In this study, we have developed a 6×6 mode-division-multiplexing communication experimental system using a photon lantern as the mode multiplexer/demultiplexer. Further, we can achieve a signal transmission rate of 6×8.5 Gbit/s in a 10-km few-mode fiber using a phase modulation and coherent detection system via offline digital signal processing. The results denote that the bit error rate of each signal can be less than 10 -3 when the received signal powers of LP01, LP11a, LP11b, LP21a, LP21b, and LP02 are -37.84 dBm, -36.47 dBm, -36.20 dBm, -35.27 dBm, -35.37 dBm, and -35.79 dBm, respectively.

In this study, based on the wave equation and material equation of a solid Raman medium and neglecting the transient effect of stimulated Raman scattering, the coupled wave equations of the interaction of the first anti-Stokes beams, the pump, and first to third Stokes beams in extra-cavity pumped anti-Stokes lasers are obtained using the plane wave approximation. Normalized parameters are introduced to normalize the equations. The effects of the normalized Raman gain coefficient, normalized pump pulse width, and normalized wave mismatch on the performances of extra-cavity pumped anti-Stokes lasers are analyzed by numerically solving the normalized coupled wave equations. The experimental data are substituted into the coupled wave equations. The theoretical estimations of the anti-Stokes optical conversion efficiencies are consistent with the reported data, which validate the correctness of the theoretical model. By analyzing the theoretical calculation results, methods to improve the conversion efficiency of extra-cavity pumped anti-Stokes lasers and associated methods of lasers design are proposed.

In this study, a 1061 nm/1064 nm dual-wavelength Nd∶YAG microchip laser is developed. By designing the transmission curve of the thin film coating at the output end of the laser, the threshold pump powers of 1061 nm and 1064 nm lasers are balanced. Using diode lasers as pump source, three groups of working materials with different thickness are tested at room temperature, and the 1061 nm/1064 nm dual-wavelength outputs are observed. The optical length of the resonator is compressed by directly coating the two ends of the microchip crystal to form a resonator. Simultaneously, the multi-longitudinal mode output at a wavelength of 1064 nm wavelength is realized.

Herein, an intracavity single-longitudinal-mode 593.5 nm yellow lasers with compact structure, stable performance, and low cost is designed. A laser diode end-pumped Nd∶YVO4 crystal is used to produce 1064 nm and 1342 nm dual-wavelength laser beams with a linear flat-cavity structure. The intracavity sum-frequency mixing of the 593.5 nm continuous yellow laser output is achieved via KTP Ⅱ-type critical phase matching. The Brewster plate (BP) and sum-frequency crystal KTP are used as a birefringent filter for frequency selection. When the pump power is 5.0 W, the single-longitudinal-mode output power at 593.5 nm is 30 mW, root-mean-square noise is 0.8%, and line width is 150 MHz. For this condition, the fundamental light at 1064 and 1342 nm is detected to be in the single-longitudinal-mode state. Experimental results show that the single-longitudinal-mode sum-frequency laser is achieved using the birefringent filter technique for a loss of more than 1.5% for the adjacent fundamental mode.

This study proposes a method based on lens array and fiber bundle for measuring frequency-modulated pulse waveforms. Grade-index fiber can provide a large receiver aperture that allows optimum laser energy transmission and performing measurements a wide dynamical range. Lens array can decrease the drift of laser spot on the fiber end face. Combining these two benefits, the system will have a high transmission efficiency from the laser near field to the fiber bundle. Results indicate that the proposed method can perform high-accuracy measurements of frequency-modulated pulse waveforms, with the measurement contrast ratio reaching 630∶1.

To realize the application of a metal grating polarizer in a polarization illumination system for photolithography, a dielectric-metal grating polarizer based on the inverse polarization effect of a resonant-domain grating is proposed. The grating comprises aluminum (Al), magnesium fluoride (MgF2), and a silicon dioxide (SiO2) substrate. In comparison with general sub-wavelength metallic gratings, the period of the proposed polarizer is close to the wavelength of the incident light (0.19--0.20 μm), and it exhibits the inverse polarization effect of transmitting transverse electric (TE) polarized light and reflecting transverse magnetic (TM) polarized light. Finite difference time domain (FDTD) simulation results indicate that the transmittance of TE-polarized light exceeds 60% and the polarization extinction ratio is greater than 180 when light having a wavelength of 0.193 μm is incident normally. In comparison with a single-layer metal grating polarizer constructed using the same materials and structural parameters, the proposed dielectric-metal grating polarizer exhibits better polarization performance in the deep ultraviolet band, with the transmittance and extinction ratio approximately increased by 10% and 4.5 times at wavelength of 0.193 μm, respectively.

Infrared broadband antireflection components were successfully fabricated using a microstructure approach combined with a coating method. First, the influence of period, filling factor, height, and film thickness on the required band transmittance was simulated using FDTD Solutions software. As such, the microstructure and film parameters corresponding to good antireflection were obtained. Then, according to these parameters, a parabolic cone microstructure was prepared onto the surface of a sapphire through laser interference lithography combined with reactive ion etching technology. Finally, a SiO2 film layer was coated onto the surface of the microstructure. Test results show that the average transmittance of samples with single-sided microstructure and composite structure at 1.5--4 μm were 92.3% and 98.7%, respectively. Therefore, the transmittance increased by 11.0% compared with double-sided polished sapphire samples. Infrared broadband antireflection of the sapphire was obtained. Finally, a high-and-low temperature cycle and humidity experiment was conducted on sapphire element with composite structure. This experiment shows that the change in transmittance was not obvious and without obvious water absorption, indicating that the component has environmental durability.

A dual-mode metasurface device is designed to achieve phase modulation with anisotropic response depending on the left and right-handed circularly polarized light. This device can control the sizes of the Si nanoblock in order to regulate the transmission phase, and require additional geometric phase by controlling its in-plane direction, simultaneously. By combining the two phases, the metasurface focuses the right-handed circularly polarized incident beam with an efficiency as high as 44.8%, while keeping the plane wavefront transmission of the left-handed circularly polarized incident beam. The designed metasurface could be used as a miniaturized polarizer in information encrypted transmission field.

A polymer Mach-Zehnder interferometer (MZI) thermo-optical (TO) switch with low-power consumption is designed and fabricated. To reduce the power consumption, a modulating arm waveguide in the heating region is designed as a suspended waveguide to reduce the heat diffusion. According to the simulation results, compared with a TO switch paired with a conventional waveguide, the suspended waveguide can significantly reduce heat diffusion. The TO switch based on suspended waveguide is fabricated by semiconductor technology. At a 1550-nm wavelength, the power consumption is about 9.3 mW and extinction ratio of the device is 21 dB. The rise and the fall times of the switch are 392 μs and 697 μs, respectively.

In this study, a scheme for measurement equipment based on hollow photonic crystal fiber was proposed to meet the measurement requirement of laser frequency tuning ratio. First, the scheme of the measurement equipment and basic structure of the photonic crystal fiber resonator (PCFR) were designed and the numerical calculation and mechanism analysis of the laser frequency tuning ratio were completed. Second, PCFR was designed and manufactured, and the laser frequency tuning ratio testing setup was constructed. Finally, the frequency tuning ratio of narrow line-width fiber laser and semiconductor laser were tested. The voltage-frequency tuning ratio of the fiber laser is 17.6 MHz/V, which is in accordance with the factory indicators; moreover, the fiber laser''s tuning ratio is consistent at the tuning range. The tuning ratio of semiconductor laser in tuning range is non-consistent, which is susceptible to environmental influences. The average current-frequency tuning ratio of the semiconductor laser is 30.9 MHz/mA. This study demonstrates that the accuracy of the proposed testing setup is much higher than that of the commercial wavelength meter, and it also displays good long-term stability. Furthermore, PCFR has technical advantages, such as high frequency measurement accuracy and small temperature drift, as a frequency reference for frequency tuning ratio detection.

In this work, the design of the mechanical structure of a precision electric mirror frame is realized. Through the design and debugging of a micro-drive device and the decoupling operation of the parallel drive mechanism, the motorized mirror frame designed in this work achieves a motion stroke of ±7.5 mrad and a single-step resolution of 0.3 μrad. The finite element modal analysis of the mirror frame was conducted to obtain the natural frequency and the corresponding mode shape. Based on this analysis, the dynamic characteristics of the electric mirror frame were tested experimentally. Using the results of the modal analysis and dynamic characteristics test, the random vibration analysis of the mirror frame was conducted, revealing that the stability index of the mirror frame under the influence of ground pulse vibrations is better than 0.5 μrad.

By studying the polarization characteristics of the birefringent Nd∶YVO4 crystal, a novel birefringent filter comprising an Nd∶YVO4 laser crystal with a wedge angle of 10° and a frequency doubling KTP (potassium titanyl phosphate) crystal is constructed in a green laser. The influences of the KTP length, incident angle of the fundamental mode in the KTP, and KTP temperature on the electing-mode function of the birefringence filter are theoretically analyzed. In the experiment, KTP crystal with lengths of 4.4, 5, and 7 mm and containing V-cavity structures are used, and 90-, 120-, 104-mW single-frequency green light is obtained, respectively. Experimental results prove that the birefringence filter comprising a wedge Nd∶YVO4 and KTP can successfully realize laser single-longitudinal-mode operation. Further, the proposed method is simple and easy. For 5-mm KTP length, the single-longitudinal-mode operating temperature range of the wedge Nd∶YVO4/KTP laser is measured to be approximately 5 ℃.

This paper proposes a method for generating an ultraviolet supercontinuum light source (SC) using highly nonlinear magnesium fluoride photonic crystal fiber (PCF). The nonlinear Schr?dinger equation was solved using the split-step Fourier method, and the generation of an ultraviolet SC in the PCF was numerically simulated. The effects of the structural parameters of the PCF and the parameters of the pump source on the ultraviolet SC were analyzed. The variations in the SC width relative to the PCF length, dispersion parameter, pump pulse peak power, and initial width of pump pulses were obtained. Simulation results reveal that the SC is extended into the ultraviolet range of 279.6--769.0 nm when the length of the PCF is 8 cm, central wavelength of the pump pulse is 450 nm, peak power is 3.1 kW, and initial pulse width is 40 fs.

This paper introduced the texture feature analysis method in digital image processing technology into laser biological tissue welding, and the microstructural changes of laser-welded skin in vitro were deeply analyzed. The second-order statistical features calculated using gray co-occurrence matrix were used as primary quantitative indexes to characterize the microstructural characteristics of isolated skin. The effects of laser parameters and their interaction on the microstructural and texture characteristics of laser-welded skin in vitro were obtained using a response surface method. The results show that the laser scanning speed is the key factor that affects the microstructure of isolated skin subsequent to welding, and the angular second moment, correlation, entropy, and contrast have significant effects on the texture feature parameters. Furthermore, the laser power and the interaction between laser power and welding speed have significant effects on the angular second moment, correlation, and entropy.

In this study, we propose a water-assisted laser micropore processing method to address the problems of severe deformation of the metal filter screen, the large tapering and poor shape consistency of the micropores when lasers are used to process massive micropores on the metal plate. The micropore processing quality can be improved based on the capillary phenomenon of the water entering the micropores, the reflection and refraction phenomena of light at the gas-liquid interface, the force and cavitation effects generated by the irradiation of laser into the water, and the cooling effect of the water. Herein, perforation experiments were conducted on a 304 stainless steel plate with different thicknesses using a picosecond pulsed laser with and without water assistance; thus, the feasibility of water-assisted laser processing can be confirmed. The experimental results denote that the micropore tapering obtained by the water-assisted method is reduced by approximately 2°--6° and that the recast layer and the heat-affected zone reduce compared with those obtained by direct laser drilling using the same processing parameters. Further, the amount of deformation of the processed filter screen is reduced, and the consistency of the micropores is improved. Finally, a filter screen containing a large number of micropores having an exit pore size of 55 μm is realized on a 60-μm-thick sheet using the process parameters optimized via an orthogonal experiment.

Unlike traditional laser drilling, the bottom-up drilling of transparent materials involves passing the laser through the material, focusing on the lower surface of the material, and removing the material from the bottom to the top layer by layer. Based on lasers with different pulse widths, this study investigates the mechanism of bottom-up drilling of glass materials. It is found that in addition of ultrashort pulse laser, the nanosecond laser can also achieve bottom-up drilling. Furthermore, it can achieve zero taper drilling. The efficiency of bottom-up drilling decreases with the decrease of pulse width, and the cut efficiency of lasers with pulse width from 2 ns to 50 ps is still higher than that of ultrafast laser. This result differs from the result of general top-down processing, which is attempted to be explained herein.

To elucidate the mechanisms of laser bending and edge effect and its control strategy, we constructed a laser thermoforming model of stainless steel-low carbon steel-stainless steel laminated metal composite plate using finite element analysis software ABAQUS. The temperature field and stress and strain fields between composite plates in laser scanning were quantitatively analyzed . The results from our study show that the difference in thermal conductivity between stainless steel and low carbon steel results in the formation of a bending angle. The main cause of the edge effect is the nonuniform distribution of temperature and plate constraints along the heating path; repeated heating improves surface temperature distribution and increases forming accuracy. Four new scanning strategies are proposed; these strategies reduce the relative change rate of surface temperature by 36% and the relative rate of change of the bending angle along the heating line by 51.8%, which improves the forming precision. Our results provide a guideline for the selection of scanning strategy for composite metal plate bending formation in industrial production.

This study considers the influence of welding speed on the dynamic behavior of the filler metal filling a molten pool. Results show that welding speed significantly influences the three-dimensional keyhole shape and flow behavior of the molten pool. When filler metal is used to fill a laser welding molten pool, an increased welding speed improves the stability of the keyhole. With the increase of welding speed, the fluctuations in keyhole depth and flow velocity at the bottom of the keyhole wall decrease, the driving force for opening the keyhole is enhanced, and the flow trend of the flow field near the keyhole from the keyhole wall to the molten pool is enhanced.

In this study, Q235 steel was welded by linear welding and oscillating welding using an IPG YLS-6000 fiber laser and oscillating welding head. The microstructures and mechanical properties of the two welded joints were compared and analyzed. Results show that laser beam oscillating welding can significantly increase the weld width from 1.34 mm to 1.60 mm when compared with linear welding; the microstructures of the two welded joints are lath martensite, lath bainite, and grain boundary ferrite; the complete austenitizing zones in heat-affected zones are made of ferrite and lath martensite, and the incomplete austenitizing zones consist of ferrite and pearlite with uneven grain sizes. The overall hardness of the oscillating welded joint is higher than that of the base metal. In addition, the tensile strength of the oscillating welded joint is 1.74 times that of the base metal, while the ductility and impact toughness of the joint are 66.7% and 90% that of the base metal, respectively. The oscillating welded joint has the same mechanical properties as the linear welded joint. Oscillating welding can effectively reduce the gap requirement associated with laser welding and has no adverse effects on the hardness, strength, and toughness of the welded joint.

In order to realize the laser additive manufacturing of complex parts with unequal wall thickness, a variable circular spot laser cladding optical system based on adaptive mirror is proposed. The system consists of reflective parabolic collimating mirror, reflective adaptive mirror and reflective parabolic focusing mirror. The air pressure proportional valve is used to adjust the gas pressure in the adaptive mirror cavity, to change the radius of curvature of the adaptive mirror surface, and make the beam incident on the reflective parabolic focusing mirror at different convergence angles or divergence angles, so as to change the spot size acting on the surface of the workpiece. Physical model is established based on adaptive mirror variable spot optical system, and using Matlab and Zemax softwares, the effects of the range of the adaptive mirror focal length, the focal length and position of the reflective parabolic collimating mirror and the reflective parabolic focusing mirror, and the parameters of the fiber laser source on the variable spot characteristics of the system are analyzed. A 6 kW fiber laser is used to carry out cladding experiment according to the scheme of variable spot, variable power, constant speed and constant power density, and an elliptical workpiece with a wall thickness of 3-8 mm is realized.

The temperature of the molten pool is one of the most important parameters in the process of metal forming based on selective laser melting. To ensure the closed-loop control of molten pool temperature, this study implementes an online detection of molten pool temperature based on the colorimetric temperature measurement method and photoelectricity technology. It proposed a type of composite operational amplifier that successfully detected the molten pool radiation signal with small amplitude and fast change. An embedded system was built to upload the radiation data of the molten pool to the industrial computer of metal forming equipment. A method of colorimetric temperature measurement was employed to eliminate the effects of the laser incident angle and other agents on the detection of radiation intensity, and furthermore, to obtain the temperature of the molten pool. During the process of melting 90%Cu-10%Sn alloy powder, it is found that the band with the strongest radiation in the molten pool of the material is 580--590 nm within the spectral range of 540--660 nm. Under the laser power range of 40--200 W, the estimated molten pool temperature range is 700--1700 ℃. The online detection of molten pool temperature during metal forming based on selective laser melting provides feedback information of molten pool temperature for the control system of metal forming equipment. Furthermore, the proposed approach of online detection also can be considered as a reference to upgrade the equipment.

To study the feasibility of laser exfoliation of the external coating of oil and gas pipelines and the effects of coating removal on the properties of the substrate, laser coating removal experiments were conducted on the epoxy resin coating on the X65 pipeline steel using a short-pulse laser. After coating removal, the pipe external surfaces were tested using a three-dimensional morphology tester. The surface and cross-section of each sample were tested and analyzed using scanning electron microscopy and energy spectrum analysis. After the two samples were irradiated using different laser technical parameters, the coating layers were almost completely exfoliated. At the same time, dense crater morphology was evident on the surface and strip-shaped and crack regions appeared below the surface. The strip-shaped regions beneath the surface were about 20 μm and 10 μm thick for samples A and B, respectively, while the crack regions were about 100 μm and 90 μm thick, respectively, and the hardness of both the samples'' surfaces increased slightly. The laser technical parameters used in this research not only meet the requirements of laser coating removal for steel pipes but also strengthen the surface. Thus, coating removal is technologically feasible for steel pipelines and valuable for further investigation.

To solve the problem of wear failure of 42CrMo steel and meet its application requirements, in situ NbC-particle-reinforced composite coatings were prepared on a 42CrMo steel surface using laser cladding with a mixed powder (Co-based, Nb,and Cr3C2 powder). The effects of NbC content on the microstructure, wear behavior, and NbC morphology of the composite coatings were analyzed. The results show that when the mass fraction of NbC is 0--15%, the coating bonds well with the substrate and no obvious defects are observed in the coating. When the mass fraction of NbC reaches 20%, microcracks appear in the coating. The matrix of the composite coating primarily comprises ε-Co and γ-Co. The primary strengthening phases are NbC, Cr23C6, and Cr7C3. NbC particles are formed by the in-situ reaction of Nb and C atoms that are dissolved in the molten pool. With the increase of NbC content, the shape of the NbC particles gradually changes from quadrilateral to petaling. The microhardness and wear resistance of the composite coatings clearly increase, and the primary wear forms of the composite coatings are abrasive wear and hard-phase spalling wear. When the mass fraction of NbC is 10%, the microhardness and wear resistance of the coatings attain their maximal values of 546.4 HV and 0.020 g/min, respectively.

Tandem pumping is characterized by high pump-source brightness and low quantum defect, which are beneficial for power scaling of fiber laser. By combining the modified chemical vapor deposition (MCVD) method with the chelate precursor doping technique, we fabricated an ytterbium (Yb)-doped aluminophosphosilicate fiber suitable for tandem pumping. Laser performances of the fiber were demonstrated in an all-fiberized master oscillator power amplifier (MOPA) laser system. Tandem pumped by 1018 nm fiber laser, 9.82 kW laser output at 1080.08 nm has been achieved with 86.8% slope efficiency. The 3-dB bandwidth is 1.62 nm at the maximum laser power. Our results indicate that the Yb-doped aluminophosphosilicate laser material is the optimal gain medium for the tandem pumping of high-power fiber laser systems. The chelate precursor doping technique combined with MCVD is an effective method for obtaining this type of material.

In a certain magnetic field, spin exchange collision is one of the limiting factors for coherence preserving in atomic magnetometers, which directly affects the detection sensitivity that the magnetometer can achieve. The light narrowing effect can be used to reduce the resonance linewidth caused by spin-exchange collisions. Previous observations of the light narrowing effect are based on atomic magnetometers with continuous pumped laser and radio frequency field, which have additional effects on the atomic resonance linewidth. Based on a pulse-pumped atomic magnetometer, the effects of atomic destructive collisions and magnetic field inhomogeneity on resonance linewidth are greatly reduced, and a direct experimental observation and verification of the light narrowing effect are realized. As pulse-pumped mode is beneficial to the realization of high atomic polarization, the study of light narrowing effect is of great importance for exploring the sensitivity limit of pulse-pumped atomic magnetometers.

In this paper, a new and fast extraction algorithm for a laser strip center with the ability to effectively resist noise or redundant points is presented. The proposed method combines the traditional barycenter algorithm and the contour tracking algorithm. Based on the distribution characteristics of laser strip in the image, the threshold contour tracking algorithm is used to avoid scanning the area that does not contain a laser fringe in the image, thereby improving the extraction speed. Therefore, the center of the laser strip can be extracted without scanning the whole image. The proposed algorithm provides advantages such as low complexity, simple calculation, and less running time. Experimental results show that the proposed algorithm can achieve fast extraction of the laser strip center. Furthermore, it is nearly 70.37 times faster than the Steger algorithm and 4.48 times faster than the traditional barycenter algorithm. Moreover, the proposed algorithm achieves excellent anti-noise effect subsequent to adding noise (redundancy) points to the light bar image.

To achieve high speed, high precision, and device miniaturization, an improved joint transform correlation (JTC) method is proposed. In this method, the correlation input image is constructed by superposing the intensity of two images to be detected. To effectively separate autocorrelation and cross correlation peaks, the method makes use of the constant image motions resulting from satellite movement. The size of each image used for the correlation operation is reduced to the size of the image to be detected, which greatly reduces the calculation of the measurement process, which is beneficial for realizing high-speed digital joint transform correlation. The simulated and experimental results show that the new method maintains the advantages of high accuracy, high robustness to image noise and low dependency on image texture of traditional JTC. Additionally, it also has the potential for high-speed image motion measurements.

Depth map acquisition, which is based on laser speckle, presents some issues, such as low matching precision, large amount of calculation, and poor robustness in different measurement environments. In this paper, a semi-dense depth map acquisition algorithm based on laser speckle is proposed to address these issues. The problem of poor robustness can be solved using the locally adaptive binarization, which preprocesses the speckle map to ensure the illumination invariance of the window descriptor. In terms of measurement accuracy, the central pixel coordinates of each speckle are extracted using a clustering algorithm, which improves the positional accuracy of each speckle. Regarding the matching success rate issue, the window descriptor is convoluted to obtain a simplified descriptor, which is able to reduce the amount of calculations and increase the matching success rate. Finally, the speckle pairing points are obtained according to the matching criterion, and then the depth values of each speckle are obtained according to the triangulation principle. Experiments confirm that the proposed algorithm is highly robust and accurate and improves the matching success rate.

Detection of the anomaly associated with the working status of satellites is an important part of space situational awareness and is an effective methodology to improve the efficiency of satellite surveillance and ensure the normal operation of the satellites. Due to the fast speed of low-orbit objects and the obvious change of observation geometry, the application of historical observation data is limited. In this study, an anomaly detection method is proposed based on the dynamic time-structured distance by combining the geometrical relation of the Sun-object-observation station, the orbit characteristics of the satellites, and the photometric relation of the space objects to quickly detect the working status of the satellites. Finally, using the proposed method, some photometric data of Fengyun, GPS and Tiangong at seven working status are simulated. The results indicate that the accuracy rate of abnormal detection with respect to the working status of the space objects is more than 90%, verifying the effectiveness of the proposed method.

In this study, a novel least-squares-based method is proposed to measure the distance using a triangular-wave amplitude modulation laser. Further, we designed two channels comprising a reference signal and a measured signal that exhibit an optical path difference. Their return signals were fitted using the improved least-squares-based straight-line fitting algorithm, and a series of fitting parameters and singular characteristic points were obtained for the triangular-wave. Subsequently, we estimated the detection distance using the proposed theoretical algorithm based on the relation of the time shift information associated with the characteristic points of two signals. The average measurement error associated with this laser ranging system is 3.2 mm at a 10-m measurement range in case of a triangular-wave modulation frequency of 10 kHz, demonstrating the reliability of the proposed method. Furthermore, the proposed method exhibits certain application potential with respect to short-scale laser ranging.

In this study, we propose a gear fault identification method based on adaptive-noise complementary ensemble empirical mode decomposition to solve the problem associated with the identification of gear faults. Initially, we used a fiber Bragg grating to extract the gear vibration signals, and uniformized the spectrum of vibration signal by adaptively adding Gaussian white noise to eliminate the mode mixing caused by the empirical modal algorithm. Subsequently, we used the correlation coefficient and the kurtosis value to obtain comprehensive evaluation indexes for selecting the effective components and extracting the features of the effective components. Finally, we used a support vector machine to identify the gear faults. The experimental results denote that the proposed method can be used to effectively identify the states of gears, including normal, mild-wear, severe-wear, pitting, cracks, broken teeth. Furthermore, the gear state identification accuracy is more than 90%.

The camera calibration method based on a two-dimensional dot array calibration plate with its ellipse center directly extracted from the target image is not the real projection image point of the circle center. Thus, the accuracy of calibration parameters is reduced by the projection deviation of the circle center. Therefore, this study proposes an iterative calibration method for detecting the real circle center image point by re-projecting the target image back to the spatial virtual matrix. First, planar camera calibration was achieved using the ellipse extraction method. Second, the virtual physical target image was obtained using calibration parameters and captured images for re-projection; moreover, the feature points of the dot center were extracted on the virtual image of the approximate circle. Third, the virtual circle center coordinate values were converted into physical coordinate values and projected onto the image that was used as the coordinate value of the real circle center image point for camera calibration. Finally, high-precision camera calibration was completed via iterative re-projection and calibration. Simulation and experimental results show that the proposed method approximately doubles the camera calibration accuracy and provides high-accuracy camera calibration parameters for three-dimensional reconstruction and visual measurement.

In this study, a peak detection algorithm based on an even-odd function to decompose fiber Bragg grating (FBG) spectra is proposed and experimentally demonstrated. An FBG spectrum is a function of wavelength λ and light intensity P(λ), which is moved along the λ axis point-by-point and intersected with the P(λ) axis. The maximum value of the odd function used for the FBG spectral decomposition under each moving point is defined as a characteristic value. The λ value corresponding to the minimum characteristic value is used as the central wavelength of the FBG spectrum. A theoretical model of the FBG spectrum is established using an asymmetric Gaussian model and additive white Gaussian noise superposition. Linearly increasing axial strain is imposed on an FBG sensor in turn. The central wavelengths of the theoretical model and experimental FBG spectra are calculated using the proposed algorithm. The results show that using proposed algorithm, the central wavelength of the FBG spectrum can be obtained more quickly and accurately than conventional algorithms.

In this study, a novel hybrid plasma waveguide with a cylindrical air gap and fully symmetrical metal and gain medium triangular ribs was designed. The two- and three-dimensional coupled electric-field-distribution models of the waveguide were built using COMSOL Multiphysics software. The mode characteristics of the waveguide were analyzed based on the finite element method. Results indicate that the designed waveguide exhibits strong optical field constraint capability, overlong propagation distance, ultra-low propagation loss, and gain threshold at 1550-nm operating wavelength. We optimize the structural parameters of the waveguide, and when the effective mode field area of the waveguide reaches 0.0022λ2, the propagation length and propagation loss are 69805 nm and 0.0017, respectively. Furthermore, the laser based on the waveguide exhibits a low gain threshold of 49.3 cm -1. Finally, the tolerance of the waveguide performance against these possible fabrication imperfections is analyzed. It is concluded that the waveguide structure exhibits good tolerance to possible fabrication imperfections. Compared with the same type of waveguide structures with air gap, the waveguide demonstrats good comprehensive performance and great potential in various integrated nanophotonic devices.

In this work, self-similar fiber laser amplification technology and nonlinear spectrum broadening are employed to overcome the gain limitation of the fiber. Using high-effectivity dispersion compensation, a high-quality pulse with the pulse width of 50 fs, average power of 30 W, central wavelength of 1030 nm, and repetition rate of 40 MHz, corresponding to a peak power of 17 MW, is realized. The system employs a saturable absorber for mode locking and chirped fiber Bragg grating for dispersion compensation in the cavity. In comparison with a chirped pulse amplification system, the proposed system has the advantages of simple and high integration. The stability and robustness of the proposed system are greatly improved through fiber thermal management technology and field programmable gate array (FPGA), which significantly promotes the applications of high-power femtosecond fiber lasers in many mainstream fields of sciences and technology.

In this study, a newly built integrated navigation system using a two-dimensional laser Doppler velocimeter (2D LDV) combined with strapdown inertial navigation system (SINS) to enhance the measurement accuracy of a vehicle being driven is proposed. The basic principle of the 2D LDV is described, and the dead reckoning process is discussed in detail with respect to the newly built system comprising 2D LDV and SINS. The theoretical and experimental results show that the proposed navigation system effectively alleviates the error accumulation effect of the pure SINS. Moreover, compared with the integrated navigation system composed of one-dimensional laser Doppler velocimeter, the 2D LDV improves the accuracy of the carrier velocity measurement, thereby demonstrating the improved navigation accuracy of the proposed navigation system. The dead reckoning position errors of the proposed navigation system in the two experiments are observed to be only 5.9 m and 5.2 m over 2.2 h.

Herein, the scattering characteristics of GF-3 full-polarization SAR (synthetic aperture radar) images were evaluated based on H/A/α polarization decomposition method, and the polarization characteristics of typical ground features in GF-3 and RADARSAT-2 full-polarization SAR images were analyzed. Evaluation method of polarization measurement accuracy based on traditional H/α characteristic scattering plane and improved intra-class aggregation and inter-class dispersion were proposed. Results show that the polarization scattering characteristics of GF-3 full-polarization SAR are clearly for different types of surface features. The average deviations of scattering entropy and scattering angle of three types of surface features are approximately 0.101 and 6.923, respectively, which is better than that of RADARSAT-2 (0.132 and 7.206, respectively). Based on the evaluation method of H/α characteristic scattering plane, the overall polarization accuracy of GF-3 and RADARSAT-2 polarimetric SAR is similar, both of which are approximately 0.7. According to the combined factors of intra-class aggregation and inter-class dispersion, the combined factors of GF-3 are 85.34 (water bodies), 28.99 (buildings), and 122.72 (plants). The recognition ability of GF-3 full-polarization SAR images for these three typical features is similar to that of RADARSAT-2. Furthermore, we analyze the characteristics of ground features with different scattering mechanisms and find that the co-polarization measurement accuracy of GF-3 full-polarization SAR is similar to that of RADARSAT-2 but the cross-polarization channel accuracy is slightly worse than that of RADARSAT-2.

Atmospheric pressure is one of the most important meteorological parameters. In this work, to realize spaceborne laser remote sensing of atmospheric pressure, ground-based lidar measurement investigations are conducted. A 532-nm laser pulse produced by the second-frequency of a single longitudinal-mode Nd∶YAG laser is used as a pump source. An optical parametric oscillator and an optical parametric amplifier using a KTP (KTiOPO4) crystal as a nonlinear conversion medium generate two laser pulses with wavelengths of 760.236 and 760.307 nm, with the pulse energy reaching 40 mJ. A ?350-mm telescope receives the backscattering of the atmosphere, the differential optical depth of two wavelengths between different altitudes and the lidar is obtained. The effective detection altitudes range of the ground-based differential absorption lidar is 500--4000 m, and the time resolution is 1--5 min. The investigations show that the differential optical depth corresponds to the pressure difference between different altitudes of the atmosphere and the lidar, and a numerical expression of the corresponding relationship can be obtained.

Exceptional points are degeneracies where at least two eigenvalues and corresponding eigenstates coalesce simultaneously in the parameter space of non-Hermitian systems. The appealing optical phenomena that occur near exceptional points, especially the ultrasensitivity of frequency splitting to tiny perturbations, have important applications in ultrasensitive optical sensing. This review first introduces exceptional points and related theories in optical sensing. Subsequently, it analyzes the difference between exceptional-point sensing and diabolic-point sensing. Finally, having jointed latest researches at home and abroad, a variety of sensing applications based on exceptional points, including nanoparticle detection, temperature sensing, refractive index sensing, optical gyroscope, and graphene biochemical or chemical sensing, are reviewed.

In this paper, to reduce the cavity volume of a cavity ring-down spectrometer (CRDS), we propose designing a Z-folding cavity that has an external dimension of 26.4 cm×8.5 cm×4.5 cm and an expanded length of up to 73.8 cm. In the study, we perform a simulation based on the parallel-Gaussian-beam-transfer-equation to match the laser mode and cavity mode. By adjusting the focal length and position of two convex lenses, we realize mode matching and identify that only the TEM00 mode existes in the cavity. In the experiment, an acousto-optic-modulator is triggered by a digital-delay-generator to switch the laser beam. The feasibility of the Z-folding cavity is verified by the successfully acquiring ring-down signals. By exponentially fitting the experimental data, the ring-down time is determined to be 0.852 μs, which provides a maximum residual of 0.004 μW. The fitted ring-down time is consistent with the theoretical calculation result. The Z-folding cavity designed in this paper is compact and can be used in a commercial CRDS setup.

The analysis of exhaled gas is a new technology for measuring the composition and content of exhaled gas, which has wide application in non-invasive detection and analysis of human health. In this paper, we build a tunable semiconductor absorption spectroscopy (TDLAS) gas-analysis system using a low-cost vertical-cavity surface-emitting laser (VCSEL) to realize online measurements of human-exhaled end-tidal carbon dioxide (CO2). The system mainly comprises a laser diode, drive control circuit, photodetector, amplifier circuit, data acquisition card, control software, lock-in amplifier, and Herriot gas cell. The gas cell has a volume of 400 mL and an effective optical path of 20 m, and the laser source has a center wavelength of 1579.57 nm. The system uses the second-harmonic amplitude in wavelength-modulated absorption spectroscopy to determine the concentration of CO2 exhaled by a human body. It achieves a fluctuation range of less than ±0.06% and a sensitivity of 0.14%, which satisfies the detection requirements for human-exhaled CO2, and it can accurately, non-destructively, and efficiently measure human-exhaled gas online. This study provides a new strategy for non-invasive detection of human-exhaled gas and related diseases by near-infrared TDLAS technology.

In this study, we used a terahertz time-domain spectroscopy system to evaluate the terahertz spectra of the baicalin mixtures and observed that the mixtures exhibited obvious absorption characteristics in 0.3--1.5 THz. Further, we applied two-dimensional correlation spectroscopy to analyze the terahertz spectra of the mixtures, which were sensitive to the variation in the concentration of baicalin. Herein, two quantitative analysis models were established using the support vector machine (SVM) and partial least squares (PLS) methods, respectively, based on the aforementioned spectral analysis. Subsequently, the KS method was applied to select the calibration set and the verification set for the two models, which were evaluated using the correlation coefficient and the root mean square error (RMSE). The experimental results indicate that a good correlation between the data predicted using two quantitative analysis models based on the SVM and PLS methods and the actual data. In this case, the RMSEs are small. Baicalin can be quantitatively detected by the quantitative analysis models based on SVM and PLS. The predictive result of the SVM model is better than that of the PLS model. This study provides a new and significant method for the quality detection of medicine.