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2024, Volume 53,  Issue 4

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Optical design
Design of automatic out-of-focus correction system for near-space lidar receivers
Li Yingchao, Zhang Qing, Liu Zhuang, Shi Haodong, Fu Qiang
2024, 53(4): 20230645. doi: 10.3788/IRLA20230645
[Abstract](5) [FullText HTML] (1) [PDF 2815KB](0)
  Objective  Lidar is the main way to obtain three-dimensional geographic information within the military, and the data results obtained through this way are also widely used in resource exploration, land use, environmental monitoring and national key construction projects, providing extremely important original information for the national economy, social development and scientific research, and has achieved significant economic benefits, showing good application prospects. The lower temperature of the near space can reach –60 ℃, the optical antenna as the core component of LiDAR, its optical components have strict requirements for temperature changes. Temperature variations can lead to thermal deformation of the element, resulting in problems of defocusing and focal plane translation, which reduces the coupling efficiency. Improving the coupling efficiency can increase the detection rate, and the off-axis reflective optical system can be realized without obstruction, which can improve the energy utilization. However, unlike conventional refractive or coaxial reflection systems, each optical element in the off-axis reflection system does not have rotational symmetry, and its temperature deformation after temperature change is not uniform, so solving the effect of temperature change on the focus of the off-axis parabolic mirror is the key to improve the coupling efficiency of LiDAR.  Methods  In this paper, the LiDAR is modeled, and the temperature field is simulated by the finite element analysis method for the model. The Zernike polynomials are used to fit the surface shape data obtained after the analysis to obtain the shape and position change of the reflector after temperature deformation, and the optical design software is used to obtain the optimal focal point position. Finally, the optical fiber position is adjusted by the focusing device to achieve the effect of focusing.  Results and Discussions  The PV value of the mirror of the designed LiDAR is less than 10/λ. Its optimal focus position curve is obtained by optical design software, and a temperature-adaptive focusing structure is designed according to this curve, through which the RMS radius of the mirror compensated by the focusing structure decreases from 26.495 μm to 15.93 μm, the spot radius is reduced by 39.9%, and the coupling efficiency is improved from 15.8% to 91%. This focusing method does not need to keep track of the changes in the temperature of the reflector and reduces a certain amount of weight and cost compared to focusing with a motor. However, the method requires a certain temperature response time, can not adjust the focus to the best position at the first time after the temperature change. If the temperature changes frequently, the motor should be used to quickly adjust. Due to the working height of the blimp is more fixed, and its ambient temperature does not change much, the method is feasible.  Conclusions  Aiming at the problem that the off-axis parabolic mirrors used in the near-space lidar will deform at low temperatures which leads to a decrease in the efficiency of the reflected light when it is coupled into the optical fiber, the self-focusing technique is investigated, and the auto-compensating lidar assembly is designed to offset the effect of temperature on the system. The deformation of the receiving system under thermal load is analyzed using the finite element method to obtain the discrete deformation data of the mirror surface, and the Zernike polynomials are used to fit the surface shape after the deformation of the mirror surface, and the simulation of the optical design software concludes that the change of the focal length of this LiDAR receiving system after optimization has a linear relationship with the temperature, and the compensation position is determined by the optical design software. A temperature adaptive adjustment mechanism is used to reduce the effect of out-of-focus amount caused by thermal deformation, which improves the coupling efficiency by more than 80%.
Adjustment and testing method of large aperture mirror Bipod support structure
Li Bin, Gao Sisi, Wang Haichao, Chen Jiayi, Chen Xi, Lu Yuting
2024, 53(4): 20230657. doi: 10.3788/IRLA20230657
[Abstract](4) [FullText HTML] (2) [PDF 3610KB](0)
  Objective  In order to effectively deal with the interference of gravity error in the installation and adjustment of large aperture remote sensing camera on the surface, the installation and adjustment mode of large aperture remote sensing camera is gradually changed from the horizontal form of optical axis to the vertical form of optical axis. Correspondingly, the support structure and assembly requirements of the main mirror of the camera are also constantly innovated. In order to meet the needs of mirror support and weight reduction, the Bipod structure is a more common structural form in the form of mirror support of 1 m and above caliber. Different from the traditional frame support form, it has a great change in the optical and mechanical structure bonding and structural position calibration needs. In order to ensure that the assembly positioning accuracy and surface shape change of the mirror under the Bipod support structure meet the requirements of the system index, a high-precision assembly method for the Bipod support structure is proposed.  Methods  In this paper, a method of assembly and adjustment of large-aperture mirror is proposed, which combines multi-objective spatial position conversion and Stewart structure motion inversion. This method can effectively ensure the accuracy of the optical and mechanical structure adhesion points, and realize the high-precision six-dimensional adjustment between the mirror and the main bearing plate only by relying on the mirror's own support structure. By decomposing the error index of the optical system of the remote sensing camera, the requirements of the reflector installation index are determined (Fig.4). Then the laser tracker was used to measure and construct the spatial coordinate system of the mirror and the support pad. The coordinate system restoration principle based on the nominal point was adopted to achieve the normalization of the coordinate system. According to the measured data of the nominal point on the support pad, the position of the support pad was corrected and fixed (Fig.8). Finally, based on the characteristics of the Bipod structure itself, the relationship between the length of the Bipod rod and the position of the mirror is calculated (Eq.14), and the position adjustment between the mirror and the main structure of the camera is realized by adjusting the length of the adjusting rod.  Results and Discussions  The bottom-up error distribution method was adopted to sort out the tolerance of various error sources in the process of optical system installation. Error distribution follows a top-down order, with the sum of the squares of the error coefficients at the lower level equal to the square of the error at the higher level. Finally, the wave aberration RMS caused by the main mirror assembly is 0.025λ, including the influence of the position deviation and surface shape change of the main mirror assembly. The laser tracker was used to measure and locate the position of the support pad and the mirror, and the assembly of the mirror and the support pad was completed. The surface shape variation (RMS) before and after the installation was 0.008λ (Fig.9), which met the requirements of the installation error index. A laser tracker was used to measure the position deviation between the mirror and the bearing plate, and the adjusting length of the Bipod rod was calculated using the Stewart structure motion inversion algorithm. After the final debugging, the setting position error of the mirror was 0.004 1 mm, which met the setting error index requirements.  Conclusions  Based on the setting process of a large aperture reflective remote sensing camera with Bipod structure, the setting error interval of the main mirror in the system is analyzed, and the shape error RMS value of the mirror assembly after setting is determined not to exceed 0.025λ. Based on this standard, the positioning, bonding, adjustment and detection scheme of the mirror Bipod structure is formulated. The practical results show that the method can effectively control the assembly positioning accuracy and surface shape error of the mirror, the positioning accuracy of the support pad can reach 0.029 mm, and the position adjustment accuracy of the mirror and the bearing plate can reach 0.041 mm. The installation results can meet the system imaging requirements of large aperture remote sensing cameras.
Miniaturization analysis and design of the common-optical path dual-wave infrared optical lens
Qi Chen, Jin Yangming, Xie Xiaoyu, Hou Huihui, Li Yongsheng
2024, 53(4): 20230722. doi: 10.3788/IRLA20230722
[Abstract](1) [FullText HTML] (1) [PDF 1934KB](0)
  Objective  This paper aims to realize the miniaturization design of medium and long wavelength common optical path optical systems.  Methods  An optical index allocation model for secondary imaging structures under size constraints based on Gaussian optics and primary aberration theory was established. Lens combinations of high, medium and low relative dispersion materials in the form of "−, +, −" structures are used as the primary mirror structure to correct the secondary spectrum; Meanwhile, field lenses and aspheric surfaces are used to correct spherical aberration to reduce the relay mirror. The light height is set to balance the residual aberration of the primary mirror.  Results and Discussions  Because of the high edge light height and small optical power of the primary mirror, its spherical aberration, chromatic aberration and secondary spectrum are the main sources of aberrations in the system. To correct the secondary spectrum, the "−, +, −" structure can be used. A lens combination of high, medium and low relative dispersion materials is used as the main mirror structure. At this time, the residual aberration of the main mirror is small. Field lenses are used to reduce the light height of the relay lens group and aspherical surfaces are used to correct spherical aberration to balance the residual main mirror Aberration.  Conclusions  Based on the above theory, a common optical path dual-wave infrared small optical system with a medium wave wavelength of 3.7-4.8 μm and a long wave band of 7.7-9.5 μm is designed. The total length of the system is no more than 135 mm, the structure is small and compact, the optical transfer function is close to the diffraction limit, and the operating temperature range covers −40-60 ℃ and is not sensitive to temperature. The analysis and design of a medium and long-wavelength common optical path miniaturized optical system based on the secondary imaging structure optical index distribution model are realized to meet the needs of a medium and long-wavelength common optical path miniaturized optical system.
Lasers & Laser optics
Optical communication and sensing
High speed demodulation of FBG based on variable step scanning strategy
Liang Lei, Zhang Yuning, Du Shangming, Tong Xiaoling
2024, 53(4): 20230671. doi: 10.3788/IRLA20230671
[Abstract](7) [FullText HTML] (1) [PDF 3007KB](0)
  Objective  Optical fiber signal demodulation is the basis of optical fiber sensing applications, and the systems based on FBG sensing are sometimes limited by the demodulation speed when measuring the high-speed dynamically changing signal. Nowadays, the high speed demodulation method of fiber Bragg grating has become one of the main research directions of fiber demodulation. The existing FBG high-speed demodulation methods have their own advantages and disadvantages. Among them, the scanning spectral analysis system has good versatility, low cost, and more balanced performance in speed and accuracy. Therefore, improving the demodulation speed of the scanning laser fiber Bragg grating demodulation system has become the research goal of this paper.  Methods  A variable step scanning strategy is designed to solve the problems such as many tuning times and long scanning time in the practical application of the laser based fiber Bragg grating wavelength demodulation instrument. The crest of the spectrum can be extracted by using the variable step scanning strategy with less wavelength tuning times. The variable step scanning strategy first uses OTSU algorithm to calculate the threshold and the coarse scanning step size in the initialization stage. In the demodulation stage, the global coarse scan is performed to extract the approximate range of the wave crest, and then the local point-by-point scan is performed in the sampling points exceeding the threshold. Only the spectral sampling data of point-by-point scanning in the local range needs to be uploaded to the supreme computer for peak searching, the amount of data required for peak searching is reduced.  Results and Discussions  A FBG wavelength demodulation system based on modulated grating Y-branch (MG-Y) laser was built (Fig.3), and demodulation test was carried out after calibration of the laser. Gaussian fitting demodulation was carried out on the host computer software for the transmitted local spectral data, and the signal-to-noise ratio simulation experiment was conducted on the variable step scanning strategy. The simulation proved that the variable step scanning strategy was suitable for the situation where the signal-to-noise ratio was higher than 9.23 dB (Fig.7). The number of the sampling points and time of laser scanning in a single round demodulation process with different fixed scanning steps and variable scanning strategies are analyzed. Experiments show that the demodulation accuracy of sample data obtained with variable scanning strategies is similar to that obtained with the smallest fixed scanning steps when there is only a single grating in the channel. In contrast, the number of points to be scanned using the variable step scanning strategy is reduced by at least 90% and the scanning time is reduced by at least 90% (Tab.1), and the demodulation speed is increased by 10 times. Although the sampling number and time of laser scanning can be reduced by using a large fixed scanning step, the demodulation accuracy is greatly reduced and has no practical application value. In summary, using variable step size scanning strategy can not only reduce the scanning time, but also have little effect on the accuracy of subsequent demodulation.  Conclusions  Through theoretical and experimental research, a variable step scanning strategy based on scanning laser type fiber grating wavelength demodulation instrument is proposed, and its high speed performance and anti-interference performance are tested. Compared with the case of using the minimum fixed step forward scanning, this strategy reduces the tuning times of single round demodulation and the amount of data collected, improves the speed of wavelength demodulation, and has similar demodulation accuracy as the case of using the minimum fixed step forward scanning. When there is only a single grating in the channel and the processing speed of the back-end demodulation algorithm is not restricted, the demodulation speed can be increased to 10 times by using the variable step scanning strategy, which can reliably meet the demand of high-speed demodulation based on the scanning laser demodulation system, and has a good application prospect.
Random modulation-based realization of optical channel measurement sample de-correlation analysis
Hou Yihao, Lou Yan, Chen Chunyi, Zhao Shengya, Li Hui, Li Qiong, Qin Xinyi
2024, 53(4): 20230731. doi: 10.3788/IRLA20230731
[Abstract](0) [FullText HTML] (1) [PDF 2188KB](0)
  Objective   Based on channel-based key extraction technology, the reciprocal random channel is treated as a public random source from which shared keys are generated. During the optical channel transmission, the received optical signals may exhibit correlation due to unstable factors such as atmospheric turbulence, indicating a degree of correlation between adjacent signal samples. In order to extract highly random key sources from the optical channel, it is necessary to reduce the measurement rate of the channel state based on the correlation time of the channel, ensuring the lack of correlation between consecutive channel measurements. To some extent, the correlation time of optical fluctuations caused by turbulence limits the number of uncorrelated optical channel measurement samples that can be obtained per second. Therefore, it is necessary to perform decorrelation on the continuous observed optical channel measurement samples obtained by the legitimate party at a sampling interval shorter than the correlation time of optical fluctuations caused by turbulence. In this paper, a simulation experiment based on random modulation is constructed to achieve decorrelation of measurement samples, and the impact of random modulation on the autocorrelation of measurement samples is analyzed.  Methods   Based on existing relevant theories, an experimental system for measuring sample decorrelation based on random modulation has been designed. The schematic diagram of the transmitting and receiving terminal principles based on random modulation is shown (Fig.1), and theoretical analysis has been conducted (Fig.2). Through analysis, the impact of the normalized variance of the modulation signal source on the correlation coefficient of consecutive measurements is explained. Utilizing OptiSystem software, a simulation experiment of the sample measurement system based on random modulation in optical channels was constructed (Fig.3).  Results and Discussions  The power samples of received signals over time were analyzed under three conditions of no modulation, single modulation, and double modulation (Fig.4). Moreover, the autocorrelation of measurement samples as a function of lag time was examined under different modulation conditions (Fig.5). In the absence of random modulation, it was observed that the autocorrelation after a lag of 100 samples is 0.676. Contrarily, under single modulation and double modulation conditions, the autocorrelation after a lag of 100 samples is 0.083 and 0.035, respectively. The utilization of random modulation effectively decreased the autocorrelation of optical channel measurement samples. Additionally, concerning the single modulation case, a separate analysis was conducted to assess the impact of varying the coherence time of the atmospheric optical channel transmission coefficient (Fig.6) and the effect of different pseudo-random code generation rates on the autocorrelation of optical channel measurement samples (Fig.7). When the sampling rate is 6.4 × 106 Hz, and the generation rate is 6 × 106 bit/s, the autocorrelation of adjacent measurement samples is 0.112. This indicates that the use of random modulation enables obtaining nearly uncorrelated consecutive observations of optical channel measurement samples at a smaller sampling interval than the correlated time caused by turbulence-induced optical fluctuations. Finally, the impact of transmission distance (Fig.8) and signal-to-noise ratio (Fig.9) on the autocorrelation of measurement samples was analyzed.  Conclusions  The impact of three scenarios, namely no modulation, single modulation, and dual modulation, on the autocorrelation of the observed optical channel measurement samples was studied. The research results show that compared to the case without modulation, the number of measurement data variations in the observed optical channel measurement samples increases within the same time interval. Moreover, applying random amplitude modulation to the transmitted optical signal reduces the autocorrelation of the observed optical channel measurement samples at a sampling interval shorter than the correlation time of optical fluctuations caused by turbulence. When other parameters remain unchanged, as the coherence time of the atmospheric optical channel transmission coefficients decreases and the delay time increases, the autocorrelation of the measurement samples decays faster and the variation becomes more pronounced. Similarly, when other parameters remain unchanged, as the generation rate of pseudo-random codes increases and the delay time increases, the autocorrelation of the measurement samples decays faster. Additionally, the impact of increasing generation rate of pseudo-random codes on the autocorrelation of adjacent optical channel measurement samples was analyzed.
Photoelectric measurement
Non-contact three-dimensional emissivity distribution measurement method of M8 LiDAR echo
Li Ronghua, Deng Yuan, Xue Haopeng, Zhou Xinchen, Zhao Mingshuo
2024, 53(4): 20230672. doi: 10.3788/IRLA20230672
[Abstract](1) [FullText HTML] (5) [PDF 4039KB](0)
  Objective  Emissivity is an important physical quantity to characterize the radiation ability of material surface. It is an important thermal physical parameter and has very important applications in many fields. Emissivity is not the intrinsic property of an object. It is a physical quantity that is difficult to measure accurately. It is related to temperature, wavelength, angle and so on. Its measurement is more complicated. At present, most of the emissivity measurement methods are only contact measurement of a single substance. It is impossible to measure the three-dimensional distribution of emissivity of complex targets. Combined with the characteristics of LiDAR, a non-contact three-dimensional distribution measurement method of emissivity is proposed based on LiDAR echo.  Methods  Firstly, the echo intensity characteristics are analyzed based on the LiDAR transmission distance equation, the main factors affecting the LiDAR echo intensity are explored, and the target point cloud data with intensity information by line array scanning of 95% reflectivity standard diffuse reflector plate are obtained by line array LiDAR (Fig.4). The stacked multi-frame single line point cloud (Fig.5) and the three-dimensional point cloud image of radar intensity with reflective spectral characteristics is obtained (Fig.8). Secondly, the piecewise polynomial model is used to fit the relationship between distance-intensity and incident angle-intensity (Tab.4). Based on the obtained piecewise polynomial model, the echo intensity under the influence of distance and incident angle is corrected, so that the measured echo intensity under different distance and incident angle can truly reflect the reflection spectrum characteristics of the target, and the validity of the correction model is verified (Fig.15-16). Finally, based on the obtained piecewise polynomial correction model, the intensity correction of the three-dimensional point cloud image of the radar intensity of the scaled satellite model with the reflectivity true value patch is carried out. The reflectivity of the target surface is calculated by using the reflectance spectral characteristics of the corrected echo intensity. The emissivity is further deduced by the reflection method, and the three-dimensional distribution of the emissivity of the scaled satellite model is obtained (Fig.21).  Results and Discussions  The correction results show that the standard deviation of echo intensity STD under the influence of distance before and after standard diffuse reflection correction is reduced from 50.58 to 3.49 (Tab.5), and the standard deviation of echo intensity STD under the influence of incident angle is reduced from 19.25 to 3.17 (Tab.5). The coefficient of variation of echo intensity under the influence of standard diffuse reflection plate distance effect and incident angle effect is reduced from 0.267 6 and 0.343 8 to 0.042 0 and 0.041 2 (Tab.5) respectively. And the consistency of echo intensity is increased by 84.31% and 88.02% respectively (Tab.6). The average emissivity deviations of the surface patches of the three satellite models can be controlled at 3.33%, 4.84% and 4.44%, respectively (Tab.7).  Conclusions  In view of the fact that most of the current emissivity measurement methods are only contact measurement of a single substance, it is impossible to measure the three-dimensional distribution of emissivity of complex targets. Combined with the characteristics of LiDAR, a non-contact emissivity three-dimensional distribution measurement method is proposed based on M8 LiDAR echo. The band of the M8 linear array LiDAR used in this paper is 905 nm, which belongs to the near-infrared band in the infrared band, and the emissivity measurement in the medium and long infrared band cannot be involved. The reflectivity of the materials used in this paper is the true value at room temperature, and the influence of temperature on emissivity measurement is not involved. In addition, due to the differences in physical factors of different LiDARs and the inconsistent expression of the unit and numerical scale of the echo intensity, the current measurement method is only applicable to specific scanning instruments. The robustness and universality of the emissivity measurement method are worthy of further discussion. Therefore, the next step will further improve the universality of the method and introduce the influence of temperature on the emissivity measurement.
Internal parameter calibration method of line-scan camera based on 2.5D calibration fan
Zhang Xu, Mao Qingzhou, Shi Chunlin, Hu Qingwu, Jin Guang, Zhou Hao, Xie Yi
2024, 53(4): 20230670. doi: 10.3788/IRLA20230670
[Abstract](5) [FullText HTML] (3) [PDF 6023KB](0)
  Objective  Aiming at the difficulty and high cost of regular calibration of line-scan camera internal parameters in industrial production lines or integrated equipment, a calibration method of line-scan camera internal parameters based on 2.5D calibration fan is proposed. The appropriate calibration object is designed, and the internal parameter calibration model of the linear array camera is established. The linear transformation theory is used to calculate the initial value of the model parameters, and the improved Levenberg-Marquardt (L-M) algorithm is used to optimize the camera parameters. The experimental results show that the internal parameters of the linear array camera calibrated by this method have high accuracy and good consistency. The maximum re-projection error is less than 0.28 pixel, and the average RMSE is 0.112 pixel.  Methods  A specific 2.5D calibration fan was designed. The internal parameter calibration model of line-scan camera including lens distortion is constructed, which takes into account the two attitude angles of the target relative to the camera. The initial value of the model parameters is solved by the equation linear transformation method, and the improved L-M algorithm is used to accelerate the optimization of the camera parameters. The detailed calculation steps and data processing process are given, and the feasibility of the method is verified according to the simulation and measured data.  Results and Discussions   The theoretical analysis and experimental results show that the linear array camera calibration method is simple and flexible, and a large number of feature point pairs with regular distribution can be obtained. The parameter calibration accuracy is not limited by the camera movement accuracy and specific direction. In addition, when the angle between the fan-bone surface and the target surface is less than 10 °, high-precision and high-consistency camera internal parameters can be obtained. The maximum value of the feature point reprojection error is better than 0.28 pixel, the average RMSE is 0.112 pixel, and the standard deviation is only 0.014 pixel.  Conclusions  Line-scan cameras are often placed inside the equipment in a modular form with other sensors, and the regular calibration of the internal parameters of such cameras is costly and difficult. In view of this, an internal parameter calibration method of linear array camera based on 2.5D calibration fan is proposed. The 2.5D calibration fan has the advantages of both the three-dimensional measurement effect of the 3D target and the low cost and high precision of the 2D target. The number of feature points is large and the distribution is regular, which avoids the problem of easy loss of features, and the feature points and image points are easy to match. The constructed linear array camera calibration model takes into account the lens distortion error and the small angle attitude between the target surface and the image surface, so that the camera movement direction and the position of the calibration object are not strictly limited. Experiments show that the internal parameters of the camera calibrated by the fan bone with $ \theta $ < ±10° are the best, and the calibration results have high accuracy and good consistency. The reprojection error of 89 % feature points is less than 0.20 pixel, the maximum error is better than 0.28 pixel, and the average RMSE is 0.112 pixel. In addition, compared with the standard L-M algorithm, the improved L-M algorithm reduces the number of iterations by half without affecting the accuracy of parameter optimization.
Dual-wavelength interferometric algorithm based on spatial-temporal conjugate complex function coupling
Cheng Jinlong, Zhu Liyan, Chen Lu, Yang Zhongming, Gao Zhishan, Yuan Qun
2024, 53(4): 20230661. doi: 10.3788/IRLA20230661
[Abstract](7) [FullText HTML] (3) [PDF 2675KB](1)
  Objective  Traditional single-wavelength interferometry is not suitable to unwrap the correct phase for measuring surface with step or groove, whose depth is larger than half wavelength. Dual-wavelength interferometry (DWI) technique employs an extra wavelength to generate a longer beat-frequency synthetic wavelength (${\lambda }_{{\rm{s}}}$). For synthetic wavelength is much longer than the optical working wavelength, DWI extends the measuring discontinuity limit of interferometry greatly. And DWI could achieve the simultaneous accurate measurements with large dynamic range for the multi-scale morphology characteristics parameters such as the macro profile and local morphology with step. Meanwhile, in the simultaneous dual-wavelength interferometry (SDWI), the two single-wavelength interferogram is captured simultaneously to accelerate the data collection, which is immune to vibration with the advantages of the time saving and higher efficiency. In practice, the dual-wavelength interferogram is usually captured by the monochrome sensor, which is more convenient and economical. And a generated dual-wavelength Moiré fringe pattern appears as the simple incoherent additive superposition of the two corresponding single-wavelength interferogram. The low beat-frequency envelope of the generated fringe pattern indirectly represents the needed synthetic-wavelength information, whose direct extraction is rather arduous. For this purpose, we present a dual-wavelength interferometric algorithm combining with spatial-temporal conjugate complex functions coupling and double phase shift strategy.  Methods  The method needs to acquire multiple groups of phase-shift dual-wavelength interferogram, and each group consists of four continuous interferogram. The phase shift step among the four frames in each group is required as π/2 at one single wavelength, while π/2 at synthetic wavelength between the adjacent groups by the designed double phase shift strategy (Fig.2). And in dual-wavelength squeezing interferometry for each group, the temporal phase shift in each group is converted into spatial carrier in the generated dual-wavelength STF. Therefore, the +1-order spectral lobes for the two wavelengths could be easily separated from others and filtered in the Fourier spectrum of the generated dual-wavelength STF without extra spatial carrier and elimination of background. After the appropriate band-pass filter and inverse Fourier transform, the single-wavelength interferometric complex functions are obtained. Subsequently, when the conjugate single-wavelength interferometric complex functions are multiplied, the synthetic-wavelength interferometric fringe pattern could be extracted directly (Fig.1). The obtained synthetic-wavelength interferogram from each group with π/2 phase-shift step at ${\lambda }_{{\rm{s}}}$ could be demodulated to retrieve the final synthetic-wavelength phase by the conventional phase-shift algorithm.  Results and Discussions  Simulations verify that the proposed method has a lower linear carrier requisition than the spatial-domain Fourier transform demodulation theory, which is merely about 0.077 times of latter numerically, even the phase-shift deviation at different wavelength exists (Fig.4). Besides, the feasibility and applicability of the proposed method are verified using simulation and experimental results. Numerical simulation indicates that the demodulated error is better than PV of 0.556 9 nm and RMS of 0.089 7 nm even when the fringe number is 1 at ${\lambda }_{{\rm{s}}}$ (Fig.3). In addition, for the test step in the experiment, the results have validated the effectiveness of the proposed method for the interferogram with lower linear carrier. And the step height deviations for the proposed method are better than 0.94% for the step with the height of 7.8 μm even for one fringe at ${\lambda }_{{\rm{s}}}$ (Fig.9).  Conclusions  To extract and retrieve the lower frequency synthetic-wavelength interferometric fringe form dual-wavelength Moiré fringe, we present a dual-wavelength interferometric algorithm combining with spatial-temporal conjugate complex functions coupling and double phase shift strategy. Several groups of phase-shift dual-wavelength interferogram are acquired with every contiguous four frames in each group. The temporal phase shift among each group is converted into spatial carrier for the spectral separation with lower spatial carrier. When the obtained spatial-temporal conjugate complex functions are coupling by multiplication, one synthetic-wavelength interferogram could be extracted for each group directly without the introduction of other wavelength interferometric information. For the designed π/2 phase shift at synthetic wavelength between the adjacent groups, the extracted synthetic-wavelength interferogram from every group is demodulated by conventional phase-shift algorithm directly. The proposed method has a lower carrier requisition than the traditional spatial-domain Fourier demodulation theory, which is merely about 0.077 times of the former numerically, even when the phase-shift deviation for different wavelengths exists.
Deep hole roundness measurement method of circular structured light system
Chen Zhenya, Ma Zhuoqiang, Li Xiang, Shen Xingquan, Yang Shangjin, Miao Hongbin, Lu Chuanjie
2024, 53(4): 20230660. doi: 10.3788/IRLA20230660
[Abstract](6) [FullText HTML] (1) [PDF 4635KB](0)
  Objective  The circular structured light based measurement system can generate circular structured light by reflecting a laser beam through a conical reflector, and utilizing laser triangulation and close-view photogrammetry algorithms to calculate three-dimensional coordinates. This kind of measurement system has been widely studied by researchers due to its advantages such as high flexibility, high accuracy and simple structure. The current circular structured light system has certain problems in measuring roundness. It is difficult for the laser, the camera and the deep hole to be measured to be parallel or coaxial, which leads to the inability to measure the accurate circular cross-section. A measurement system based on circular structured light is constructed, the mechanism of systematic error is analyzed and a method based on the circular structured light system to measure circularity is proposed, which provides some compensation for systematic errors.  Methods  The mechanism that generates systematic errors in the measurement of roundness by the circular structured light system is analyzed (Fig.3). A high-precision electric linear slide is used to move the deep hole parts to be measured to complete the full-field measurement, and a high-resolution point cloud is obtained and fit the axis. Through the Rodrigues formula, the inner surface point cloud rigid body will be transformed to the point cloud axis and z-axis parallel to the location of the point cloud axis, searching for the z coordinate near the point as a cross-section of the roundness of the assessment point (Fig.4). The roundness evaluation is completed by the grid search algorithm (Fig.5).  Results and Discussions  The compensation by the proposed method works well in roundness measurement experiments, in which the measurement uncertainty is 4.78 µm (Tab.3). For the circular structured light measurement system, a long rod can be assembled after the laser to increase the axial measurement range of the system. For the proposed circular structured light roundness measurement method, the resolution of the 3D point cloud can be improved by reducing the step length of the motorized linear slide to further improve the measurement accuracy.  Conclusions  A three-dimensional measurement system of circular structured light is built, the error generated when the circular structured light system measures roundness is analyzed, and a method based on the circular structured light system to accurately measure roundness is proposed. It is verified that the use of the method to measure roundness has a good compensating effect through the measurement experiments.
3D shape measurement of transparent objects by phase deflection based on multi-frequency phase shift
Su Chaoyang, Wang Zhangying, Ni Yubo, Gao Nan, Meng Zhaozong, Yang Zeqing, Zhang Guofeng, Yin Wei, Zhao Hongwei, Zhang Zonghua
2024, 53(4): 20230702. doi: 10.3788/IRLA20230702
[Abstract](1) [FullText HTML] (1) [PDF 5345KB](0)
  Objective  Phase Measuring Deflectometry (PMD) in optical 3D measurement is widely used in optical surface measurement, rapid detection and other fields because of its advantages of rapid speed, high precision, stability and anti-interference. Due to the refraction and reflection of the upper and lower surfaces of transparent objects, the camera collects mixed fringes with different surface reflections. It is difficult for traditional PMD to measure them effectively in three dimensions. The existing phase extraction methods require high accuracy of the initial phase and need to collect a large number of fringe patterns. In order to solve this problem, a PMD method based on multi-frequency phase-shifting is proposed to measure the 3D morphology of transparent objects surface.  Methods  This study proposes a multi-frequency phase-shifting-based PMD for measuring the 3D surface morphology of transparent objects. Firstly, the display screen shows a variety of sinusoidal fringes with different frequencies combined with multi-step phase shift, and the camera collects the mixed fringes reflected and superimposed on the surface of the object from another angle. Subsequently, the mixed fringes are separated iteratively by the least square method, and the wrapped phase of the upper and lower surfaces are obtained, and the unwrapping phase is obtained by the optimum three-fringe selection method in temporal phase unwrapping. Then, the relationship between phase and gradient is determined by gradient calibration, and the gradient of the measured object relative to the reference plane is determined according to the unwrapped phase. Finally, the gradient integral is used to restore the 3D morphology of the transparent object surface.  Results and Discussions  In order to prove the effectiveness of the proposed method, a glass plate with a thickness of 3 mm and a plano-convex lens with a radius of curvature of 515.09 mm were measured, and a comparative experiment was conducted between the multi-frequency phase-shifting method and the multi-frequency method to verify the effectiveness of the proposed phase separation method (Fig.14). The experimental results show that the proposed method can effectively measure the 3D morphology of transparent objects surface (Fig.16, Fig.19). Compared with the existing methods, the average error of measuring the upper surface of transparent glass plate is reduced from 32.4 μm to 5.1 μm (Tab.2). This method can effectively avoid the influence caused by the large deviation of initial phase value, shorten the calculation time, and is suitable for 3D shape measurement of transparent objects with different shapes.  Conclusions  A method for measuring the 3D morphology of transparent objects surface based on multiple frequencies is proposed. This method makes up for the deficiency of phase separation in traditional phase deflection measurement and can effectively measure the 3D morphology of transparent objects surface. Through the combination of different frequency fringes and multi-step phase-shifting, the upper and lower surfaces of transparent objects are separated by using the least square method for multiple iterations, which reduces the accuracy requirements of initial phase values, improves the numerical stability, is easy to realize phase convergence operation and shortens the calculation time. Compared with the traditional methods, it can be concluded that the proposed method of measuring the 3D topography of transparent objects based on multiple frequencies improves not only the realizability and numerical stability of iterative algorithm, but also the accuracy of topography measurement.
Atmospheric optics
Analysis of aerodynamic optical effects in airborne laser communication optical windows
Jiang Yiyuan, Meng Lixin, Zhang Lizhong, Liu Zhi, Dong Keyan
2024, 53(4): 20230680. doi: 10.3788/IRLA20230680
[Abstract](6) [FullText HTML] (2) [PDF 7424KB](0)
  Objective  Wireless laser communication technology has advantages such as high communication speed, good security, and portability, making it widely applicable in both military and civilian communication fields. Aircraft, as one of the crucial platforms for wireless laser communication research, can effectively enhance the reliability and applicability of laser communication, playing a significant role in the future integrated aerospace network. However, the practical application of airborne laser communication faces various challenges. Apart from common atmospheric laser communication issues like atmospheric absorption, scattering, platform vibrations, and cloud cover, aerodynamic optical effects pose a significant constraint on the application of airborne laser communication.  Methods  This paper addresses the issue of selecting the shape of the optical window for the airborne laser communication optical terminal. It designs streamlined fairings with conformal and planar optical window shapes (Fig.3). Fluent is used to simulate and analyze the flow field around the fairings under different flight altitudes, speeds, and azimuth angles. The refractive index distribution at the front end of the optical window is obtained based on the density and refractive index relationship (Fig.7). Then, using the ray tracing method, the wavefront distortion of the two windows under different conditions is obtained (Fig.6), verifying the independence of the optical grid and tracing length in ray tracing (Fig.8-9). Finally, the far-field free diffraction spot changes are analyzed using phase screen analysis (Fig.13).  Results and Discussions  The results show that, at different azimuth angles, RMS increases first and then decreases, reaching a minimum at 90°, and rapidly increasing after exceeding 90°, reaching the maximum at 180° (Fig.10). Overall, the RMS caused by the spherical window is slightly larger than that caused by the planar window. When changing the pitch angle, the spherical window exhibits good flow field consistency, while the planar window has some influence on the flow field near the window (Fig.11). Flight altitude and speed are also crucial factors affecting window aerodynamic optical effects. Increasing flight altitude and decreasing flight speed can weaken aerodynamic optical effects, and the RMS change caused by the planar window is smaller than that caused by the conformal window (Fig.12). To gain a deeper understanding of the performance of the optical system under actual working conditions and observe the impact of wavefront distortion on the optical system, we conducted far-field diffraction analysis of the calculated wavefront distortion. With increasing azimuth angle, the changes in intensity and offset become more drastic, resembling the trend of RMS changes. However, for the planar window, after azimuth angle exceeds 150°, its wavefront distortion becomes more severe compared to the conformal window (Fig.14). Increasing flight altitude can weaken the window's impact on aerodynamic optics, and increasing flight speed results in noticeable differences between the two, with speed having a more significant impact on aerodynamic optical effects than altitude (Fig.15).  Conclusions  When the azimuth angle is less than 90°, there is little difference in the optical transmission performance between the two window shapes. However, beyond 90°, the RMS value of the conformal window consistently exceeds that of the planar window. Nevertheless, after wavefront aberration and far-field diffraction, the distortion of the spot from the conformal window is noticeably smaller than that from the planar window, especially at azimuth angles of 180°, where the planar window's far-field spot shows significant peak intensity reduction and spreading. Changes in pitch angle demonstrate better stability for the conformal window compared to the planar window. Variations in flight altitude and speed significantly affect the peak intensity of the far-field diffraction spot, while having a smaller impact on the spot's displacement. Increasing flight altitude weakens the influence of the window shape on aerodynamic optical effects, while increasing flight speed exacerbates the differences between the two window shapes, with flight speed having a stronger impact on aerodynamic optical effects compared to flight altitude. The wavefront aberrations caused by aerodynamic optical effects are highly complex, and relying solely on wavefront aberration RMS values cannot fully define their impact on the optical system. For instance, although the RMS value of the planar window is smaller than that of the conformal window at azimuth 180°, the spreading degree of the spot during far-field diffraction is stronger than that of the conformal window. Overall, the conformal window exhibits more stable flow fields during variations in pitch, altitude, and speed, showing a more consistent trend and performing better in far-field diffraction. Conversely, the planar window's RMS value is lower than that of the conformal window under conditions of large azimuth angles and low-speed flight.
Infrared technology and application
On-chip integrated polarization spectral imaging device for atmospheric infrared band
Zhou Jianyu, Li Quanmin, Wang Jin, Huang Shengdi
2024, 53(4): 20240012. doi: 10.3788/IRLA20240012
[Abstract](2) [FullText HTML] (2) [PDF 2212KB](0)
  Objective  Infrared detection and remote sensing are the core technologies of meteorological observation. As an important payload of meteorological satellites, infrared radiation detectors are mainly used for quantitative detection of atmospheric temperature and humidity. Their detection accuracy depends on the number of spectral and polarization measurement channels. The common technical solution is to achieve spectral and polarization detection by combining filters and polarizer wheels, which leads to problems such as large system volume, high power consumption, and few channels. The development of on-chip integrated polarization spectral imaging devices is an effective method to solve the above problems. Previous studies have mainly used arraying schemes of thin film resonant cavities or resonant microstructures, but both cannot meet the requirements of spectrum and polarization selection. To solve these problems, this article proposes a new design approach based on coupling regulation of thin film microstructures, which can provide an on-chip integrated polarization spectral imaging device.  Methods   An on-chip integrated polarization spectral imaging device based on thin film microstructure, which combines a subwavelength grating broadband reflector and a multi-layer high reflection film is built in this paper (Fig.1). By constructing matching conditions for phase and amplitude at the interface between multilayer films and microstructures, narrow band transmission peaks with polarization selective characteristics can be excited through the coupling of them (Fig.2). Specifically, firstly one can design a broadband high reflectivity microstructure with a central wavelength of λ0, then design a high reflection film stack centered on λ0, with the outermost layer being a low refractive index interlayer, next adjust the appropriate spacing layer thickness Dspacer to make a narrow band transmission peak appears near the center λ0, last scan the transmission spectrum with changes in scanning period and duty cycle, find the parameter combination corresponding to the peak, and achieve multi-channel design (Fig.3). Meanwhile, by changing the lateral parameters of the microstructure, multi-channel polarization filtering can be achieved at different wavelengths, thus enabling on-chip integrated spectral imaging (Fig.4).  Results and Discussions   Taking the atmospheric infrared band around 13 μm as an example, an on-chip integrated polarization spectral imaging device with 6 channels is designed, the average transmittance of them is over 94% and the extinction ratio is about 30 (Fig.6 , Tab.1). In addition, research and exploration on the physical mechanisms (Fig.5) and fabrication schemes of the devices (Fig.7) are also conducted. Meanwhile, as the device is not sensitive to the refractive index of the substrate, the selection of the substrate could also be more flexible. This new design approach has opened up new doors for on-chip integrated spectral imaging devices. With the improvement of fabrication technology and further optimization of structure, it is expected to achieve better performance and be successfully applied in the field of polarization spectral imaging. In addition, in recent years, some low dimensional infrared detection materials such as GaSb nanowires have also shown excellent infrared detection performance, and due to their dimensional advantages, they can achieve the detection of polarized infrared light. Assigning spectral selection function to infrared detection materials with polarization selective properties will also be a new research approach in the future.
Afterpulse suppression scheme of InGaAs/InP high speed sinusoidal gated single photon detector
Cao Yang, Su Yang, Jiang Lianjun, Liu Ming, Guo Shuyang, Zhang Wenzhe, Fang Yuqiang, Gao Song, Chen Zunyao, Chen Zhitong, Yu Lin, Tang Shibiao
2024, 53(4): 20230701. doi: 10.3788/IRLA20230701
[Abstract](2) [FullText HTML] (1) [PDF 1361KB](0)
  Objective  Quantum key distribution (QKD) system based on the basic principles of quantum mechanics can reach the level of information theory security. Single photon detector is an important component of QKD system, and the afterpulse probability has an important effect on the performance of QKD system. In this paper, an InGaAs/InP high sinusoidal gated detector afterpulse suppression scheme is designed to meet the requirements of QKD system.  Methods  In this paper, according to the law that the probability of the afterpulse of the InGaAs/InP high-speed sinusoidal gated detector shows an exponential decreasing distribution with time, the detection pulse is measured by using the "Start-Stop" time interval measurement method. Each detection pulse is time-marked separately, and the probability of the afterpulse of the detector is reduced by discarding the detection pulse within a certain period of time (Fig.2).  Results and Discussions  This paper actually verifies the relationship between the afterpulse probability and the discard time of a single photon detector (Fig.5). The main afterpulse distribution area is pointed out and the reason for the depression in the range of discard time is explained (Fig.6). The afterpulse probability under different discard time conditions was calculated.The afterpulse probability is 2.46% when the discard time is 500 ns, and 1.97% when the discard time is 5 μs. Furthermore, it is pointed out that distinguishing the arrival time of detection pulse by time measurement can also improve the ability of QKD system to resist quantum hacker attacks such as avalanche transition region attack and behind door attack.  Conclusions  According to the law that the afterpulse probability of the InGaAs/InP high-speed sinusoidal gated detector is exponentially decreasing with time, a afterpulse suppression scheme based on time measurement is proposed in this paper. The afterpulse probability of the detector is reduced by marking the detection event with time and discarding the detection pulse in a period of time. The proposed afterpulse probability suppression scheme has the characteristics of clear principle and easy engineering implementation, and has no direct influence on the working process of single photon detector, but the detector saturation count rate will decrease with the increase of discard time. At the same time, distinguishing the arrival time of detection pulse by time measurement can also improve the ability of QKD system to resist quantum hacker attacks, and can support the application of high-speed sinusoidal gated single photon detector in QKD system.
Optical imaging
Depth extension technique of spectral domain optical coherence tomography
Shu Yukang, Zhao Chao, Du Xiaoyu, He Xiang, Zhao Hang, Shi Xiaofeng, Ma Jun
2024, 53(4): 20230720. doi: 10.3788/IRLA20230720
[Abstract](1) [FullText HTML] (1) [PDF 5590KB](0)
  Objective   Spectral domain optical coherence tomography (SD-OCT) is a technique used for tomography and three-dimensional imaging of the internal microstructure of materials and biological tissues. By performing Fourier transform on the collected interference signal, the SD-OCT system can reconstruct image depth information, providing a detailed view of the sample's structure. However, the resulting image includes a mirror image with zero phase delay symmetry, which can be problematic. To address this issue, the sample is positioned on the zero phase delay side during scanning to eliminate the influence of the mirror image. This approach, while effective, only utilizes half of the imaging range space. Improving the detection depth of OCT systems by eliminating complex conjugate images is crucial for enhancing the accuracy and reliability of the imaging process.  Methods   The deconjugated imaging system was developed from the original SD-OCT system (Fig.3). The system utilizes a broadband light source, with the light being split by a beam splitting cube into the reference arm and sample arm. The reference arm consists of a collimator and a reflector, while the sample arm includes a two-dimensional galvanometer, scanning lens, and sample stage. The two-dimensional galvanometer enables scanning of the sample in both X and Y directions by deflecting the reflector. The entire sample arm system is finely tuned using an electronic displacement platform to adjust the relative position, create beam offset, and add carrier frequency. The sample stage can be moved axially to focus imaging at different depths within the sample, allowing for the superposition of focusing planes and enhancing the imaging depth.  Results and Discussions   To eliminate the influence of the complex conjugate image, the distance between the beam and the center of the mirror is adjusted to 1.2 mm. Before the spectral data undergoes the standard image reconstruction process, the complex conjugate image is removed using the added carrier frequency through Hilbert transform (Fig.5). This results in a doubling of the imaging range compared to the original image. However, the focal depth of the lens is much smaller than the imaging depth, leading to a decrease in lateral resolution outside the depth of field (DOF) range. By focusing on imaging samples at different depths, extracting the focal plane, and using Gaussian weighted fusion (Fig.6), the imaging depth of the system is doubled from 1.97 mm to 3.94 mm, without being limited by the lens depth of focus. The sample structure information is clearly visible (Fig.7). When applied to biological tissues, the beam penetration depth is limited by the scattering and absorption properties of the tissues. The imaging depth is increased from 0.51 mm to 0.65 mm, with well-connected tissue signals in the vertical direction. The strip-like structure of the colon wall recess is clearly visible (Fig.8).  Conclusions   A method has been developed to improve the depth and quality of OCT imaging by combining the carrier frequency method to remove conjugate images and beam focusing at different depths of the sample. This method was tested on various samples, including multi-layer transparent films and biological tissues. Results showed that the effective imaging depth for transparent films doubled to 3.94 mm, while for colon tissue, the imaging depth increased from 0.51 mm to 0.65 mm, a 27.5% increase. The extended depth also allowed for clearer display of deep structural information in the sample.
Materials & Thin films
Newsletter
Verification of demodulation method for differential optical Doppler velocimetry data
Zhang Zhijun, Song Ran, Jiang Lili, Zhang Xinyu, Li Bingbing, Chen Shenggong, Su Juan, Wu Chi
2024, 53(4): 20240094. doi: 10.3788/IRLA20240094
[Abstract](26) [FullText HTML] (8) [PDF 1007KB](6)
  Objective  In the field of physical oceanographic research, seawater flow velocity is one of the key parameters, primarily measured using acoustic Doppler velocimeters. In recent years, laser Doppler technology has made significant advancement in seawater flow velocity measurement. Laser Doppler velocimetry, with its simple and integrable structure, is expected to be a complementary technique with acoustic Doppler velocimeters in marine applications. Compared to acoustic velocity measurement techniques, laser Doppler velocimeters offer several advantages: their shorter wavelength (in the micron range) allows for the study of smaller-scale water features, and they can resist noise interference generated by underwater vehicles when used with unmanned underwater vehicles. However, due to seawater absorption and scattering, the detected signal is extremely weak and buried in strong noise, posing challenges for Doppler signal demodulation. Moreover, limited by the sampling frequency, there exists an error between the peak position of the obtained data spectrum and the true frequency. Therefore, effectively removing noise interference and improving measurement accuracy are crucial for laser Doppler velocimeters. In this paper, an adaptive filtering algorithm is employed to denoise the collected signal, followed by fast Fourier transform to enhance the signal-to-noise ratio. Three peak-finding algorithms are compared, and the Gaussian-LM algorithm is selected to process the power spectrum of the signal, bringing the peak position closer to the real peak value and thereby improving the demodulation accuracy of the Doppler signal and significantly reducing the error caused by noise.  Methods  The principle of laser Doppler velocimetry is illustrated in Fig.1(a). A laser beam is split into two equal beams by an optical fiber splitter after passing through a single-mode optical fiber. These two beams are then collimated into parallel beams by a collimator and directed onto a plano-convex lens at the end, which focuses the parallel beams onto a specific point outside the instrument, generating interference fringes at this focal point. When particles in the water pass through these interference fringes, they scatter light, which is collected by the plano-convex lens and converted into parallel light. This scattered light is then collected by an avalanche photodetector and converted into an electrical signal, which is acquired by an oscilloscope. The acquired signal undergoes algorithm processing to demodulate the flow velocity. Fig. 1(b) is a field photo of the optical system prototype being tested in the Marine environment off Qingdao. The key to signal processing is accurately extracting the Doppler frequency shift from a large amount of noise, and the noise in the Doppler signal is non-stationary. Therefore, the least mean square error algorithm can be utilized to effectively denoise the Doppler signal. Fast Fourier transform shifts the focus of the research from the time domain to the frequency domain, where it is easier to analyze the regularity of the Doppler frequency. Further, the Gaussian-LM algorithm is employed to perform peak finding on the Doppler signal, obtaining accurate frequency information.  Results and Discussions  Through simulation, the optimal peak finding algorithm was selected. The Monte Carlo algorithm, Gaussian fitting algorithm, and Gaussian-LM algorithm were employed to perform peak finding on Gaussian signals with added noise, and their measurement accuracies were compared, as shown in Fig.2(a). Peak finding calculations were conducted on multiple datasets, and their standard deviations are illustrated in Fig.2(b). The results indicate that the Monte Carlo algorithm exhibited the lowest peak finding accuracy, while the Gaussian-LM algorithm demonstrated the highest accuracy. Moreover, the Gaussian-LM algorithm exhibited smaller standard deviation compared to other algorithms, with a lower fluctuation range, indicating greater stability. Therefore, the Gaussian-LM algorithm was chosen for peak finding in the Doppler signal. A comparative experiment on seawater velocity was conducted at the Zhongyuan Tourist Dock in Qingdao, China, using a home-made optical Doppler velocimetry (LDV) and an acoustic Doppler velocimeter (ADV model: SonTek Argonaut-ADV). Algorithmic research was carried out on the obtained seawater velocity measurement data. Considering the different sampling rates of the two instruments, the data were first averaged over 30 minutes. From Fig.3(a), it can be observed that the data before algorithm processing roughly align with the trend of velocity values measured by ADV, but there are still discrepancies. However, the data after algorithm processing shows a higher degree of fitting with the data measured by ADV. Fig.3(b) illustrates the errors obtained by ADV for the data before and after processing, and presents the calculation of the average error. Through error analysis, it shows that the average error between the pre-processed LDV and ADV velocity measurements was 0.2905 cm/s, while the average error between the post-processed LDV and ADV velocity measurements was 0.2163 cm/s, indicating a reduction in error of 25.5%.  Conclusions  The signal of light scattering from suspended particles in seawater is extremely weak. Extracting signals submerged in noise and demodulating them to obtain velocity information poses a challenge for accurate measurements with laser Doppler velocimeters. In this paper, demodulation algorithms based on velocity data obtained from experiments in the near-shore of Qingdao were studied. Initially, through simulation and optimization, the Gaussian-LM algorithm was selected as the peak finding algorithm. Subsequently, signal denoising was performed based on the Least Mean Square (LMS) algorithm on the actual velocity data obtained during sea trials, combined with the Gaussian-LM algorithm for peak finding, achieving high-precision demodulation. Comparative experiments between home-made laser Doppler velocimeter and a well-known commercial acoustic Doppler velocimeter indicate that the post-processed velocity measurement error based on this algorithm is 0.21 cm/s, representing a 25.5% error reduction compared to the pre-processing velocity measurement error.
Metal water-triple-point automatic reproduction control system for in-situ online calibration of temperature sensors
Qiao Zhigang, Gao Dexin, Zhang Muzi, Zhao Shanshan, Wu Jiali, Su Juan, Chen Shenggong, Jing Chao, Liu Hailing, Yang Bo, Wu Chi
2024, 53(4): 20240096. doi: 10.3788/IRLA20240096
[Abstract](44) [FullText HTML] (13) [PDF 960KB](10)
  Objective  The triple point of water refers to the state where water, ice, and vapor coexist simultaneously, with an equilibrium temperature of 273.16 K (0.01 ℃). In the International Temperature Scale, the triple point of water serves as the sole reference point for defining the thermodynamic temperature unit Kelvin, and it is one of the most important fixed points in ITS-90 [1-2]. The thermodynamic temperature reproduction of water's triple point is crucial for practical temperature measurements [3].The reproduction of water's triple point is achieved by freezing an ice mantle inside a triple point of water cell. Widely used in the ITS-90 guidelines are triple point of water cells with borosilicate glass or fused silica shells. Traditional reproduction methods include the ice-salt mixture cooling method, dry ice cooling method, and liquid nitrogen cooling method. These methods all require the cooling of the triple point of water cell using dry ice, liquid nitrogen, or other cryogenic media, followed by freezing the high-purity water inside the cell and then storing it in an ice bath. While these traditional methods offer high reproduction accuracy and good results, they are complex, operationally difficult, and demand high standards for operators and the environment, making them inconvenient for on-site calibration and integrated applications [2-3]. Addressing the limitations of traditional triple point of water cells and reproduction methods for in-situ applications, such as the on-site calibration of temperature sensors in the deep sea, this paper investigates a miniaturized triple point reproduction control system suitable for the automatic calibration of temperature sensors, based on a self-developed miniature metal water triple point cell.  Methods  This control system utilizes the principle of spontaneous phase transition of high-purity water in a metal water triple point container, combined with a thermoelectric cooler (TEC) based on the semiconductor Peltier effect and a temperature control circuit, to achieve the automatic reproduction and maintenance of the water triple point. Temperature phase transition monitoring is achieved through the use of thermistors and temperature detection circuits. By employing a dual thermistor setup and TEC in a closed-loop control, the system adjusts the driving power of the TEC based on the temperature difference detected by the feedback resistors, thereby realizing the automatic reproduction and maintenance of the water triple point.  Results and Discussions  Figures 1(a) and (b) respectively illustrate the control schematic of the automatic reproduction system for the metal water triple point bottle and a photograph of the actual metal water triple point bottle. The research employed a miniaturized metal water triple point bottle, utilizing the principle of spontaneous phase transition of high-purity water, along with a thermoelectric cooler (TEC) based on the semiconductor Peltier effect and a temperature control circuit, to achieve the reproduction and maintenance of the water triple point. High sensitivity thermistors combined with a temperature detection circuit were used for monitoring the phase transition of high-purity water. A closed-loop control consisting of dual thermistors and the TEC was utilized. Based on the temperature difference detected by the feedback resistors, the study investigated the cooling demand of the high-purity water phase transition and established a thermodynamic model for the triple point bottle cooling system. By appropriately adjusting the TEC's driving power, the state of the water triple point was reproduced and maintained for an extended period. The measurement results in Figure 2 indicate that, significant supercooling of the high-purity water inside the metal water triple point bottle was observed. It remained unfrozen at the liquid-solid phase equilibrium temperature (0 ℃) and suddenly underwent a phase transition when the temperature reached the transition temperature (approximately −7.3 ℃), causing a rapid increase in the internal trap temperature, which then stabilized, with a stability duration of 20 minutes and a temperature fluctuation of ±1mK. The analysis of the experiment demonstrates that the miniaturized triple point temperature automatic reproduction control system based on the metal water triple point bottle can achieve spontaneous phase transition of high-purity water and maintain a stable temperature plateau for a certain period, facilitating high-precision in-situ temperature calibration of temperature sensors.  Conclusions  This study indicates that combining the metal water triple point bottle with properly arranged temperature monitoring sensors, a TEC cooling system, and a refrigeration control circuit and algorithm can automatically reproduce and maintain the high-purity water triple point state for 20 minutes, with a temperature fluctuation of ±1 mK. This provides an accurate, stable, and sustainable environment for in-situ calibration of temperature sensors, serving high-precision in-situ temperature calibration in deep-sea and deep-space environments.
Invited paper
Silicon based hot electron short wave infrared detection technology (cover paper·invited)
Wen Xinhao, Jia Yu, Yu Leyong, Shao Li, Chen Hui, Xia Chaojie, Tang Linlong, Shi Haofei
2024, 53(4): 20240116. doi: 10.3788/IRLA20240116
[Abstract](2) [FullText HTML] (1) [PDF 6513KB](0)
  Significance  Short wave infrared detectors, as a very important type of detector, play a crucial role in sensing and obtaining target image information. Their notable features include the ability to penetrate smoke, high spatial recognition, all-weather working ability, and applicability in harsh weather conditions, making it widely applicable in multiple fields of national major needs and national economic development. In the military field, shortwave infrared detectors, with their unique night vision and covert reconnaissance functions, have become a key tool for enhancing combat capabilities at night and in adverse weather conditions. In the field of security monitoring, it provides strong technical support for video monitoring under low or no light conditions, significantly enhancing security capabilities. In terms of environmental monitoring, these detectors provide valuable data support for environmental protection and climate research by accurately measuring specific components in the atmosphere. In addition, in the medical field, the application of shortwave infrared detectors in disease diagnosis has opened up new paths for medical technology innovation. Therefore, in-depth research on shortwave infrared detectors has important practical significance.  Progress  This article systematically reviews the photoelectric conversion mechanism of Schottky photodetector, and summarizes and analyzes recent research results at home and abroad around the basic physical processes of hot electrons. This article first introduces the formation and basic characteristics of metal silicon Schottky junctions, and explores the three core processes of hot electron generation, transmission, and injection. Next, in terms of the generation of hot electrons, a review is conducted on the relevant work of researchers to improve the efficiency of hot electron generation through methods such as light absorption enhancement and thermal loss suppression. In terms of the transfer of hot electrons, the current proposed methods to control the initial position, initial energy and momentum, and mean-free path of hot electrons have been summarized to improve the transfer efficiency of hot electrons. In the injection method of hot electrons, strategies to improve injection efficiency such as multiple Schottky junctions and interface engineering were introduced. In addition, considering the crucial impact of dark current on detector performance, this article also explores current methods for suppressing dark current. Finally, this article provides an outlook on the future development direction of this field.  Conclusions and Prospects  Silicon-based hot electron detection technology holds the potential to broaden the response band of silicon to include the short-wave infrared band, while maintaining compatibility with silicon-based semiconductor processes. Its advantages, including low cost and high uniformity, bode well for its significant role in diverse fields such as military applications, security, and environmental monitoring. Looking ahead, it is imperative to delve deeper into the research of novel materials, structures, and mechanisms to further enhance the detector's performance. By focusing on developing new materials that can enhance the mean-free path of electrons and optimize the density of states, the transport efficiency of hot electrons can be boosted. Concurrently, the pursuit of innovative structures that efficiently absorb wide-spectrum infrared light, coupled with the optimization of the Schottky interface to increase hot electron injection efficiency and minimize dark current, is paramount. Moreover, exploring novel photoelectric conversion mechanisms that transcend the constraints of classical frameworks offers a promising avenue for pioneering advancements in infrared detection technology.
Advances of laser range-gated three-dimensional imaging (invited)
Wang Xinwei, Sun Liang, Zhang Yue, Song Bo, Xia Chenhao, Zhou Yan
2024, 53(4): 20240122. doi: 10.3788/IRLA20240122
[Abstract](2) [FullText HTML] (0) [PDF 8522KB](0)
  Significance   Traditional light detection and ranging (LiDAR) can obtain point cloud data of three-dimensional (3D) scenes, but it is often difficult to obtain high-quality intensity images. Therefore, a technical solution that combines LiDAR and cameras is usually used, where LiDAR senses 3D spatial information and cameras obtain high-definition texture images of the scene. However, this composite technical solution faces the problem of heterogeneous data fusion. For example, in self-driving and driver assistance systems there are different working distances of the two sensors under severe weather or low light level conditions, and it is hard to achieve effective data fusion, which leads to performance degradation or failure. With the advent of the artificial intelligence era, light ranging and imaging (LiRAI) that simultaneously obtains high-resolution intensity images and dense 3D images of targets and scenes, has become a development trend of LiDAR. That means a single sensor can realize light ranging and imaging instead of light detection and ranging, and thus the heterogeneous data mismatch problem of LiDAR and camera composite technology can be solved. In essence, laser range-gated 3D imaging (Gated3D) technology is a kind of gated LiRAI, since it can utilize a single gated camera to simultaneously obtain high-quality 2D intensity images and high-resolution 3D images. Gated3D has gained much attention in the applications of long-range surveillance, advanced driving assistance system and underwater imaging, owing to its long working distance, fast imaging speed, high resolution and the ability to suppress medium backscattering noise. Unlike traditional imaging methods that indiscriminately capture targets and backgrounds within the field-of-view, laser range-gated imaging selectively captures targets within a specific distance range-of-interest (ROI), which filters out medium backscattering noise in the imaging chain, as well as background noise outside the ROI, thereby increasing the imaging distance and enhancing the image quality. Moreover, different from traditional scanning LiDAR, the Gated3D technology employs gated cameras beyond megapixels, and thus offers spatial resolutions surpassing mechanical scanning LiDAR and outperforming flash LiDAR based on avalanche photodiode (APD) arrays. Over the past decade, there has been significant progress domestically and internationally in the development of Gated3D technologies. These advancements have led to the achievement of super range resolution 3D imaging, and promoted their applications.  Progress   This paper systematically reviews the advances of Gated3D technologies in conjunction with its applications across various fields. It introduces the working principles of different technologies such as time slicing, gain modulation and range-intensity correlation methods. Their imaging characteristics of working distance, range resolution, imaging speed and depth of field are discussed. In recent years, the applications of Gated3D technologies have been explored in remote surveillance, automatic driving, vegetation measurement, marine life observation, underwater obstacle avoidance and so on. The results indicate that the technology readiness level (TRL) of range-intensity correlation 3D imaging technology is relatively high, generally reaching TRL5-7. It can fully utilize the correlated information between target distance and image intensity in gated images, enabling real-time super-resolution 3D imaging with fewer gated images. The application of deep learning techniques has further improved the performance of range-intensity correlation method. Finally, the paper analyzes the challenges and further development directions and application prospects faced in laser range-gated 3D imaging technology.  Conclusions and Prospects   We believe that LiRAI will be the trend of LiDAR. LiRAI refers that with the help of active illumination, it does not rely on ambient light level, and uses a single sensor to simultaneously obtain high-resolution intensity images that reflect the radiation characteristics and texture characteristics of targets, as well as dense point cloud data/3D images that reflect the 3D spatial information of targets and their scene, and has long working distance with a certain ability of imaging through scattering medium. The Gated3D technology utilizes a single gated camera to simultaneously obtain high-quality 2D intensity images and high-resolution 3D images. The pixels in 2D images correspond one-to-one with the voxels in 3D images, inheriting the technical advantages of laser range-gated imaging through scattering medium. It has great potential to achieve high-performance LiRAI. The development trends of Gated3D are expected to focus on long-distance imaging in fog, rain, snow, smoke, dust, and underwater conditions, high-resolution fast 3D imaging in large depth of view, and high-performance color LiRAI. In the future, with the support of computational imaging and artificial intelligence, Gated3D will achieve faster, higher precision, longer working distance, more imaging functions, higher sensing dimensions, stronger adaptability to complex environments, and thus meet diverse scenario task requirements.
Lasers & Laser optics
Polarization manipulation in a quasi-continuous c-cut Er,Yb:YAl3 (BO3)4 laser
She Kai, Xie Pengjian, Zhou Pengfei, Wei Yong, Xu Shan, Li Bingxuan, Zhang Ge
2024, 53(4): 20230693. doi: 10.3788/IRLA20230693
[Abstract](3) [FullText HTML] (3) [PDF 2055KB](0)
  Objective  By constructing a resonant cavity model, the theoretical calculations reveal the spot singularity feature after coherent superposition of two intrinsic modes in an isotropic solid-state laser. And it is experimentally verified for the first time that the polarization state of the 1.6 μm output laser can be effectively manipulated in diode-pumped quasi-continuous c-cut Er,Yb: YAl3(BO3)4 lasers without using any specific intracavity optical polarization selector element. The transformation from a partially polarized state to a stable line-polarized state with a switchable orthogonal special case of line polarization direction is achieved, all with a polarization extinction ratio of 21 dB. It is also verified that the line-polarized output of the laser originates from the coherent superposition of two orthogonal eigenmodes by spot comparison. This thesis provides a reliable scheme for the direct output of line-polarized light from c-cut Er,Yb: YAl3(BO3)4 lasers with the modulation of polarization states.  Methods  The resonant cavity model is shown (Fig.1), where θ is denoted as the angle between the coordinate systems of the intracavity loss anisotropy ∆φ and the phase anisotropy ∆t. The experimental principle diagram of the c-cut Er,Yb: YAl3(BO3)4 polarization controlled laser is shown (Fig.3). The laser cavity has a length of approximately 32 mm, and the laser crystal is mounted on a water-cooled copper heat sink at 288.1 K. The output laser beam is split into two paths using a beam splitter (Splitter). In one of these paths, a Glan-Laser prism is employed as a polarizer, and a laser power meter (Thorlabs, Inc.) is used to measure the laser power after passing through the polarizer. Both measurements are utilized to determine the polarization state of the laser output. The other output laser uses an infrared camera and a spectrometer to detect the beam quality and emission wavelength of the output laser with different polarization states. In the experiment, by re-aligning the resonator, the output coupling mirror is slightly tilted, and the polarization state of the output laser is transformed from unbiased to linear polarization state, and the maximum polarization extinction ratio of linear polarization output is up to 21 dB.  Results and Discussions  By re-aligning the resonator, the output coupling mirror is slightly inclined, and the output laser polarization state is linearly polarized, and the polarization direction can be a set of orthogonal directions, and the polarization extinction ratio can reach 21 dB (Fig.4). Measurement of output laser wavelengths in different polarization states is carried out by spectrometer. When the polarization state changes, the center wavelength of the output laser fluctuates around 1 602 nm, with a maximum variation of less than 1 nm (Fig.6). The laser spot is detected after passing through the analyzer. When the angle of the Glan-prism is 0°, the laser power through the Grand prism is minimum. And when it is rotated and shifted by 0°, the line polarized light passes through the Glan-prism and the spot shows an elliptical distribution. And when the offsets are equal but in opposite directions, the ellipsoid-like spots can be observed, whose long axes are orthogonal to each other. The essence is that two orthogonally polarized eigenmodes undergo coherent superposition to produce a linearly polarized light output (Fig.7). Measurement of the beam quality factor M2 of different polarization states of the output laser is carried out by beam quality analyzer. The beam quality factor M2 of the partially polarized laser output when the laser is running freely and the linearly polarized laser output in two different polarization directions is maintained at about 2.0, and there is no obvious difference in beam quality degradation when the laser output linearly polarized laser (Fig.8),  Conclusions  In this study, theoretical calculations are performed by constructing a resonant cavity model to simulate the singular features of the spot profile after coherent synthesis of the intrinsic mode. And it is experimentally verified for the first time that in a quasi-continuously pumped c-cut Er,Yb: YAl3 (BO3)4 laser operated at 1.6 μm, the polarization state of the output laser can be switched from a partially polarized state to a stable linearly polarized state by adjusting the degree of tilt of the output coupling mirror, and the polarization extinction ratio reaches 21 dB, with a pair of orthogonal linear polarization directions that can be switched. In the process of adjusting the angle of the output coupling mirror, the output band of the output light is not changed, and the loss of output power of the generated linearly polarized beam is around 7%, while the quality of the beam in different polarization states does not change significantly. By spot comparison, it is verified that the direct output of line-polarized light from the laser is achieved by coherent synthesis of two orthogonal eigenmodes. This method provides a reliable solution for the direct output and modulation of line polarization of c-cut Er,Yb: YAl3 (BO3)4 and other isotropic lasers.
High-precision temperature control system design for laser diode
Ye Mao, Du Ensi, Wang Qiuwei, Zhao Yiqiang
2024, 53(4): 20230713. doi: 10.3788/IRLA20230713
[Abstract](1) [FullText HTML] (1) [PDF 3112KB](0)
  Objective  Laser diode has been widely used in laser scanners, optical storage, laser printers, optical fiber communications, laser pointers, laser spectroscopy and other fields because of their light weight, high efficiency, small size, low power drive, high conversion efficiency, and direct modulation. As a high-efficiency photoelectric converter, temperature has a great impact on its performance and life; If it is serious, it will cause the increase of threshold current, the shift of emission wavelength, the reduction of service life and other adverse effects. In order to prolong the service life of laser diode and stabilize the functional parameters, a temperature control system with high precision, fast response, high stability and good reliability must be designed to control the temperature of semiconductor lasers, so as to meet the needs of different environments of semiconductor lasers. Therefore, a digital analog hybrid temperature control system based on FPGA is designed.  Methods  The temperature conditioning method of variable temperature control zero point is adopted. This method is based on iteration and multi-objective optimization algorithm to determine the optimal number of zero points, the optimal position of zero points, and the minimum number of ADC bits (Fig.3). A three-wire Wheatstone bridge is adopted to reduce the influence of wiring resistance and connection point resistance (Fig.2). The control method of anti-windup PID (AWPID) is adopted to reduce overshoot and speed up response (Fig.6). The Full bridge Synchronous Buck drive circuit (H-BUCK) adopts T-type capacitance network to reduce the ripple (Fig.7).  Results and Discussions  After six iterations of the multi-objective optimization algorithm, the constraint conditions for the temperature conditioning method of variable temperature control zero point were met. The optimal number of zero points within the current temperature control range was determined to be six, with the optimal zero point positions at −33 ℃, −12 ℃, 8 ℃, 29 ℃, 49 ℃, and 70 ℃. Additionally, the minimum number of ADC bits required was found to be 10 (Fig.5). The temperature conditioning method of variable temperature control zero points reduces the requirements for amplifier and ADCs. The test results show that the accuracy in the whole temperature range is 0.02 ℃ (Tab.4), which is an improvement of 89.7% compared to the accuracy of a single fixed zero point (with the zero point set at 25 ℃) (Tab.4). The control strategy of AWPID reduces the overshoot from 9.13% to 1.5%, and shortens the stabilization time from 41 s to 30 s.  Conclusions  In order to improve the accuracy, integration, response speed and reduce the cost of laser diode temperature control system, a digital analog hybrid temperature control system based on FPGA is designed. A three-wire Wheatstone bridge is used. Aiming at the nonlinear error of thermistor and bridge, a temperature conditioning method of variable temperature control zero point is adopted. The method is based on iteration and multi-objective optimization method to improve the temperature control accuracy. The measured results indicate that the temperature control system employing variable temperature control zero points achieves a temperature control accuracy of ± 0.02 ℃ within the temperature range of −45 ℃ to 75 ℃. This accuracy is an 89.7% improvement compared to the maximum temperature control accuracy of 0.1951 ℃ achieved by the temperature control system with a single temperature control zero point. Another advantage of the variable temperature control zero point is that it reduces the system cost. By reducing the voltage range of the Wheatstone bridge, it reduces the requirements for the common mode range of the instrumentation amplifier. By increasing the number of zero points, it reduces the ADC bits. By reducing the temperature range at each zero point, it reduces the signal-to-noise ratio (SNR) requirements of the ADC. The H-BUCK uses a T-type capacitor network to further reduce the ripple and increase the stability, Extend the service life and accuracy of thermoelectric cooler (TEC). According to the characteristics of large temperature lag and high delay, the temperature control strategy adopts the AWPID automatic control method to reduce overshoot and accelerate speed. The test results show that, compared with the PID control strategy, the AWPID control strategy reduces the overshoot from 9.13% to 1.5%, and improves the stability time from 41 s to 30 s. The stability test shows that the temperature control system can maintain the temperature control accuracy of ± 0.02 ℃ for a long time, meeting the requirements of high accuracy. The system has the characteristics of high precision, high integration and low cost, and can meet the multi field and requirements of high precision laser diode temperature control system.
Research on distributed maximum power point tracking system for laser wireless power transmission (inner cover paper)
Chen Yuchao, Deng Guoliang, Yang Huomu, Sun Yanfengxu, Gou Yudan, Wang Jun, Zhou Shouhuan
2024, 53(4): 20230689. doi: 10.3788/IRLA20230689
[Abstract](6) [FullText HTML] (2) [PDF 2867KB](0)
  Objective  In Laser Wireless Power Transmission (LWPT) system, Maximum Power Point Tracking (MPPT) technology is a key factor to improve the power transfer efficiency by adjusting the circuit parameters at the receiver in real time, so that the output power of the laser photovoltaic cell reaches the maximum value. Due to the error of the aiming system and the obstruction of the object, the photovoltaic cell array will be subjected to uneven laser irradiation, and the photovoltaic cell array will generate different photogenerated currents, which leads to the current mismatch between the photovoltaic cells in the array, and the output power-volt characteristic curves is showing multi-peak, and the output power will be significantly reduced. The Distributed Maximum Power Point Tracking (DMPPT) technique can effectively reduce the current mismatch between the photovoltaic cells in the string of the array. Aiming at the problem of current mismatch in LWPT system, DMPPT technology is applied to LWPT system according to the characteristics of high laser irradiation power density but low power of Gallium Arsenide (GaAs) cell. It is proposed to replace the traditional Boost circuit with a Parallel-Type Boost circuit (PT-Boost) to reduce the input current ripple, so as to effectively improve the tracking accuracy of each MPPT submodule of DMPPT system and the overall circuit efficiency of DMPPT.  Methods  The working state of PT-Boost circuit is theoretically analyzed (Fig.5). A theoretical model of the DMPPT system is established and its output characteristice is analyzed; The above theoretical model is used to build a set of DMPPT simulation system, simulate and analyze the input current ripples of the PT-Boost circuit relative to the Boost circuit in the MPPT sub-module of the DMPPT system, and set up different LWPT scenarios to verify the feasibility of the DMPPT system. Finally, the DMPPT experimental platform (Fig.8) is built to test the specific performance of the DMPPT system.  Results and Discussions  The simulation results show that under the same GaAs cell model parameters, the current ripple is reduced and the MPPT accuracy is higher when utilizing the PT-Boost circuit as a DC/DC converter compared to the traditional Boost circuit (Fig.6). And under different shading conditions, the difference between the maximum power point of the DMPPT output and the total power of the GaAs battery array is within 2%(Fig.7). which shows that it is feasible to utilize the DMPPT system to reduce the current mismatch of each cell among the strings of the GaAs battery array. A DMPPT test system was established, and the experimental results showed that the tracking efficiency of the DMPPT system based on the PT-Boost circuit was improved by 3.6% to 93.5% compared with the traditional Boost circuit (Fig.9). Based on the above study, laser wirelessenergy transmission scenarios with 0%, 25% and 50% shading were set up, and the overall circuit efficiency of DMPPT reached 93%, 92.6% and 90.3%, respectively (Fig.10).  Conclusions  In the study of LWPT system, the non-uniform distribution of laser irradiation leads to the occurrence of multi-peak conditions in the output of photovoltaic cells array, and the output power decreases. In this paper, The DMPPT system is used to reduce the current mismatch between cells in the photovoltaic cell array string, thus reducing the occurrence of multi-peak conditions and improving the output power. In addition, the PT-Boost circuit is utilized to replace the traditional Boost circuit and the phase relationship between the branch currents is changed by shunting in parallel, which reduces the current ripple of the input current of circuit and effectively improves the tracking accuracy of the DMPPT. The simulation verifies the feasibility of the DMPPT system to reduce the output current mismatch of each cell between battery array strings under different laser wireless energy transfer scenarios. The DMPPT test system was built, which simulates different shading scenarios by blocking part of with the sunshade. The test results show that the overall circuit efficiency of the DMPPT reaches 93%, 92.6% and 90.3%, respectively. Compared with using a traditional Boost circuit as the DC/DC converter of the DMPPT system, the DMPPT system has an improvement of about 2% in the laser irradiation non-uniformity scenario by using a PT-Boost circuit as the DC/DC converter of the DMPPT system. The results are instructive for the maximum power point tracking of laser wireless power transmission under uneven laser irradiation scenarios.
Experimental research on laser detection and tracking of unmanned aerial vehicles under flame and smoke
Yang Zhen, Guo Qianqian, Liu Manguo, Jiao Dan, Chen Haohui, Zhang Yong, Zhang Jianlong
2024, 53(4): 20230700. doi: 10.3788/IRLA20230700
[Abstract](1) [FullText HTML] (2) [PDF 2764KB](0)
  Objective  In recent years, with the gradual opening of low altitude airspace and the continuous development of unmanned aerial vehicles (UAVs) technology, rapidly developing drones have been widely used in military, agriculture, transportation, public safety and other fields. However, due to the characteristics of simple operation, low cost, difficulty in supervision, and strong breakthrough ability, UAVs have caused many safety accidents and violent threats to social security and stability. Therefore, there is a strong demand for countermeasures against UAVs. With the development of laser weapons, the technology is becoming increasingly mature and has huge advantages in anti-UAVs. Under the action of high-energy laser, the target is prone to burning and catching fire, resulting in the target being blocked by flame smoke. Currently, visible light and infrared detection methods are easily affected by severe interference from flame and smoke, resulting in poor or unclear imaging of targets. Therefore, aiming at the problem that visible light and infrared detection methods cannot achieve stable tracking and aiming under flame and smoke, the paper studies a high-precision aiming scheme based on active lidar system.  Methods  The paper proposes a high-precision aiming scheme based on the active lidar system. Firstly, the laser characteristics under the flame and smoke were analyzed. Secondly, a lidar detection system based on APD single photon detector was designed. Then, the theoretical curve of the detection probability was simulated and analyzed as a function of the laser pulse energy. Finally, a photon imaging system based on InGaAs-SPAD was built, and the imaging system was tested and experimentally conducted indoors.  Results and Discussions  In the absence of the influence of fire and smoke, the X and Y coordinate trajectory curves of the target centroid based on visible light image tracking and range image tracking are basically consistent. By calculating the angular radian size of the target centroid position based on distance image tracking relative to the visible light tracking target centroid position, the X-coordinate angular radian curve and Y-coordinate angular radian curve were obtained (Fig.8). It is known that the average angular radian of the X-coordinate is about 0.55 mrad, and the average angular radian of the Y-coordinate is 0.53 mrad. By using a lidar system to collect range profiles under smoke, when the gating delay is 0 ns, the target is obstructed by smoke on the collected range profiles, making it impossible to obtain the image of the UAV (Fig.9). This is because when photons scattered by smoke cause detector response, photons reflected by the target will not cause detection starting point response, thus unable to obtain the range profiles of the target. Subsequently, through debugging under the same conditions, we obtained the range profiles for filtering out smoke obstruction when setting the gating delay to 43 ns (Fig.10). So active laser imaging can filter out the influence of smoke. In the experiment, the UAV was exposed to strong light during its movement, and under the sudden change conditions, the target lost track (Fig.12). Due to the correlation between the loss of target tracking and the impact of the blue part of the range profile mutation, when tracking based on the range profile, the influence of firelight noise on the target is filtered out, and the target is tracked (Fig.13). The deviation of the X and Y coordinates of the tracking rectangle center from the target centroid is 0.58 mrad and 0.39 mrad, and the target can be stably tracked within the range of the tracking rectangle under sudden changes in firelight.  Conclusions  The analysis of the optical characteristics of the firelight background and the laser attenuation characteristics of smoke shows that the radiance at 1 064 nm wavelength within the 10 nm narrowband filter passband is approximately 0.014 W/cm2; The transmittance range of laser in smoke is 60%-85%; In the case of backward scattering, fr is approximately 0.2-8 in the rough range of 0.2-0.8 μm. A lidar detection system is designed based on APD single photon detector, simulation analysis of the theoretical curve of detection probability changing with laser pulse energy. Under the conditions of this experiment, the optimal range for obtaining laser pulse energy is 0.75-4 mJ. Based on the above analysis, experimental verification of UAV lidar detection was conducted under smoke and fire backgrounds. Under the background without the influence of fire and smoke, the relative offset of the target centroid position based on range profile tracking to the target centroid position based on visible light image tracking is less than 0.55 mrad. Under simulated smoke conditions, clear contour target distance profiles were obtained by filtering out smoke effects through gating delay; Under simulated firelight mutation conditions, the X and Y coordinates of the tracking box center deviate from the target centroid by 0.58 mrad and 0.39 mrad. The experimental results show that this scheme can achieve imaging of unmanned aerial vehicles in the background of flame and smoke, and the Mean-Shift algorithm is used for range profile tracking, which compensates for the shortcomings of lighting mutation and easy loss of tracking when the background is similar to the target based on visible light image sequence tracking.
Temperature adaptability of optical middle cabin of airborne high-energy laser system based on cage-type structure
Li Xiang, Zhou Chen, Zhu Yongqi, Dong Keyan, Gao Liang, An Yan, Xi Wenqiang, Liu Yuhai
2024, 53(4): 20230663. doi: 10.3788/IRLA20230663
[Abstract](4) [FullText HTML] (1) [PDF 2792KB](0)
  Objective  The high-energy laser system is a directed energy system that realizes damage to a target by firing a high-energy laser beam at the target and generating a very high energy density on the surface of the target. Installed on a carrier aircraft, it can realize rapid detection and precise strike on the target by using the high mobility of the carrier aircraft, and plays an important role in the field of modern optoelectronic confrontation. The optical middle cabin is an important part of the high-energy laser system, and its main role is to realize the capture and tracking of the target. Due to the size limitations of the optical middle cabin for a typical small diameter cylindrical envelope, we can't use the traditional single-layer layout of the optical path, and put forward the "optical path stacking", cage-type layered structure (Fig.3), through the 45° mirrors layered transmission of the optical path, to solve the problem of the system volume is too large and meet the system requirements. However, the optical middle cabin contains the fine tracking and the main laser branch, and it is difficult to ensure the consistency of the optical axis when the temperature changes, it is necessary to simulate and analyze the optical middle cabin to explore the temperature adaptability of the optical middle cabin.  Methods  The optical-mechanical-thermal integration simulation analysis is carried out for the optical middle cabin, and the finite element simulation analysis model is established (Fig.4) to obtain the mirror distortion (Fig.5). Because the mirror deflection in the mirror distortion will cause the shift of the optical axis, the method of chi-square coordinate transformation is used to calculate the mirror deflection amount, and the mirror deflection amount of a single mirror is obtained (Tab.4). Later, the error transfer matrix of the optical axis by mirror deflection is proposed according to the theory of prism mounting error transfer matrix, and the deflection amount of each mirror is brought in to obtain the azimuthal deviation of the optical axis of the system's fine tracking branch and the main laser branch to be 109.634 μrad, and the pitch deviation to be 132.952 μrad.  Results and Discussions   The optical axis deviation of each optical axis in the optical middle cabin should be less than 150 μrad, system wave aberration should be less than 1/12λ@632.8 nm. After simulation analysis, the system optical axis deviation is obtained to meet the index requirements. In order to verify the accuracy of the simulation results, the optical middle cabin is tested by using a large diameter collimator to build a light path, and using an air conditioner to warm up the overall environment to 25 ℃, the position of the fine tracking spot and the main laser spot is detected, and the azimuthal deviation of both is calculated to be 104.019 μrad, and the pitch deviation is 125.009 μrad, which is 5.40% and 6.35%, respectively, and verify the reasonableness of the simulation results. After that, the optical middle cabin is placed between the interferometer and the high-precision reflection, and the wave aberration of the optical middle cabin system is detected by changing the environment temperature, and the worst wave aberration is 1/15λ, which meets the requirements of the index.  Conclusions  In order to meet the lightweight and miniaturization requirements of airborne equipment, we adopt the idea of "optical path stacking" and propose a cage-type layered structure, and carry out structural design of the optical middle cabin to meet the system volume requirements. In order to investigate the temperature adaptability of the optical middle cabin, finite element simulation analysis of the optical middle cabin is carried out to obtain the mirror distortion, and then the displacement of the rigid body in the distortion is calculated to obtain the deflection of the mirror. According to the theory of prism mounting error transfer matrix, the error transfer matrix of the optical axis deflection by mirror deflection is proposed, and the optical axis consistency deviation between each branch is calculated, and the optical axis azimuthal deviation of the fine tracking branch and the main laser branch is obtained to be 109.634 μrad, and the pitch deviation is obtained to be 132.952 μrad. Experiments are carried out in the laboratory, the experimental results show that the azimuthal deviation of the fine tracking branch and the main laser optical axis is 104.019 μrad, and the pitch deviation is 125.009 μrad, which are 5.40% and 6.35% of the simulation results, respectively, and verify the reasonableness of the simulation results.
Experimental study on the jamming of high-repetition rate laser to rangefinder
Zhang Lan, Wan Yong, Liu Quanxi, Long Xing, Zhang Xiaochuan, Wu Huanxin, Yi Xuebin, Lu Delin, Gao Jing
2024, 53(4): 20240002. doi: 10.3788/IRLA20240002
[Abstract](2) [FullText HTML] (2) [PDF 2889KB](0)
  Objective  Because of the advantages of small size, fast measurement speed, low consumption and high ranging accuracy, pulsed laser rangefinder is widely used in a wide range of military equipment. Therefore, it is crucial to ensure that it is impossible to obtain the correct target distance information. Jamming methods for laser rangefinder can be divided into two kinds: deception jamming and blinding jamming. When the distance between enemy and us is far away, the blinding jamming has higher requirements on the performance of laser and the accuracy of matching tracking equipment, which will lead to an increase in cost and difficulty. So deception jamming is more used in the interference of rangefinder. And high-repetition rate laser technology is always employed in it. At present, most of research on this technology are theoretical analysis and lack of experimental verification. To fill the gap in this part, the influence of high-repetition rate laser power and frequency on jamming efficiency of pulsed laser rangefinder at a fixed distance has been explored in experiments. Meanwhile, the effective jamming rate of laser with different repetition frequency at different distances has also been researched. These studies have a certain reference value for practical engineering application.  Methods  The experimental device is composed of a laser rangefinder, a high-repetition rate laser jammer and a protected target (Fig.3). Because the high repetition rate laser jammers are positioned near the protected target, the distance between them and the protected target can be ignored when long-range jamming is carried out. The distance between target and rangefinder without jamming has been measured first. When the frequency of jamming laser is 150 kHz, this distance is measured at different pump currents. Above tests have been repeated respectively at frequencies of 30 kHz and 50 kHz. Effective jamming rate of laser with different frequencies and distances has also been investigated.  Results and Discussions  The output power and effective jamming probability can be obtained by changing the pump current when the frequency of jamming laser is 150 kHz (Tab.2). With the pump current increasing to 11.5 A and above, the effective jamming probability increases to 100%. At this time, the minimum value of ranging results is distributed near the blind area of rangefinder, and the maximum value gradually decreases with the power growing (Fig.5). By fitting the distribution of ranging results, it is found that jamming results follow a normal distribution（Fig.6）. And with the increase of output power, the distribution center gradually approaches to the minimum jamming distance which is 1km. Adjust the repetition frequency of the jamming laser to 50, 30 kHz respectively to repeat the above experiments. The minimum distance is still distributed around the blind area of the rangefinder and doesn’t change with the change of frequency. But the maximum value decreases with the increase of frequency (Fig.7 & Fig.8). The distribution of jamming ranging results still follows normal distribution, and the distribution center also tends to the minimum jamming distance which corresponds to laser frequency（Fig.9）. When the pump current is 12 A, the frequency of laser is changed to explore the effective jamming probability at different jamming distances. The higher repetition frequency of jamming laser is, the minimum jamming distance will be shorter and the effective jamming range will be larger (Tab.3).  Conclusions  Combining theoretical analysis and field test, the effective jamming probability is taken as the criterion of efficiency evaluation, and a method to judge whether high-repetition rate laser jamming interferes effectively with pulsed laser rangefinder is proposed. The influence rules of the power and repetition frequency of high-repetition rate laser on distance measurement of pulsed laser rangefinder at different distances are obtained. When the actual jamming distance is farther than the minimum jamming distance, if the power is insufficient, there won’t be any jamming effect. When we continue to increase the power of the jamming laser in the successful jamming state, the minimum measured distance is still distributed around the blind area of the rangefinder, and ranging results tend to follow a normally distributed pattern. The bigger the power is, the distribution center will be closer to the minimum jamming distance which corresponds to laser’s frequency. When the jamming power is sufficient, the higher the jamming laser’s frequency is, the effective jamming range will be larger. Therefore, in practical engineering applications, it is necessary to improve the output power and frequency of jamming laser, the aiming accuracy of jamming equipment as much as possible.
The factors influencing the success probability of near-infrared space debris laser ranging detection
Zhang Mingliang, Wen Guanyu, Fan Cunbo, Guan Bowen, Song Qingli, Zhang Haitao, Wang Shuang
2024, 53(4): 20230695. doi: 10.3788/IRLA20230695
[Abstract](4) [FullText HTML] (3) [PDF 1511KB](0)
  Objective  More and more space debris has seriously threatened the safety and normal operation of spacecraft in orbit. The removal of space debris is imperative, and the primary task is to conduct high-precision orbit determination. So in recent years, space debris laser ranging has become a research hotspot. At present, the main problems of space debris laser ranging are poor orbit accuracy and few observation time. Therefore, a theoretical formula for the success probability of space debris laser ranging detection is established in this paper. Combining with the solar orientation information and target orbit prediction information, the relative difficulty of target detection can be judged in real time, providing a certain reference for observation. This method can improve the observation efficiency and orbital accuracy of space debris, and increase its total observation time.  Methods  Based on the RADAR equation and the daytime noise estimation formula, a calculation method for the success probability of space debris laser ranging is derived. The influence of atmospheric transmission and sky background noise in this method is simulated. On this basis, the effects of sun altitude angle, target zenith angle, target orbital distance and target cross-sectional area on the success probability of target detection are simulated.  Results and Discussions  The success probability of target detection decreases with the increase of the sun altitude angle, and the relationship is basically linear. That is, the higher the sun altitude angle is, the greater the system noise and the higher the false alarm rate is, resulting in a lower success probability of target detection (Fig.6). The success probability of target detection gradually decreases with the increase of target zenith angle. When the target zenith angle increases to 70°, the success probability of target detection begins to decrease rapidly. Therefore, in the actual measurement of laser ranging, it is difficult to detect the target when the elevation angle is lower than 20° (Fig.7). The success probability of target detection of the system decreases sharply with the increase of orbital distance (Fig.8). When the orbital distance of the target is fixed, the success probability of target detection increases with the increase of the cross-sectional area of the target, which is approximately linear (Fig.9).  Conclusions  The success probability formula of target detection established above, combining with the sun orientation information and target orbit prediction information, can determine the relative difficulty of target detection in real time, and provide a certain reference for observation. This method can improve the observation efficiency and orbit accuracy of space debris, and increase its total observation time. At the same time, the method can be extended to other stations, especially in the new automatic space debris observation station, which has great application value. Recently, Europe has built a lot of low-cost, miniaturized and intelligent mobile stations, aiming to comprehensively improve the total observation time of space debris. For automatic unattended stations, this method will help to select observation targets, thus greatly improving the observation efficiency of the system.
Materials & Thin films
Design and fabrication of wide angle phase control mirror
Li Daqi, Liu Baojian, Yu Deming, Duan Weibo, Liu Dingquan
2024, 53(4): 20230721. doi: 10.3788/IRLA20230721
[Abstract](1) [FullText HTML] (1) [PDF 1718KB](0)
  Objective  In quantum communication, such as quantum key distribution, quantum entanglement, and quantum teleportation, photons are controlled to a specific polarized direction for transmission and decoding. There is a need to establish an effective and stable link of quantum channels between the transmitter and receiver to maintain max channel efficiency and reduce quantum bit error rate. Therefore, it is necessary to do phase control on the thin film optical components in the optical system. In published literatures, most phase control mirrors are designed with a single incidence angle (such as 10°, 22.5°, 35° and 45°), which cannot meet the requirement of optical-mechanics system used in new generation of quantum communication working at medium to high orbit any longer. This research developed a dielectric reflector with high reflectivity and wide angle range for efficient energy transfer and precise phase control, which has wide application prospects for this type of thin film component in next generation of quantum communication systems working at medium to high orbit.  Methods  The design method is as below. Two materials with different refractive index are selected, the values of which are referred as H and L respectively. (HL) ^ n is used as the basic film system mechanism, and the design value of wavelength is determined based on the signal channel at the center of the cutoff band. Multiple layers equivalent to d1L d2H d3L or d1H d2L d3H for phase control are applied on the surface layer of the film base structure. The initial film structure is: G | (HL) ^ 14 d1H d2L d3H d4L d5H d6L d7H d8L | Air, where G is the substrate , d1-d8 are the thickness coefficients of each film layer respectively. Optimal goals are set based on task indicators, and optimization algorithms are used such as Global Modified LM or Global Simplex for optimal iterations to change film thickness to obtain the best design result. The preparation of this product was completed on Lab900-plus vacuum deposition machine produced by Leybold Company in Germany. The equipment is equipped with two e-type electron guns, with SiO2 using a circular crucible and Nb2O5 using a seven hole crucible. Equipped with a 4-probe quartz crystal oscillator physical thickness control system, OMS5100 optical automatic control system, and a Veeco RF ion source with a grid aperture of 12 cm. The sample is Φ 50 ×5 mm quartz substrate.  Results and Discussions  During the deposition, precisely controlling the layer of high sensitivity in the film system is needed. So the film thickness fitting analysis is carried out in terms of the evaporation consumption of the crucible material and crystal oscillator parameter correction, which ensured the success of development in the end. The results show that the reflectivity of the reflector reaches over 99.3% at 780 nm at incidence angles of 37.5°, 45°and 52.5°, and the phase difference is less than 3° (Fig.6), meeting the task requirements. It also passed the environmental reliability and firmness tests (Tab.6).  Conclusions  The wide angle phase control mirror uses Nb2O5 and SiO2 as high and low refractive index materials, quartz as the substrate, and a combination film system of high reflectivity film layer and multi layer phase control film as the initial film system. Optimization algorithms such as Global Modified LM or Global Simplex are used to design high reflectivity and phase control mirror in wide angle range. On Lab900-plus, the equipment of Leybold in Germany, by combining electron beam evaporation with Veeco RF ion source assisted deposition, deposition process was optimized. By combining optical monitoring and crystal oscillator monitoring, the development was successful. The results show that at 780 nm, with an incidence angle range of 45° ± 7.5°, the reflectivity is greater than 99.3%, and the phase difference is controlled within 3°. The product passed environmental reliability tests. The development of this product can enable the design of quantum communication optical systems with a wide angle field of view. How to ensure the product performance over the lifetime is the focus of next stage of work.
Preparation of MCNO thin films by solid-state reaction of oxide-metal multilayers
Zhao Yuanyuan, Wang Yuanyuan, Wang Rongxin, Wang Zhipeng, Zhu Yu, Song Helun, Xiang Yang
2024, 53(4): 20230723. doi: 10.3788/IRLA20230723
[Abstract](1) [FullText HTML] (1) [PDF 1680KB](0)
  Objective  Microbolometers offer advantages such as wide working range, fast response time, and simple device structure. The working principle is that infrared radiation incidents on the absorption layer, generating thermal energy which is then transferred to the thermosensitive material. The thermosensitive material changes its electrical resistance upon absorbing the heat and thus produces a change in output signal detected by the readout circuit. The thermosensitive layer has a significant impact on the performance of microbolometer. Currently, the main industrial materials for the thermosensitive layer are vanadium oxide and amorphous silicon, with a general temperature coefficient of resistance (TCR) of −2%/K. Compared to vanadium oxide and amorphous silicon, manganese cobalt nickel oxide (MCNO) with high TCR coefficients has great potential in uncooled infrared detector applications. Unlike most MCNO thin films prepared by magnetron sputtering of a stoichiometric compound, this paper reports the synthesis of MCNO by combining magnetron sputtering and electron beam evaporation. MnO2 and Co2O3 were co-sputtered initially, followed by e-beam evaporation of Ni. Multilayers of MnO2-Co2O3 oxides and Ni were thus deposited alternatively and subjected to in-situ and/or post-annealing to promote interdiffusion and the formation of MCNO compound. The objective was to investigate the feasibility of the new fabrication method based on solid state synthesis. The prepared thin films showed good crystallinity and negative temperature coefficient of resistance, indicating that the proposed method could be applied to further fine-tuning the compositions of MCNO thin films in the future.  Methods  Thin films were prepared using a magnetron sputtering-electron beam evaporation hybrid method using a laboratory-developed high vacuum coating unit, where an e-gun and two magnetron sputtering targets coexisted in the same chamber. The sample stage was controlled by a stepper motor for 180-degree rotation, thus enabling alternative up-facing magnetron co-sputtering and down-facing electron beam evaporation in a single fabrication process. Sapphire substrate was cleaned with acetone, ethanol, and deionized water sequentially, dried up with blowing N2 gas, and pre-heated in the vacuum chamber at 450 ℃ for one hour prior to the deposition. To study the effect of post-annealing on the performances of MCNO thin films, the as-deposited thin film samples were first annealed in-situ in the vacuum chamber for an hour, then annealed in a tube furnace in the air at 750 ℃, 850 ℃, and 950 ℃ respectively. The surface morphology of the resultant MCNO thin films were characterized using scanning electron microscopy (SEM). The crystal phase structure was characterized using X-ray diffraction analysis. The resistance at variable temperature from 220 K to 300 K was measured using a two-probe setup on a UV-THz full-spectrum photoelectric test probe stage. The transmittance and absorbance of the thin films were examined using Fourier-transform infrared spectroscopy (FTIR) in the 2.5-25 μm range.  Results and Discussions  XRD characterization showed that the thin films exhibited polycrystalline structures with different preferential orientations as the annealing temperature increased. The primary crystal orientation obtained was (111), and the peak intensity increased with increasing annealing temperature. SEM characterization showed that the grain size of the thin film increased with increasing annealing temperature, consistent with the XRD results. FTIR characterization showed that the absorbance of the thin films increased toward the long wavelength in the 6-8.5 μm range and reached 1.50 for the sample annealed at 850 ℃. XPS quantification showed that the ratio of Mn4+/Mn3+ in the film increased with increasing annealing temperature. All thin films exhibited ohmic characteristic curves measured with four-point probe method at room temperature. It was shown that the samples annealed at 750, 850, and 950 ℃ respectively exhibited characteristic negative temperature coefficients, with calculated TCR of −1.95%/K, −4.20%/K, and −4.14%/K accordingly. The high TCR could be attributed mainly to the fabrication method, as solid phase synthesis of MCNO thin films via layer-by-layer deposition could result in metastable states in the thin films, thereby achieving better TCR and absorbance unobtainable in systems of thermal equilibrium.  Conclusions  This paper explored a magnetron sputtering-electron beam evaporation hybrid method based on solid phase synthesis to fabricate MCNO thin films. The impact of post-annealing on the properties of the thin films was investigated. Thin films prepared with optimized processing parameters showed high absorbance and excellent negative temperature coefficient of resistance. This study demonstrated the feasibility of fabricating MCNO films using the proposed method, offering a new approach for further development of MCNO thin films with optimal composition-property relationship.

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