Dispersive Fourier transform offers a powerful real-time way to measure the transient, complex, and non-repetitive nonlinear dynamics of the soliton in a passively mode-locked fiber laser. With this technique, we investigate the real-time fission dynamics of a dissipative soliton in a passively mode-locked fiber laser based on the nonlinear polarization rotation technique. The results show that the dissipative soliton fissions occur when the pump power is 280 mW and that a single soliton pulse is generated by the background noise pulse before the dissipative soliton fissions. Different from the single soliton mode-locking state when the pump power is 180 mW, a new spectral component is observed on the spectrum of the single soliton generated before fission. The new frequency component amplifies and stretches with the increase in the number of round trips and ultimately evolves into a new dissipative soliton pulse. This research is significant for understanding the formation and fission mechanism of the dissipative soliton in a passively mode-locked fiber laser.

To obtain fiber devices with large stretched amount, we designed and fabricated two kinds of chirped fiber Bragg grating (CFBG) stretchers with the phase mask inscription technology. According to the theories of the phase mask inscription technology and dispersion compensation with CFBGs, the fabrication methods for the two stretchers were proposed and the optical paths were optimized to obtain CFBGs with high reflectivity and wide reflection bandwidth. The reflective resonance wavelength of CFBG was controlled by the tension sensor, and the inscription method was improved. A cascaded CFBG stretcher with large dispersion and a series CFBG stretcher with large reflection bandwidth were fabricated. Fiber sources were then built to test the two kinds of stretchers. The stretched amount provided by the cascaded CFBG stretcher was directly measured to be 345 ps, which was consistent with the theoretical result. The stretched amount provided by the series CFBG stretcher was indirectly calculated through direct and reverse connections. The calculated result, being about 278.7 ps, was smaller than the theoretical result.

The Gaussian beam propagation theory is employed to analyze the effects of alignment errors including radial offset, end face tilt offset, and axial offset on the efficiency of coupling between the space light and single-mode fiber (SMF). The numerical and experimental results show that when the angle Ω between the end face and radial direction is 90° and 270°, the effects of the three alignment errors on the coupling efficiency are independent from each other. When the angle Ω is 180°, the coupling efficiency reaches the maximum value. To better compensate the influence of alignment errors on the SMF coupling efficiency, we design an optical fiber coupler with a 5 degree-of-freedom coupling structure on the basis of piezoelectric ceramics and the stochastic parallel gradient descent algorithm with a variable gain to find the optimal alignment attitude for coupling space light into SMF. The experimental results show that the 5 degree-of-freedom optical fiber coupler can effectively correct different alignment errors and that the SMF coupling efficiency reaches 53.2% when the system is in closed-loop state.

In order to realize the long-term safety monitoring of the structure, a fiber Bragg grating tilt sensor with the combination of lever principle and pendulum structure is designed. Based on this, theoretical analysis, numerical calculation and performance test of the sensor are carried out. The results of numerical calculation and experiment are compared and analyzed, and it is found that they are in good agreement with each other. In order to increase the sensitivity of the sensor, the size parameters of the sensor are designed. Considering the anchorage efficiency between the fiber Bragg grating and the metal material, and the manufacturing process requirements of the sensor, the best combination value is given comprehensively. The experimental results show that the effective monitoring range of the proposed fiber Bragg grating tilt sensor is -5°~5°, the sensitivity of the sensor is 359.04 pm/(°), the linear correlation is 0.999, and the measurement repeatability error is 3.433%. In addition, the proposed fiber Bragg grating tilt sensor reduces the influence of temperature on angle measurement, is suitable for the working environment with large temperature variation and high precision, and has a good application prospect.

In this paper, a denoising method of variational mode decomposition and permutation entropy is proposed, the key parameters and thresholds in permutation entropy are analyzed and set, and then the decomposition layer value of variational mode decomposition is determined by permutation entropy, and the decomposed modes are reconstructed to achieve denoising of vibration signals. The advantages of this method in orthogonality, completeness, signal-to-noise ratio, and efficiency are verified by simulation tests. Finally, the actual vibration signals collected by the system are denoised. The experimental results show that, compared with the existing empirical mode decomposition-correlation coefficient and full empirical mode decomposition-correlation coefficient methods, the proposed method has the best denoising signal-to-noise ratio (the ratio of noise signal to noise reduction) for three kinds of vibration signals (contact, wheel rolling, and rain), which are 32.5358 dB, 30.5546 dB, and 29.3435 dB, respectively, and the time-consuming is also less, which is 1.4432 s, 1.6320 s, 1.2349 s, respectively, and the accuracy of signal pattern recognition is the highest, all above 99%.

The analysis of the theoretical model of few-mode fiber is not perfect. The channel model of few-mode fiber based on the theory of displacement and rotation of fiber segment is established, which takes into account the factors such as mode crosstalk and differential mode delay. In order to improve the system performance, the influence of probability shaping technology and interleaved encoding technology on signal transmission performance are studied. The simulation results show that after the 16-order QAM (Quadrature Amplitude Modul) signal is transmitted through a few-mode fiber with a length of 50 km, when the forward error correction limit is 3.8×10 -3, the optical signal-to-noise ratio is reduced by 3 dB, and the transmission distance is effectively improved.

As a key module in the orthogonal frequency division multiplexing (OFDM) system, the synchronization module is the basic premise for data demodulation and channel estimation. Due to the sensitivity of OFDM-based visible light communication (VLC) systems to synchronization errors, the accuracy of symbol timing offset (STO) estimation directly affects the system performance. An STO estimation method based on odd-even symmetry is presented for direct current biased optical OFDM (DCO-OFDM) systems. By designing training symbols with an odd-even symmetric structure, this method can generate an ideal pulse-like timing metric, thereby achieving excellent STO estimation accuracy. The performance of the proposed method is assessed by measuring the root-mean-square error (RMSE), mean absolute error (MAE), and the bit error rate (BER) of timing offset estimation through simulation. It is then compared with the performance of two baseline methods (Park method and Guerra method). The simulation results show that this novel symbol timing synchronization approach is superior to the above methods in the additive white Gaussian noise (AWGN) channel and the multipath fading channel, which verifies the effectiveness of this method in DCO-OFDM systems.

In order to improve the performance of optical fiber macrobending temperature sensor, a novel optical fiber macrobending temperature sensor based on polyimide (PI) coating is proposed. The optical fiber bending radius of the sensor is determined by the optical fiber bending loss-temperature measurement method based on the core-cladding-infinite coating structure, and a new type of optical fiber macrobending temperature sensor is obtained by coating the PI film on the 1060-XP optical fiber cladding. The temperature sensing experimental results of the sensor show that PI coating can not only improve the mechanical properties and heat resistance of optical fiber, but also significantly improve the temperature sensitivity and temperature measurement resolution. The novel optical fiber macrobending temperature sensor can achieve a wide temperature measurement range of -20 ℃ and 100 ℃, and the temperature sensitivity is 0.072 dB/℃ and the resolution is 0.14 ℃. Compared with other optical fiber macrobending temperature sensors, the temperature sensing performance of the designed sensor is significantly improved.

In order to solve the problem of low radiation energy at the edge of the output spot of the equal curvature optical integrator, which leads to the poor uniformity of the radiation surface, the differential and integration method of radiation energy based on the diffraction theory is proposed, and a variable curvature optical integrator is designed. The aperture and number of the integrator channel are determined according to the Fresnel number, and the radiation distribution curve is divided equidistant. According to the difference of the focal length of each eye lens in the integrator, the radiation distribution curve is superimposed step by step. Based on the diffraction theory, the mathematical model of variable curvature optical integrator in two-dimensional plane is established, and the mathematical function of light intensity distribution on the working face is deduced. The aspheric optimization design of each circle eye lens in the field lens group is carried out by using Zemax software to improve the imaging quality and eliminate the sidelobe effect. The variable curvature optical integrator and the constant curvature optical integrator are simulated by LightTools software, and their performance differences are compared and analyzed. The results show that the variable curvature optical integrator can significantly improve the edge radiation energy of the output spot of the solar simulator, up to 56% higher than that of the equal curvature optical integrator, the irradiation non-uniformity in the Φ100 mm radiation plane is better than ±0.5%, and the radiation inhomogeneity in the Φ200 mm radiation plane is better than ±1%.

A novel high resolution light field display method based on polymer dispersed liquid crystal film is proposed. First, the virtual camera array is built to collect the light field information of the target object, so as to obtain its element image array. Then, according to the photoelectric characteristics of the polymer dispersed liquid crystal, high resolution light field display results can be obtained by adjusting the applied voltage of the polymer dispersed liquid crystal film and using the transient/afterglow effect of the human eye. The experimental results show that compared with the traditional method, the proposed method is simple and has higher display quality, and the peak signal-to-noise ratio of the presented image is improved by about 11%. In addition, the system built by the proposed method is less difficult and practical.

As the main component of photovoltaic power station, photovoltaic cells have defects, such as hidden cracks, scratches, hot spots, and broken gates, which affect the performance of photovoltaic cells and the operation status of photovoltaic power stations, so it is very important to carry out defect detection of photovoltaic cells. A pulsed electric infrared thermography (PEIT) experimental system is established, and the system is used to carry out detection experiments of photovoltaic cells with different types of defects and to collect infrared thermography sequences. Two kinds of supervised learning algorithms, linear discriminant analysis (LDA) and quadratic discriminant analysis (QDA), are used to process thermography sequences, and compared with principal component analysis (PCA) and fitting correlation coefficient (FCC). The experimental results show that the PEIT algorithm can effectively detect the defects of photovoltaic cells, and the QDA algorithm is better than LDA, PCA, and FCC algorithms in signal-to-noise ratio, information entropy, and mean square error, and it can effectively identify all kinds of defects in photovoltaic cells.

The spectral observation coding scheme based on compressed sensing and deep learning theory has some problems, such as the complex process of filter design and spectral reconstruction, and the difficulty of hardware implementation of the designed spectral transmittance. Therefore, in order to simplify the spectral observation system, considering the manufacturing difficulty of common interference filters, a spectral transmittance observation coding scheme based on symmetrical tridiagonal Toeplitz matrix is proposed. The well-posedness of spectral observation matrix is discussed by matrix theory, and its tolerance is studied by numerical simulation. The theoretical analysis results show that with the increase of the size of the spectral observation matrix, the condition number of the symmetric tridiagonal matrix increases slowly and the upper limit can be controlled. The numerical simulation results show that the non-negative least squares algorithm is used for spectral reconstruction, and under certain constraints, increasing the size of the observation matrix has little influence on the suitability of the spectral observation coding scheme of the symmetric tridiagonal matrix, which can still guarantee the high accuracy of spectral measurement reconstruction.

This paper presents an imaging method of a liquid crystal (LC) lens using a 90° twisted nematic LC (TNLC) cell instead of a polarizer. Two images are captured with the 90° TNLC cell in the 0° and 90° optical rotatory states. By overlapping the two images and subtracting the defocused image obtained when the LC lens is not working, we can reduce the introduction of undesired low-frequency components of non-modulated light. In this way, the contrast of the final image is enhanced, and the noise generated by the original imaging processing without a polarizer is lessened. The proposed method is verified by experiments, and the results show that clear and well-focused images can be obtained without any polarizer.

It is required to carry out on-line quality detection on the processing components in advanced manufacturing. The fringe projection three-dimensional measurement technology is adopted because it meets the requirements of on-line and non-contact. However, due to the large difference of surface reflectivity of metal materials, the phenomena of overexposure and underexposure will occur in the captured structured images and result in the missing or mistaken of the final measuring data. Aiming at this problem, the point cloud data of the estimated shape of the measured object in the camera coordinate system is obtained by iterative nearest point (ICP) point cloud registration based on the incomplete point cloud data measured directly and the designed data of the components obtained by computer-aided design (CAD). Taking the data as the estimated shape, combined with the system calibration parameters, the image intensity correspondence of the same point between projection and imaging is established. The projection gray range and the minimum projection grayscale are calculated by using the intensity imaging response curve of each point on the measuring element by camera. Finally, the three-dimensional shape measurement of high reflective elements can be realized by using a projection grating with non-equivalent coefficient. The experimental results show that the proposed method can completely measure three-dimensional shape of high reflective elements without changing the structural parameters of the measurement system nor increasing the structural complexity of the measurement system.

In color-encoded phase measurement profilometry, the obtained phase shifting fringe pattern is distorted caused by the color crosstalk and uneven intensity responses among multiple optical channels in the optoelectronic device and other factors. Consequently, large phase errors arise when phases are solved by the classical phase technique. A two-step correction method is proposed by analyzing the characteristics of the fringe patterns in the red, green, and blue channels obtained by the color imaging device with a mathematical model of color-encoded fringe patterns. The first step is to normalize the intensities of the images in the three channels with their mean value and standard deviation. In the second step, the actual phase shifts after distortion are searched with the probability density function curve, and the influence of inaccurate phase shift on the measurement result is reduced. The propsoed method does not need to pre-correct the coupling coefficients and phase shift offsets of the system, and it can achieve simple and fast phase error compensation. Simulation and experimental results verify the effectiveness of the proposed method.

To meet the demand for high-precision detection and location of surface defects in industrial inspection, this paper proposes a defect feature reconstruction method. A texture camera is added to the three-dimensional (3D) reconstruction system of binocular grating projection to realize the texture mapping of the reconstructed point cloud model. The 3D reconstruction of surface features can be realized using the feature regions extracted from the image captured by the texture camera. Since the object needs to be reconstructed from multiple perspectives, a precision turntable is introduced. The rotation axis calibration method is used to obtain the projection relationship between the texture image and the point cloud data at different rotation positions, and the judgment method based on the distance criterion is employed to eliminate the occluded part of the point cloud. Finally, the pixel coordinates of the texture image are located in three dimensions by four-quadrant proximity search and linear interpolation based on distance weighted average. The experiments complete the reconstruction of the pixel points in the marked defect contour in the image and realize the accurate size calculation and location of surface features. The reconstructed defect size and position are calculated which are then compared with the results measured by an optical image measuring instrument. The results show that the measurement error of the proposed solution for the 3D sizes and positions of defects is no more than 0.2 mm, and the areas of defects can be calculated more accurately.

The precision measurement of the temperature field is of great significance in many fields, such as machinery, aerospace, biomedicine, food and chemical engineering, electric power, energy, and environment. A three-dimensional temperature field measurement method based on multi-step phase shift method and polarization interference optical tomography optical path is proposed. First, the polarization interference optical tomography measurement system is designed by combining the Mach-Zehnder interference optical path structure and optical tomography technology, and the multi-step phase shift is realized by using the rotation of the polarization device to achieve high-precision signal detection. Then, the three-dimensional refractive index distribution of the measured medium is restored by the exponential filtering backprojection algorithm, and the three-dimensional temperature field distribution is obtained. Finally, the measurement formulae are derived and the experimental system is built. The error analysis shows that the system measurement uncertainty is about 0.8 ℃ under the existing experimental conditions. The measurement experiment and comparison results show that the measured temperature field is consistent with the actual situation, and the result is less than 2 ℃ compared with the calibration temperature of the platinum resistance thermometer.

Intensity correlation interferometry, which exploits the high-order correlation characteristics of optical fields to obtain the spatial angles of astral bodies, is expected to achieve high-precision measurement of angular positions of pulsars. However, conventional intensity correlation measurement requires coherent detection, which imposes an extremely high requirement on the temporal resolution of detectors. In this paper, intensity correlation interferometry for astral observation angle based on spatial modulation is proposed. The optical field is spatially modulated by a modulation screen located in front of the detector, and the modulator screen is rotated to obtain second-order interference fringes. A second-order correlation function is derived theoretically for the case when there is an angle difference between the screen modulators of the two optical paths. On the basis of the theoretical results, a two-mirror experimental scheme is designed to verify the results obtained by visible light experiments. The experimental results are consistent with the theoretical analysis results. This method, greatly reducing the temporal resolution requirement on detectors, is significant for China’s fulfillment of spacecraft autonomous navigation in the future.

The efficient coupling of fiber and planar optical waveguide is an important link in the interconnection design of optical waveguide devices. The commonly used inverted tapered waveguides have solved the problem of mode mismatch between fibers and waveguide chips. However, the coupling of high refractive index difference waveguides and fibers usually requires a small tip size (<180 nm), the processing is complicated and the waveguide is easy to collapse. A broad-band (visible and near infrared bands) fiber-waveguide horizontal coupler is designed. By introducing the polymer SU-8 tapered structure, the linewidth of the three-port branch waveguide is improved, and an effective bandwidth of 1dB over 300 nm in 850 nm wave band is achieved.

In an off-axis pumped laser, the off-axis thermal lens effect gives rise to the Hermite-Gaussian (HG) mode hopping. Namely, with the pump power increasing, the running laser turns from a single HG mode into mixed modes and then into the adjacent low-order HG mode. Our analysis shows that this is due to the decrease in the effective off-axis amount caused by the off-axis thermal lens-induced optic axis shift of the resonant cavity for off-axis pumping. The experimental results demonstrate that a higher-order mode has a lower pump power for mode hopping. Moreover, we can return the laser to the original HG mode by modifying the off-axis amount in practice. A larger adjustment is required as the pump power increases. We experimentally demonstrate pulsed outputs at a high peak power in sixteen HG modes, from mode HG1,0 to mode HG16,0. At a repetition rate of 10 kHz, a pulse width of 32 ns and a peak power of 4.1 kW are realized for the mode HG1,0, and a pulse width of 79 ns and a peak power of 0.7 kW are realized for the mode HG16,0.

A Dy∶PGS laser with 4.3 μm high-power continuous laser generated by cascade oscillation is numerically simulated. The whole process of stable continuous output of Dy∶PGS laser generated by cascade oscillation is simulated, the laser power and the spatial distribution of population density in the resonant cavity are calculated, and the effects of pump power, crystal length and reflectance of output mirror on 4.3 μm laser output are analyzed. The calculation results show that the self-termination effect of Dy∶PGS crystal can be eliminated by cascade oscillation, and a 4.3 μm laser output with high power and efficiency can be realized. The output power of 4.3 μm laser can reach to 2.535 W with slope efficiency of 29% when the 1.7 μm pump power is 10 W. The optimal length range of the crystal is 12--24 mm, a higher reflectance of the idler outpot mirror is needed, and the optimal range of the reflectance of the signal output mirror reflectance is 0.8--0.9.

Based on a spin-flip model (SFM) of vertical-cavity surface-emitting lasers (VCSELs), this paper theoretically investigates the characteristics of polarization switching (PS) and polarization bistability (PB) in the VCSEL subject to continuous variable-polarization optical injection. The results show that in the case of reasonable injection parameters, PS and PB effects of the polarization components of the VCSEL can be induced by the polarization angle of the injection light when the angle is continuously scanned along the forward and reverse routes. Meanwhile, the forward and reverse PS points shift with the scan period, leading to a variable width of the PB region. For a given injection intensity, the forward and reverse PS points exhibit opposite changing patterns with the increase in scanning period. Moreover, a shorter scanning period and a larger frequency detuning are both conducive to a wider PB region. When the frequency detuning is fixed, the change in injection light intensity also has a great influence on the PS position and the PB width. A weak injection intensity and a short scanning period are both conducive to expanding the PB width. When injection intensity and frequency detuning are fixed, both the polarization angles corresponding to forward and reverse PS points demonstrate an approximate uptrend with an increasing bias current, whereas the width of the PB region experiences violent fluctuations. Furthermore, a longer scanning period leads to a smaller PB width. Additionally, the spin-flip rate also has an impact on the PS and PB characteristics of the output polarization components of the VCSEL. A lower spin-flip rate is more liable to result in a larger PB width for given injection parameters.

Single-crystal silicon is an important semiconductor material. Generally, the surface of single-crystal silicon after ingot slicing is prone to deep surface defects, such as grooves, pits, and cracks, etc. To solve this problem, a two-step laser irradiation method is proposed, which can repair surface defects and decrease surface roughness at the same time. First, the repairable defect depth is forecasted by finite element method (FEM) simulation under different laser parameters. Then, surface defects with various depths can be repaired by using the deeper surface layer melting at a higher energy density of 0.50 J/cm 2. However, the thermal capillary flow caused by high energy density can easily lead to the residual high-frequency features on the surface, which will lead to the increase of surface roughness. Subsequently, the residual high frequency features can be effectively eliminated by re-radiating the same surface with a low energy density of 0.20 J/cm 2. Finally, the surface with the original surface roughness of 1.057 μm is irradiated by the two-step laser to obtain a defect-free smooth surface with a surface roughness of 26 nm.

At present, many domestic aero-engines are bought from abroad. Only physical objects and installation dimensions are provided for such aero-engines, and the lack of three-dimensional digital models brings great difficulties to the assembly coordination design of aircraft and aero-engines. Therefore, aircraft design departments urgently need to quickly reconstruct geometric models of aero-engine profiles. To enable a reconstructed geometric model of the aero-engine profile to retain exact structural features, this paper proposes a feature segmentation method of the aero-engine profile point clouds based on deep learning. It divides the whole point clouds into feature data and non-feature data, which is conducive to the subsequent reconstruction of various complex structural features by different methods. An iterative density equalization algorithm designed to create a feature segmentation dataset provides a basis for the training, testing, and performance evaluation of the feature segmentation network. A feature segmentation network is designed to collect the shape structure and local neighborhood information from multi-scale patches and thereby determine whether the center is a feature point. The trained feature segmentation network model is then applied to the profile point cloud of an aero-engine. The verification results show that the accuracy of feature segmentation reaches 95.16%, which means the proposed algorithm achieves high-precision semantic segmentation.

Laser speckle contrast imaging (LSCI) technology is widely used in tissue surface blood flow imaging with large field of view. When real-time in vivo monitoring of blood flow distribution and changes in deep tissue or lumen tissue is needed, the combination of LSCI and endoscopic imaging is an effective way to solve the problem of LSCI imaging depth. For this reason, a commercial laparoscopic LSCI imaging system is built to image microfluidic replicas and rabbit large intestine. The experimental results show that the system can correct the static scattering and eliminate the influence of system noise on speckle contrast, and the quantitative monitoring of blood flow can be realized by using the speckle contrast measurement under single exposure. The commercial laparoscopic LSCI imaging system will have important clinical application potential.

Because of its incomparable advantages in light field phase control, multifunctional composite, micro-nano integration, and other aspects, metalens has great application potential in many fields. However, the design of metalens requires professional knowledge and rich experience, which makes it difficult for non-professionals to master it quickly, thus hinders the large-scale preparation of hyperlens. By means of MATLAB and finite difference time domain (FDTD) hybrid programming, the design process of dielectric metalens independent of preset physical model is studied, and the automatic design of dielectric metalens is realized. By inputting the required metalens parameters on the software interface written by MATLAB and calling FDTD design simulation program in the background to build the nanostructure, the relationship among the size of the structure, phase, and transmittance can be calculated. According to the required phase distribution, the superlens is constructed and its performance is evaluated by numerical simulation. The design process and software can greatly facilitate the design of metalens by non-specialists.

The fields-of-view of volume holographic waveguide display systems are often limited by the grating diffraction response bandwidth, which still cannot meet people’s needs for large field-of-view display. In order to expand the field-of-view of the waveguide display, the influencing factors of the field-of-view of the waveguide display are analyzed based on the strictly coupled wave theory model, and a double-layer volume grating waveguide structure that can effectively expand the diffraction response bandwidth is proposed. The grating is prepared by the variable-angle fractional exposure method, and the holographic waveguide display system is built. The imaging results show that the horizontal and vertical fields-of-view of the display system can be expanded to 33.4° and 22.6° respectively, and the diagonal viewing angle is 40.3°.

Taking 4T PPD (4 transistor clamped photodiodes) type CMOS image sensor as the research object, the damage simulation of neutron irradiation fluence of 1×10 11, 3×10 11, 5×10 11, 7×10 11, 1×10 12 neutron/cm 2 are carried out, and the device model of CMOS image sensor and the defect model of displacement damage after different neutron irradiation fluence are established. The correlation double sampling technique is used to measure the output value of floating diffusion (FD) under two continuous pulses from bright to dark, and a simulation method for measuring charge transfer loss (CTI) is established. The relationship between CTI and neutron irradiation fluence is obtained, and the variation of CTI with neutron cumulative fluence is analyzed. Combined with neutron irradiation effect experiments to verify the validity of the theoretical simulation results of neutron irradiation-induced CTI degradation. The results show that the sensitive region of displacement damage of CMOS image sensor is space charge region, and displacement damage defects will be introduced into space charge region after neutron irradiation. Through continuous capture and emission of carriers, these defects make the signal charge can not be quickly transferred to FD, resulting in charge transfer loss, and the charge transfer loss increases with the increase of neutron irradiation fluence, and there is a linear relationship between them in a certain range.

In order to improve the practicability of compressed sensing ghost imaging and solve the problem of associative imaging failure caused by the loss of sampled data and the inability to repeat sampling in the scene, a expansion method of packet loss data in ghost imaging is proposed. First, the influences of different packet loss data on imaging performance are analyzed. Then, the image quality is improved by grouping the sample data and extending the sample results with missing phenomena. The simulation and experimental results show that, compared with the traditional method, the grouping expansion method can reduce the influence of packet loss data on the imaging quality, which is beneficial to further promote the practical application of ghost imaging.

The continuous development of laser technology has put forward higher requirements for the optical properties, laser-induced damage threshold, and mechanical properties of laser coatings. Laser coatings with low absorption loss have very important applications in the fields of high-power laser and precision measurement. In this paper, the research progress of laser coatings in absorption loss control is reviewed from two aspects, including the deposition processes of E-beam evaporation and ion beam sputtering and coating materials. The absorption loss control methods in different manufacture processes during deposition, as well as the absorption mechanism and control methods of pure material and mixture material coating are introduced in detail.

The spectral information of the scene is affected by different illumination conditions, hence the spectral reflectance reconstruction of multispectral images taken under scenes with uncontrollable illumination requires illumination spectrum estimation. Therefore, a general method based on a single multispectral image is proposed to accurately predict the illumination spectrum of the scene. First, by analyzing the response features of each pixel, the chroma weight map is designed and calculated to find the pixels that contain more illumination information. Then, the component analysis of the weighted image is carried out to extract the illuminant response features in the channel domain. Finally, benefiting from the innovative introduction of the dictionary learning method trained by illumination spectrum library, the relative spectral power distribution of the scene illuminant can be estimated. The average angular errors of the illumination spectrum estimation obtained by the proposed method on simulated data and real data are 0.29 and 3.42, respectively. Compared with the existing counterparts, the proposed method shows better accuracy and robustness.

According to the preliminary requirements of hard X-ray free electron laser (HXFEL) for offset mirror group (the meridian shape error is less than 0.5 μrad before bending and after clamping), the traditional mechanical bending is difficult to meet and can be corrected in real time, so the structure of piezoelectric side-placed piezoelectric bimorph mirror (PBM) is adopted. The mirror is placed horizontally, and the voltage needed to correct the low frequency surface shape error caused by dead weight is calculated by singular value decomposition (SVD) method, and the feasibility of the scheme is verified by finite element analysis method. Different bending surface shapes are solved and analyzed, and then thermal analysis method is added to solve the surface shape mutation caused by local radiation. The simulation results show that in the case of dead weight, the initial slope error of the mirror is 242.43 nrad (root mean square, RMS), and the slope error can be greatly reduced to 7.743 nrad (RMS) after voltage bending correction. When adjusting the target shape, the slope error between the actual bending shape and the target shape is only 0.0024 nrad (RMS).