A single mode-multimode-single mode (SMS) optical fiber structure is adopted, and a pattern recognition classification method is proposed based on the combination of short-time Fourier transform (STFT) and convolutional neural network (CNN) to deal with the intrusion signals which are applied on the multimode fiber. The proposed method initially performs STFT on the intrusion signal to obtain the time-frequency map and subsequently creates a training set and a test set. Further, the training set is input into three network models for training, and a reasonable network model is selected according to the engineering application index. Finally, the identification result of the intrusion signal is made to the test set through the network model; furthermore, the validity and real-time performance of the method are verified using four intrusion signals. The results denote that the proposed method can effectively identify artificial and non-human intrusion signals; in addition, the robustness of this method can be verified by increasing the types and quantities of intrusion signals with noises, thereby reducing the alarm failure and false alarm rate of the intrusion signals and improving the application value of the SMS fiber structure in perimeter defense area pattern recognition.

Aiming at the spatial-spectral correlation characteristics of multispectral images, we propose an end-to-end multispectral image compression method using a convolutional neural network. At the encoding end, multispectral data are fed into the multispectral image compression network, and the main spectral and spatial features of the multispectral image are extracted using convolution. The size of the feature data is reduced by downsampling. The entropy of the spatial-spectral feature data is controlled by the rate distortion, and a dense distribution of spatial-spectral feature data is obtained. The intermediate feature data are quantized and encoded using lossless entropy coding to obtain a compressed bitstream. At the decoding end, the bitstream can be used to reconstruct the multispectral image through an inverse transformation process that involves entropy coding, inverse quantization, upsampling, and deconvolution. Experimental results denote that the proposed method can effectively preserve the spectral information contained in the multispectral images at the same bit rate and improve image reconstruction quality by 2 dB than that of JPEG2000.

When joint bilateral filtering is used to repair depth images, hole-filling effect is poor because the filtering neighborhood range and weight parameter cannot be estimated accurately. To address this problem, we propose an adaptive hole-filling and optimization algorithm for depth images. The proposed algorithm reduces the input parameters and restores each missing depth value. First, the filtering neighborhood range of each hole pixel is determined based on the effective pixel proportion. Then, the parameter value of spatial distance weight is calculated based on the neighborhood size. Finally, the structural similarity is introduced as a parameter calculation index of the color similarity weight. The performance of the proposed algorithm is tested on the Middlebury stereo-matching dataset and the registered Kinect RGB-D dataset, and qualitative comparison and quantitative analysis are performed to compare the performance of the proposed algorithm with those of other methods. The experimental results show that the developed algorithm can effectively fill in missing depth values, reduce the image noise, and improve the quality of depth images meticulously and accurately.

To study the limits of improvements of the high-power large-field fiber power and brightness caused by the thermo-optic effect and thermal-induced mode instability under strong pumping conditions, we analyze the influence of the variation of absorption coefficient on the thermal deposition, thermal-induced refractive index, and numerical aperture of a fiber. Results show that a fiber with high absorption coefficient causes a high heat load density, and its numerical aperture increases under the thermal-optic effect modulation, thereby reducing the mode instability threshold. Based on the theoretical research, fibers with two different pump absorption coefficients are designed and fabricated,and high-power mode instability experiments are performed. Experimental results show that the mode instability threshold power is 800 W for the high pump absorption fiber with an absorption coefficient of 1.71 dB/m. However, for the fiber with a low pump absorption coefficient of 1.20 dB/m, the mode instability is not observed when the output power reaches 1700 W. Thus, the mode instability threshold can be considerably improved by decreasing the pump absorption coefficient of the active fiber. These results are significant to the development of high-power active optical fibers and provide a novel and effective technical approach for obtaining high-power fiber laser output.

Quantum cascade lasers (QCLs) in terahertz frequency emit terahertz radiation by intersubband optical transitions in conduction band of semiconductor quantum wells. Terahertz QCLs are the terahertz sources which are able to be electrically pumped and operated in continuous-wave. Terahertz quantum cascade lasers are small, light, compact, and easy for integration, and they are the solid-state terahertz sources that have the highest output power. This article will review the recent developments of double-metal terahertz QCLs in terms of narrow beam pattern, frequency tunability,and high output power.

The interaction of an intense femtosecond laser with gas to produce high-order harmonics is an important ultrafast coherent light source. Our simulation shows that a mid-infrared femtosecond laser pulse can enhance the multi-photon resonance between the ground states and excited states of the atom by the ac Stark effect and generate high-brightness monochromatic high-order harmonic radiation. By solving the time-dependent Schr dinger equation, we obtain unusual resonantly enhanced high-order harmonics below the threshold, the intensity of which is strongest at an optimal laser intensity. Further, the time-frequency analysis shows that the resonance enhancement is related to the second-order ac Stark effect in a high field, while insensitive to the laser wavelength. This new mechanism makes mid-infrared femtosecond laser pulses more conducive to the generation of ultra-fast monochrome ultraviolet/extreme ultraviolet light sources with high brightness and has important application prospects in condensed matter physics and materials science.

Herein, we report an efficient wavelength-locked 914-nm laser diode in-band pumped intracavity frequency doubling green laser. In our experiment, a wavelength-locked 914-nm laser diode is used as the pumping source to realize good pump uniformity and efficiency and substantially reduce the thermal effect of the laser. A 532-nm laser output with high beam quality is obtained. At a pump power of 18 W and a repetition rate of 130 kHz, the maximum power output of 6.7 W for green laser is obtained. For this pumping power, the conversion efficiency is 37.2%, which corresponds to a conversion efficiency of 60% for absorbed pumping.

By taking the prism laser gyroscope as the research object and using the finite element analysis method, the problems of the generation and amplification of the residual stress are studied in temperature variation process under the condition of existing scratch on the surface of optical cement. Based on the change of the fused silica's dielectric tensor with stress, the relationship between the stress and gyroscope measurement value is analyzed. The analysis results show that the micro scratch on the optical cement region produces the residual stress in the area whose size is several times of the scratch width, and the frequency division and polarization change are caused. The lock-in threshold of the gyroscope increases, the test precision of the gyroscope and working stability are affected. Finally, by variable temperature experiment, the gyroscope detects the normal angular velocity component of the Earth's rotation, and the combined light spot morphology and precision curve stability are used to verify the correctness of the theoretical analysis.

A 780-nm high spectral resolution lidar was designed herein based on a laser diode. A distributed feedback semiconductor laser was used as the seed of the light source system. A tapered semiconductor optical amplifier driven by pulse current was used to output the narrow line width pulse laser. The optical splitting system mainly comprised a narrow-band interference filter, a Fabry-Perot etalon, and a 87Rb absorption cell. The bandwidth of the narrow-band interference filter was 0.5 nm. The full width at half maximum of the Fabry-Perot etalon was 2.8 GHz. Solar background light was filtered by their combination. The Mie scattering signal was absorbed by the 87Rb cell at the temperature of 338 K, which had an inhibition rate of up to 33 dB. The Rayleigh scattering signal was then effectively extracted. Based on the American standard atmospheric model, the detection capability of the high spectral resolution lidar was verified via numerical simulations. This research was highly significant to the detection of aerosol optical parameters.

A repetition rate laser amplifier is the key component of a repetition rate laser; here, the repetition rate of the amplifier is determined by precisely controlling the heat produced during pumping and amplifying. To achieve a repetition rate laser exhibiting kilojoule output energy, a repetition rate laser amplifier prototype with a diameter of 130 mm is developed based on flash-lamp pumping, a neodymium glass gain medium, and liquid cooling. After ensuring the rational mechanical design and selecting a coolant with suitable properties, the developed amplifier prototype operates at 1 shot per minute and exhibits a double pass small signal gain of 1.3189. At 1053 nm, the PV (peak to valley) of 10-shot average double pass wavefronts for the amplifier prototype are 0.2718λ and 0.3223λ when the temperatures of coolant are 20 ℃ and 30 ℃, respectively.

975 nm and 1975 nm dual-wavelength pumped 3.5 μm Er∶ZLBAN fiber lasers have been numerically modeled. Laser power and particle density are estimated in both time and space dimensions and simulation results match well with the experimental reports. The whole process of stable continuous laser oscillation in a dual-wavelength pumped 3.5 μm Er∶ZBLAN fiber laser is showed by simulation. 3.5 μm laser characteristics are estimated and studied with different fiber parameters, such as pump power, 1975 nm pump mode overlap factor, reflectivity of output lens, and interionic interaction. The condition in which ESA2 process has great influence on laser power is also investigated. Simulation promotes the understanding of 3.5 μm Er∶ZLBAN laser dynamics and provides valuable insight for laser designing.

Semiconductor laser bars packaged with different thicknesses of solders and WCu submount are simulated using the multi-physical field simulation software of COMSOL Multiphysics. Results demonstrate that the maximum thermal stress of an In or AuSn solder occurs at the interface between the WCu submount and Cu heat sink. Thermal stresses of laser dies packaged using an In solder and an AuSn solder with the same thickness are 3.57 GPa and 3.83 GPa, respectively, and the corresponding wavelengths at the peak of spectrum are 800.5 nm and 798 nm, respectively. Reducing the solder's thickness is beneficial for reducing the thermal stress and temperature in the laser die. However, if the solder's thickness is too thin, it may cause weak welding of the laser core or uneven distribution of the solder, forming voids in the solder layer; hence, the selection of solder thickness should be considered as a whole. With increasing thickness of the WCu submount, the thermal stress of the laser die decreases; however, the temperature of the core rises. The optimal thickness of the WCu submount is 380 μm. This study provides a basis for optimizing the packaging of high-power semiconductor lasers and has guiding significance to practical production.

A gradient wettable surface can make a droplet autonomously flow in a pre-determined direction. Therefore, this method is important for various applications. In this study, a method is developed to quickly fabricate a gradient wettable surface on the surface of 304 stainless steel using nanosecond laser ablation and heat treatment. The surface microstructure, chemical composition, and contact angle are observed and characterized via scanning electron microscopy, energy spectrum analyzer, and contact angle measuring instrument. A high-speed CCD camera is used to observe the flow of liquid on the gradient wettable surface. The experimental results show that the content of the carbon (C) on the surface is an important factor that affects surface hydrophobicity after laser processing. For 304 stainless steel, short-time heat treatment at temperature of 200 ℃ accelerates the increase of the C content, realizing fast curing of contact angle. The uniformly wettable surfaces with different contact angles can be obtained by designing the surface microstructure of the target. Then, the surfaces with different wettability gradients can be obtained by designing the surface microstructure distribution. The flow distance and velocity of the liquid droplet on the target surface can be controlled by changing the surface microstructure distribution.

Well formed bulk CoCrW alloys are fabricated by selective laser melting (SLM) and the effect of the heat treatment process on the phase composition, microstructure, and microhardness of the CoCrW alloy is studied. The equilibrium phase diagram of CoCrW alloy is simulated and calculated by JMATPro software, and the X-ray diffractometer, scanning electron microscopy, and microscopic hardness tester are used to test the phase composition, microstructure, and microhardness of the CoCrW alloy before and after heat process. Results show that the original SLM sample mainly comprises γ phase and a trace amount of ε phase. After heat treatment, a large portion of γ phase is transformed into ε phase, and the lump- and strip-like precipitate phases are produced. The size of the precipitated phase at the fusion line is large under low temperature heat treatment, and it is clearly different from those of other regions. The grain boundary and grain size of the precipitated δ phase grow with the increasing heat treatment temperature. Simultaneously, the difference in the precipitated phase size at the fusion line decreases. The microhardness of the CoCrW alloy decreases after heat treatment, especially for heat treatment at 1100 ℃ followed by water cooling.

TC4 titanium alloy specimens are prepared via laser three-dimensional (3D) printing and the effect of solution-aging state on the microstructures and mechanical properties of the specimens is investigated. The experimental results show that an increase in solution temperature results in more obvious coarsening of the primary α phase. As the time in solution increases, more of the α phase transforms into the β phase. With respect to aging, as the temperature increases, the length and width of the secondary α phase precipitated from the residual β phase increase, and as the aging time increases, the volume fraction of the secondary α phase increases. During the aging treatment, the secondary reticular β phase is precipitated from the primary α phase, and the secondary α phase is precipitated from the β phase between the primary α phases. Owing to the precipitation of the two secondary phases, the properties of the specimens are affected. With treatment conditions at 950 ℃/1 h/WQ+550 ℃/4 h/AC, the TC4 titanium alloy shows the best mechanical properties. The room-temperature tensile strength is 990 MPa, the yield strength is 920 MPa, the elongation is 11.5%, and the area reduction is 27.5%. The fracture morphologies of the laser-3D printed TC4 titanium alloys in both the as-deposited and solution-aging states are laden with dimples and plastic fractures. Overall, the national standards for forgings have been met.

The temperature field is studied and the process parameters are optimized for the laser cladding based on the internal wire feeding of three beams. A heat source model of the three beams is established. The temperature field of the molten pool is simulated by using the ANSYS software. The effects of defocusing amount, laser power, and scanning speed on the morphology of the cladding layer are analyzed by experiments and simulations. The simulation and experimental results demonstrate that the defocusing amount considerably impacts the width of the cladding layer and the morphology of the dilution zone. The laser power mainly affects the dilution rate of the cladding layer, and the scanning speed considerably impacts the height of the cladding layer. Further, the change of the scanning direction impacts the dilution area of the cladding layer. The experiments are performed under a defocusing amount of -2 mm, a laser power of 1500 W, and a scanning speed of 5 mm·s -1. The cladding layer is smooth and exhibits no obvious cladding defects. The cladding layer structure is martensite, the hardness is 411 HV, and the hardness distribution is relatively uniform.

Herein, 4Cr5MoSiV1 steel samples are fabricated via selective laser melting (SLM), and the effect of tempering on microstructure and mechanical properties is investigated. Results show that tempering of SLM-fabricated 4Cr5MoSiV1 steel samples involves martensite decomposition, retained austenite transformation, and carbide precipitation, thus producing stable ferrite and alloy carbides. Post tempering, the sample's grain morphology disappears, and its grain size increases. In addition, the sample's microhardness and ultimate strength decrease, whereas its elongation increases. Post tempering at 450 ℃, fine and uniformly distributed carbides precipitate, contributing to precipitation strengthening and causing secondary strengthening of the sample. The sample's elongation after tempering twice at 600 ℃ increases to 18.6%. The fracture of unheated and low-temperature tempered samples is attributed to a brittle cleavage fracture. The fracture of the medium-temperature tempered sample is found to be a quasi-cleavage fracture. Finally, the fracture of the high-temperature tempered sample and twice-tempered sample exhibit a quasi-cleavage fracture dominated by ductile fracture.

The femtosecond laser, which induces periodic microstructures on the surfaces of metallic materials, is extensively applied in photovoltaic power generation, self-cleaning, and biomedical fields. In this paper, we fabricate periodic microstructures on the surface of Ti by utilizing a green femtosecond laser with a power of 75 W, a pulse width of 800 fs, and a wavelength of 515 nm. First, the ablation threshold of Ti under the line-scan condition is discussed herein. The Ti surface microstructures are induced based on a 90° cross-surface scan processing. We derive the formula for the calculation of the effective-pulse number per unit point in the surface scanning and summarize the microstructural evolution. The results obtained demonstrate the following: 1) the ablation threshold of Ti by utilizing the green femtosecond laser is significantly lower than that of the previously reported long-wavelength infrared laser; 2) the surface microstructure of Ti can be attributed to the effective-pulse number per unit point and the laser fluence in the surface scanning. Due to the increase in the effective-pulse number, the microstructure changes from nonuniform distribution of hump to uniform columnar array structure when the laser fluence is low, or to interconnected hilly structure when the laser fluence is high. The similar structure can be obtained either by applying a high laser fluence and small effective-pulse number, or a low laser fluence and large effective-pulse number. The former can significantly increase processing efficiency.

Laser-induced TIG arc welding is used to weld 1.5-mm dissimilar aluminum alloys 6061-T6 and 5083-O to systematically study the influence of the addition of a laser on the collapse and mechanical properties of welded joints. We find that the welded joints appear inconsistent owing to the difference in chemical compositions and thermophysical properties of the two aluminum alloys used. Compared with traditional arc welding, the addition of a laser can not only effectively reduce the degree of collapse of the welded joint, but also greatly reduce the inconsistent collapse depth ratio of the welded joint. Moreover, the inconsistent collapse of the welded joints shows a decreasing trend with the increasing laser power used for welding. It causes the joint stress to change; therefore, the joint strength first increases and subsequently decreases.

A noncontact and high-precision detection method based on laser ranging sensing is investigated for measuring the surface profiles of stainless steel lap laser weld joints. The influences of roughness and inclination of the workpiece surface on detection results are also studied. Moving-mean filtering and image tilting correction methods are adopted, and a detection data filtering and correction algorithm is developed. Based on feature point recognition of the detected profile curves of laser weld joints, a method for detecting and calculating the residual and collapse heights of laser welding joints is established. Results show that the profile curves of laser welding joints obtained by these methods are in good agreement with the corresponding metallographic detection results; additionally, weld-surface conditions are well reflected

Laser cladding experiments on 1-mm-thick Ni60 alloy coatings on a small-modulus gear-tooth surface are conducted using a continuous fiber laser. The analysis on the reason for causing cladding defects by the thin small-modulus tooth top and single-pass laser concentrated in the middle of the gear is performed. A double-pass laser processing method is proposed for the fabrication of nicked-based alloy coatings on the small-modulus gear-tooth surface. Under the condition of constant laser energy input, the energy of the original single-pass laser is distributed to two small spots according to a certain energy ratio scheme. The gear's tooth top and bottom are then sequentially processed. Results show that by using two 1-mm spots with an energy ratio of 4∶6, the proposed method not only reduces the ablation of the gear's tooth top, but also solves the problem of excessive energy being concentrated in the middle of the gear. Additionally, the coatings' dilution rate significantly reduces. Low precipitation of crystalline austenite containing Fe and Ni is observed in the microstructure in contrast to the high precipitation of granular cementite rich in Cr. The wear resistance and strength of the cladding layer are improved compared to those of single-pass laser cladding. Overall, the double-pass laser cladding process significantly promotes the quality of the nickel-based alloy coatings prepared on the small-modulus gear-tooth surface.

Herein, a tempered 27SiMn hydraulic support substrate with 431 stainless steel corrosion-resistant coatings is successfully prepared by 50 m/min extreme high-speed laser cladding (EHLA) and 1.5 m/min conventional laser cladding (CLA) methods, respectively. The macroscopic features, microstructure, and corrosion resistance of the coatings prepared by both methods are comparatively investigated. Results show that the coatings prepared by both methods form good metallurgical bonds without microcracks and blowhole defects. Compared to the CLA coating, the EHLA coating has a multi-layer structure similar to the “dominoes” and a low dilution of 4%. Cr achieves a high atomic number fraction of 19% in the EHLA coating. The macroscopic structure of the CLA coating is relatively large, and the direction of dendrite growth is disordered due to the change of temperature gradient at the overlapping zone. The macroscopic structure of the EHLA coating has a high degree of uniformity and compactness, and the microstructure at the bottom/substrate interface is planar crystalline. The dendrites in the middle overlapping zone and the surface of the EHLA coating are finer and slightly roughened at the overlapping area. The corrosion resistance of the EHLA coating is significantly superior.

A hole defect in the ZTC4 titanium alloy plate is repaired by the coaxial powder-feeding laser-cladding technology. The effects of multiple laser repairs on the microstructure, heat-affected zone size, and hardness distribution of repaired parts are investigated. The result shows that the heat-affected zone of the repaired specimen exhibits a microstructural transition from a basketweave structure to a colonies structure,and then transition to a needle-shaped martensite. The repaired area primarily comprises a coarse β-columnar crystal and widmanstatten structure, which is longer at the top of the repaired area. During multiple repairs, the size of the heat-affected zone considerably increases. However, the hardness of the heat-affected zone only increases slightly.

A nanosecond-pulsed-laser paint stripping experiment is performed to reveal the influences of laser parameters on the surface morphology of laser paint stripping. The surface morphology and oxygen content are observed; the three-dimensional morphology and surface roughness are evaluated; the residual-paint-layer area is measured to characterize the cleaning rate. The results show that the roughness of the paint-stripped surface increases with the increasing laser power. With a decrease in the cleaning speed, the roughness first decreases and subsequently increases. Coking occurs with an increase in the pulse frequency. The paint removal mechanism changes from the vibration and ablation effects to the ablation effect with a decrease in the cleaning speed; this results in a decrease followed by an increase in the cleaning rate. Under the experimental condition of 50 W laser power, 90 kHz pulse frequency, and 7000 mm/s cleaning speed, the best laser cleaning is achieved, and the paint stripping rate is 99.4%.

Femtosecond laser direct writing technique is used to fabricate microelectrodes on flexible substrates with a cheap copper ion pre-coating. The copper nanoparticles are obtained using laser-induced reduction and in situ bonding, and the as-written copper electrodes exhibit excellent electrical conductivity. Further, the effects of laser power and scan speed on the microstructure and electrical conductivity of the electrodes are studied. The results denote that the conductivity generally increases with the increase of laser energy density. The obtained copper electrodes are mostly network-like metallic copper and exhibit the lowest sheet resistance of 2.74 Ω·sq -1 at a laser power of 1210 mW and a scan speed of 1 mm/s. This research provides a new technique for developing low-cost and high-efficiency flexible devices, which can benefit the application of copper nanomaterials in the electronics industry.

Standard compact tensile (CT) samples were tested to compare the fatigue crack growth rates (FCGRs) of the arc dominated zone (ADZ), laser dominated zone (LDZ), heat affected zone (HAZ), and base metal (BM), which were sampled on the laser arc hybrid welding joints of an ultra-low-carbon bainitic steel. The growth features of the fatigue crack in each microcell were summarized, and the reason for the deflected path of crack growth was explained. The result shows that the FCGRs increase but the accelerated velocities of the HAZ and BM decrease with the growth of the stress intensity factor (SIF). At the low SIF, the FCGRs decrease in the following sequence: BM, HAZ, ADZ, and LDZ. When the SIF is sufficiently large, the sequence changes to: LDZ, ADZ, BM, and HAZ. The crack-path deflection in the HAZ is observed clearly, and the secondary cracks increase when the SIF increases. However, the crack deflection in the weld zone is not observed clearly. This difference is attributed to the non-uniformity in the material composition and microstructure. The observation of the fracture explains that the generation of the secondary cracks is associated with the brittle phases. Further, the secondary cracks are primarily responsible for the change in the FCGRs.

Ga-doped ZnO (GZO) transparent conductive thin films are deposited on glass substrates via the pulsed laser deposition method; further, the influence of oxygen pressure on the structure, surface morphology, and photoelectric properties of the GZO thin film is systematically investigated using X-ray diffractometer, ultraviolet-visible spectroscopy, atomic force microscopy, and Hall test system. Results show that all the samples exhibit a hexagonal wurtzite structure with a preferred orientation along the c-axis. Homogeneous, dense, and compact surfaces are obtained for all the GZO films. The crystal size initially increases and then decreases with the increasing oxygen pressure; optimum crystallinity is observed at an oxygen pressure of 0.5 Pa. The prepared GZO films exhibit a transmittance higher than 91.97% in the visible region; the band gap of the GZO film is 3.492-3.576 eV. The carrier density and Hall mobility initially increase and then decrease with the increasing oxygen pressure. The resistivity initially decreases when the oxygen pressure increases. However, with a further increase in the oxygen pressure, the resistivity increases. The minimum resistivity of 2.95×10 -4 Ω·cm is observed when the oxygen pressure is 0.5 Pa.

ZnSe is an important semiconductor material of II-VI group. It exhibits a zinc blende (ZB) structure at room temperature and pressure, and the phase transition into a rock salt (RS) structure occurs under high pressure. Doping transition metal ions (TM 2+) into the ZnSe crystal can effectively produce laser gain media and photoelectric materials in the mid-infrared region, which exhibit important research significance. This study mainly deals with the effect of high pressure on the properties of the Cr 2+-doped ZnSe semiconductors. Further, the effect of dopant (Cr 2+) on the phase transition pressure of ZnSe and the changes of electronic structures, optical properties, and mechanical properties of ZnSe and Cr 2+∶ZnSe are calculated under high pressure using the first-principles calculations based on the density functional theory. The introduction of the dopant (Cr 2+) reduces the phase transition pressure of ZnSe from ZB structure to RS structure, and this trend continues with a further increase in the doping concentration. The electronic structures and optical properties of ZnSe and Cr 2+∶ZnSe are evaluated under high pressure, and ZnSe is found to shift from exhibiting semiconductor properties to exhibiting metal properties under high pressure. The calculated elastic constants of the crystals satisfy the stability conditions under both ambient and high pressure. Meanwhile, the large bulk modulus, shear modulus, and Young's modulus of the crystal under high pressure indicate that the RS structure exhibits considerable hardness and stability and is remarkably resistant to deformation under external influences.

Herein, a novel method for measuring axicon surface based on annular sub-aperture scanning is proposed. This method successfully performs the general measurement of axicon surfaces with a large diameter and different cone angles. A micro-motion scanner is used to move the axicon step by step while the wave measuring interferometer records several ring phases in the normal direction of axicon. The surface is computed by extracting effective pixels and the interpolation-stitching. The proposed method successfully measures an axicon with a cone angle greater than 96.4° and a diameter above 100 mm. Results in the normal direction of the surface are exclusively used, and a theoretical measurement accuracy up to λ/4 PV (λ is wavelength; PV is peak valley) is achieved. In the experiments, the top surface of a convex axicon (with a nominal angle of 140°) is measured by a 4-inch (1 inch=2.54 cm) DynaFiz interferometer of ZYGO company. Two measurements are performed with the axicon rotated at 90°. Both results are in good agreement with the measurement results of the LuphoScan profiler of Taylor-Hobson company, and the difference in PV values is 0.54 μm. These results demonstrate the good feasibility of the proposed method.

In order to solve the problem that the existing laser scanning projection system needs at least 4 cooperation target points to realize the coordinate transformation relationship calibration, and it often takes multiple times and much time to complete the coordinate transformation parameter solution for the non-convergent iterative algorithm, we proposes and studies the laser scanning projection system calibration technique that combines laser ranging. The mathematical models of laser scanning projection system with and without laser ranging module are studied. The particle swarm optimization algorithm is used to simulate the calibration accuracy. Aiming at the problem that the ranging error has a great influence on the calibration accuracy of the system, a particle swarm derivative calibration algorithm is proposed to improve the calibration algorithm. Finally, a rapid calibration of only three cooperative target points is realized and the accuracy is increased to 10 -7 mm.

Defocusing dither optimization techniques can eliminate the nonlinear error and overcome the limitation of refresh rate of a projector. However, the dithering algorithm is only a simple matrix transform and the dithering fringe after defocusing does not approximate the ideal sinusoidal fringe enough, inducing a measurement phase error. This paper proposes a binary particle swarm optimization (BPSO) method,and the binary defocusing dither technique is optimized. To speed up the BPSO process, binary patches are first optimized; then, the patches are joined together with respect to symmetry and periodicity to generate the full-size pattern. The simulation and experimental results demonstrate that the proposed method can obtain high quality measurement results under various extents of defocusing.

A star sensor thermal drift calibration system based on error decoupling is proposed. According to the calibration principle and the simulation requirements of the star, a self-collimation optical calibration system and an error decoupling optical path are designed. According to the characteristics of common optical path, the installation deformation error of the star sensor and the coupling error caused by the thermal deformation and the installation deformation are obtained by the autocollimator and the CCD, removing the installation deformation error from the coupling error to obtain accurate star sensor attitude shift quaternion. The simulation results show that the maximum variation errors of the star sensor around the axes are 0.2638, 0.1317, 0.0472 (″)/℃ in the environment range of -25~60 ℃, respectively, and the errors are controlled in the range of 0.02 (″)/℃ with the ideal result. This provides a new idea and method to reduce the coupling error generated in the calibration of star sensors and improve the accuracy of thermal drift calibration system.

Scanning slit is one of the most important elements in step-and-scan lithographic tools for controlling exposure doses. As the technology of lithography extends to 90-nm (and lower) process nodes, the lithographic-tool illumination subsystem has presented significant requirements for the precision and repeatability of the penumbra measurement. Based on this, a blade edge's penumbra measurement technique for scanning slits of lithographic tools based on the pupil image is proposed. The corresponding relationship between the penumbra of the mask surface and pupil image is deduced by analyzing the imaging optical path of the coplanar scanning slit. A penumbra measurement system is developed, and the scanning slit edges' penumbra of a 90-nm lithographic-tool illumination system is measured. The experimental result shows that the proposed measurement method can effectively reduce the impact of light-intensity fluctuation on the penumbra measurement. The repeatability of the penumbra measurement is 0.026 mm, which is 3.46-times higher than that of the conventional scanning method. This technique can be used to measure the optical penumbra parameters of high-numerical-aperture immersion-lithographic-tool illumination subsystems.

The chaos generated by the external-cavity feedback semiconductor laser has an obvious relaxation oscillation and the energy of the low-frequency band is low, limiting the dynamic range of chaos optical time domain reflectometry (OTDR). An optical fiber ring structure is proposed herein to improve the dynamic range of the chaos OTDR. Experiments and numerical simulation find that multibeam interference occurs when the chaotic light passes through the fiber ring. Moreover, the energy of the low-frequency band is improved by delaying the self-beat frequency effect. The experimental detection results show that the dynamic range of the chaos OTDR increases by 5 dB when the fiber ring is used under a 200-MHz detection bandwidth.

To improve the application adaptability of the method used for solving the echo time of pulse laser ranging, this study transforms the echo time solving problem into a waveform classification problem and uses a novel deep learning method to solve the echo time. Further, a one-dimensional convolutional neural network model is trained by simulating the sample echo data containing different distances, signal amplitudes, waveform shapes, and noises with a time resolution of 0.1 ns, and a classification accuracy of 99.85% is obtained using the sample test set. Using the deep learning method and the Gaussian fitting method to process the airborne lidar echo data, the correlation coefficient of the wall surface sweep measurement results is 0.99981. Further, the plane fitting residuals of the field flight test data are approximately 20 mm; the effects of the two methods are observed to be equivalent. The results denote that the proposed method can satisfy the requirements for solving the echo time of airborne pulse laser ranging and can improve the solution accuracy and adapt to several application scenarios.

Temperature change affects the accuracy of the radiometric calibration of the directional polarimetric camera (DPC). The first step in scientific remote-sensing data processing is temperature correction. Herein, we analyze the influence of temperature change based on the DPC working principle and the polarization-measurement model. Combined with the thermal-control design and thermal vacuum test, we develop a temperature-compensation-correction method aiming at the influence of radiation measurement. We design a temperature-response test scheme for in-orbit temperature conditions at different stages to monitor external conditions which affect test data and effectively eliminate the influence of the detector-frame transfer effect, and obtain the relationship between the radiation calibration coefficient and the temperature-change response. According to the analysis of experimental data and in-orbit data, the radiation measurement error caused by temperature change reduces to less than +0.1% through real-time measurement of the dark-background channel and through the temperature-correction method of radiation-response-coefficient compensation, which meets the requirements of in-orbit radiation correction.

We develop a mathematical model of a fiber optic current sensor and deduce a mathematic equation of measurement accuracy of the sensor. The influence of the loop gain variation on the measurement accuracy is theoretically investigated. Further, we develop a method for online monitoring of the loop gain. The loop gain value is demodulated and transmitted to the monitor in real-time as one of fault monitoring points for system monitoring. A stable loop gain controlling technique is developed to automatically adjust the loop gain, thereby eliminating the accuracy degradation caused by the loop gain change in the long-time system running. Results show that loop gain value monitored online as well as the scale factor error and phase error change as the preamplifier gain changes, consistent with the simulation results. Furthermore, the loop gain value monitored online remains stable, and appropriate measurement accuracy can be achieved while utilizing the proposed stable controlling technique. These observations denote the effectiveness of the proposed online monitoring and stable controlling technique for loop gain.

To study the influence of the resonant frequency in sound pressure transfer function on the frequency response characteristics of distributed feedback fiber laser hydrophone, a sensitivity enhanced structure through polyurethane end surface pulling is designed. The sound pressure transfer function model is established based on electro-acoustic theory. Then the relationship between the structure and material parameters with the sound pressure transfer function is simulated, and prototypes of hydrophones are fabricated and tested. The results show that the average sound pressure sensitivity reaches to -142.8 dB with the fluctuation less than ±1.5 dB in the frequency range of 10~2000 Hz and the resonant peak appears near 3150 Hz, which agrees well with the simulation result. The frequency response characteristics of fiber laser hydrophone can be effectively predicted,refined and optimized, which is of great guiding significance for the development of small array elements used in small-sized underwater combat platform.

Femtosecond laser direct writing technology has been extensively used for the preparation of functional microfluidic chips because of ultrashort pulse duration and extremely high peak intensity of femtosecond lasers. This study summarizes the following three research directions based on the direct writing technology of femtosecond lasers for microfluidic chips: the integration technology of functional devices fabricated by femtosecond lasers in microfluidic chips with different materials, the multi-functional applications of microfluidic chips integrated by femtosecond lasers, and the rapid processing of microfluidic chips using femtosecond lasers. Furthermore, according to the summaries on the research results of femtosecond laser direct writing technology in the field of microfluidics, this study provides a reference for the research, application, and future development of the microfluidic chips prepared using femtosecond laser direct writing technology.

A carbon nanotubes/Ag nanoparticles (CNTs/AgNPs) surface-enhanced Raman scattering (SERS)-active substrate is prepared by the chemical reduction method, and a polydimethylsiloxane (PDMS) microfluidic system exhibiting SERS activity is constructed by using the molding method. Further, experiments are conducted by using the rhodamine 6G (R6G) solution at a concentration of 10 -7 mol/L as probe molecules. The experimental results verify the feasibility of the Raman test of the CNTs/AgNPs-enhanced substrate in the microchannel; in addition, the variation of the Raman signal of the probe molecules in the microchannel has also been verified and analyzed at different positions and time.