control, making microstructure fabrication more flexible, stable, and cheaper.

A novel harmonic mode-locked fiber laser based on nonlinear multimode interference (NL-MMI) in a microfiber-assisted ultrafast optical switch is proposed in this Letter. The microfiber-assisted ultrafast optical switch can be obtained by tapering the splicing point of the graded-index multimode fiber (GIMF) and single-mode fiber, which not only helps to shorten the self-imaging period in GIMF to relax the strict requirement of NL-MMI on the length of multimode fiber, but also improves the harmonic order. In the experiment, with the waist diameter of ∼15 µm, the repetition rates of the fiber laser can be stably locked at 285 MHz, corresponding to the 16th-order harmonic mode-locking, with the pulse duration of 1.52 ps. Our results provide novel insight into the design of a high-repetition-rate laser and the application of microfibers in the mode-locking device.

We found the beam quality factor M2 of the fundamental mode as a function of wavelength is U-shaped in the working photonic bandgap (PBG) of an all-solid PBG fiber (AS-PBGF) for the first time, to the best of our knowledge, and our simulation results also match well with the phenomenon. The normal band that is near the high-frequency edge of the third PBG integrates the lowest M2 and single-mode operation simultaneously, while the other two edge regions suffer from anomalous variation of M2 versus wavelength. The general applicability of this finding can be further extended to other PBGs and also other representative structures in the AS-PBGF field.

We propose a new experimentally verified ghost imaging (GI) mechanism, derivative GI. Our innovation is that we use the derivatives of the intensities of the test light and the reference light for imaging. Experimental results show that by combining derivative GI with the standard GI algorithm, multiple independent signals can be obtained in one measurement. This combination greatly reduces the number of measurements and the time required for data acquisition and imaging. Derivative GI intrinsically does not produce the storage-consuming background term of GI, so it is suitable for on-chip implementation and makes practical application of GI easier.

An active ultrafast formation and modulation of dual-band plasmon-induced transparency (PIT) effect is theoretically and experimentally studied in a novel metaphotonic device operating in the terahertz regime, for the first time, to the best of our knowledge. Specifically, we designed and fabricated a triatomic metamaterial hybridized with silicon islands following a newly proposed modulating mechanism. In this mechanism, a localized surface plasmon resonance is induced by the broken symmetry of a C2 structure, acting as the quasi-dark mode. Excited by exterior laser pumps, the photo-induced carriers in silicon promote the quasi-dark mode, which shields the near-field coupling between the dark mode and bright mode supported by the triatomic metamaterial, leading to the dynamical modulation of terahertz waves from individual-band into dual-band PIT effects, with a decay constant of 493 ps. Moreover, a remarkable slow light effect occurs in the modulating process, accompanied by the dual-transparent windows. The dynamical switching technique of the dual-band PIT effect introduced in this work highlights the potential usefulness of this metaphotonic device in optical information processing and communication, including multi-frequency filtering, tunable sensors, and optical storage.

A key limitation in the observation of instruments used in operations and heart sutures during a procedure is the scattering and absorption during optical imaging in the presence of blood. Therefore, we propose a novel real-time fiber-optic infrared imaging system simultaneously capturing a flexible wide field of view (FOV) and large depth of field infrared image in real time. The assessment criteria for imaging quality of the objective and coupling lens have been optimized and evaluated. Furthermore, the feasibility of manufacturing and assembly has been demonstrated with tolerance sensitivity and the Monte Carlo analysis. The simulated results show that the optical system can achieve a large working distance of 8 to 25 mm, a wide FOV of 120°, and the relative illuminance is over 0.98 in the overall FOV. To achieve high imaging quality in the proposed system, the modulation transfer function is over 0.661 at 16.7 lp/mm for a 320×256 short wavelength infrared camera sensor with a pixel size of 30 µm.

In this paper, a novel liquid level sensor with ultra-high sensitivity is proposed. The proposed sensor is configured by a slice-shaped composite long period fiber grating (SSC-LPFG). The SSC-LPFG is prepared by polishing two opposite sides of a composite multimode–single-mode–multimode fiber structure using a CO2 laser. The method improves the sensitivity of the sensor to external environment. Based on the simulation calculation, a liquid level sensor with a length of 3 mm is designed. The experimental transmission spectrum agrees well with the simulation result. The experimental results show that the sensitivity reaches 7080 pm/mm in the liquid level range of 0–1400 μm in water. The temperature sensitivity is 24.52 pm/°C in the range of 20°C–90°C. Due to the ultra-high sensitivity, good linearity, and compact structure, the SSC-LPFG has potential application in the field of high-precision liquid level measurement.

In this paper, we present a suppression method for the thermal drift of an ultra-stable laser interferometer. The detailed analysis on the Michelson interferometer indicates that the change in optical path length induced by temperature variation can be effectively reduced by choosing proper thickness and/or incident angle of a compensator. Taking the optical bench of the Laser Interferometer Space Antenna Pathfinder as an example, we analyze the optical bench model with a compensator and show that the temperature coefficient of this laser interferometer can be reduced down to 1 pm/K with an incident angle of 0.267828 rad. The method presented in this paper can be used in the design of ultra-stable laser interferometers, especially for space-based gravitational waves detection.

Based on the Vernier effect of the cascaded double ring resonator (CDRR) sensor, a sensor consisting of a microfluidic control system, a sensing ring, and a reference ring with a micro-heater for thermal tuning is proposed in this paper. In wavelength interrogation, a broadband spectrometer or a large tunable range laser is not required. The shift of the output spectrum caused by the refractive index change of the sample is converted into the change of the electric power by the thermal tuning heater. Due to the Vernier effect, the sensitivity of the sensor is 20 times higher than that of the single ring. The spectral envelope of the thermal-optic tuning of the sensor was fitted by a Gaussian function in the wavelength range of 8 nm. The experimental results showed that the sensitivity was 33.703 W/RIU, and the limit of detection was 1.34×10-5 RIU.

Here, we designed a broadband, low loss, compact, and fabrication-tolerant silicon-based four-mode edge coupler, composed of a 1×3 adiabatic mode-evolution counter-taper splitter and a triple-tip inverse taper. Based on mode conversion and power splitting, the proposed structure can simultaneously realize efficient mode coupling from TE0, TM0, TE1, and TM1 modes of multimode silicon waveguides to linearly polarized (LP), LP01,x, LP01,y, LP11a,x, and LP11a,y, modes in the few-mode fiber. To the best of our knowledge, we proposed the first scheme of four LP modes coupling, which is fully compatible with standard fabrication process. The 3D finite-difference time-domain simulation results show that the on-chip conversion losses of the four modes remain lower than 0.62 dB over the 200 nm wavelength range, and total coupling losses are 4.1 dB, 5.1 dB, 2.1 dB, and 2.9 dB for TE0-to-LP01,x, TM0-to-LP01,y, TE1-to-LP11a,x, and TM1-to-LP11a,y, respectively. Good fabrication tolerance and relaxed critical dimensions make the four-mode edge coupler compatible with standard fabrication process of commercial silicon photonic foundries.

We report an electro-optically (EO) tunable microdisk laser fabricated on the erbium (Er3+)-doped lithium niobate on insulator (LNOI) substrate. By applying a variable voltage on a pair of integrated chromium (Cr) microelectrodes fabricated near the LNOI microdisk, electro-optic modulation with an effective resonance-frequency tuning rate of 2.6 GHz/100 V has been achieved. This gives rise to a tuning range of 45 pm when the electric voltage is varied between -200 V and 200 V.

As a universal phenomenon in nonlinear optical systems, intermittency is usually accompanied by the coherence loss such as soliton explosions in fiber lasers. Based on real-time spectroscopy, we revealed the coherent dissipative soliton intermittency in normal-dispersion fiber lasers. By increasing the pump strength, the intermittency transforms from the transient pulsation to the bi-stable soliton. It is demonstrated that the slow-gain effect dominates such coherent intermittency. Our results provide novel insights into laser physics, offering a promising approach for studying the bi-stable dissipative soliton.

A tunable multi-wavelength erbium-doped fiber laser with precise wavelength interval control is reported theoretically and experimentally in this paper. It is made up of a Mach–Zehnder interferometer (MZI) filter and a Sagnac filter and supplemented by the four-wave-mixing effect. Compared with other filters, the proposed MZI filter based on the fused taper technology can change the wavelength interval more flexibly. The experiment result shows that wavelength tuning can be achieved, and the tuning range can reach ∼15 nm. Moreover, the variation in the number of wavelengths is also realized. The maximum side-mode suppression ratio can reach 39 dB.

In this paper, a high-power and high-efficiency 4.3 µm mid-infrared (MIR) optical parametric oscillator (OPO) based on ZnGeP2 (ZGP) crystal is demonstrated. An acousto-optically Q-switched Ho:Y3Al5O12 laser operating at 2.1 µm with a maximum average output power of 35 W and pulse width of 38 ns at a repetition rate of 15 kHz is established and employed as the pump source. A doubly resonant OPO is designed and realized with the total MIR output power of 13.27 W, including the signal and idler output power of 2.65 W at 4.07 µm and 10.62 W at 4.3 µm. The corresponding total optical-to-optical and slope efficiencies are 37.9% and 67.1%, respectively. The shortest pulse width, beam quality factor, and output power instability are measured to be 36 ns, Mx2=1.8, My2=2.0, and RMS1.9% at 8 h, respectively. Our results pave a way for designing high-power and high-efficiency 4–5 µm MIR laser sources.

We report on a mid-infrared fiber laser that uses a single-walled carbon nanotube saturable absorber mirror to realize the mode-locking operation. The laser generates 3.5 µm ultra-short pulses from an erbium-doped fluoride fiber by utilizing a dual-wavelength pumping scheme. Stable mode-locking is achieved at the 3.5 µm band with a repetition rate of 25.2 MHz. The maximum average power acquired from the laser in the mode-locking regime is 25 mW. The experimental results indicate that the carbon nanotube is an effective saturable absorber for mode-locking in the mid-infrared spectral region.

The generation of supercontinuum (SC) often requires ultrashort pulsed lasers with high peak power and gain media with large nonlinear coefficients, such as a long piece of fiber or photonic crystal fiber. In this Letter, we propose and demonstrate that high-power SC can be generated through a simple narrow-bandwidth fiber Bragg gratings (FBGs)-based laser cavity without any modulation, based on the mechanism of intense nonlinear effects induced by the inherent self-pulsation generated inside the cavity. In the experiment, an ∼80 W SC laser with the spectrum range from 600 nm to 1600 nm was achieved. To the best of our knowledge, this is the first report about SC generation through a simple fiber laser cavity. This work enriches the research content of SC and provides a cost-effective method for high-power SC lasers.

Optical levitation technology is a new levitation technology for trapping micro/nano-particles. By taking advantage of the mechanical effect of light, it has the characteristics of non-contact and high sensitivity. However, the traditional optical levitation system is large in volume, complex in adjustment, and greatly affected by the external environment. Herein, a miniature optical levitation system based on a laser diode, miniature lenses, and a micro-electro-mechanical system (MEMS) particles cavity is proposed. First, we analyze the output spot characteristics of the laser diode. Being compared the characteristics of different kinds of laser diodes, the type, wavelength, and power of diodes in the levitation system are determined. Then, the micro-particles cavity is fabricated based on the MEMS process. The MEMS process is widely used in the manufacturing of micro-electronic devices because of its advantages of small size, high precision, and easy mass production. The particle cavity processed in this way can not only ensure the advantage of small volume, but also possesses high processing repeatability. The volume of the entire package including the light source, focusing lenses, and MEMS cavity is just Φ 10 mm×33 mm, which is the smallest optical levitation system reported, to the best of our knowledge. After the entire levitation system is designed and set up, one silica particle of 10 µm diameter is stably trapped in the atmospheric environment. Finally, the micro-displacement and vibration signal are detected by a four-quadrant photoelectric detector to evaluate the stiffness of the optical levitation system.

In this paper, an effective method is proposed to generate specific periodical surface structures. A 532 nm linearly polarized laser is used to irradiate the silicon with pulse duration of 10 ns and repetition frequency of 10 Hz. Laser-induced periodic surface structures (LIPSSs) are observed when the fluence is 121 mJ/cm2 and the number of pulses is 1000. The threshold of fluence for generating LIPSS gradually increases with the decrease of the number of pulses. In addition, the laser incident angle has a notable effect on the period of LIPSS, which varies from 430 nm to 1578 nm, as the incident angle ranges from 10° to 60° correspondingly. Besides, the reflectivity is reduced significantly on silicon with LIPSS.

Transient response of an erbium-doped fiber amplifier (EDFA) is studied in an externally-modulated analog link. Double tones represented as transmitted radio frequency and dither signals are introduced. Extra modulation is generated owing to the EDFA’s transient response caused by a low-frequency dither signal. Therefore, the parasitic modulation is superposed to the output signals and may significantly affect in-band electrical spectra. Analytical and numerical solutions are both given, which agree well with experimental results. This work indicates that a suitable dither signal should be selected to maximize the carrier to intermodulation ratio. In-band spurious free dynamic range is optimized in the meantime.

Metalenses are expected to play an increasingly important role in miniaturized and integrated optical imaging components/systems. However, devising broadband achromatic metalenses with high focusing efficiencies is still quite challenging. In this work, we proposed an aperture-shared partition phase cooperative manipulation approach for designing a high-efficiency broadband achromatic metalens composed of two concentric sub-metalenses. As a proof-of-concept, an achromatic polarization-independent metalens is successfully designed for the visible and near-infrared range from 450 nm to 1400 nm with the focusing efficiency over 70% for the wavelength range of 600 nm to 1400 nm. The approach reported here provides a possibility for designing a high-performance metalens, which has great potential applications in integrated optics.

We proposed a multifunctional terahertz metasurface based on a double L-shaped pattern and a vanadium dioxide (VO2) film separated by polyimide. When the VO2 film is an insulator, a dual-band electromagnetically induced transparency effect is obtained, and the physical mechanism is investigated based on the current distribution and “two-particle” model. When the VO2 film is a metal, a dual-band linear-to-circular polarization converter, in which the y-polarized linear wave can be effectively converted to left-handed circularly polarized (LCP) and right-handed circularly polarized simultaneously in different bands, can be achieved. By arranging the metal pattern rotating 30°, a multifunctional antenna can be obtained. When the VO2 is an insulator, the radiation of the LCP wave is divided into four beams, with two beams reflected and two beams transmitted. When the VO2 is in the metallic state, we can only get the co-polarized reflected wave with a 21° angle. Moreover, in our design, the VO2 film does not need lithography to obtain certain patterns, which improves the convenience of fabrication and experiment. Our design opens a new way for the development of multifunctional terahertz devices and has potential applications in the terahertz communication field.

A new unsaturated wind-chime model is proposed for calculating the formation time of the diffraction rings induced by spatial self-phase modulation (SSPM) in molybdenum disulfide suspension. To optimize the traditional wind-chime model, the concentration variable of 2D materials was introduced. The results of the unsaturated wind-chime model match quite well with the SSPM experimental results of molybdenum disulfide. Based on this model, the shortest formation time of diffraction rings and their corresponding concentration and light intensity can be predicted using limited data. Theoretically, by increasing the viscosity coefficient of the solution, the response time of the diffraction ring, to reach the maximum value, can be significantly reduced. It has advanced significance in shortening the response time of photonic diodes.

We demonstrate integrated lithium niobate (LN) microring resonators with Q factors close to the intrinsic material absorption limit of LN. The microrings are fabricated on pristine LN thin-film wafers thinned from LN bulk via chemo-mechanical etching without ion slicing and ion etching. A record-high Q factor up to 108 at the wavelength of 1550 nm is achieved because of the ultra-smooth interface of the microrings and the absence of ion-induced lattice damage, indicating an ultra-low waveguide propagation loss of ∼0.0034 dB/cm. The ultra-high Q microrings will pave the way for integrated quantum light source, frequency comb generation, and nonlinear optical processes.

We report the observation of second order Bragg resonance (2nd-OBG) produced by tilted fiber gratings (TFGs) fabricated using phase mask UV inscription. The theoretical analysis has revealed that the generation of high order Bragg resonance of gratings is induced by a square-shape refractive index profile, which is caused by over-saturated UV exposure. In the experiment, we have studied the TFGs with different tilt angles under over-saturated UV exposure, in which all gratings have showed the 2nd-OBR, and the larger tilt angle of the grating has the stronger 2nd-OBR. When the tilt angle of the grating is ∼45°, the Bragg resonance exhibits very strong polarization dependence, because the 2nd-OBR wavelength is located within the polarizing bandwidth of 45° TFG. Finally, we have demonstrated an erbium-doped fiber laser with >99.9% degree of polarization, and, by applying mechanical stretching on the grating, a wavelength tunable laser output has been achieved. The output laser shows ∼0.2 dB amplitude variation within 1 h continuous monitoring of the laser.

It is well known that cats have fascinating eyes with various colors, such as green, blue, and brown. In addition, they possess strong night vision ability, which can distinguish things clearly even in a poor light environment. These drive us to reveal the secrets behind them. In fact, cats’ eyes can be considered as special lenses (which we would like to mimic by using a Luneburg lens). We make an analysis of the role of photonic crystals behind the lens and demonstrate that the integration of photonic crystals into Luneburg lens can be regarded as a retroreflector and greatly improve the light focusing intensity of the lens in a broad band of frequencies. This wonderful bioinspired phenomenon is expected to design more interesting and serviceable devices by combining photonic crystals with transformation optics.

Different post-treatment processes involving the use of ammonia and hexamethyldisilazane (HMDS) were explored for application to 351 nm third harmonic generation SiO2 antireflective (3ωSiO2 AR) coatings for high power laser systems prepared by the sol-gel method. According to experimental analysis, the 3ωSiO2 AR coatings that were successively post-treated with ammonia and HMDS at 150°C for 48 h and again heat-treated at 180°C for 2 h (N/H 150 + 180 AR) were relatively better. There were relatively fewer changes in the optical properties of the N/H 150 + 180 AR coating under a humid and polluted environment, and the increase in defect density was slow in high humidity environments. The laser-induced damage threshold of the N/H 150 + 180 AR coating reached 15.83 J/cm2 (355 nm, 6.8 ns), a value that meets the basic requirements of high power laser systems.

Large area and uniform monolayer MoS2 is of great importance for optoelectronic devices but is commonly suffering from rather weak photoluminescence. Here, by engineering the concentration profiles of gaseous chemicals through extra trace amounts of water, we demonstrate the uniform dendrite-type growth of monolayer MoS2 unraveled by spatially resolved fluorescence spectroscopy, which exhibits macroscopic monolayer flakes (up to centimeter scale) with photoluminescence intensity of orders of magnitude higher than conventional chemical vapor deposition monolayer MoS2. Both spectroscopic evidence and theoretical models reveal that the fast-fractal dendrite growth can be ascribed to the extra introduced water sources that generate sufficient aqueous gas around the S-poor regions nearby the central-axis zone, leading to highly efficient Mo sources transport, accelerated S atom corrosion nearby grain edges, and/or defect sites, as well as enhanced photoemission intensity. Our results may provide new insight for high throughput fabrication of MoS2 monolayers with high yield photoluminescence efficiency.

Recent advances in the research of vortex beams, structured beams carrying orbital angular momentum (OAM), have revolutionized the applications of light beams, such as advanced optical manipulations, high-capacity optical communications, and super-resolution imaging. Undoubtedly, the methods for generation of a vortex beam and detection of its OAM are of vital importance for the applications of vortex beams. In this review, we first introduce the fundamental concepts of vortex beams briefly and then summarize approaches to generating and detecting the vortex beams separately, from bulky diffractive elements to planar elements. Finally, we make a concise conclusion and outline that is yet to be explored.

Multi-band signal propagation and processing play an important role in quantum communications and quantum computing. In recent years, optical nonreciprocal devices such as an optical isolator and circulator are proposed via various configurations of atoms, metamaterials, nonlinear waveguides, etc. In this work, we investigate all-optical controlled nonreciprocity of multi-band optical signals in thermal atomic systems. Via introducing multiple strong coupling fields, nonreciprocal propagation of the probe field can happen at some separated frequency bands, which results from combination of the electromagnetically induced transparency (EIT) effect and atomic thermal motion. In the proposed configuration, the frequency shift resulting from atomic thermal motion takes converse effect on the probe field in the two opposite directions. In this way, the probe field can propagate almost transparently within some frequency bands of EIT windows in the opposite direction of the coupling fields. However, it is well blocked within the considered frequency region in the same direction of the coupling fields because of destruction of the EIT. Such selectable optical nonreciprocity and isolation for discrete signals may be greatly useful in controlling signal transmission and realizing selective optical isolation functions.

A patterned polymer dispersed liquid crystal transparent display using one-time UV exposure is demonstrated. The device is fabricated by exposing the cell with a uniform UV light through a mask with selective attenuation of the UV light. The voltage-transmittance response of the device is different for the corresponding regions. Thus, three different states of total scattering, patterned transparent, and total transparent can be realized by controlling the applied voltages. The proposed device used in shutter mode as smart windows and in projected display mode is demonstrated, which shows great potential for smart windows with customized patterns or logos and has potential application in the field of projected transparent displays.