The echo signal of light detection and ranging (LiDAR) is nonlinear and non-stationary and is easily disturbed by various noises. In order to filter out noises and extract effective signal information, it is necessary to select appropriate methods for noise reduction processing. In this study, Poisson noise was added to the simulated LiDAR echo signal, and then de-noising experiments were carried out by wavelet transform (WT), empirical mode decomposition (EMD), variational mode decomposition (VMD), and their improved and combined algorithms. Afterward, we selected the optimal de-noising method for LiDAR echo signal through comparative analysis. The experimental results showed that the WT-VMD joint algorithm has the maximum output signal-to-noise ratio (SNR) and the minimum root-mean-square error (RMSE) under different original SNRs, with a small smoothness of the de-noised curve, and therefore it can restore the original LiDAR echo signal well and improve the accuracy of subsequent signal inversion.

This paper utilizes the observed data at three sites, namely Jinhua, Hefei, and Lanzhou sites, for the same period to carry out planetary boundary layer height (PBLH) inversion, statistics, and analysis. The research results show that the PBLHs over Jinhua, Hefei, and Lanzhou have characteristics of seasonal and diurnal variation. The PBLHs over Jinhua and Hefei are lower than that over Lanzhou in spring, summer, and autumn. The starting time of the rise and fall of boundary layer height over Lanzhou is in well correspondence with the time of sunrise and sunset, whereas the height over Hefei does not increase significantly until a few hours after sunrise. Jinhua, Hefei and Lanzhou have the highest monthly mean diurnal PBLHs in September, August, and June, respectively, and the corresponding monthly mean maximum mixed layer heights are in September, August, and July, respectively. Studying the spatial and temporal distribution pattern of the PBLH can provide further reference for research of air pollution prevention and control, weather forecasting, and climate prediction.

Atmospheric scattering and turbulence during the transmission of ultraviolet beams can cause severe intersymbol interference and transmission attenuation in a nonline-of-sight wireless ultraviolet optical communication system. To prevent these problems, a wireless ultraviolet scattering channel estimation method based on deep learning is proposed. In the training stage of deep learning model, the deep neural network (DNN) is optimized using differential evolution algorithm to accurately estimate the channel characteristics based on the optimal network output. Then, channel attenuation is compensated at the receiver. Simulation results show that compared with least-squares estimation, the mean-square error of the proposed method is increased by one order of magnitude, and the bit error rate is increased by two orders of magnitude. Compared with minimum mean-square error estimation, the mean-square error of the proposed method is increased by 38%, and the bit error rate is increased by 78%. In addition, differential evolution algorithm during DNN training accelerates learning convergence and promotes global optimization. Furthermore, the proposed method maintains stability in different environments with varying turbulence intensities in the channel model.

To deal with the fiber nonlinear impairments in coherent optical communication systems, this paper proposes a nonlinear compensation (NLC) algorithm based on deep neural network (DNN) and improved principal component analysis (IPCA) by using the triplets derived from the first-order perturbation solution of the nonlinear Schr?dinger equation. The simulation systems of a single-channel 32 GBaud polarization-division-multiplexing 16-ary quadrature amplitude modulation (PDM-16QAM) optical transmission system are built to verify the feasibility of the proposed NLC algorithm. Compared with the DNN-NLC scheme, the IPCA-DNN-NLC scheme reduces the computational complexity by 90.7% with only a 0.06 dB Q-factor penalty, which means that the new algorithm enables similar NLC performance with much lower complexity. Compared with the digital back propagation (DBP) scheme, the IPCA-DNN-NLC scheme realizes a 0.91 dB Q-factor improvement over 800 km transmission. The proposed scheme can work normally without prior knowledge of the link parameters, which is versatile and robust.

We experimentally verified the wired transmission of the probabilistically shaped 8-level pulse amplitude modulation (PS-PAM8) signal over terahertz hollow-core fibers in a photonic-aided W-band communication system by using the probabilistic shaping technology. Via the envelope detection scheme, we carried out the transmission of the 5.75-GBaud PS-PAM8 signal over 1-m hollow-core fibers at the W-band from 97 GHz to 105 GHz. The net transmission rate in the experiment reaches 13.63 Gbit/s, which is within the soft-decision forward error correction threshold of 2.4×10 -2 with a 20% overhead. The experimental results prove the potentiality of THz hollow-core fibers to be a new high-speed W-band transmission medium.

This paper proposed a copper-ion sensor with a Mach-Zehnder interferometer structure based on an ion-imprinted composite film. A section of single-mode fibers (SMFs) was spliced between two sections of three-core fibers (TCFs) and another two sections of SMFs were coupled to the ends of the two sections of the TCFs to form a TCF-SMF-TCF sensing structure. A chitosan (CS)/polyvinyl alcohol (PVA) composite film, a copper-ion-imprinted composite film, and an optimized copper-ion-imprinted composite film were coated on the surface of the TCFs, respectively. The copper-ion concentration was accurately detected according to changes in the relative refractive index of the TCF cladding caused by the specific copper-ion adsorption of the composite films. The experimental results demonstrate that as the copper-ion concentration increases, the monitored troughs in the transmission spectra of the fiber-optic sensors coated with the CS/PVA composite film, copper-ion-imprinted film, and optimized copper-ion-imprinted film exhibit red shift, red shift, and blue shift, respectively. A comparative analysis shows that the sensor coated with the optimized copper-ion-imprinted film displays the optimal response to copper ions. It has a response sensitivity of 62.258 pm·μmol -1·L and a detection limit of approximately 0.602 μmol·L -1, with good selectivity and high pH stability. This sensor, with the advantages of easy preparation and simple structure, has application potential for high-selectivity copper-ion detection in water.

Based on the principle of edge ray, the theory of differential geometry, and the law of geometrical optical reflection, a general mathematical model of non-imaging solar compound parabolic concentrator (CPC) surface structure is constructed for non-concave surface absorber, and the geometric boundary conditions for solving the model are obtained. The applicability of the general mathematical model is verified for the common circular and upper plate absorbers. The results show that the general principle equation is intuitive and convenient in the process of obtaining the structural parameter equation of CPC surface shape. Based on this method, the CPC structural equation of semi-circular absorber with new structure is obtained, and the correctness of the CPC surface equation of semi-circular absorber is verified by laser experiment.

In order to improve the laboratory geometric calibration accuracy of directional polarimetric camera (DPC), the influence of collimator divergence angle on image point positioning accuracy is analyzed, the error model is established, and the image point positioning error is corrected by improving the target equation when fitting the geometric model parameters. The results of the comparative verification experiment show that the proposed method improves the calibration accuracy of the geometric calibration method based on collimator, and the improvement is more obvious when the field of view is larger. When the divergence angle of the collimator is 2° and the incident field of view is 50°, the calibration accuracy is improved by at least 0.15 pixel. The high-precision laboratory geometric calibration will provide accurate initial geometric parameters for the on-orbit geo-positioning and the registration of multi-angle, multi-spectral, polarization images of the DPC.

This paper introduces a polarization state generator into an imaging polarimeter to calibrate the instrument and proposes a double-calibration method for the imaging polarimeter. The standard polarization states generated by the polarization state generator are measured by the polarimeter to obtain the polarimeter response matrix and thereby to calibrate the potential errors in the polarimeter for the first time. The errors of the fast-axis azimuth angle and the linear dichroism coefficient of the quarter-wave plate in the polarization state generator can then be solved with the optimization algorithm to achieve a double-calibration of the standard polarization states and the polarimeter response matrix. The method is tested in the laboratory, and the errors in the polarization state generator and the calibrated polarimeter response matrix are solved. Double-calibration of the imaging polarimeter is thus achieved. The experimental result shows that the method can improve the measurement accuracy of the imaging polarimeter.

A strong-pulsed-field terahertz camera based on light emitting diode (LED) is reported in this paper. It is based on the operational principle that LED can generate nanosecond pulse-duration photovoltaic signal with a reproducible response to picosecond pulse-duration intense terahertz irradiation due to impact ionization when terahertz electric field strength is larger than 50 kV/cm. By employing this effect, we fabricate scanning and array LED-terahertz cameras. These devices have successfully captured images of focused terahertz beam profile generated in lithium niobate via the tilted pulse front technique. The proposed camera has characteristics of low cost, strong photovoltaic signal, rapid response, and large imaging area. Meanwhile, it would give a new idea in developing terahertz imaging technology based on strong-field nonlinear effect.

In the radiometric calibration of a large military infrared imaging simulator when the field stop changes, the infrared radiometer needs to be used under the measurement mode of the underfilled detector. Yet, existing studies have neglected the adverse influences of the image inconsistency between calibration and measurement under this mode on the measured results that lead to large errors in radiometric calibration of large military infrared imaging simulators when the field stop changes. In response to the above problem, theoretical research is conducted on the effect of detector surface heterogeneity on the response voltage in different imaging states. Then, a radiometric calibration method based on the image consistency of calibration and measurement is proposed to reduce the measurement error caused by image inconsistency. Finally, experiments are carried out on calibration of the infrared radiometer and radiation measurement of a certain type of infrared imaging simulator in different imaging states. Experimental results show that if the imaging states of calibration and measurement are not consistent, a large measurement error will be produced. The measurement error introduced by the image inconsistency between calibration and measurement when the field stop changes can be effectively avoided by the proposed radiometric calibration method based on the image consistency of calibration and measurement, which ensures the effectiveness of radiometric calibration of large military infrared imaging simulators.

A control point extraction and optimization method for laser scanning projection graphics based on feature adaptation was proposed. It achieved the adaptive distribution of projection control points for different graphics features by dividing the feature region and non-feature region of the to-be-projected graphics according to the curvature of each adjacent point and the normal vector angle. Meanwhile, to ensure the shape accuracy of the projected graphics after projection control points were diluted, we also studied the dilution and optimization of the number of control points based on the spatial resolution of human eyes. Finally, the optimized control points of the projected graphics were used as the scanning projection points to complete the cyclic scanning projection of the to-be-projected graphics contour. The experimental results show that the proposed method, when applied to a laser scanning projection system, can reduce the number of projection control points by more than 20% and solve the “flicker” problem in the scanning projection of complex graphics.

The existing ellipse fitting algorithm uses the Lissajous figure to solve the demodulation error caused by the non-ideal 3×3 coupler. However, it is difficult to demodulate the signal when it is weak and the corresponding Lissajous figure deviates from the ellipse. This paper proposes a modified ellipse fitting algorithm on the basis of a fixed Lissajous figure for the same system. The algorithm uses the reference signal to determine ellipse parameters and thereby accurately demodulate the weak signal with small phase changes. The experimental results show that compared with the conventional ellipse fitting algorithm, the modified algorithm can accurately demodulate the signals with weak vibration. The detection resolution is increased by 400 times and the demodulated phase has a good linear relationship with the strain. The results prove that the modified algorithm solves the problem that the conventional ellipse fitting algorithm cannot demodulate weak signals and improves the measurement resolution of the system.

To reduce the phase error caused by the nonlinear response in the measurement system, this paper proposed a fast self-calibration method of the Gamma factor based on the Fourier transform. In this method, Fourier transform of the captured grating fringes was performed to convert the image from the gray-scale domain to the frequency domain. Then, the high-order harmonic components and fundamental components of the distorted grating fringes were found in the frequency domain. The optimal pre-encoded Gamma value within the real number range was searched by an optimization function to minimize the power ratio of the high-order harmonic components to the fundamental components and complete the Gamma value self-calibration of the measurement system, which thereby effectively reduced the phase error in the actual measurement process. On standard planes and actual measurement objects, the method in this paper was verified by experiments and compared with classic phase-error correction algorithms. Experimental results prove that this method can suppress Gamma nonlinear response to a great extent. It is simpler and more efficient than existing phase-error correction methods and improves measurement accuracy and measurement efficiency.

The proper calibration of the model values is crucial for high-precision ellipsometer measurement, and random error is critical in determining calibration accuracy. This study discusses random error estimation in the ellipsometer. The random errors encountered in measurement results are numerically evaluated for dark noise-limited, shot noise-limited, and light-source fluctuation-noise-limited systems. To reduce the random errors in measurement results, this study adopts the enumeration method and a genetic algorithm to find the best configuration for the aforementioned three noise-limited systems. Numerical analysis and experimental results show that compared with the commonly used configuration, the random error under the proposed optimal design can be reduced by more than 1/3. Although only the optimal configurations of three noise constrained systems are given in this paper, however, the noise estimation results and optimization methods can be employed in real-world models with various noise models.

The Risley-prism system has a great application prospect in target tracking with the aim of both large field of view and high precision due to its extension of the field of view. Although increasing the prism vertex angle enlarges the magnification of the field of view, it also expands the influence of the system assembly error on the target pointing accuracy. Considering the low pointing accuracy of the Risley-prism system with a large vertex angle, this paper proposes a method of correcting the pointing error of the Risley-prism system based on the particle swarm algorithm. A Risley-prism imaging system is built with a prism vertex angle of 14.85° and a prism refractive index of 1.515. A mathematical model of an experimental prototype based on assembly error analysis is built by using pointing test results of the ideal model. Parameter identification of the setting values of the prism vertex angle and the prism refractive index in the reverse solution algorithm is conducted. Finally, a pointing test is carried out on the experimental prototype. The experimental results demonstrate that the proposed method effectively improves the pointing accuracy of the Risley-prism system with a large prism vertex angle. When the method is applied, the maximum pointing error decreases by 52.4%, and the average pointing error reduces by 43.3%. The root-mean-square error decreases by 44.0%, and the fitting radius of the least squares method decreases by 44.7%.

We propose an on-chip optical flat-top filter based on silicon-on-insulator with low ripple factor and high shape factor. Our scheme is based on a racetrack microring resonator-assisted asymmetric Mach-Zehnder interferometer, and we realize a filter with the ripple factor (the ratio of the maximum power to the minimum power in passband) of 0.11 dB, the shape factor (the ratio of 3 dB bandwidth to 20 dB bandwidth ) of 0.79, the sidelobe suppression ratio of 22.78 dB, insertion loss of 0.22 dB, and the free spectral range (FSR) of 5.22 nm. The FSR can be tuned by reducing the lengths of relevant waveguides proportionally without any changes in the structure. When the size is 0.58 of the original one, the FSR increases to 11.17 nm, and the shape factor is 0.81. The insertion loss is 0.21 dB, and the ripple factor and sidelobe suppression ratio are about 0.14 dB and 16.83 dB, respectively. Finally, we simulate the effect of the fabrication error on the performance of the smaller filter, and further verify that we can eliminate the effect of the fabrication error by embedding micro-heaters in the filter. The flat-top filter has advantages of small size, light weight, low fabrication process, large fabrication tolerance, low ripple factor, high shape factor, and low loss. It can be widely used in frequency combs generation, optical switch and some other optical signal processing fields.

A Fano resonant structure based on the coupling of the photonic crystal nanobeam with the photonic crystal slot nanobeam cavity is proposed to improve the sensing performance of the photonic crystal resonant cavity. A slot structure is designed in the nanobeam cavity with conical holes to obtain a highly localized light field that produces strong light-matter interaction and ultimately to improve the sensitivity. The model is simulated by the three-dimensional finite-difference time-domain method, and it is found that the sensitivity of the refractive index sensor with the proposed structure reaches up to 897 nm/RIU, and the size of the proposed structure is only 21.4 μm ×1.55 μm. In light of the coupled-mode theory, the mechanism of Fano resonance is discussed and the variation of the transmission characteristics of the proposed structure with the structural parameters is analyzed qualitatively.

The photon cross-correlations of different light fields, namely coherent light, mixed coherent and chaotic light, and chaotic light, in a changing process of light feedback intensity are investigated theoretically and experimentally via an unbalanced fiber interferometer, and the above-mentioned light fields are distinguished. The second-order photon cross-correlations of the light fields are analyzed as functions of delay time and coherence time. The results show that the second-order photon cross-correlation g(2x)(τ) of coherent light only has a minimum value of 0.5 at zero delay time. The g(2x)(τ) of chaotic light has two maximum peaks of 1.25 at symmetric delay time. The g(2x)(τ) of mixed coherent and chaotic light not only has a minimum value at zero delay time but also has two maximum peaks at symmetric delay time. Meanwhile, as the proportion of chaotic light increases, the peak values of g(2x)(τ) at symmetry delay time rise from 1 to 1.25 and the g(2x)(0) at zero delay time increases from 0.5 to 1, indicating that the mixed light is transiting from coherent light to chaotic light. The g(2x)(τ) of coherent light is measured experimentally under no light feedback, and the corresponding coherent time obtained is 65 ns. When the feedback intensity is 2.5%, the g(2x)(τ) of the chaotic laser is measured. The proportion of chaotic light is 75%, and the corresponding coherent time is 0.7 ns. When the feedback intensity is 18%, the g(2x)(τ) of chaotic laser in coherence collapse regime is measured, and its coherent time is 0.8 ns. The experimental results are in good agreement with the theoretical ones. It indicates that this method can clearly distinguish light fields with different proportions of chaotic light and their corresponding coherent time and thereby provide a theoretical and experimental basis for further revealing and monitoring the quantum statistics of output light fields.

In order to achieve an efficient solar pumping system, it is necessary to design a pumping cavity with both low thermal shock and high light concentration. In this paper, the two-stage pumping system of Fresnel lens and vase shaped pumping cavity is established by using TracePro software, and the optimum diameter of entering window, position of entering window, caliber of exit window, diameter of vase waist, position of vase waist and length of crystal rod are optimized for high efficiency and more homogenous pump energy distribution. Through theoretical calculation, the pump threshold power of 60 mm crystal rod is 4.568 W/m 2, the output power of the optimal system structure is 18.21 W, and the focal length of thermal lens is 31.0 cm. In comparison with the conic cavity, the axial temperature distribution curves of the crystal rod under the two kinds of cavity shape are obtained. By comparison, it is found that the vase cavity has obvious advantages in reducing thermal shock and improving pump light uniformity. The optimal design of this paper provides a new idea for follow-up experiments.

A two-camera vision system with far and near sight distance for global target positioning and accurate local measurement is usually required when an industrial robotic arm works in multiple stations. Lacking a common field of view, the intrinsic and extrinsic parameters of the vision system cannot be calibrated using traditional methods; therefore, a new calibration technology based on point-array coded targets and multi-view geometry is proposed. First, two sizes of calibration panels with point-array coded targets matching the field of view size are prepared for the two-camera vision system with far and near sight distance. A sub-pixel edge detection method based on gray gradient is proposed to improve the positioning accuracy of the ellipse centers. Then, a decoding algorithm of the point-array coded target is improved for robust image feature matching. Based on this, using high-precision central pixel coordinates of point-array coded targets under different perspectives and the corresponding relationship of targets between different images, the intrinsic parameters of two cameras and the transformation matrices between cameras and calibration panels can be calculated using multi-view geometry technology. Then, a hand-eye calibration equation, AX=ZB, is constructed from the transformation matrix, and the extrinsic parameter X between two cameras can be obtained by solving the equation. Finally, the suppression of calibration error by two kinds of optimization methods is analyzed. Experimental results demonstrate that the calibration accuracy is improved after optimization. The angle error is reduced to 0.05°, and the position error is reduced to 0.36 mm.

Simultaneous localization and mapping (SLAM) based on visual-inertial information fusion is a hot topic in the field of robot navigation. Multimodal data synchronization is a premise of the data fusion algorithm, and the key to it is to accurately obtain the timestamps of the acquisition time of different sensors. While the timestamp of the vision sensor can be accurately obtained by the hardware method, that of the inertial sensor is usually roughly replaced by the output time, which lowers the accuracy of the visual-inertial fusion algorithm. In this paper, a method of time synchronization calibration of the visual-inertial sensor was proposed. A calibration device based on a planar simple pendulum was designed. The vision sensor and the inertial sensor captured data independently in the motion of the planar simple pendulum, and timestamps were added according to the same clock. Then, a method of estimating the angular displacement function and the angular velocity function of the pendulum’s center of gravity was proposed by the least squares method. Finally, the output delay and timestamp of the inertial sensor were obtained by comparing the phase differences of the two functions. Experimental results show that the standard deviation of repeated calibration by the proposed method is 0.018 ms, which proves the effectiveness of the method.

Most mucosal lesions originate from superficial epithelial tissues. In view of this, it is of great significance to study the distribution characteristics of spatially resolved diffuse reflection spectra for the early identification of mucosal lesions in the sub-diffusive regime. Under the circumstance, the second-order scattering orientation factor γ also greatly influences the diffuse reflectance of tissues, besides the absorption coefficient μa and reduced scattering coefficient μ's. For the quantitative and simultaneous determination of the three optical parameters, this paper firstly establishes a forward Monte Carlo numerical model based on optical fiber geometry and Gegenbauer kernel (GK) phase function for practical applications. Then, the 3D look-up table (3D-LUT) method and back-propagation neural network (BPNN) are used to determine the optical parameters and verified by numerical simulation and phantom experiments. The numerical simulation results show that small errors are produced by both methods, and BPNN has higher accuracy, faster calculation speed, and better robustness to the random noise than the 3D-LUT method. The phantom experiments demonstrate that the two methods can effectively determine the three optical parameters in the presence of the experimental system noise and the phantom calibration error, which proves the feasibility and effectiveness of the proposed method.

In order to simplify the actuation scheme of on-orbit high resolution imaging of space camera, a dual actuation structure of active deformation mirror based on the form of variable thickness is proposed. First, according to the theory of elasticity, the theoretical relationship between the wavefront aberration representation method and the variable thickness of the deformed mirror is established, and the thickness distribution required by various aberration modes can be solved. Then, the effect of aberration compensation is quantitatively evaluated through structural modeling, finite element analysis, surface fitting calculation and optical performance evaluation. Finally, the structure scheme and the driving scheme of the variable thickness deformation mirror optical machine are designed. Two actuators are used to realize the correction of spherical aberration and astigmatism. The deformation experiment results show that the dual-actuated deformation mirror can produce better spherical aberration and astigmatism, avoid the impression effect, and effectively compensate its own machining error and wavefront error of optical system.

A direct construction method for freeform surface optical system is proposed to address the problem that the initial structure is difficult to obtain in the design of such system. With the ideal object-image relationship and the Fermat principle as the criteria of single module iteration, an initial structure with favorable optimization potential can be directly obtained from a plane optical system without optical power by tracing light rays point by point. It can be solved faster by sampling feature light rays with different densities at different iteration stages. After the initial optical structure is obtained, optical design software can be used to further optimize the image quality. A compact off-axis two-mirror optical system with a 150 mm focal length and a physical size less than 40 mm×70 mm×60 mm is designed by the proposed method, which verifies the feasibility of this method in the design of freeform surface optical systems.

In order to solve the problem that the portable eye tracker is susceptible to the external environment and the changes of the physiological and psychological state of user, a compact sclera angiography device for eye movement tracking is proposed. The imaging device consists of a main lens, a microlens array, and an imaging detector, whose focal length of the main lens is equal to the distance from the surface of the eye to the main lens, and the imaging detector is located at one focal length of the microlens array. In order to further reduce the volume of the whole imaging device, a rectangular prism is used to turn the optical path twice, which is the imaging object, the main lens, the rectangular prism, the reflective surface 1, the reflective surface 2, the microlens array, and the imaging detector in order of the optical path. The results show that the device can achieve a clear image of scleral vessels at close range based on microlens array, and can obtain a multi-aperture image array of scleral vessels, which has the characteristics of small size, large depth of field and high resolution.

In order to solve the problems of large size, heavy quality, and high cost of traditional photoelectric imaging equipment, two independent design "nodes" of optical system design and image restoration on the whole link of photoelectric imaging are integrated based on the idea of global optimization of computational imaging. First, the difficulty of image restoration with different geometric optical aberrations is analyzed. Then, an optical transfer function that is easy to be processed by the image restoration algorithm is designed through the iterative method of optical-image global automatic optimization. Finally, the output blurred image of single lens and three lenses optical system with global optimization and their restoration quality are analyzed and evaluated. The results show that compared with the optical output blurred image, the contrast, sharpness, and edge retention coefficient of the restored image have been improved to a certain extent. The correction pressure is allocated to the image restoration process to reduce the design pressure of the optical system, thereby simplifying the structure of the optical system.

The chirp characteristics of the converted signals in an all-optical wavelength conversion system based on cross gain modulation of a semiconductor optical amplifier are analyzed. By means of numerical simulation, the chirp and waveform changes of a return-to-zero code and a non-return-to-zero code after wavelength conversion are studied. The relationships of the chirp wirh optical power, signal rate, wavelength, signal extinction ratio, symbol waveform parameters, and bias current of the semiconductor optical amplifier are also studied. The results show that when the number of symbols is small, the return-to-zero code has a larger chirp than the non-return-to-zero code. When the proportion of rising edge (falling edge) is less than 10% of the signal cycle, the converted waveforms of return-to-zero and non-return-to-zero codes both appear obvious the overshoot (down-shoot) phenomenon. The overshoot (down-shoot) causes the chirp peak value of the converted light to be larger than the chirp valley value. Increasing the bias current can reduce the overshoot and enhance the cross gain modulation effect. The effect of bias current on chirp is different at different stages. The increase of data transmission rate accelerates the change of carrier concentration and leads to the enhancement of chirp peak value (valley value).The red shift of the gain peak under light injection makes the chirp sensitive to the reference light and signal wavelengths. The change of extinction ratio can also change the carrier concentration in the semiconductor optical amplifier, which affects the chirp of the converted light. Increasing the reference optical power or decreasing the signal optical power can obtain a lower chirped signal. The results are of great significance to the practical application of an all-optical wavelength conversion system based on the cross gain modulation effect of a semiconductor optical amplifier.

For high absorption of long-wave infrared spectra, a long-wave infrared ultra-broadband perfect absorber is designed in light of the impedance matching theory by the finite-difference time-domain method in this paper. First, a metamaterial absorber with a metal-insulator-metal structure is analyzed, and the absorber has an average absorptivity of greater than 91% in the range of 7--14 μm. Then, on the basis of the metal-insulator-metal structure, an ultra-broadband perfect absorber with an embedded structure is designed. It has almost perfect absorption characteristics in the range of 7--14 μm with an average absorptivity of over 97.55% and appears to be insensitive to polarization. Its average absorptivity remains over 90% (90.5% in the transverse magnetic mode and 93.7% in the transverse electric mode) when the incident angle is 50°. The research results show that broadband perfect absorption is mainly caused by the combined action of multiple modes such as surface plasmon and the Fabry-Perot resonator. The proposed absorber, with excellent absorption in the range of infrared spectra, holds a promising application prospect in fields such as energy harvesting and infrared sensors.

The laser altimeter of the Gaofen-7 satellite is equipped with a laser surveillance camera and a laser footprint camera to obtain laser spots and optical footprint images, so as to assist laser positioning. In this paper, we proposed a method of extracting the footprint spot centroid that took into account the influence of the external environment on the image quality of the footprint spot. The least squares image matching between the footprint spot and the surveillance spot was carried out to obtain the conversion model. Then, the surveillance spot centroid was extracted by the gray centroid method. Finally, the surveillance spot centroid was converted to the footprint spot centroid by the conversion model. The data of 100 groups of simulated spots and 1 track of Gaofen-7 satellite footprint spot were used for the test. The results show that the offset standard deviation of the spot centroid extracted by the proposed method was less than 0.08 pixel and 0.1 pixel larger than those of the gray centroid method and the Gaussian fitting method. 30 tracks of Gaofen-7 satellite footprint spots over a time span of seven months were tested. The results show that both the two footprint spot centroids have a jitter scope of fewer than 1.2 m on the ground, with good long-term stability. The experimental results demonstrate that there is a stable geometrical relationship between the Gaofen-7 laser altimeter laser and the optical footprint image and that the optical footprint image can be used to assist the in-orbit pointing calibration and geometric positioning processing.

For the multiple imaging by the microlens array and the distributed phase modulation by the micromirror array, the snapshot Fourier transform imaging spectrometer can acquire image and spectrum information in real time. Limited by the fabrication accuracy of array devices, the structural parameters of the units in the microlens array and the micromirror array display different degrees of deviations, which affects the system performance. The structural parameter deviations of the lens units in the microlens array can be equated with the non-uniformity of the focal length, and the differences among the step units in the micromirror array are mainly reflected in the non-uniformity of the step length. A model of optical field propagation with non-uniform errors was built according to the phase modulation characteristics of the microlens array and the micromirror array on the optical field. The Monte Carlo method was used to synthesize the non-uniform errors of the focal length of the microlens array and the step length of the micromirror array. Statistical analysis showed that the relative spectrum error increased monotonously with the increase in the standard deviations of the focal length and the step length. Given the non-uniform error of the step length, a spectrum correction method of discrete spectrum phase compensation was proposed. The method effectively reduced the fabrication accuracy requirement on the micromirror array and improved the quality of the recovered spectrum.

To effectively suppress the system and cavity mode noises in gas sensing based on off-axis integrated cavity output spectroscopy and therefore improve the signal-to-noise ratio (SNR) and gas detection sensitivity, this paper proposes an improved empirical mode decomposition (EMD) filtering algorithm on the basis of the traditional EMD method. In the process of hierarchically decomposing noisy signals, the Savitzky-Golay (SG) filtering algorithm and cross-correlation operation are combined, and a reconstructed filtering signal is obtained by using the filtering signals and correlation coefficients. Simulation and experimental results of methane gas samples show that the EMD-SG filtering method can significantly improve the SNR and reduce the lower limit of gas detection. In addition, compared with traditional wavelet-denoising and Kalman filtering, the EMD-SG filtering algorithm has obvious advantages in processing the Gaussian white noise and the non-linear and non-stationary random noise in the system noise and achieves a better filtering effect. After treatment with the EMD-SG filtering algorithm, the SNR of the absorption signal is increased by 1.9 times, and the lower limit of the detection is reduced from 8.7×10 -6 to 4.6×10 -6. The proposed EMD-SG filtering algorithm based on off-axis integrated cavity output spectroscopy has a high SNR and favorable denoising effect and can effectively improve the detection performance of the system. It provides a new method and a basis for developing low-noise gas sensors based on the off-axis integrated cavity to monitor the atmospheric environment.