In this study, we employ optical spatial filtering to improve the quality of the Bessel beam generated by a deformable mirror. An annular filter is designed to filter the spectral components produced by the round tip error of the deformable mirror on the spatial spectrum plane of the generated Bessel beam, making the axial intensity distribution of the Bessel beam increasingly uniform. Furthermore, a filter that combines a sectorial filter and an annular filter is designed, which directly transforms a circularly symmetric Bessel beam into an elliptic Bessel beam. The aspect ratio of the main lobe of the generated elliptical Bessel beam is approximately 1.72. The axial full width at half maximum exhibits a maximum of 247 mm, denoting an excellent non-diffraction property. The proposed optical spatial filtering method can effectively improve the performance of the Bessel beam generated by a deformable mirror and advance its applications.

Traditional beam-quality measurement methods face problems such as excessive time consumption and low efficiency of optical field information acquisition on the equidistant propagation plane; to combat these, a non-equidistant and bidirectional propagation method to determine beam quality is proposed. A non-equidistant and bidirectional propagation model is established by updating the condition of non-equidistant propagation step, while the iteration of the propagation step in the fitting of laser propagation profile is realized to determine beam quality. Experimental results show that the error of beam-quality measurement is ±1.9% in the non-equidistant and bidirectional propagation model. The proposed method can effectively improve the efficiency of laser beam-quality measurement.

In this study, an analytical expression is derived based on the extended Huygens-Fresnel principle for the spectrum of a Gaussian Schell-model (GSM) beam that propagates in biological tissues. The spectral change of the GSM beam during propagation is studied based on the normalized spectrum and the relative spectral shift. Results show that the spectral blue shift, red shift, and rapid transition can be observed when the GSM beam propagates in biological tissues, and they are dependent on the off-axis distance, propagation distance, type of biological tissue specimen (specifically the refractive-index structure constant of tissue turbulence), and spatial correlation length. As the propagation distance increases, the refractive-index structure constant increases, meaning that the turbulence of biological tissue becomes stronger. Meanwhile, as the spatial correlation length increases, the position where spectral rapid transition occurs is farther and the transition qualities correspondingly decrease; furthermore, the spectral rapid transition becomes increasingly weak and the propagation position where a transition can be observed from the spectral red shift to blue shift becomes increasingly distant. With increasing values of the refractive-index structure constant and the spatial correlation length, the off-axis distance associated with the spectral rapid transition will also increase, i.e., the distance between the observation position and the propagation axis will increase.

In comparison with the traditional suturing, laser biological tissue welding has the following advantages: shorter operation time, faster wound healing, less tissue damage, and no thread removal. To improve the strength and quality of laser soldering of biological tissues, increase the laser absorptivity in the direction of full-thickness, and explore the effects of dyes on the strength and thermal damage of laser soldering of biological tissues, we use the bovine serum albumin solution as the matrix, chitosan as the stabilizer, and indocyanine green and methylene blue as dyes. We use a Nd∶YAG laser (1064 nm) to weld the skin in vitro with flux and compare the welding effect of flux. Results show that indocyanine green and methylene blue are significant organic chromophores for laser welding. They promote the efficiency of photothermal conversion, improve the welding strength, and reduce thermal damage.

This study experimentally and theoretically examines the response characteristic of twisting the second-azimuthal-order few-mode long-period fiber grating (FM-LPFG). The theoretical analysis shows that the twisting responsivity of the coupled resonant wavelength of the high-order FM-LPFG is closely related to the azimuthal order of the grating, i.e., the twisting responsivity of the resonant wavelength of the second-azimuthal-order FM-LPFG is almost two times larger than that of the first-azimuthal-order FM-LPFG; the twisting-induced phase mismatching leads to an almost linear decay of the intensity of the resonant peak in the transmission spectrum of the grating with the increase of grating twisting rate in a relatively small twisting-rate domain. This decay rate is inversely related to the value of the phase mismatch. Further, the experimental results show that the twisting responsivity of the resonant wavelength of the second-azimuthal-order FM-LPFG is approximately 1.5 times larger than that of the first-azimuthal-order FM-LPFG, which reaches 0.72 nm·(rad·m -1) -1 and 0.82 nm·(rad·m -1) -1 in the cases of clockwise and counter-clockwise twisting, respectively. The intensity of the resonant peak varies linearly with the increase of the twisting-rate in the small twisting-rate domain, and the linear sensitivities are 0.81 dB·(rad·m -1) -1 and 0.72 dB·(rad·m -1) -1 in the cases of clockwise and counter-clockwise twisting, respectively; however, the corresponding intensity of resonant peak has the characteristics of small variation amplitude and fluctuation in the large twisting-rate domain, indicating that experimental results are generally in agreement with the theoretical analysis. These twisting response characteristics of the wavelength and intensity of the resonant peak of the second-azimuthal-order FM-LPFG have potential applications in high-precision sensing of twisting mechanical parameters (such as twisting capacity, twisting speed, and acceleration) and simultaneous measurement of multiple parameters in the same monitoring twisting-rate domain.

In this study, we propose an improved genetic algorithm, i.e., an interference suppression subchannel allocation (IGA-ISSA) scheme, to solve the channel interference problem that can be attributed to the layout of multiple access points in heterogeneous VLC/WiFi networks. In IGA-ISSA, decision-making is designed for the user to select a VLC or WiFi network in accordance with the VLC channel quality. Further, the users are classified into three priority levels according to the user location characteristics in VLC. Subsequently, a subchannel allocation scheme is designed based on the user conflict graph with respect to different levels for suppressing the interference. Furthermore, IGA-ISSA is introduced to optimize the subchannel allocation for the VLC network. Finally, a WiFi subchannel allocation scheme is designed based on the different user-required rates with respect to the allocated WiFi subchannel in case of users accessing a WiFi network. The simulation results denote that the IGA-ISSA scheme can improve the throughput and user satisfaction with respect to a VLC/WiFi network.

In this study, an integrated resonant fiber optic gyroscope (R-FOG) scheme that employs a hybrid photonic crystal fiber (PCF) resonator is proposed to suppress the polarization-fluctuation noise of R-FOG. The hybrid PCF resonator, which exhibits a high polarization extinction ratio and good temperature stability, is primarily composed of a specially designed PCF and single polarization fiber (SPF). To reduce the resonator loss, the mode field diameter of PCF is modified to become consistent with that of SPF. Further, the fusion loss of the two types of fibers is controlled within 0.1 dB. In addition, the fineness of the hybrid PCF resonator is 13.2. A double closed-loop R-FOG system is set up based on the hybrid PCF resonator, and its performance is experimentally studied. The results denote that the R-FOG system exhibits small polarization-fluctuation error characteristics. The white noise dominates the output at an integration time of 300 s when a bias stability of 0.25 (°)·h -1 is achieved. Within the dynamic range from -240 (°)·s -1 to 240 (°)·s -1, the scale-factor nonlinearity of the double closed-loop R-FOG system is observed to be 2.3×10 -4. The performance of the double closed-loop R-FOG system is better than that of a single closed-loop R-FOG system.

This study develops a slider sensor in which a fiber Bragg grating is considered to be the sensing element and combined with a silicone rubber and a bracket using polymethyl methacrylate, to measure the coefficient of friction (COF) between silicone rubber and a contact object. Further, the relationship between the strain measured by the fiber Bragg grating and COF is theoretically analyzed. The COF is measured based on the mean value and standard deviation of the strain measured by the fiber Bragg grating when the slider is sliding. Experimental results show that the mean strain decreases with the COF, and the maximum sensitivity is -443.7481 με/unit when the COF is 0.34-0.435. Furthermore, the standard deviation of strain increases exponentially with the COF, and the maximum measuring sensitivity is 284.5672 με/unit. The method for measuring the COF can be used in the field of tactile perception with respect to mechanical fingers and has broad application prospects.

The hogel-based effective perspective image segmentation and mosaicking (EPISM) method utilizes a two-step approach for holographic stereogram printing. This method can generate a holographic stereogram with high-quality reconstructed images using only a few sampled images. However, this method needs to establish an accurate correspondence among hogels to obtain effective perspective image segments, and it can be utilized only when certain position constraints are satisfied. To solve this problem, we propose a general hogel-based EPISM method for holographic stereogram printing in this study. The proposed method uses an approximate correspondence of hogels instead of an accurate correspondence of hogels to obtain several effective perspective image segments, so it can be applied to general cases. Furthermore, using optical experiments, we verify that the proposed method can generate a holographic stereogram with high-quality reconstructed images by employing a few sampled images. Although the way to obtain the effective perspective image segments is different, the reconstructed images of the holographic stereograms obtained by the two methods are with the same good quality. The printing efficiency of the proposed method is higher than that of the EPISM method, and the proposed method can also be utilized to improve the “pixel correspondence” holographic stereogram.

The characteristics of the discharge shock waves in high-repetition-rate excimer laser discharge chambers are experimentally investigated using the shadow method. This results represent the generation and evolution of the discharge shock waves. Thus, three typical forms of discharge shock waves, i.e., transverse, longitudinal (the main electrode shock wave), and preionization shock waves, are obtained. Further, the velocities of these shock waves are estimated, and the repetition rate at which the shock waves may effect the discharge is analyzed. The results of this study support the design of discharge-shock-wave suppression devices in high-repetition-rate discharge chambers.

In this study, the influence of the gas pressure ratio of helium to neon on the characteristics of a ring laser gyro is investigated. The relation between the power tuning curve and the gas pressure ratio in a ring laser cavity is theoretically derived from the plasma dispersion function. A specific type of a ring laser gyro is used for experimental measurements. An inflatable experimental device with strong operability,adjustable gas pressure ratio, and real-time monitoring of the gas parameters is presented. The relation between the output gain of the laser gyro and gas pressure ratio is determined. The optimal working gas pressure ratio of helium to neon is also determined. Experimental results indicate a positive relation between the laser output gain and gas pressure within a certain range. The output gain reaches its maximum at a certain pressure value and then falls with an increase of the pressure to the point, where the laser output becomes zero.

In this study, we analyze the vertical distribution characteristics of aerosols in the Langfang city on clean, hazy, and cloudy days based on the ground-based lidar and CALIPSO satellite data and determine the possible sources of pollutants. The analysis results denote that the extinction coefficient profiles of the two lidar inversions are consistent under clear, hazy, and cloudy conditions and that the correlation coefficient is large. Small amounts of dust aerosols below 2 km can be observed on clear days, with an extinction coefficient of less than 0.2 km -1, a depolarization ratio concentrated in 0.10-0.30, a color ratio concentrated in 0.5-1.0, high particle non-sphericity, and large particle size. Below 500 m, there exists a collection of fine particles exhibiting high sphericity on hazy days, with a maximum extinction coefficient of greater than 2 km -1, small depolarization and small color ratios. Additionally, the polluted continental aerosols and the polluted dust are observed to simultaneously exist. On cloudy days, the extinction coefficient is large at the vertical height, where cloud layers appear, and the profile shows a sharp peak. The depolarization ratio is mostly less than 0.02, and the color ratio is concentrated at approximately 0.10. Furthermore, a small amount of dust aerosols is distributed under the influence of an updraft below 7 km.

Aiming at the differences in sweep speed, step size, range, accuracy, and signal-to-noise ratio (SNR) of linear sweep lasers, this study proposes a scheme for directly generating an acousto-optic modulation-based frequency-scanning laser. It adopts a ring-shaped cavity structure, which has the characteristics of fast sweeping speed, high frequency sweeping precision, and high SNR. The existing frequency-scanning laser is optimized by analyzing the effects of the laser's SNR and length of the erbium-doped fiber in the erbium-doped fiber amplifier on the performance of the generated frequency-scanning laser.

Herein, the influence of gamma-ray radiation on laser power characteristics is investigated experimentally by irradiating the gain fiber of a fiber laser with gamma rays in off- and on-line modes. In the off-line irradiation experiments, it is found that the slope efficiency of the fiber laser remains unchanged when the pump power is larger than a certain value. In the on-line irradiation experiments, with a total radiation dose of 2580 Gy, it is found that the laser output power decreases exponentially during the initial stages of irradiation until it reaches the lowest value, then increases slightly, and thereafter tends to be stable. The variations of power degradation rates with radiation dose under two different irradiation modes are compared, and the power degradation rate of the on-line irradiation laser is found to be higher during the initial stages of irradiation. With the accumulation of radiation dose, the power degradation rates of fiber lasers irradiated in both on-line and off-line modes tend to be consistent. These preliminarily experimental results are explained using color center theory of fiber radiation-induced loss.

Some problems, such as distortion and cracking, can be observed in the laser cladding layers because of their non-equilibrium microstructure and residual stress. Herein, the tempering heat treatment is used to improve the comprehensive properties of a 420 martensitic stainless steel coating. Further, the effect of tempering temperature on the microstructure, mechanical properties, and corrosion resistance is investigated. The results denote that the as-cladded specimen mainly comprises martensite, austenite, and M23C6 carbide and that reversed austenite can be observed after the tempering treatment is conducted at 200 ℃ and 400 ℃. The tempering temperature affects the mechanical properties. The tensile strength (approximately 1800 MPa) and microhardness (approximately 530 HV) of the as-tempered specimens at low (100 ℃ and 200 ℃) and medium (400 ℃) temperatures are comparable with those of the as-cladded specimen. Furthermore, the elongation of the specimens increases with the increasing tempering temperature. However, the strength and microhardness considerably decrease and the elongation slightly decreases after high-temperature tempering treatment (600 ℃). Therefore, the as-tempered specimen at 400 ℃ exhibits optimum comprehensive mechanical properties but slightly lower corrosion resistance when compared with those of the as-cladded specimen and the commercial 420MSS martensitic stainless steel.

In this study, Inconel 625 superalloy (IN625) samples were formed using the selective laser melting (SLM) technology. The effects of the process parameters on defects, such as pores and cracks, were investigated. Then, the microstructure and mechanical properties of the samples under the optimal relative density were analyzed. Results show that the samples has a relative density of over 99.5% when the energy density is 50-78 J·mm -3. An energy input of approximately 75 J·mm -3 is required to approach the completely dense state. The sample displays cracks and un-melting powder owing to the low energy density. When the energy density is high, irregular and fine pores appear. The IN625 sample morphology shows cellular and columnar grains. Meanwhile, the average size of grains is 10-30 μm and the number of subgrains is small. The crystallographic orientation is the preferred direction for the IN625 samples. A large number of small dimples appear on the mixed fracture. The microhardness, tensile strength, yield strength, and elongation are (327±3) HV, (930±5) MPa, (700±5) MPa, and 29.5%, respectively.

The Lc-Sr-31 (Fe-based) alloy powder was deposited on the surface of the cast-steel material of a high-speed train-brake disk via laser cladding to obtain a composite coating with high temperature oxidation resistance. Optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffractometer were used to study the oxide film microstructure and phase composition of the Fe-based cladding layer, which was produced at different oxidation time at 650 ℃. Furthermore, the mechanism of high-temperature oxidation corrosion resistance was obtained. The results show that the Fe-based cladding layer and matrix material are metallurgically bonded and there are no defects such as cracks and pores. After 100-h oxidation treatment at 650 ℃, the dense Cr2O3 and SiO2 oxide films are formed on the surface of the cladding layer and there is no clear oxidation mass gain. When the high temperature oxidation time is extended to 200 h, several discontinuous FeCr2O4 spinel oxides appear on the surface of the cladding layer, which provides continuous oxidation resistance for the matrix material.

The 6106-T6 aluminum alloy hollow extrusion profiles having a lock bottom structure were welded via fiber laser-cold metal transfer (CMT) arc hybrid welding, fiber laser-variable polarity tungsten inert gas (VPTIG) hybird welding, and fiber laser-melt inert gas (MIG) hybrid welding. Subsequently, a hybrid welding joint with good forming properties and without clear defects was obtained using optimized welding parameters. Furthermore, the joint microstructure, tensile, and fatigue properties were studied, and the fatigue fracture mechanism and fracture morphology were analyzed. The results denote that the sizes of equiaxed grains at the center of the laser-CMT and laser-VPTIG hybrid welding joints gradually decrease from the upper part of the bead to the bottom. However, the sizes of the coarse equiaxed grains in the upper and lower parts do not change considerably, and the sizes of grains at the center of the laser-MIG hybrid welding joint are large. Furthermore, the tensile strengths of the laser-CMT, laser-VPTIG, and laser-MIG hybrid welded joints are 213.0, 198.0, and 200.0 MPa, respectively. These values denote a certain degree of strength loss when compared with that of the base metal. The fatigue limits of the three hybrid welded joints are 105.00, 100.83, and 113.50 MPa, respectively. All the fatigue fracture positions are located in the columnar crystal zone at the fusion line of the welding joints. In addition, the fractures are dimpled, indicating a typical ductile fracture.

Herein, the effect of temperature on the frequency of the zero-group-velocity (ZGV) Lamb wave of an aluminum alloy sheet is studied. Further, an experimental system is established for the all-optical detection and excitation of the S1-ZGV mode in an aluminum alloy sheet under different temperatures using a pulsed laser based on Doppler vibrometer. The S1-ZGV mode frequency is observed to vary with the temperature in an aluminum alloy sheet. Subsequently, the experimental results denote that the frequency of the S1-ZGV mode decreases by 10.4% when the temperature increases from 20 ℃ to 370 ℃, whereas the theoretical results denote that the frequency of the S1-ZGV mode decreases by 10.9%. Therefore, the experimental results are observed to be consistent with the theoretical results. In addition, when the temperature increases from 20 ℃ to 370 ℃, the effects of the sheet thickness and shear wave velocity on the frequency of the S1-ZGV mode are observed to be the smallest and largest, respectively, by considering the influences of the sheet thickness, Poisson ratio, and shear wave velocity of the aluminum alloy sheet on the frequency of the S1-ZGV mode.

A low-power laser is used to induce a TIG arc for improving the surface quality and mechanical properties of the stainless steel components produced via TIG arc additive manufacturing; the arcs before and after laser radiation are compared. Further, the tensile strength and microstructure of the components are investigated at different laser powers. The results denote that the laser induced effect can compress the arc and stabilize the deposition process, resulting in high-speed deposition. The proposed methodology results in low heat input and improves the mechanical properties of the wall. A columnar dendrite microstructure of the wall is considered. The distance between the dendrites from the bottom to the top gradually increases, whereas the hardness decreases. The top of the wall exhibits an equiaxed dendrite microstructure. The 316 stainless steel produced by laser-induced arc additive manufacturing comprises austenite and ferrite.

In this study, various morphological features of plasma were studied by using the visual observation during fiber laser-TIG arc hybrid welding. The results show that based on the non-uniform distribution of light intensity of plasma, approximately 3-4 images of plasma can be obtained simultaneously by using the multi-imaging method and the metal and arc plasma can be distinguished. Compared with the Ar shielding gas, the metal and arc plasma can be distinguished more easily under the He atmosphere. Under the same arc current and imaging conditions, the area of the hybrid welding plasma is obviously larger than that of the single arc plasma, and the area of the plasma in hybrid welding with an TIG arc-laser mode is obviously larger than that of the plasma in hybrid welding with a laser-TIG arc mode. Further analysis shows that the metal vapor and shielding gas are not fully mixed during the hybrid welding. In addition, metal plasma significantly affect the morphology and stability of the hybrid-welding plasma.

The laser-cladding-deposited TC4 titanium alloy is considerably anisotropic, and its mechanical properties are lower than the forging standards. In this study, boron (B) with different mass fractions is uniformly mixed into the TC4 powder by mechanical stirring, refining the microstructure and improving the comprehensive mechanical properties of the alloy. Results show that at a B mass fraction of 0.050%, the mechanical properties of the deposited TC4 titanium alloy exceed the forging standard, and the yield and tensile strengths become 1049 MPa and 1136 MPa, respectively. Furthermore, the elongation and area reduction become 11.50% and 33.5%, respectively, though the plastic anisotropy is larger than 30%. After solid solution and aging heat treatment with 950 ℃×1 h/air cooling +500 ℃×4 h/air cooling, the comprehensive mechanical properties are further optimized and the anisotropy is reduced to less than 5%. While we ensure suitable mechanical strength, the elongation and area reduction are enhanced to 16.70% and 37.3%, respectively.

The types of surface scratches on the fused-silica workpiece during the polishing process and the length of the scratches caused by impurity particles having different particle sizes in the ending stage are analyzed by introducing α-AL2O3 particles into the polishing process of fused-silica glass. Further, the effects of the polishing-powder mass fraction in the polishing solution and the polishing pad structure on the polishing quality of the fused-silica glass are studied. Results show that the impurity particles require sufficient support force provided by the polishing pad to produce scratches on the surface of the workpiece; the support force of the polishing pad on which the impurity particles are subjected depends on their positional height and the number of matrix particles participating in the polishing process. It is difficult to control the positional height of the impurity particles; however, at a relatively high polishing-powder mass fraction of 6%, the microporous pitch-polishing pad effectively reduces the scratching probability without introducing surface roughness or excessively affecting the polishing efficiency under identical process conditions. These observations exhibit guiding significance for processing and production.

In our study of the removal of the protective epoxy coating from the surface of the aramid-fiber-reinforced resin matrix composites based on the usage of an infrared pulsed laser, we analyze various removal mechanisms using lasers and determine the cleaning methods using theoretical calculation and experimental analysis. From the calculation and test data, we obtain technological parameters for the laser removal of many different coatings; thus, we find that the effective removal of an epoxy protective layer from the surface of the resin matrix composites can be achieved using the laser for initial removal followed by wiping the surface using alcohol. The test results show that the laser-treated aramid-fiber-reinforced resin matrix composites show no damage and can satisfy the industrial application requirements.

The fabrication process of holographic lithography micro-nano gratings using metal masks is designed and optimized. First, an 860 nm periodic grating is prepared using holographic lithography and inductively coupled plasma (ICP) dry etching on the GaAs substrate. The hard metal mask grown by magnetron sputtering is introduced into the etching process as a barrier layer for grating etching and the Ni mask is fabricated by the lift-off method. It has been shown that when photoresist, SiO2, and Ni are used as masks for ICP dry etching, they determine the etching depth and morphology of the grating. Results show that the Ni mask has strong etch resistance. Scanning electron microscopy demonstrates that Ni with a thickness of 50 nm can be used as a hard mask to create a grating with an aspect ratio of about 4.9. The grating has a groove width and etching depth of 300 nm and 1454 nm, respectively, with steep sidewall morphology and good periodicity and uniformity.

To meet the technical requirements of the 278-nm laser system, a separation film for frequency doubling is developed to separate the double frequency laser and quadruple frequency laser. Combined with the laser damage thermal effect and intrinsic absorption of the materials, Hafnium and UV-SiO2 are chosen as film materials and the well-cleaned JGS1 quartz glass is used as the substrate. According to the thin film theory and laser damage field effect mechanism, the periodicity variation in electric field intensity distribution of the film is studied and design of the frequency separation film is completed by Macleod software. The chemical composition, optical properties, and surface roughness of the film are analyzed using X-ray photoelectron spectroscopy, spectrophotometer, and ZYGO interferometer, respectively. In comparison with experimental results, the deposition technology of the film is optimized to improve the film growth process and reduce external absorption. Experimental results show that the transmittance of films is 98.82% at 278 nm and reflectance is 99.80% at 556 nm when the incident angle is 45°. In the wide incident angle range of 40°-50°, spectral performance can meet the requirements of the target. The laser-induced damage threshold is 12.53 J/cm 2, which meets the needs of use.

When a frequency-modulated continuous-wave interference technique is used to measure the distance to a moving target in real time, the Doppler effect causes deviations in the measurement results. To solve this problem, we propose a method that employs triangular wave modulation on the optical frequency of the laser light source. By generating two frequency-swept signals with opposite frequency-modulation directions in one modulation period, the Doppler effect can be eliminated using the opposite frequency shifts between the two frequency-modulation directions. Experimental results show that triangular wave modulation at a frequency of 2 Hz and an amplitude of 15.57 μm can overcome a 5-mm measurement deviation caused by the Doppler shift for single-direction motion; the standard deviation of the target distance measurement results is 0.035 mm. Simultaneously, we realize a distance measurement for a target moving with a velocity of 1 mm/s over a measurement range of 800 mm with good linearity. This technique also achieves a speed measurement of up to 9 mm/s at 800 mm, and the minimum relative error can reach 0.067%. The method can be used for real-time tracking measurement of the distance to the moving target, and it is useful for a wide range of applications of frequency-modulated continuous-wave interference-ranging technology.

When a length is measured using a plane array device, reaching submicron-level accuracy is difficult because of the limitation imposed by the pixel size of the plane array device and subdivision technique. Therefore, we propose a method for measuring two-dimensional submicron displacements based on the multibeam interference principle of the Fabry-Perot (F-P) etalon. A two-dimensional micro-displacement in the focal plane is obtained by calculating the variation of the center coordinate of a concentric interference ring. The virtual plane array pixel subdivision technique and peak-position coordinate local subdivision technology are used to process the massive information of the plane array. In this way, the influence of undetermined systematic error is reduced, which allows an accurate calculation of the center coordinate of the concentric interference ring. The experiment uses an F-P etalon with an interval of approximately 2 mm and a optical lens with focal length about 50 mm. The center of the imaging concentric interference ring is calculated at different positions in the focal plane. The results show that the measurement range can reach 3 mm. The experiment uses a laser phase-modulating homodyne interferometer for the comparison measurements. The results show that in the range of 34 μm, the linear fitting standard deviation of the measured results is 0.0154w″ and the extended uncertainty is 0.036w″ when the coverage factor is 2.45, where w″ is the relative pixel interval. These results confirm the accuracy of the measurement method.

A novel birefringence measurement method based on a ptychographic iterative engine (PIE) in a planar polarimeter is proposed in this paper. The complex amplitudes formed by a sample in a dark field under two different polarization states are reconstructed using the proposed PIE method. The phase retardation and azimuth angle are simply and accurately extracted from the probe phase and ratio of two complex amplitudes, respectively, and the two-dimensional quantitative measurement of birefringence samples is realized. A birefringence resolution target is used to verify the proposed method, and the obtained results are completely consistent with the resolution target's actual distributions. The maximum phase retardation error is no more than 23.9 nm, and the azimuth angle error is 0.49°. This method has a simple structure and enables a traditional planar polarimeter to quantitatively measure birefringence. The proposed method reduces the number of required PIE scans and shortens the data collection time and processing process, providing a practical method for the birefringence measurement of large-aperture optical devices.

In this study, a measurement system is constructed based on binocular vision to enable round-hole profile measurement of the thin-walled parts in the industrial field. Using a binocular camera as the vision sensor, the brightness of light source can be automatically adjusted by detecting the gray value of the hole's inner portion or the part's exterior, enabling the left and right cameras to simultaneously obtain the clearest round-hole image. Three-dimensional reconstruction can be realized through round-hole profile detection and matching based on epipolar restriction; further, the aperture information and three-dimensional coordinates can be acquired. The experimental results denote that the system exhibits high accuracy and that its stability and accuracy satisfy the requirements of industrial applications.

The second-order (defocusing and astigmatism) error of the reference surface is the major factor that results in accumulated stitching errors, whereas the higher-order errors of the reference surface will result in high-frequency shape errors. Subsequently, the law of stitching errors caused by the second- and higher-order errors of the reference surface is analyzed, and the relation between any two subaperture stitching errors that can be attributed to the reference surface error is studied. Further, an algorithm is proposed to effectively reduce the influences of the higher-order errors of the reference surface on the subaperture stitching results. The stitching errors of the higher-order errors of the reference surface are isolated using this algorithm to eliminate the stitched surface data of any two subapertures. Furthermore, the validity and effectiveness of this algorithm are verified using simulations and experiments.

We demonstrate a high-power, linearly polarized all-fiber amplifier with a narrow linewidth and near-diffraction-limit beam quality by suppressing the stimulated Brillouin scattering and self-pulsing effects simultaneously. The maximum laser power is 2.62 kW with 86.7% optical-optical efficiency. Beam quality (M2), polarization extinction ratio (PER), and linewidth are well maintained during power amplification with M2<1.3, PER ~96.3%, a 3 dB linewidth of 32 GHz, and a 20 dB second-order moment linewidth of 30 GHz. To the best of our knowledge, this is the largest demonstration of an all-fiber laser with narrow linewidth, linear polarization, and beam quality near the diffraction limit.

Based on home-made 100/400 double clad Yb-doped active silica fiber, a high power nanosecond Q-switched fiber laser with master oscillator power-amplifier (MOPA) structure and tunable repetition rate of 30-60 kHz is built. The center wavelength of system is 1064.08 nm. Using the 976 nm laser pumping, the output of average power of 761 W is achieved at the repetition rate of 60 kHz with a single pulse energy of 12.6 mJ, a slope efficiency of 83.4%, and a beam quality of Mx2=9.82, My2=9.02. At the repetition rate of 30 kHz, the average power is 526 W, the single pulse energy is 17.5 mJ, the slope efficiency is 85%, and the beam quality is Mx2=8.67, My2=8.57. This fiber is the reported domestic fiber with the highest average power and single pulse energy obtained at high repetition rate.

A hydrogen sensor based on a cross spiral-micro-structured fiber Bragg grating (FBG) is developed. We employ a femtosecond laser to fabricate the cross spiral microstructure on the fiber cladding and then coat the surface of the FBG with a Pt-WO3 film prepared by a hydrothermal method. The hydrogen sensitivity of the spiral micro-structured probe is 1.55 times higher than that of a non-micro-structured FBG hydrogen sensor. We determine the sensitivity of the sensor from numerical simulations and discuss the difference between the simulation values and the experimental results. The sensor has the characteristics of high sensitivity, fast response, and good repeatability, and it has significant application prospect for monitoring hydrogen leakage.

Ultrashort pulse (USP) lasers have spearheaded great innovations in science and technology, as evidenced in their wide range of applications from chirped pulse amplification to optical frequency comb-associated precision spectroscopy and from super-resolution fluorescence microscopy to femtosecond chemistry. These outstanding scientific achievements (respectively awarded by the 2018 and 2005 Nobel Prizes in Physics, and 2014 and 1998 Nobel Prizes in Chemistry) have sufficed to evidence the profound and far reaching influence of USP laser on the innovations of science and technology, and the cognitive power of humankind. Herein, we review some of the fundamental knowledge regarding the main characteristics, characterization methods, and generation and amplification methods of USP lasers. Specifications and optical schematics of industrial USP laser systems, as well as the subject of how to select USP laser parameters for practical applications, are specially discussed. A pedagogical “ultrafast laser optics” learning map is also introduced for providing an overall picture of this fast advancing and extremely fascinating field.

Based on the tunable diode laser absorption spectroscopy (TDLAS), a fuzzy adaptive proportional-integral-differential cycle control algorithm is introduced to achieve precise control of temperature and pressure inside the cavity, and the simultaneous measurement of stable carbon isotopes in 12CH4 and 13CH4 gases has been achieved by utilizing TDLAS via monitoring the near infrared absorption line at 1658 nm. Examination of gas samples from various coal seams and differing locations in coal mines allows for differentiation of gas source and elucidation of the origin type of the gas based on the observed isotope abundance. The detectable concentration range for the methodology is 0.94%-83.91%, and the isotopic abundance varies from -66.75‰ to -48.32‰. This technology lays the foundation for distinguishing coal seam gas sources and judging the gas types according to the isotopic abundance, and provides a valuable tool for areas such as gas transport channel in mine, early warning for gas disaster source, coalbed methane research, and environmental atmospheric studies.

Laser-induced fluorescence (LIF) spectroscopy combined with principal component analysis is employed to detect and quantitatively analyze frying-oil contamination in a variety of vegetable oils. Olive and peanut oils adulterated with frying oil are analyzed using LIF spectroscopy; the intensities and positions of fluorescence peaks are utilized to determine the relative concentrations of the components and the collected data is processed and analyzed by combining principal component analysis with a partial least-square model. The results show that the intensities of fluorescence peaks around 500 nm increase with increasing concentrations of frying oil, while the intensities of fluorescence peaks around 670 nm are reduced with increasing amounts of contamination. The vegetable oils are classified utilizing this methodology, and the frying-oil contaminant concentrations are predicted within 2% error.

Herein, the long-term continuous observation of the vertical structure of tropospheric ozone in Guangzhou is conducted by using differential absorption lidar. The diurnal variation characteristics of ozone vertical structure in Guangzhou are provided. The characteristics of two different ozone pollution modes of local pollution and regional transmission are analyzed, and the determination method of the ozone pollution pattern is proposed. Results show that the daily variation of ozone in Guangzhou is obvious, and that the single peak characteristic is prominent. The seasonal variation law of ozone concentration is the following: ozone concentration is highest in summer and lowest in winter;there is little difference between spring and autumn. Local pollution analysis shows that the ozone with high concentration is mainly concentrated near the ground, and gradually decreases with an increase of the height. The decreasing rate of ozone concentration below 0.7 km increases rapidly with the increasing height, and for 1.5-2 km tends to be evenly distributed. The study provides the characteristic values of ozone transport for the three height intervals. The correlation between the extinction of particles and ozoneconcentration at the same height is analyzed. Combined with the meteorological field, the dominant direction of ozone transportation in each season is analyzed.

Herein, a new imaging-transformation method for acquiring a measurement matrix is proposed. A visible light camera is used to capture amplitude-modulated plate pattern information, then a coherent imaging algorithm is used to obtain the simulated light field, and finally the simulated light field is transformed into a ghost imaging measurement matrix. We validate the feasibility of this imaging-transformation method using an established terahertz ghost imaging system. Experimental results show that the measurement matrix obtained using the proposed method can enable terahertz ghost imaging to reach diffraction limit resolution. The resulting image quality is superior to that obtained using the classical scanning method. In addition, we experimentally study the influences of sampling frequency, pupil function, and matrix resolution on the final image quality.

A terahertz polarization-insensitive broadband absorber is designed. Its absorption spectrum's full width at half maximum (FWHM) is 2.29 THz, and the bandwidth is about 1.65 THz when the absorptivity is more than 97%. Further, the absorptivity becomes 99.9% at some resonant frequencies, indicating that it can achieve a perfect absorptivity of approximately 100%. The absorber stacks three layers of periodic units exhibiting the same structure but different sizes, and it can effectively expand the bandwidth. This study analyzes the absorption mechanism and discusses the influences of different parameters on its performance. The simulation results denote that the absorption characteristics of the absorber are polarization-independent and that it can maintain a wide bandwidth over a certain incident angle range. This polarization-insensitive broadband terahertz absorber that exhibits high absorptivity has significant research value in terahertz imaging, terahertz wave detection, electromagnetic stealth, and other such applications.

A terahertz absorber comprising a circular resonant ring structure with four response frequency bands has been designed based on the principle of different responses of resonant rings exhibiting different sizes and shapes to electromagnetic fields. In this study, the absorber characteristics are studied using the finite-difference time-domain method. The terahertz absorber is designed and simulated by changing the geometrical size of the top metal ring pattern, the thickness of the dielectric layer in the middle layer, and the change rate of the silicon conductivity at the top metal ring. When the terahertz absorber is coupled with a multi-frequency absorber, the absorption at low frequencies reaches perfect absorption and the absorption rate at high frequencies increases from 70% to 94%; further, the low-frequency shifts from 0.775 THz and 1.064 THz to 0.697 THz and 1.017 THz, respectively, with a change in conductivity. The frequencies of 78 GHz and 47 GHz are shifted to achieve continuous frequency tuning.