An optoelectronic oscillator (OEO) based on a double-Brillouin-frequency shifter (DBFS) is proposed and experimentally demonstrated. Two orders of stimulated Brillouin scattering are realized in the DBFS, and the narrow gain bandwidth of the second-order Stokes wave is utilized to implement the phase modulation to intensity modulation conversion. Only one laser is used to provide both signal and pump waves. A k-band microwave signal with a harmonic suppression ratio of 61 dB and a side-mode suppression ratio of 45 dB is experimentally generated, and 400-MHz tunability is realized by adjusting the laser wavelength. Because of the simple structure and high-frequency oscillation, the DBFS-based OEO can find applications for wireless communications and civil radar systems.

We numerically simulate and optimize a high-power fiber Raman amplifier cladding pumped by spectrally combined diode lasers at wavelengths from ∼0.9 to ∼1  μm in the continuous-wave regime. This amplified a signal at the first-Stokes wavelength of 1024 nm. We found that it was possible to add pumps over an increasingly wide wavelength span up to ∼90  nm, while still maintaining an incremental conversion efficiency higher than 60%, even though the Raman linewidth is only ∼15  nm. We investigated the dependence on the power of individual diode lasers and on the wavelength spacing and found that the total conversion efficiency reaches ∼70  %   with realistic pump sources based on state-of-the-art diode lasers. We believe this study shows the potential for high-power fiber Raman lasers pumped by spectrally combined multiwavelength diode and fiber laser sources.

The transient theory of stimulated Brillouin scattering (SBS) in optical fibers is used to investigate the effects of weak feedback of incident broadband laser fields exhibiting different spectral shapes. The proper boundary conditions are given for the forward- and backward-propagating electric fields inside the fiber with a finite amount of reflectivity at the fiber ends. When the linewidth of the laser approaches the Brillouin frequency shift, a weak reflection of the laser field overlaps with the SBS gain and can be amplified in the fiber, resulting in an apparent reduction in SBS threshold. It is shown that the spectral shape of the incident field and its overlap with the SBS gain spectrum affects the SBS signal. Numerical examples are presented for incident laser fields that (1) simulate a pulsed, multilongitudinal-mode laser and (2) are a constant amplitude with a sinusoidal phase modulation. For the pulsed field with mode spacing much less than the Brillouin linewidth, calculations show that an end reflection of 0.01% (−40  dB) of the laser field with a linewidth equal to 0.75 of the Brillouin frequency shift can reduce the effective SBS threshold by a factor of 1.5. When the incident laser field is constant amplitude and phase modulated with a sinusoid, calculations predict sharp and distinct enhancement of the reflected power when the ratio of the Brillouin frequency shift to the modulation frequency is an integer for end reflections as low as 0.001% (−50  dB). Experiments performed with a sinusoidal phase modulated signal confirm the theoretical predictions. These results have implications on the design of high-power fiber laser systems that utilize spectral broadening to suppress SBS.

We suppress stimulated Brillouin scattering in an erbium-doped optical fiber amplifier for 50-ns-long transform-limited signal pulses by counter-directional pumping with a pulse burst. The pump pulse burst is codirectional with the parasitic Brillouin Stokes wave, which, therefore, undergoes cross-phase modulation and thus spectral broadening due to the intensity-modulated pump. The broadening inhibits its growth. We experimentally study the effect of pump pulse parameters and improve the SBS threshold by up to 4 dB when amplifying signal pulses at a wavelength of 1565 nm with pumping at 1536 nm.

The fast active distant binocular filter (DBF) with two complementary nematic liquid crystal (NLC) π-structures for observing the frame-sequential stereoscopic images is described. The DBF is characterized by steep front and rear edges of the pulse optical response defined by the fast (about 50 to 200  μs) rise time of the π-structures. Their relatively long decay time (about 2 ms) during relaxation processes does not affect the transition times of the DBF optical response. The corresponding long phase shifts in two π-structures are mutually compensated with use of the developed algorithms of electronic control. The DBF optical characteristics are studied theoretically and experimentally for selection of white light fluxes with duration in several milliseconds typical for the common stereoscopic system. The minimum duration of the selected view images can be a fraction of a millisecond. When compared stereo glasses with conventional NLC shutters, the described active DBF allows to provide the increased optical efficiency and absence of mechanical stress on the face (head) of the observer to improve the comfort of stereo image observation.

Reflective tomography reconstructs a scene from calibrated reflective images, using algorithms from x-ray tomography. Many works on the subject are based on analytical formulas, such as the filtered backprojection. However, these formulas require constraints on the acquisition geometry, such as a circular rotation. We want to avoid such constraints; they may be seriously violated in some practical cases. To tackle this problem, we tune the algebraic reconstruction technique from x-ray tomography. More precisely, we look for a model of the scene such that the x-ray projections of the model approximate recorded calibrated reflective images. The model is computed by an iterative algebraic method: a Kaczmarz algorithm. In this way, we perform incremental supervised learning in optics, where the hypothesis space emulates reflective tomography. We get a flexible method for multiple-view reconstruction based on linear algebra. It accepts a general calibrated acquisition, such as several cameras arbitrarily located/oriented, with visible near-infrared wavelengths. It could reconstruct a scene using several devices simultaneously, such as air–ground cameras combined with ground–ground cameras. The relevance of the approach is numerically shown from calibrated CCD images of the Middlebury datasets. In particular, we get reconstructions from 16 views.

We show how to construct and calibrate a full-Stokes imaging polarimeter system by combining the video data from two separate polarization cameras with a nonpolarizing beamsplitter and a waveplate. As a result, this system can capture the full Stokes vector at each pixel for 3 megapixel images at up to 60 Hz. To demonstrate some of the advantages of measuring the s₃ Stokes vector component that is normally not measured in polarization cameras, we show three experiments: viewing three-dimensional glasses, detecting a scarab beetle in a natural environment via the circular dichroism of its shell, and mixing an optically active liquid with a neutral liquid.

A new infrared preprocessing modality called sequence differential processing is proposed. Different from the cold image subtraction which only decreases the background noise, the proposed modality takes the parallel test into account to preprocess the raw thermal data sequence prior to applying the advanced postprocessing techniques such as principal component thermography and pulsed phase thermography. The results show that the preprocessing method not only effectively reduces the influence of uneven laser energy distribution on detection efficiency but also enhances internal defect information. Moreover, the combination of the proposed preprocessing method with the pulsed phase thermography and principal component analysis algorithms improves the ability of laser infrared thermography to detect defects inside aviation carbon-fiber-reinforced plastics.

To compensate for disturbances in an aircraft’s forward flight and variations in its attitude and angular velocity in the imaging process of photoelectric imaging system, a method of image motion compensation using a fast steering mirror was put forward. First, the working sequence of the image motion compensation system was designed. The position of the fast steering mirror is established by increasing the oblique wave to allow the fast steering mirror to move at the desired speed and compensate for the image motion speed. Then, a mathematical model of the fast steering mirror was established. A linear extended-state observer is designed to estimate the disturbance in real-time, and disturbance compensation is generated to offset the disturbance. Finally, a step response experiment and a tracking experiment were performed to test the characteristics. The step response curve shows that the system’s stable time is 1.8 ms. The tracking characteristic is determined by a given scan curve. The fast steering mirror can satisfy the requirement for a high-speed and highly precise tracking system. It improves the system’s interference rejection performance and reduces the difficulty of controller design.

The architecture and calibration of a hyperspectral imaging sensor based on an exponentially continuously variable narrow-band transmission filter is described. The system design allows for great flexibility in choice of sensors and lenses to be used. Spectral and radiometric calibration using lenses of different focal length and vignetting characteristics is described. The point-spread-function at different wavelengths depends on the lens design and the f-number. The advantage of using a tilt/shift lens is demonstrated. Low f-number lenses show vignetting, which influences both the spectral and radiometric calibration. Retroeffects in the microlenses of the focal plane array are observed but to a large extent will be remedied by future improvements in the optical filter. Noise properties of the sensor system are discussed, and signal-to-noise ratios estimated. From the model, it is possible to obtain parametric performance variations based on the properties of key components. Finally, the sensor performance is indicated by demonstrating a spectral image.

We propose a new structure of Smith–Purcell (SP) emitter for increasing its power radiation. Injection of an electron beam passing over the metallic surface causes excitation of the surface mode for metal grating and subsequently electron acceleration and electromagnetic radiation. This radiation can be achieved in the terahertz spectrum applying suitable parameters in the structure. The grating is in half-cylindrical form, which helps to increase the interaction of a particle with the surface, and because the electron beam moves within the grating, intensity of radiation power, and electric field dramatically accumulate. The proposed geometry utilizes a single instead of the 10-point electron source with smaller dimensions and higher emitted power values compared to the traditional SP emitters. Furthermore, it comes with about 2- and 32-time increase, respectively, in radiation power and electric field. For our simulations, 3-D particle in cell and eigenmode solvers have been used.

We study the effect of viewing directions in projection-based multiviewer display systems. Such systems display different images to several users watching a common screen. While different separation techniques have been used in cinema, we are interested in applications that require a more demanding viewing angle and more independent views, such as multisided immersive environment. We compare the luminous efficiency, crosstalk and colorimetric properties of polarization and spectral multiplexing, as a function of the viewing direction. We find that, whereas for spectral separation the crosstalk does not depend on the viewing angle, for polarization separation, the crosstalk increases with the viewing angle due to the depolarization induced by conventional screens.

Blind pixels are an inevitable problem in short-wave infrared (SWIR) InGaAs focal plane arrays (FPAs). An experiment using a laser beam-induced current method was set up to characterize the blind pixels in InGaAs FPAs. According to the result, the blind pixels are classified into four types: single overhot pixels, single dead pixels, crossing dead pixels, and paired blind pixels. Additionally, the causes of the different types of blind pixels are analyzed. It is indicated that the blind pixels are caused by defects and impurities, disconnection of the readout circuit, unformed PN junctions, and a wrong connection between the PN junction and readout integrated circuit, respectively. Based on the characterization, a blind pixel calibration method that greatly improved the quality of images taken by InGaAs FPAs is proposed.

An edge collection function is proposed for characterizing the optical efficiency of an energy-harvesting system that utilizes photoluminescence (PL) in a waveguide. We assume that a single spot in a waveguide is excited and that PL is isotropic. For the photons to be collected by one edge of the waveguide, they must be emitted toward the edge, trapped in the waveguide and they must survive self-absorption on the way. The optical efficiency is formulated as the product of these probabilities. When this function is calculated for every spot on the waveguide and for each wavelength of the PL spectrum, the efficiency of the system is given by superposition. Its validity is checked by a Monte Carlo simulation for the case of no self-absorption loss. In experiment, we fabricate a 5-cm² waveguide with a thin layer of Lumogen F Red 305 and measure its efficiency by placing a photodiode array in the vicinity of its edge with a small air gap. The formula roughly reproduces the efficiency and its dependency on the position of the excitation spot. This analytical approach allows one to estimate the optical efficiency for an arbitrary incident light distribution with small computational complexity.

A null phase measuring deflectometry (PMD) is presented to measure the three-dimensional shape of aspheric mirrors with radial symmetry. Curved fringes displayed on a mobile screen are reflected by the surface under test, and a camera captures the fringe patterns. By analyzing the captured fringe patterns, the deviations along the normal of the mirror’s surface can be restored by integrating the slopes of these deviations, and then the surface under test can be reconstructed. Compared with non-null PMD, the qualitative information of surface in null PMD can be shown fast and easily for a surface with large deviations in the grinding stage. Computer simulation and experiment are presented to validate the presented method.

Absolute phase retrieval has been widely studied, as it is not much influenced by the discontinuity and occlusion of objects in phase-measuring profilometry. In the practical measurement, however, fringe order determination is by no means easy when the grayscale of deformed patterns is degraded because of uneven reflectivity or the large curvature of measured objects. We introduce a phase-shifting coding (PSC) method to overcome that problem. In the PSC method, fringe order is encoded by the phase-shifting amounts of additional fringe patterns, and the decoding algorithm to retrieve absolute phase is designed on the basis of phase difference and neighborhood consistency. The special statistical filtering and self-correcting filtering methods are utilized to eliminate absolute phase errors. Experiment results demonstrate that the proposed PSC method is effective and has strong robustness in grayscale degradation regions.

An optical indoor positioning system using an optical receiver and camera is proposed. Compared with previous works in this field, our system improved the coverage without sacrificing accuracy. Thus, the proposed indoor positioning system is more suitable under environments with dense obstacles, such as warehouses, hospitals, and factories. Since the camera is deployed at the receiver side, the angle of arrival (AoA) can be estimated by recognizing the light source in an image. The optical receiver is able to measure received signal strength (RSS). Combining the information in AoA and RSS, the relative position of the receiver to the light source can be estimated. In this system, the position of the light source can be sent to the receiver applying visible light communication. Thus, both communication and positioning can be enabled using a single light-emitting diode on the transmitter side.

Driven by the growing demand for large-size x-ray mirrors used in synchrotron radiation and free-electron laser facilities, a linear deposition system based on magnetron sputtering technique is built. The layer growth quality and thickness uniformity are optimized by adding masks in front of the cathode to reduce the oblique-incidence particles and tune the sputtering flux distribution. Based on this, a 0.5-m-length W/Si multilayer mirror is demonstrated. The multilayer has a d-spacing of 3 nm and a bilayer number of 60. The surface roughness of the multilayer is only 0.33 to 0.25 nm in the spatial frequency range of 3.0  ×  10^−  3 to 1.0  μm^−  1, and the average interface width is 0.32 nm. The hard x-ray reflectivity and uniformity of the large mirror were characterized at the Optics Beamline in Shanghai Synchrotron Radiation Facility. A maximum reflectivity of 62.5% and 62.7% was measured at 8 and 18 keV, respectively, with an angular resolution of Δθ  /  θ  =  2.7  %  . The d-spacing uniformity over the 0.5-m-length and 60-mm-width area of the mirror is 1.0% and 1.2% (peak-to-valley), respectively. These results indicate a good and uniform quality of the nanoscale multilayer over the large mirror area. The measured second- and third-order reflectivities of the multilayer are more than 2 orders magnitude lower than the first order, implying a good suppression of high harmonics in the monochromator application.

A cost effective folded Fabry–Perot (F–P) interferometer is developed using a He–Ne laser source tuned by acousto-optic modulators (AOM). The two AOMs are employed to expand the frequency tuning range of the tracking He–Ne laser. The configuration and diffraction efficiency of each AOM is fully investigated to maintain balanced efficiency along the whole frequency tuning range. The optical phase modulated Pound–Drever–Hall frequency locking technique is employed in this system to minimize the locking error of the F–P cavity resonance. The displacement of the target corner cube reflector is converted to the tracking frequency change and measured by beating with an iodine-stabilized laser. A self-developed xPC target system is employed to accomplish the frequency locking, data acquisition, and air refractive index compensation in the overall system. A displacement comparison experiment between the F–P interferometer and a commercial interferometer shows the residual error is within ±4  nm in the range of 500 nm.

At present, new approaches for the use of drones in high-precision optical applications are rising, especially with those known as multirotor. However, the optical turbulence effects generated by multirotor drones are not entirely understood. These optical effects can reduce the performance of the optical instruments that they transport. We present measurements of the wavefront deformation generated by the temperature fluctuations and the airflow of a drone’s propulsion system. To do so, we used a single arm of a DJI S800 EVO Hexacopter (professional drone) and measured its operating temperature with a commercial infrared camera. The resulting temperature variation, between a switched-off propulsion system at room temperature and one running at its maximum performance, was 34.2°C. Later, we performed two different interferometric tests: Takeda’s method and the phase-shifting technique, using a ZYGO interferometer. These tests show that the total deformation over an incident wavefront to the propeller airflow is lower than 0.074  λ PV and 0.007  λ RMS (HeNe laser, λ  =  633  nm). We determine that the optical turbulence produced by a drone propulsion system is negligible.

We propose an acousto-optically tuned external-cavity laser diode (ECLD) based on the Littrow configuration to expand the wavelength-scanning range and eliminate mechanical motions. The scanning range and tuning rate of our prototype were 68.85 nm and more than 100 kHz, respectively, without any mode hop. Moreover, we implemented this improved ECLD in an optical coherence tomography imaging system and conducted a measurement of the two-dimensional thickness distribution of a thin glass plate via continuous wavelet transform analysis.

We previously presented a technique calibrating a multiaperture array projector (MAAP) for performing high-speed three-dimensional (3-D) surface topology measurements with aperiodic sinusoidal fringe structured illumination. Although a stereo-camera setup used to be required for triangulation, the presented MAAP calibration technique re-enables photogrammetric 3-D measurements to be made using a single CCD or CMOS camera. For such a monocular-view photogrammetric system, intrinsic camera calibration, intrinsic projector calibration, and extrinsic system calibration are vital to obtain high-measurement performance. However, there is currently no such technique that can comprehensively perform the required extrinsic system calibration when using an MAAP and a single-camera setup. A comprehensive optimization-based method is proposed and the resulting measurement performance is evaluated. With the new calibration technique, we are able to achieve a surface standard deviation of ∼50  μm.

We experimentally demonstrate coincidence measurements as well as g⁽²⁾  (  nτ  )   correlation function measurements with only one single-photon detector (SPD) with the aid of field-programmable gate array and 2  ×  2 fast optical switches. Both experiments produced similar results compared to conventional experimental methods, requiring more than one SPD. When detecting correlations between multiple single photons, this technique can be employed to reduce demands for the number of SPDs and potentially would be able to offer new options to researchers and institutions requiring coincidence measurement systems with lower complexity as well as smaller form factors.

The performance evaluation of infrared (IR) imaging systems plays a vital role prior to effective deployment. The minimum resolvable temperature difference (MRTD) is the primary key parameter of an IR imaging system, because it describes the overall system performance including that of a human operator. MRTD defines the minimum temperature difference required to resolve the four-bar target of defined spatial frequencies and is used to estimate the recognition ranges of a thermal imaging system before deployment for field evaluation. Conventionally, MRTD measurements use the temperature difference of a four-bar target that is exposed to the minimum resolvable condition. In current practice, subjective MRTD measurement employs a human operator to observe the minimum resolvable condition and therefore suffers from poor repeatability. It is a tedious procedure to evaluate large numbers of thermal imaging sights during production. In this work, an accurate objective MRTD measurement method is described by measuring the modulation transfer function and the noise equivalent temperature difference (NETD) of the thermal imager. Experiments were carried out to measure both subjective and objective MRTD. The results of the subjective and objective MRTD measurements are presented along with a comparison of the results. It is observed that the percentage error in objective MRTD is very close to that from a conventional subjective MRTD measurement.

Although autocollimators enable noncontact measurement, their performance is limited in large-scale and long-distance applications because of errors caused by nonideal point light sources. Therefore, we analyze this type of measurement error, including the error source and the equations used to describe the irradiance distribution of the light spot. A two-dimensional exponential approximation formula is used to express the light irradiance distribution and compensate the spot imaging errors. Experimental results show that the measurement error could be reduced by sixfold. Therefore, the proposed compensation algorithm can be applied to autocollimator measurements over distances.

Initial configuration is important for optical system design, especially for systems with high requirements in specifications. The number and complexity of optical surfaces needed in an optical system largely depend on the choice of the initial configuration. It is essential to achieve a good initial configuration with minimum aberrations before using complicated surfaces. We propose an initial configuration design method for off-axis reflective optical systems. With this method, a good initial configuration with conic surfaces can be obtained directly. Nodal aberration theory and matrix method are used to calculate the aberration coefficients for the off-axis reflective optical system. Genetic algorithm is applied to search for the initial configuration with minimum aberrations. The validity of this design method is demonstrated by designing an off-axis three-mirror optical system.

To realize an efficient, low-cost, and two-way heat-driven deformable mirror, a deformable mirror-integrated structure using a thermoelectric cooler is proposed. The bidirectional heat-driven stroke, thermal field distribution, and deformation were obtained by finite element analysis. At the same time, the error correction effect of the deformed mirror is verified by experiments: the mirror peak value is corrected from 7.087 to 0.386  μm and the root-mean-square value is corrected from 1.347 to 0.081  μm. For four special aberrations—convex divergence, concave divergence, 45-deg astigmatism, and 90-deg astigmatism—the correction error can be controlled below 5%. More importantly, the cost of the deformed mirror structure designed is under 10,000 yuan; the cost is greatly reduced compared to other deformed mirror structures.

To meet the requirements for the safety of the space environment and the increasing demand for the detection of faint targets in deep space, using a space-based detection optical system with a wide field of view (FOV) has become a major trend. However, such systems are very susceptible to stray light. Based on the above background, we design a baffle with angled and parallel vanes under the limitation of a miniaturized baffle, and we propose a vane angle optimization method. The method is proposed by establishing an irradiance transfer model based on the bidirectional scattering distribution function with Gaussian statistics on the surface of baffle and vane. In addition, the result of the point source transmittance (PST) obtained by Monte Carlo simulation in LightTools indicated that the PST is improved by 2 to 5 times in the FOV of 30 deg and 70 deg when the angle of incidence is at the stray light exclusive angle compared to the traditional method. Meanwhile, the method establishes an analytical expression; therefore, the optimization process is more efficient, and the time required for simulation can be significantly reduced.

In the field of high-speed railways (HSRs), electromagnetic interference (EMI) to the wireless train-ground communication system (e.g., the GSM-R system for train scheduling and dispatching) severely reduces the operational efficiency and even results in accidents. The direction-of-arrival (DoA) estimation of EMI is crucial for detection and removal. A photonics-assisted system is proposed to estimate the DoA of EMI for the GSM-R system along the HSRs in a remote and real-time mode. The proposed scheme is featured by simultaneous photonics-assisted detection and location (DoA estimation) of the EMI, with the advantages of strong EMI immunity, low loss transmission, and remote distribution. Here, an integrated optical multichannel transceiver operating as the photonic front end is used to acquire multiple delayed replicas of the electromagnetic signals, and then, a multiple signal classification algorithm is used to estimate their DoAs at the center station. In the indoor experiments, the DoAs ranging from −60  deg to +60  deg are successfully estimated with errors less than 1.2 deg in a 10-km radio-over-fiber link. In the field trials, the GSM-R signal accompanied by a suspected EMI is detected with DoA errors less than 1.8 deg.

The laser damage thresholds of optical coatings can degrade over time due to a variety of factors, including contamination and aging. Optical coatings deposited using electron beam evaporation are particularly susceptible to degradation due to their porous structure. In a previous study, the laser damage thresholds of optical coatings were reduced by roughly a factor of 2 from the years 2013 to 2017. The coatings in question were high reflectors for 1054 nm that contained SiO₂ and HfO₂ and/or TiO₂ layers, and they were stored in sealed PETG containers in a class 100 clean room with temperature control. At the time, it was not certain whether contamination or thin-film aging effects were responsible for the reduced laser damage thresholds. Therefore, to better understand the role of contamination, the coatings were recleaned and the laser damage thresholds were measured again in 2018. The results indicate that the contamination played the most dominant role in reducing the laser damage thresholds of these optical coatings, even though they were stored in an environment that was presumed to be clean.

We propose a serial-parallel combined relay orthogonal frequency division multiplexing (OFDM) free space optical (FSO) communication system in which the relay protocol of the system is decode and forward. Considering the joint attenuation effect of the light intensity scintillation caused by atmospheric turbulence, path loss and pointing error between the transmitter and receiver on the atmospheric channel fading model, we derive the Meijer G closed-form expressions of outage probability and symbol error rate (SER) of the system over the Málaga (M) distribution model. Furthermore, the effects of key factors such as weather conditions, mapping orders, serial-parallel combined structure parameters, and number of subcarriers on the outage performance and SER performance of the proposed system are analyzed through simulations. Numerical results show that under certain conditions, compared with serial relay or parallel relay, the serial-parallel combined relay structure can further expand the communication coverage and bring greater advantages to the performance of the OFDM-FSO system.

The investigated Q-switching technique is based on the spectral overlap between a Fabry–Perot (F–P) filter and a fiber Bragg grating. This Q-switching concept is implemented in a ring erbium (Er)-doped fiber laser configuration. We use a traveling-wave analytical model to study the peak power and the time duration of the output pulses with respect to the cavity parameters, such as the Er-doped fiber length, the pumping power, and the scanning frequency of the fiber F–P tunable filter. Experimental results are used to validate the proposed Q-switched fiber laser model.

A stable multiwavelength erbium-doped fiber laser has been proposed and experimentally demonstrated based on black phosphorus quantum dots (BPQDs) packaged by polytetrafluoroethylene. The laser mainly consists of a polytetrafluoroethylene packaging device of black phosphorus quantum dots (PTFEPD-BPQDs) and a Lyot filter. PTFEPD-BPQDs have the function of suppressing the mode competition and stabilizing the multiwavelength output. In the experiments, for the unpackaged BPQDs device, BPQDs will degrade and lose its function within 48 h, whereas for the PTFEPD-BPQDs, BPQDs can keep their function for more than 3 weeks. Fifteen stable multiwavelength lasing signals (MWLSs) with the wavelength space of 0.35 nm are achieved. A 10-min optically driven deposition is more conducive to the stable output of MWLSs. The wavelength offset and peak power fluctuation of the output spectrum at central wavelength are 0.1 nm and 0.9 dB, respectively.

Space division multiplexed (SDM) elastic optical network (EON) is considered to be a promising scheme for large-capacity optical communication networks. The static routing, modulation, spectrum, and space allocation (RMSSA) in SDM-EONs with bundles of single-mode fiber is studied. Considering the computational complexity of resource allocation formulation, a path-based integer linear programming (ILP) formulation with fewer variables and constraints is modeled to solve the static RMSSA problem. Then a heuristic algorithm named local optimal RMSSA (LO-RMSSA) is proposed to be applicable in large-scale network scenarios. The calculated metrics are the maximum index of utilized frequency slots, the local spectrum resource utilization, and the runtime of algorithms. The results show that the proposed ILP model and LO-RMSSA algorithm get higher computational efficiency with other metrics no worse than the existing one.

We have demonstrated direct and stable flat-top beam emission from a passively Q-switched microchip laser, featuring an intracavity spot-defect for mode selecting. By adjusting the distance between the focusing lens and the microchip laser, the Gaussian, flat-top, and doughnut-shaped mode can be achieved separately. The flat-top mode is obtained due to the equally weighted superposition of Gaussian and doughnut-shaped mode. By using the self-injection seeding technique, the timing jitter of these three kinds of mode can be reduced effectively.

We theoretically and experimentally demonstrated an all-fiber, hundred-watts-level, linearly-polarized, narrow spectral linewidth laser amplifier at a central wavelength of 1018 nm based on master oscillator-power amplifier configuration, which is composed of a laser oscillator and one stage of the fiber amplifier. The laser system can generate 104-W output power with 3- and 20-dB spectral linewidth of ∼0.073 and ∼0.25  nm, respectively, and a higher polarization extinction ratio of ∼17.89  dB at 1018.3 nm was obtained. Theoretical analysis based on the rate equations was used to optimize the parameters of 1018-nm ytterbium-doped fiber laser system for the maximum suppression of amplified spontaneous emission (ASE). The ASE was well depressed based on the optimization for the parameters of the laser system including the seed power, seed spectrum, gain fiber length in the amplifier, etc. And   ∼  27-dB signal-to-noise ratio was achieved at the maximum output power. The slope efficiency for the amplifier stage can reach 79%, and near-diffraction-limited beam quality (Mx2∼1.617 and My2∼1.635) was obtained.

The Ti-6Al-4V polished by femtosecond laser processing is first investigated. The surface nanoparticles of Ti-6Al-4V induced by femtosecond laser have been characterized by scanning electron microscopy and energy dispersive spectrometry, and the influence of laser fluence and the overlapping rate of laser beam on surface roughness have also been analyzed. Moreover, the relationship between the laser parameters and the surface roughness of Ti-6Al-4V has been revealed, and the fine surface roughness of Ti-6Al-4V is obtained based on the optimized femtosecond laser processing parameters.

For channel estimation in the relay-assisted multiple-input multiple-output and orthogonal frequency division multiplexing indoor visible light communication (VLC) system affected by light source, there is a problem that the received signal cannot be separated by simple filtering. To solve this problem, a channel estimation by PARATUCK2-PARAFAC-based noise compensation which introduces tensors to construct the suitable model of VLC is proposed. First, the transmitting pilots for channel estimation and noise estimation are constructed separately with the Hermitian symmetry structure. Then the tensor-based transmission model of the indoor VLC system is established, and the signals for channel and noise estimations are received. Finally, an asymptotic estimation for separating signals from different sources and noise is proposed by PARATUCK2-PARAFAC. Simulation results show that the proposed algorithm provides a more effective scheme for channel estimation comparing with the state-of-the-art algorithms.

We describe the investigation into tailoring the optical properties of ammonium dihydrogen phosphate (ADP) crystal by the influence of mild shock waves (Mach number 1.7) produced by a shock tube. An ADP crystal was grown by employing the Sankaranarayana–Ramasamy method with (101) orientation. The test crystal was subjected to different numbers of shock pulses such as 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and 13, respectively. Pre- and post-shock wave-loaded test crystals were experimented with a ultraviolet-visible spectrometer for recording the optical transmission. From the observed results, it is known that, on increasing shock pulses, optical transmission is enhanced from 55% to 66% in such a way that optimization is attained. Thereafter, it reduces to 47% by further increasing the number of applied shock waves. Following the optical transmission, optical constants such as extinction coefficient, optical density, skin depth, real and imaginary parts of the dielectric constant, optical conductivity, and electrical conductivity are also greatly altered if the ADP crystal is exposed to shock pulses. From this experiment, an alternative way to boost the optical properties of an ADP crystal without modifying the original crystal system and without adding any dopants is proposed.

Extreme ultraviolet lithography (EUVL) is recognized as a leading technology in next-generation lithography. Achieving spectral purification while ensuring extreme ultraviolet (EUV) reflectance is one of the key technologies for industrializing EUVL. An EUV collector mirror with phase grating can be used in the spectral purification of an EUVL source. However, it also induces a considerable loss of EUV. We propose a deposition model for calculating the multilayer coating profile on the surface of an EUV collector with grating based on a geometric line tracing method. In addition, it also analyzes the coating defects that influence the EUV reflectance, and the evolution of the coating defects with the grating positions. Experimental results reveal that the model accurately predicts the multilayer coating profile deposited on the surface of the collector, which helps improve how EUV collector is deposited.

Multimode waveguides on lithium niobate-on-insulator (LNOI) and silicon-on-insulator (SOI) platforms are numerically investigated in buried, rib, and strip configurations. Performance of waveguides is compared in terms of waveguide cross-sectional area, dispersion, mode hybridization, and power confinement for both quasi-transverse electric and quasi-transverse magnetic modes. Tall waveguides with single mode in the horizontal direction, supporting higher order modes in the vertical direction, are analyzed. Also, wide waveguides with single mode in the vertical direction, supporting higher order modes in the horizontal direction, are studied. Designs that overcome mode hybridization are proposed, which are well suited for applications such as optical interconnects. LNOI waveguides were found to exhibit lower dispersion in all the configurations, with power confinement and physical dimensions comparable to those of SOI waveguides. The results are instrumental in design optimization of multimode components for on-chip mode-division multiplexing schemes and multiparameter sensing applications. Electro-optic effect is also illustrated in a buried multimode LNOI waveguide that is largely useful in modulation and switching applications.

A fiber-optic carbon monoxide (CO)-sensing system with high sensitivity was proposed by combining Fabry–Perot interferometer (FPI) sensor with sensing material Au/ZnO. The sensitivity is 814  pm  /    %   within the CO concentration range of 0% to 4%. The FPI sensor was fabricated by inserting single-mode fiber into a partially polymer-filled glass capillary to form an air cavity; the length of the air cavity is 76.83  μm. The temperature sensitivity of the FPI is −2.7  nm  /    °  C, which provides better conditions for measurement of CO. The catalyst Au/ZnO was prepared by coprecipitation, which could catalyze the oxidation of CO at ambient temperature and humidity. X-ray diffraction and scanning electron microscope analysis were employed to reveal the crystalline structure and morphology of the catalyst. To eliminate the influence of ambient temperature change on the accuracy of the concentration test, a fiber Bragg grating with central wavelength of 1550.24 nm was introduced in the experimental setup for the temperature compensation.

A lensless light field image sensor configuration is presented to realize a proximity light communication system from multi-light-emitting diode (LED) clusters with different incident angles. The light field image sensor is fabricated on a standard complementary metal–oxide–semiconductor (CMOS) process and utilizes the Talbot effect. Two metal layers are stacked above the photodiode acting as diffraction gratings at the top and a masking layer at the bottom placed at Talbot depth to either pass or block the incident light. In particular, we introduce a proximity optical communication scheme to achieve dual light communication from different incident angles with various wavelengths. To simplify the classification between different combinations of incident angles and wavelengths, a deep neural network (DNN) is utilized. We reduced the number of light field image sensors by 50% and increased the transmission data bits per clock by 114% compared with our previous work. A test environment for deciphering three incident angles and three types of wavelength combinations is devised. For each incident angle, three LEDs, which are red, green, and blue (RGB), are located at precomputed locations. A 3  ×  3 chip LED array consisting of three sets of RGB LEDs is used to transmit signals, and a light field image sensor chip acts as a receiver 3.5 cm away. A range of 46 different types of blink combinations are tested from this single LED array considering up to dual lights communication. The light field image sensor chip consists of six photodiodes with different grating pairs. In particular, three types of grating layer combinations with the photodiodes can detect nine different single blinks and 36 different dual blink combinations, thus totaling 46 types of spatial signals. We achieved a maximum data rate of 251  kbit  /  s with the proposed prototype at a 45.5-kHz sample rate. The proposed proximity optical communication scheme had power dissipations of 30.6 mW on a 1.8-V power supply for the sensor chip and 27.5 mW on a 3.3-V power supply for the LED array. This can be applied in the low-power data input/output communication between a compact cluster of chips.