2023 Vol. 52, No. 2
column
- Invited paper
- Special issue-Laser cleaning technology
- Infrared technology and application
- Lasers & Laser optics
- Photoelectric measurement
- Optical devices
- Optical communication and sensing
- Optical imaging
- Optical design
- Image processing
- Newsletter
- Special issue-Laser cleaning technology
- Lasers & Laser optics
2023, 52(2): 20210870.
doi: 10.3788/IRLA20210870
Based on the background that picosecond pulse width laser with multi-pulses damaging the solar cells, we use three methods that are the surface morphology, voltammetry characteristics and electrolumi-nescence of solar cells to obtain the damage characteristics of solar cells with the laser before and after laser ablation. A three-junction GaAs solar cell with pulse width of 15 ps and wavelength of 1 064 nm is irradiated by picosecond pulsed laser. By changing the laser irradiation power through repetition frequency regulation, the damage characteristics of grid line and non-grid line of solar cell under laser irradiation are analyzed. The experimental results show that, although the laser spot is small, the material inside the battery has been damaged. It's mainly because the damage of the ordered structure of the material inside the battery is gradually increased. When the laser power is higher, the internal damage area is larger. When the gate line is irradiated by laser, the fusion of the gate line will greatly affect the absorption of the carriers by the solar cell, thus reducing the photoelectric conversion ability of the solar cell, and then affecting the electrical performance of the solar cell, so that the damage effect of the gate line is stronger than that of the non-gate line.
2023, 52(2): 20220835.
doi: 10.3788/IRLA20220835
Laser with the high brightness and good direction is irradiated to the object to be cleaned. After laser beam is absorbed by the pollution layer and/or substrate, with such processes as light stripping, vaporization, the adhesion force between pollutants and substrate is overcome so that pollutants leave the surface of the object, to achieve the purpose of cleaning, while causing no damage of the object itself. After the 1980s, laser cleaning technology has received more and more attentions. In the past 40 years, as a new efficient environmental cleaning technology, laser cleaning technology has developed rapidly and has been used in electronic components cleaning, cultural relics maintenance, paint and rust removal and other fields. Since 2010, further research on laser cleaning has been carried out, and great achievements have been made in the detection and intelligent control of laser cleaning. Breakthroughs have been made in the mechanism of laser cleaning. Chinese scientists have also made great contributions. The paper summarizes the principle, characteristics, origin and development of laser cleaning technology in the past 40 years, and introduces the development of laser cleaning in China in recent years.
2023, 52(2): 20220784.
doi: 10.3788/IRLA20220784
Laser cleaning technology has many advantages, such as non-contact, high cleaning accuracy, minor damage to the substrate, and being environmentally friendly. It plays an increasingly important role in intelligent manufacturing. With the development of laser cleaning technology, the need for rapid detection and accurate evaluation of laser cleaning quality is becoming increasingly urgent. The laser interacts with the layer to be cleaned and the substrate during laser cleaning. By collecting and analyzing the light, sound, and other signals and surface characteristics changes during the interaction between laser and substance, real-time characterization of the cleaning process and results can be realized, which is gradually widely used in the automated laser precision cleaning process. This review summarizes the working principle and research progress of laser cleaning monitoring technologies such as acoustic wave monitoring, spectral, and image monitoring. The possible future development and trend of laser cleaning monitoring technology are also discussed.
2023, 52(2): 20220780.
doi: 10.3788/IRLA20220780
Laser paint removal technology has the advantages of high efficiency, environmental protection, high safety, real-time monitoring and easy automation, and has become one of the main ways of aircraft paint removal. In this paper, the main methods and mechanisms of laser paint removal technology were analyzed. Secondly, the influence of process parameters on the laser paint removal effect of aircraft skin was summarized, and the design idea of engineering laser paint removal process parameters was proposed. The effectiveness of the design idea was verified by the result of the paint removal of the metal skin of an aircraft. The evaluation methods of laser paint removal effect and the existing technical standards were introduced. Finally, the application examples of laser paint removal for aircraft skin were summarized, and the future research direction and emphasis of laser paint removal technology were prospected.
2023, 52(2): 20220753.
doi: 10.3788/IRLA20220753
With the continuous transformation and upgrading of China's industry, higher requirements have been put forward for the quality, efficiency and service of industrial cleaning. The traditional cleaning methods cannot meet the application needs of industrial fields due to its high pollution, high energy consumption and other defects. As a green, environmentally friendly and non-destructive new cleaning method, laser cleaning has rapidly become a hotspot technology in the field of industrial cleaning. This paper presents three typical laser cleaning methods and summarizes the relevant mechanism of laser cleaning. At the same time, the application progress of laser cleaning in aerospace, shipbuilding, rail transit and other fields is also stated. According to the scientific and technological achievements of the enterprises and institutions of higher learning in laser cleaning systems and equipment at home and abroad in recent years, the difficulties in the promotion and application of laser cleaning in China are put forward, and the development direction of laser cleaning technology in the future is prospected.
2023, 52(2): 20220369.
doi: 10.3788/IRLA20220369
High order active vibration suppression technology has been gradually applied to the multi-order vibration of space infrared mechanical cryocooler to improve the working life of cryocooler and the working performance of infrared load. When the mechanical cryocooler is driven by high-order working frequency, it will produce serious harmonic distortion, which will affect the active suppression effect of high-order vibration. By analyzing the working mechanism of the driving circuit and the high-frequency characteristics of the load, the method of using the control freewheeling path to suppress the harmonic distortion in high-order driving is put forward, and the corresponding implementation method according to the required freewheeling path is also presented. For one of the freewheeling methods, simulation and experiments show that the scheme can effectively suppress the waveform distortion and reduce the high-order harmonic distortion by more than 75%.
2023, 52(2): 20220355.
doi: 10.3788/IRLA20220355
Wall spectral emissivity solution is one of the key techniques for infrared stealth of aircraft. Firstly, the wall reflected light path and light source were designed, and the brightness sequence of wall reflected radiation was obtained by spectral radiometer. In order to eliminate the influence of external interference on the solving accuracy of spectral emissivity as much as possible, Bi-LSTM brightness regression network model was designed based on bidirectional LSTM network, and the test samples were trained and learned. The wall emissivity solution model based on BRDF and the luminance regression model based on Bi-LSTM network were used to solve the wall emissivity. The calculation results show that the relative error of the proposed wall emissivity solution method based on BRDF is 12.21%, which meets the requirements of engineering test.
2023, 52(2): 20220263.
doi: 10.3788/IRLA20220263
In order to investigate the infrared extinction properties of graphene, graphene was prepared by redox method. The structure of graphene was also confirmed by scanning electron microscope images and X-ray diffraction mapping. The infrared extinction properties of graphene were tested using the smoke box test and potassium bromide compression method, and compared with those of graphite and carbon fibres under the same conditions. The results show that the infrared extinction performance of graphene in the far infrared band is excellent. For the 8-14 μm far infrared band, its average mass extinction coefficient is approximately 2.10 m2/g, which is 2.39 times of that of the the average mass extinction coefficient of graphite and 3.56 times that of carbon fiber under the same conditions, providing better infrared interference than conventional carbon material smoke screens. Potassium bromide press tests also show that graphene exhibits very good infrared extinction in both mid and far infrared bands, outperforming traditional carbon material smoke screens.
2023, 52(2): 20220445.
doi: 10.3788/IRLA20220445
In order to meet the space application requirements of the large format Mosaic infrared detector, ultra-scale cold platform need to work at low temperatures, and the cold platform support structure requires high rigidity to meet the components' anti-vibration performance, and a high structure thermal resistance to reduce its conduction heat leakage. A symmetrical eight-bar structure is proposed as the cold platform support, which adopts a new type of zirconia ceramic material with high strength and low thermal conductivity. Based on the finite element software, the influence of the height of the support structure, the installation inclination angle, the aspect ratio and the material on the modal fundamental frequency of the module, the thermal resistance of the support structure and the maximum stress of the module under 30 g static load are analyzed. A set of parameters is used to design the actual test component, the thermal resistance of the support structure reaches 220 K/W, and the components are subjected to a 5-2 000 Hz sine frequency sweep test, a total root mean square of 9 g RMS, and random vibration in the three directions of XYZ and other mechanics. In the environmental test, the final component passes the experimental verification of space environment adaptability. The fundamental frequency of the component reaches 560 Hz, and the test results are in good agreement with the simulation results. The results show that the symmetrical eight-rod zirconia support structure solves the problem that the cold platform of the large format infrared detector requires both high mechanical properties and low heat leakage, and meets the needs of engineering applications.
2023, 52(2): 20220338.
doi: 10.3788/IRLA20220338
In order to improve the quality of automatic fiber placement and assist on-site personnel to quickly detect defects, this paper proposes a real-time instance segmentation network named Trans-Yolact, which is based on Transformer. The Trans-Yolact is used to detect, classify and segment multi-spectrum images of composite material defects. Based on Yolact, aiming at the characteristics of composite material defects, Trans-Yolact's detection ability of composite material defects is enhanced from the two dimensions of space domain and channel domain. In the spatial domain, the convolution kernels have the limitation of spatial scale. The detection of narrow, long, large-size defects is not effective. Therefore, this paper adopts the BoTNet of the CNN+Transformer architecture as backbone; at the same time, the Transformer is introduced into the FPN structure of the Yolact network to enhance the network's ability to obtain information from non-local spaces. In the channel domain, the infrared and visible simultaneous detection method is adopted, and the shallow structure of the backbone is improved, which is divided into visible channel, infrared channel, and mixed channel. Channel domain attention mechanism is introduced in mixed channel. Enhance the comprehensive judgment ability of the network for infrared and visible images. The results show that the mAP of Trans-Yolact for composite defect detection is 88.0%, which is 5.5% higher than Yolact network, and the AP of narrow defects such as miss and twist are increased by 15.2% and 5.1%. The AP of foreign defects including some large-scale defects is increased by 9.1%. Finally, the Trans-Yolact network is pruned. After pruning, the amount of floating-point operations per second (FLOPs) and parameters are reduced by 26.5% and 44.7% compared with Yolact network. The number of detection frames is increased by 58%, reaching 57.67 fps. And the online test is carried out on the large-scale gantry composite material automatic laying equipment, which can meet the real-time detection and segmentation of composite material defects under the maximum laying speed of 1.2 m/s in the production process.
2023, 52(2): 20220593.
doi: 10.3788/IRLA20220593
In order to improve the measurement accuracy, stability and efficiency of the existing 3D coordinate positioning technology, a deep-learning-based point-diffraction interferometer for 3D coordinate measurement method was proposed. A deep neural network was designed for coordinate reconstruction of the point-diffraction interference field. The phase difference matrix was used as the input to construct the training dataset, and the coordinates of point-diffraction sources were used as the output to train the neural network model. The well-trained neural network was used to process the measured phase distribution initially and the phase information was converted to the coordinates of point-diffraction sources. According to the obtained coordinates of point-diffraction sources, the initial particles of the particle swarm optimization algorithm were further modified, and then the high-precision three-dimensional coordinate was reconstructed. This neural network provides a feasible method to establish the nonlinear relationship between the phase distribution of the interference field and the coordinates of the point-diffraction sources, and significantly improves the accuracy, stability and measurement efficiency of the 3D coordinate positioning. In order to verify the feasibility of the proposed method, numerical simulation and experimental verification were carried out, and different methods were used for repeated comparison and analysis. The results show that the single measurement time of the proposed method is about 0.05 s, and the experimental accuracy can reach the submicron magnitude. The mean and RMS values of the repeatability experiments are 0.05 μm and 0.05 μm, respectively, which proves the feasibility of the proposed method and its good measurement accuracy and stability. It provides an effective and feasible method for 3D coordinate positioning.
2023, 52(2): 20220488.
doi: 10.3788/IRLA20220488
The resolution and light collection ability of telescope were directly proportional to its aperture. With the increasingly strict requirements of human beings for the resolution of telescopes, the size of telescope mirrors would be also increasing. With the increasing size of the mirror, the mirror seeing became more and more important. Mirror seeing mainly referred to the degradation of image quality caused by turbulence on the mirror surface. When the mirror size exceeded the local atmospheric turbulence scale, we had to consider the influence of this factor on imaging or processing. The working environment of the system would affect the mirror seeing to a certain extent, so the mirror seeing was also of great significance to the integrated detection process. Therefore, in order to improve the surface accuracy of mirror processing and the integration effect of the detection system, it was necessary to accurately measure the mirror seeing of the instrument, so as to provide judgment for its processing detection and application integration. In our work, one-dimensional detection (autocollimator method, etc.), two-dimensional detection (slope/curvature method, holographic wavefront sensing method and shearing interference method, etc.) and three-dimensional detection (holographic particle velocimetry and temperature field method, etc.) were described from three aspects: principle, research status and application in mirror seeing. By introducing the detection methods for different scenes and detection requirements, it had a good guiding significance for the detection of mirror seeing.
2023, 52(2): 20220367.
doi: 10.3788/IRLA20220367
The planar-array-based imaging radar can achieve transient 3D detection and is suitable for pose measurement of moving platforms or non-cooperative targets. A multi-view point cloud auto-registration method for pose measurement of spatially non-cooperative targets was proposed for non-uniform grid point clouds with crosstalk characteristics between adjacent pixels. Based on the principle of improved coherent point drift (CPD), the method treats the target point cloud as the data distribution set and the source point cloud as the set of center-of-mass points of Gaussian mixture model (GMM). The likelihood function of the constructed GMM model is solved by using Bayesian posterior probability formula and Expectation-Maximum (EM), and the weight of the point set are adaptively adjusted by the overlap of the point clouds in the optimization process. The distance residuals between source point set after one EM iteration are ranked, the optimal transformed point cloud pair is selected, and the local perturbation quantity is established using the nearest neighbor method to obtain the spatial transformation matrix for each drift iteration. To avoid getting into local solutions, the attributes of the point set involved in the drift operation are alternated by supervising the mean square error update rate of the point cloud. For spatially targets, two simulation conditions are established to obtain multi-view non-cooperative target point cloud datasets. The results show that the method is robust under the strong noise and pixels blurring interference, and the average largest common point set corresponding is improved by approximately 61% compared with the other coarse-fine registration strategy, which can be applied to the non-cooperative target pose measurement under the spatial planar-array-based 3D imaging platform.
2023, 52(2): 20210813.
doi: 10.3788/IRLA20210813
With the development of photoelectric measurement technology, infrared gas detection technology is widely used in many fields. Temperature has an important influence on the detection of gas concentration and isotopic abundance. The traditional temperature control system using proportional integral differential (PID) control algorithm has the disadvantages of overshoot, slow response time and low precision. Firstly, COMSOL software is used to determine the heating structure by finite element analysis. Secondly, the STM32 single chip microcomputer is used to collect real-time temperature data through 16 bit AD chip LTC1864. Finally, the linear auto disturbance rejection algorithm (LADRC) is used to adjust the PWM wave that achieve the high-precision and real-time dynamic adjustment of the system temperature by controlling the semiconductor cooler (TEC). Under the temperature of 19.8 ℃ condition, an temperature control experiments with a target temperature of 32 ℃ is carried out. The results show that the standard deviation of temperature fluctuation is 0.0357 ℃. Compared with the temperature control system using PID algorithm, it has the advantages of no overshoot, fast response time and high precision.
2023, 52(2): 20220451.
doi: 10.3788/IRLA20220451
To improve the spectral matching accuracy of the star simulator light source system, firstly, a star simulator light source system based on digital micromirror is designed and built. Secondly, the spectral fitting of the genetic algorithm is performed according to the regional spectrum calibrated by wavelength, The results show that the scheme exits a certain matching error between the fitted spectrum and the target spectrum. Finally, in order to improve the accuracy of spectral matching, an error feedback and accuracy improvement method is proposed to divide the region into two-dimensional wavelength and energy. The experiment simulates light sources with color temperatures of 2550 K, 4766 K, 6576 K, and 8910 K. The results show that, compared with the feedback method of one-dimensional division in the wavelength direction, the maximum error of spectral matching decreases by 55.7%, 50.6%, 45.2%, and 42.2%, respectively, which significantly improves the spectral matching accuracy of the star simulator light source system. The study aims to compensate for the angle measurement error caused by the spectral match error, which improves the star sensor's calibration accuracy.
2023, 52(2): 20220408.
doi: 10.3788/IRLA20220408
With the continuous progress of science and technology, military targets are developing towards miniaturization, ultra-high speed and low detectivity, so the detection and identification ability of targets is also put forward higher requirements. Based on the deep analysis of point target imaging process and on the basis of energy distribution, based on abnormal Rayleigh distribution of point target energy calculation method, the infrared radiation characteristics and has a better precision than the experimental verification, it is concluded that the measurement deviation of radiation intensity can be controlled under 8%, and measuring the degree of discrete numerical smaller, can effectively distinguish between the point target and background. It is proved that the calculation method has good engineering applicability, high measurement accuracy and wide application prospect.
2023, 52(2): 20220304.
doi: 10.3788/IRLA20220304
Terahertz (THz) wave has broad application prospects because of its many unique properties. However, due to the relative lag in the development of related materials and devices, there are still many limitations in the practical application of terahertz technology. Metamaterials and metasurfaces can effectively manipulate the phase, amplitude, and polarization of terahertz waves, providing many new possibilities for the development of terahertz technology. One of its important functions is to reflect incident waves in non-specular directions through phase discontinuities, commonly known as generalized Snell's law. However, most of the previously reported anomalous reflection devices are relatively inefficient and have limitations in practical applications. To solve this problem, we propose a terahertz metasurface anomalous reflector that reflects normal incident light to a 40° direction without changing its polarization. We theoretically expound the idea of improving efficiency and demonstrate its effectiveness through numerical simulation. We construct a metasurface using all-dielectric materials to eliminate material losses and exploiting the coupling of different Bloch waves to provide a non-local response, the operating efficiency of the device can be increased to more than 99%. In addition, our design concept can be generalized to polarization-independent devices and could be useful for other similar devices. This work has the potential to be applied to practical terahertz devices such as terahertz lasers and terahertz cavity resonators.
2023, 52(2): 20220471.
doi: 10.3788/IRLA20220471
To meet the great demand of multi polarization millimeter-wave remote sensing imaging, a highly integrated dual polarization millimeter-wave detector is reported. The detector adopts the solid-state electronics scheme. A horn antenna is used to collect the millimeter-wave radiation signal. An ortho-mode transducer structure (OMT) is utilized to orthogonal polarization states of the incident signal into the vertical and horizontal polarization channels. Dual polarization signal power detection is realized by Schottky diode detector after two stage low noise amplifier. The detector, which shows a high level of integration, has two same channels while each channel can receive two orthogonal polarization states of the incident millimeter-wave radiation simultaneously. The measurement results display that the detection band is W-band (75-110 GHz), the average Noise Equivalent Temperature Difference(NETD) reaches 0.3 K(@10 ms), the detector’s size is less than 155 mm×15 mm×20 mm, and the detector’s weight is less than 0.3 kg. Based on the dual polarization detector, an outdoor area in the city has been imaged and various polarization parameter images are generated to analyze the polarization characteristics. The results show that, compared with single polarization remote sensing, dual polarization remote sensing can obtain more polarization information, common objects in urban environment can be highlighted, which is helpful for automatic object recognition, contour extraction in the future, and remote material component recognition.
In order to overcome the common frequency-dependent behavior of metasurface (MS), an ultrathin and multifunctional MS that based on geometric phase is proposed to control wavefront independently of transmitted circular polarized (CP) terahertz (THz) waves at two different THz frequencies. The unit-cell of MS consists of two layers metal structure separated with dielectric substrate layer. The metal layer pattern is the same, which is made of double-C-shaped-ring (DCSR) resonator structures and the middle metal ring and rectangular metal patch (RMP) resonator structure. By rotating the metal resonators ring on the top layer separately, the cross-polarized transmitted wave can be controlled to have the same amplitude and correspondingly different phases. By appropriately arranging the array of unit-cell, the designed MS has ability to arbitrarily manipulate the wavefront. For example, the vortex beam, which carry orbital angular momentum (OAM) with topological charge of l = +1, +2, +3 and +4, can be generated and the corresponding all mode purities are greater than 60% at the lower frequency of f1 = 0.701 THz; the CP wave focusing effect can be achieved and its error is only 0.04 at the higher frequency of f2 = 1.663 THz. The simulation results show that the designed MS is good at manipulating electromagnetic waves at two different THz frequencies.
2023, 52(2): 20220371.
doi: 10.3788/IRLA20220371
Graphene and other two-dimensional (2D) materials have attracted extensive attention in the field of science and engineering because of their unique physical and chemical properties. To explore novel 2D material system and expand its application range is a hot research hotspot. Among them, the 2D group-VA monoelemental Xenes (phosphorene, arsenene, antimonene and bismuthene) have narrow adjustable energy band width, high charge carrier mobility, excellent light transmission and outstanding photonics properties, which obtained a great attention in 2D materials. Similarly, bismuthene has also attracted attention in their optoelectronic applications. In view of this, this paper analyzes the relevant theory and experimental research progress of bismuthene from the aspects of basic physical structures, material preparation methods and optoelectronic applications. In the aspect of controllable preparation of materials, electrochemical exfoliation method of bismuthene is mainly discussed. Finally, the application status of bismuthene in optoelectronic field are discussed, including ultrafast optoelectronic devices, and the future development of bismuthene is prospected.
2023, 52(2): 20220324.
doi: 10.3788/IRLA20220324
A pulse sensor based on cascaded grating is proposed. Cascaded gratings composed of two fiber Bragg gratings with the same parameters are implanted into the silicone wristband. One fiber Bragg grating is used as a sensing grating to sense the pulse signal, and the other grating is used as a matching grating for demodulation. Because the parameters of the two gratings are exactly the same, their reflection peaks overlap, and the reflection spectrum of the sensor shows a single reflection peak. The pulse signal modulates the strain state of the sensing grating under the action of stress, resulting in the shift of the central wavelength of the reflection peak of the sensing grating, which leads to the change of the reflected light intensity of the sensor. By monitoring the reflected light intensity of the sensor, the measurement and extraction of pulse are realized. In the experiment, the pulse of healthy young and old male subjects, and the same healthy male subjects at 8 am/pm are tested. The results show that the sensor can effectively extract the pulse feature points of different subjects, and the test results are in line with the pulse theory of traditional Chinese medicine. When the ambient temperature changes, the reflection peaks of the two gratings shift in the same direction at the same rate, and the reflected light intensity of the sensor remains stable, so the sensor is not affected by the ambient temperature. In addition, the sensor has the advantages of simple structure, convenient demodulation and low cost. There are potential applications in pulse measurement.
2023, 52(2): 20220370.
doi: 10.3788/IRLA20220370
The lithium niobate crystal optical electric field sensor has the advantages of wide bandwidth and negligible interference to the original electric field due to its all-dielectric structure. But it has low sensitivity to measure the electric field. The influence mechanism of crystal geometry size on sensor sensitivity is analyzed. The sensitivity of the sensor is improved by increasing the crystal size along the direction of the applied electric field and reducing the crystal size along the vertical direction of the applied electric field on the crystal cross-section. The influence of different thickness, width and length of the crystal on the internal electric field intensity has been analyzed using COMSOL simulation, and it is concluded that the internal electric field intensity of the crystal increases by about 5.1 times and 12.3 times when the thickness decreases from 15 mm to 3 mm and the width of the crystal increases from 3 mm to 22 mm, respectively. When the length of the crystal increases from 15 mm to 55 mm, the internal electric field intensity of the crystal changes by only about 5%. Three lithium niobate crystal electric field sensors with crystal sizes of 3 mm×3 mm×42.2 mm (x×y×z), 3 mm×6 mm×42.2 mm (x×y×z) and 6 mm×6 mm×42.2 mm (x×y×z) have been designed and developed, the sensitivities are 0.243 mV/(kV·m−1), 0.758 mV/(kV·m−1), 0.150 mV/(kV·m−1), respectively. When thickness and length of the crystal are constant, the sensor sensitivity is increased 3 times with the crystal width increased from 3 mm to 6 mm. When width and length of the crystal are constant, the sensor sensitivity is increased 5 times with the crystal thickness decreased from 6 mm to 3 mm. Combining the simulation and experimental results, it is concluded that a higher sensitivity electric field sensor can be developed by designing the crystal with wider width and thinner thickness under a certain crystal length.
2023, 52(2): 20220318.
doi: 10.3788/IRLA20220318
Time-of-Flight (ToF) imaging senses the depth information of the scene using the time of light traveling from the target to the camera, with the advantage of compact construction, low cost and real-time imaging. In the scattering scene, the depth measurement of ToF imaging is affected by multipath interference due to the light scattered by the scattering medium, which results in large depth measurement errors, and limits the application of ToF imaging in the scattering scene. ToF imaging through scattering media can effectively correct the multipath interference caused by scattered light via separating the target component from the mixed signal received by a ToF sensor for recovering the depth information of a scattering scene, which is promisingly applied in foggy autonomous driving, underwater surveys, biomedicine, and other fields. Herein, the principle of PL-ToF and CW-ToF imaging is introduced in detail according to the difference of ToF imaging system, and the mechanisms and characteristics of ToF stable and transient imaging in a scattering scene are introduced and analyzed. The researches of ToF imaging through scattering media in stable and transient imaging are reviewed and summarized, respectively. In addition, the potential applications of ToF imaging through scattering media are presented. Finally, the future development trend is prospected according to the advantages and disadvantages of existing ToF imaging through scattering media.
2023, 52(2): 20220308.
doi: 10.3788/IRLA20220308
To reduce the influence of bad pixels in large-aperture interferometric imaging shortwave infrared high-speed hyperspectral imagers on recovering spectra, a bad pixel identification template was established using a hyperspectral imager test process to effectively raise the efficiency of identification of bad pixels. First, image data were collected based on the gain template and frame-rate template for the hyperspectral imager test. Then, the judgment threshold Th1 was set reasonably according to the gain response of normal pixels to identify abnormal pixels under different gains and record the corresponding coordinate values, and the judgment threshold Th2 was set reasonably according to the frame-rate response gray values of normal pixels to identify abnormal pixels under different frame rates and record the coordinate values. Finally, after the abnormal pixels identified by the gain template and those identified by the frame-rate template were compared, they were used together to identify bad pixels. The experimental results show that the identification method based on the gain and frame-rate templates can effectively identify bad pixels in a shortwave infrared hyperspectral imager detector without increasing the cost of equipment development and testing, while providing an economical, practical, efficient, and reliable technical means for correcting bad pixels in a shortwave infrared hyperspectral imager. The method also offers a useful reference to improve the accuracy of inversion of interferometric imaging hyperspectral imager spectral data.
2023, 52(2): 20220442.
doi: 10.3788/IRLA20220442
In order to realize the miniaturization of the laser/infrared dual-mode seeker imaging system and simplify the optical structure, a dual-mode common optical path annular aperture ultrathin imaging system with four reflections is designed. The design principle of the optical splitting path of the system is studied, and the relationship between the obscuration ratio and the field of view is given. The long-wave infrared (LWIR) 7.7-9.5 μm and laser 1.064 μm dual-mode seeker imaging system is realized by a single optical element. The focal length of the dual-mode annular aperture system in the LWIR is 70 mm, the equivalent F number is 1.3, and the full field of view is 8°. The modulation transfer function (MTF) values of each field of views are greater than 0.136 when the spatial frequency is 41.7 lp/mm. The focal length of the dual-mode annular aperture system at the laser wavelength is 53.8 mm, the equivalent F number is 1, the full field of view is 10°, and the spot diagram in the full field of view is uniform. When the ambient temperature ranges from −40 ℃ to 80 ℃, the MTF value of each field of view is greater than 0.13 in the LWIR, and the energy distribution is basically unchanged in laser wavelength. As a result, the optical passive athermalization is realized. According to the tolerance results, this system is machinable.
2023, 52(2): 20220397.
doi: 10.3788/IRLA20220397
Underwater optical imaging is an important underwater measurement method. The existing underwater camera imaging resolution measurement method is affected by the water body and the measurement method, and it is difficult to accurately measure the imaging resolution. The underwater camera imaging resolution measurement technology based on an underwater collimator is proposed to directly measure the underwater camera imaging resolution by generating parallel light beams in the water. Through simulation, the modulation transfer function (MTF) of the underwater collimator in visible light in water and single wavelength in the air are basically the same. In conclusion, a method for adjusting the underwater collimator in the air is proposed. The experimental test is carried out for an underwater camera developed in the laboratory, and its resolution under the condition of visible light in water and a 635 nm light source in the air is the same. The experimental results show that the proposed underwater camera imaging resolution measurement method based on the underwater collimator can effectively eliminate the influence of water on the measurement and realize the accurate measurement of the underwater camera imaging resolution.
2023, 52(2): 20220390.
doi: 10.3788/IRLA20220390
Due to the limitations of scanning equipment or the complexity of the model structure, holes appear in the point cloud model, which seriously affects the subsequent processing of the model. To address the problem of point cloud hole repair, this paper proposes a point cloud hole repair method with an additional value-added conditional moving least squares method. Firstly, the closed hole boundary is extracted and iteratively sliced through density analysis, which not only reduces the impact of the uneven distribution of the point cloud, but also improves the retention standard of detailed features of the model; Besides, the discrete group of points is projected onto the fitted surface, and the projected point set is fitted twice to obtain the nodes of the fitted surface to ensure that there are enough boundary neighborhood nodes for hole repair; Finally, the holes are repaired by using the additional value-added conditional moving least squares method. Meanwhile, the curvature of the value-added point cloud is constrained, so as to achieve the reconstruction that fits the spatial characteristics of the original model. In the test, different types of holes are artificially made on four point cloud models, and the effectiveness of proposed method is verified by comparison with the existing four methods. The results show that, compared with the four existing methods, the completeness and accuracy of this method are improved by more than 1.83%, and the root mean square error of the alignment and the root mean square of the curvature are reduced by more than 68%, which proves the applicability of this method for point cloud model holes, which can provide reliable information for the reconstruction of 3D point cloud models.
Aiming at the problem of low accuracy of DEM reconstruction and deformation analysis due to the lack of data during the scanning of high and steep slope point cloud based on light and small unmanned airborne LiDAR in the topographic environment of high-altitude gorge, this paper proposed an optimized data acquisition scheme using UAV image/point cloud data acquisition of high and steep slope flying perpendicular to the ridge, and DEM reconstruction scheme using UAV LiDAR point cloud with the aid of dense point cloud derived from multi-view oblique images. The accuracy and integrity of DEM reconstruction of high and steep slope and the accuracy of landslide deformation measurement in different periods have been improved. This method uses the point cloud data generated from the collected tilted image and the LiDAR point cloud data of the same period, in order to compensate the LiDAR point cloud missing data based on the iterative nearest neighbor point algorithm. Then, high-precision DEM products of landslides are constructed using the compensated LiDAR point cloud; Finally, the deformation of the landslides in elevation direction is obtained by comparing the current DEM with the DEM generated from the oblique images acquired in the past. Taking the landslides located at high and steep slopes of Qinghai Longyangxia Hydropower Station as the study area, an experiment is carried out and verified using the measured GNSS ground control points. It is concluded that the accuracy of LiDAR point cloud after fusion is 0.063 m, which is 0.018 m higher than that before fusion. The elevation accuracy of the reconstructed DEM of three typical landslides is 0.08 m, which improves the completeness and accuracy of DEM of slopes. The deformation results of typical landslides in the area are obtained by a comparison of DEM from two different epochs. The conclusions can be drawn that many slopes in the landslide group have different degrees of soil sliding, and the deformation in the elevation direction is up to more than 50 m, heavy deformations happened to the landslides.
2023, 52(2): 20220376.
doi: 10.3788/IRLA20220376
Compared with conventional optical remote sensing data, which contain only multispectral information and can only realize two-dimensional land cover classification, the advantage of airborne multispectral light detection and ranging (MS-LiDAR) is that it contains both multispectral and spatial information and can realize three-dimensional land cover classification. However, the existing land cover classification methods for airborne MS-LIDAR data require too high feature dimension to distinguish all kinds of objects simultaneously and have high algorithm complexity. So, a stepwise classification algorithm combining spatial and normalized difference ratio index (NDRI) features is proposed. Firstly, the multi-band independent point clouds of airborne multispectral LiDAR are merged to obtain the merged point cloud data with spatial location and their multispectral information. Secondly, based on the elevation consistency of urban ground spatial adjacent points, a ground filtering algorithm under spatial neighborhood growth is used to separate ground and non-ground points. Thirdly, based on the difference of laser reflectance characteristics of different objects, the NDRI index is designed to separate the grass (tree) from the ground (non-ground), and the adaptive optimal NDRI index under the principle of maximum inter-class variance is used to achieve the fine classification of ground and non-ground points. Finally, 3D majority voting is used to alleviate the noise in the previous classification result in order to further optimize classification result. The proposed algorithm makes comprehensive use of the spatial and multispectral features contained in multispectral LiDAR data, and the step-by-step processing method is more convenient to design simple and effective rules according to the characteristics of specific separation objects. The effectiveness and feasibility of the proposed algorithm are tested by using Optech Titan airborne multi-spectral LiDAR data of different scenes. The experimental results show that: (1) The average overall accuracy and Kappa coefficient of the proposed algorithm can reach 90.17% and 0.861, this demonstrates that the proposed algorithm can realize the accurate three-dimensional land cover classification of the multi-spectral LiDAR data in urban areas. (2) The step-by-step processing method adopted by the proposed algorithm is more convenient for designing simple and effective rules according to the characteristics of specific separation objects. The algorithm design is simple and the complexity is low. (3) The availability of NDRI features designed to distinguish between grass (trees) and road (buildings) can provide theoretical support for the design and selection of salient features of other machine learning algorithms.
2023, 52(2): 20230049.
doi: 10.3788/IRLA20230049
This paper introduces the main task of the fifth sub-topic project "Key Technologies in Laser Engineering and Cleaning Application" of the National Key Research and Development Project of the Ministry of Science and Technology (2017YFB0405100) from 2017 to 2021: the key technologies of quasi-continuous all-solid-state kilowatt level laser cleaner prototype. Laser cleaner prototype with three-dimensional large gantry frame forms laser cleaning industrial platform. A laser cleaning mechanism model for rail surface rust removal and aircraft composite skin paint removal is introduced. The methods of laser cleaning monitoring are introduced from sample identification, process monitoring and result detection. The laser cleaning parameters of 60 rail rust removal and aircraft aluminum skin depainting are obtained by author's research group, and the cleaning efficiency index passes the expert test. During the implementation of the project, developed prototypes are applied to more than 10 units such as China National Petroleum Corporation, Harbin Institute of Technology, Academy of Armored Force Engineering, Lianbo Heyi Technology Co., LTD. At the end of project execution, the project is awarded as the outstanding project of the key R&D plan of the Ministry of Science and Technology.
2023, 52(2): 20230073.
doi: 10.3788/IRLA20230073
High efficiency and quality is the main focus direction within the laser cleaning area, and also the key to realize the large-scale industrial application. The National Key Research and Development Program "Additive Manufacturing and Laser Manufacturing" Key Special Project "High Energy High Repetition Rate Pulsed Laser Intelligent Cleaning Technology and Equipment" (common key technologies) aims at the urgent demand for precision, green, intelligent and controllable new cleaning technology in aerospace equipment manufacturing and repair. This project is dedicated to understanding the multi-beam laser cleaning multi-field coupling mechanism, and breaking key technologies such as cooperative control of high energy high repetition rate nanosecond laser with multi-dimensional gain, residue location identification and path planning on complex surfaces, multi-beam splicing and energy control in ultra-wide format. At the same time, the research on high energy high frequency nanosecond pulsed laser, high efficient and high quality cleaning mechanisms and key technology, and ultra-wide format cleaning process and equipment are carried out. Finally, the research results will enable some typical applications for removing thermal control coatings from spacecraft surfaces and paint from aircraft radome surfaces.
2023, 52(2): 20220739.
doi: 10.3788/IRLA20220739
In high-power laser facilities, contaminants on the surface of optical components can reduce beam quality and even induce damage to optical components. For the contaminated large-aperture vacuum separator (430 mm×430 mm) coated with SiO2 sol-gel antireflection film in the facility, a Nd:YAG pulsed laser with a wavelength of 355 nm was performed in laser cleaning experiment. The experiment adopted a single-shot laser dry cleaning and a laser cleaning system assisted by an airflow replacement system. The influence of key characteristic parameters on in-situ laser cleaning was studied, and the process parameters that could be used for laser in-situ laser cleaning were obtained. The processing of optical elements was characterized by microscope, dark field imaging and image processing software analysis. The experimental results suggest that there is an optimal process window for laser in-situ cleaning of optical components. After the laser cleaning method assisted by airflow replacement, the laser cleaning ability is greatly improved compared with the simple single-shot dry laser cleaning. Therefore, the single-shot laser cleaning assisted by the airflow displacement system can effectively improve its cleaning ability and provide an effective means for the in-situ removal of contaminants on the surface of large-aperture optical components in high-power laser facilities.
2023, 52(2): 20220836.
doi: 10.3788/IRLA20220836
The light-gray erosion protection coating and anti-static finish coating on the surface of glass fiber-reinforced polymer composites were removed using a nanosecond laser with a wavelength of 1 064 nm employed in both pulsed and continuous-wave (CW) modes. The effect of the laser parameters at different laser modes on cleaning quality was investigated. SEM was used to observe micro morphology of material surface and cleaning products after processing. EDS and FTIR were used to detect the element content and chemical functional groups on the surface of materials before and after cleaning respectively. The temperature field during cleaning was analyzed by COMSOL Multiphysics. The results reveal that when removing the single anti-static coating, the pulsed laser can completely remove the top coating while just slightly harming the primer. When employing pulsed laser technology to remove the double paint, there is still a residual paint on the substrate's surface. The rainproof prime paint, resin and fiber layer of the substrate are also easily damaged. While the continuous laser can fully remove double paint and provide a clean surface. The pulsed laser removal process predominantly involves thermoelastic vibration effect, whereas the CW laser removal procedure is mostly thermal ablation. These results can serve as a guide for choosing the laser mode when removing paint from aeronautical composite surfaces.
2023, 52(2): 20220782.
doi: 10.3788/IRLA20220782
For the contamination problem of micro-particles on glass surface, the cleaning threshold of pulsed laser and its cleaning effect on particle contaminants are studied in this paper. The particle adsorption and deformation model was used to calculate the particle adsorption force. The relationship between thermal stress and laser energy density was analyzed, and the cleaning conditions and theoretical thresholds for particle contaminants were obtained. The cleaning effect of forward irradiation and back irradiation of particles on glass surface were compared in experiments, and the effect of parameters such as power density, scanning speed and cleaning times on the cleaning rate were studied. The results show that the back irradiation cleaning of large particles can be removed, and forward irradiation cleaning of small particles is more effective. The experiments of single-factor and three-factor show that the laser power density has a greater impact on the cleaning rate than the scanning speed, and the laser cleaning times have little impact.a greater impact on the cleaning rate than the scanning speed, and the laser cleaning times have little impact.
2023, 52(2): 20220821.
doi: 10.3788/IRLA20220821
The aging paint removal of aircraft skin is an important step in aircraft maintenance and repair. Compared with traditional chemical cleaning methods, laser cleaning has the advantages of high efficiency and low pollution, but the problem of substrate damage caused by excessive cleaning of high-energy laser exists, so it is very important to monitor the cleaning process in real time so as to give feedback to the laser in time. Aiming at this problem, a real-time monitoring method based on acousto-optic composite method is proposed, and the optical and acoustic signals are collected and analyzed respectively in the cleaning process. For the spectral detection method, the laser-induced breakdown spectrum is used to calibrate the different characteristic peaks of different elements in different paint layers to realize the inversion of different paint layers; The acoustic signal detection method is to determine the corresponding paint layer by comparing and analyzing the intensity and frequency of the acoustic signal generated by the laser ablation of different paint layers, and then the cleaning condition and removal effect of the paint layer can be deduced by comparing the two methods. The research shows that the acousto-optic composite method can achieve accurate real-time monitoring of the laser paint removal process.
2023, 52(2): 20220534.
doi: 10.3788/IRLA20220534
The processes, testing and application technology research were carried out for the welding synchronous laser cleaning of aluminum alloy for rail transit vehicle carbodies. The process window of laser cleaning for oxide film of aluminum alloy was obtained through the study of process. The prediction model of laser cleaning parameters before matching different welding speeds was established. And the effect of laser cleaning on the superficial oxide film of aluminum alloy was analyzed. According to the actual working conditions, an integrated laser cleaning and arc welding synchronous device was designed. And the pre-welding synchronous laser cleaning process verification was carried out. High-quality and high-efficiency removal of oxide film before welding was achieved. By analyzing the mechanical properties of welded joints, detecting weld defects and observing the microstructure of the cross-section of the welded joint, the quality of welding synchronous laser cleaning welds was evaluated. The research shows that with 200 W pulsed laser, the cleaning speed is adjustable from 0.5 to 1.1 m/min. Welding synchronous laser cleaning of oxide film with a line width of 45 mm can be effectively achieved. After laser cleaning, the primary oxide film is completely removed and the effect of regenerative oxide film can be avoided. The shear strength and strain of the welded joint after laser cleaning before welding increased by 26.4% and 9.98% compared with the untreated joint. The shear strength and strain of welded joints after laser cleaning increased by 3.53% and 1.43% respectively compared to those after manual grinding. And the central microstructure of the weld was mainly composed of α matrix and β (Mg2Al3) phase. The performance of the welding met the requirements of rail transit vehicle carbody manufacturing. Laser cleaning with welding can effectively replace manual grinding technology.
2023, 52(2): 20220779.
doi: 10.3788/IRLA20220779
In order to study the influence of laser parameters on the performance of laser cleaning based on thermal ablation effects, considering the working mechanism of thermally-induced ablation, Fourier heat conduction equation, and energy conservation theorem, a finite element analysis (FEA) software is used to establish a two-dimensional laser cleaning simulation model of dynamic energy conservation during the thermal ablation. In the simulation model, there is a mathematical relationship between the laser ablation intensity of the thin layer-base system and the normal velocity of the paint layer boundary, by using the virtual Robin boundary condition in the Fourier heat conduction equation. The simulation reflects the dynamic conservation of energy between the laser ablation energy and the latent heat consumption inside paint layers, which confirms the mathematical relationship between the mass loss and the energy consumption, and the simulation results become more precise and reliable. Such a simulation model is used to analyze the effects of the laser power, spot diameter, scanning speed, and scanning energy density on the cleaning performance. The simulation results show that the average residual thickness of a paint layer decreases with the increase of the laser power and scanning energy density, and the decrease tendency gradually slows down. When the laser power becomes too large, the metal base will be damaged. The average residual thickness of the paint layer increases with the spot diameter and scanning speed. However, reducing the spot diameter and scanning speed is harmful to enhance the cleaning efficiency. According to the relationship between the physical parameters of the paint layer and the scanning energy density, the optimization protocol of laser cleaning parameters has been proposed by considering both the cleaning performance and the cleaning efficiency. The simulation model is also used to analyze the cleaning performance of each scanning of an uneven surface paint layer by the uninterrupted multiple scanning. The study in this paper could be helpful for the design of laser cleaning devices as well as the optimization of laser parameters.
2023, 52(2): 20220155.
doi: 10.3788/IRLA20220155
In order to meet the requirements of removing rust on the surface of complex space materials in ship cabins, this paper adopts the pulsed fiber laser cleaning platform to carry out the optimization of laser cleaning process parameters and the experimental study on the surface properties of EH36 Marine steel plate after cleaning. Based on the macroscopic and microscopic morphology observation and surface chemical composition analysis, and combined with the derusting standard specification of the shipbuilding industry, the quality evaluation criterion of Marine steel laser rust removal is established, and the optimal process parameters of material laser rust removal are determined. On this basis, the surface quality, microhardness, roughness and comprehensive mechanical properties of the materials after laser derusting are tested and evaluated. The results show that, under the optimal derusting process parameters (average power 210 W, repetition frequency 250 kHz, pulse width 300 ns, scanning speed 4000 mm/s), the surface quality of Marine steel plate can reach Sa2.5 grade, and the mechanical properties remain basically unchanged, which can meet the requirements of derusting process on the surface of complex space materials in ship hold. It has excellent applicability in the field of surface rust removal of shipbuilding materials.
2023, 52(2): 20220164.
doi: 10.3788/IRLA20220164
A 200 W nanosecond pulsed fiber laser is used to study the laser cleaning of surface area carbon and oxide of GH3030 nickel base superalloy. The effects of laser spot overlapping rate on the cleaned surface morphology, element composition, phase composition, surface roughness and microhardness are analyzed. The results show that as the overlapping ratio of laser spot increases from 58.33% to 70.83%, the content of carbon and oxygen decreases first and then increases, the content of nickel increases first and then decreases, and the surface roughness decreases first and then increases. When the spot overlapping rate is 66.67%, the contents of carbon and oxygen decrease to the lowest value, which are 5.01 wt% and 1.40 wt% respectively. At the same time, the content of nickel reaches the peak, which is 72.96 wt%. The surface roughness Ra decreases to 0.229 μm and Rz reduces to 1.47 μm. The change of laser spot overlapping rate will not have a significant impact on the surface microhardness of GH3030 superalloy.
2023, 52(2): 20220412.
doi: 10.3788/IRLA20220412
In order to continuously detect the tropospheric and stratospheric wind field with high accuracy, a direct wind lidar system is built to detect the tropospheric and stratospheric wind field. Based on the Rayleigh scattering Doppler wind measurement principle of the double edge Fabry-Perot etalon, the system uses a rotary table detection structure, and tracks the frequency drift by means of frequency tracking to ensure the accuracy of wind measurement. The experimental results show that the system has a good detection performance on the tropospheric and stratospheric atmospheric wind field, and the frequency tracking range is ±50 MHz, which can greatly reduce the wind speed error caused by frequency drift. After the stable operation of the system and long-time observation, the random error of radial wind speed measured at 40 km is 8 m/s. After the radial wind speed is combined into horizontal wind speed, the maximum random error at 38 km is about 10 m/s. The detection altitude of the system is 25 km in the daytime and 38 km at night. Compared with the sounding data, the wind speed error is less than 10 m/s, and the data volume within the range of ±5 m/s accounts for about 75.8%. The detected wind direction error is basically consistent with the trend of the sounding balloon, the error range is of 10° to 20° and the data volume within the range of 15° accounts for about 58.6%. The measured data and sounding data are statistically analyzed, and the results have good consistency. The system can provide data support for the detection of tropospheric and stratospheric atmospheric wind fields.
2023, 52(2): 20220366.
doi: 10.3788/IRLA20220366
The Advanced Topographic Laser Altimeter System (ATLAS) carried on the Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) launched by NASA in 2018 is the only spaceborne photon counting lidar in the world so far. It has a high orbital spatial sampling rate, which makes it possible to detect ocean wave elements by remote sensing. The premise of photon counting lidar for wave detection is to accurately extract signal photons from the sea surface and determine the instantaneous sea surface profile. Up to now, there are few reports on the detection of sea surface morphology and wave elements by spaceborne photon counting lidar, and there is also a lack of extraction methods specifically for sea surface signal photons. Based on the distribution characteristics of signal photons on the sea surface, a new signal extraction algorithm is proposed in this paper. First, the rough denoising of sea surface echo photons is completed through histogram statistics and adaptive threshold selection; Then, based on the lidar spot size and sea surface fluctuation characteristics, an appropriate search neighborhood is selected to calculate the density of signal points and noise points. The point density difference is used to classify signal photons and noise photons; Finally, the backscattered noise photons with higher density are further removed by Gaussian function fitting, and the signal photons reflected from the sea surface are obtained. Using the above algorithm, the sea surface signal photons and instantaneous sea surface profiles of seven different sea state regions in the Pacific Ocean are extracted, and the peak wavelength and peak wave period of local ocean waves are further calculated. Comparing the calculation results with the global atmospheric reanalysis ECMWF Re-Analysis5 (ERA5) data of the European Centre for Medium-Range Weather Forecasts (ECMWF) during the same period, the results are basically consistent in sea areas with different wind speeds and water depths. As a result, the relative error of the wave period in more than half of the areas is within 5%, which preliminarily proves the feasibility of calculating ocean wave elements with the spaceborne photon counting lidar observation results.
2023, 52(2): 20220406.
doi: 10.3788/IRLA20220406
Without the turbulence of atmosphere, spaceborne inverse synthetic aperture lidar (ISAL) can obtain high-resolution images of orbit objects at a long distance. Therefore, ISAL plays an important role in spaceborne imaging. The cross-range echo signal model of spaceborne ISAL is established. Generally, the motion of an orbital object is considered a second-order rotation after translational compensation, i.e. uniformly accelerated rotation. Under this condition, the ISAL cross-range echo signals can be equivalent to multicomponent linear frequency modulation (LFM) signals with various chirp rate and initial frequency. Therefore, it is difficult to obtain a well-focused image when using the traditional FFT method. A fast cross-range imaging algorithm based on Radon-Wigner transform is proposed. The Radon-Wigner transform combined with a successive elimination procedure are performed to estimate and separate the cross-range multicomponent LFM signals one by one from strong to weak, in each range unit. After prominent peak points are estimated and extracted in all range units, rearrangement is performed by linear superposition and a focused 2D ISAL image can be obtained. The proposed method avoids to obtain signal parameters and conduct instantaneous Doppler imaging, which is simplified and greatly improved in efficiency. Simulation results show that, compared with the traditional range instantaneous Doppler imaging algorithm, the average processing time of the proposed algorithm is reduced from 222.66 s to 23.51 s. In the case of low signal-to-noise ratio, the target contour in the image is more significant and easier to detect and recognize.
2023, 52(2): 20220159.
doi: 10.3788/IRLA20220159
The spectra of the lateral laser radiation in nematic liquid crystal (NLC) laser devices are studied, and the characteristics of the laser radiation are deeply analyzed. Two types of NLC cells of standard NLC cell and SU-8 grating based NLC cell are fabricated and injected with a mixture of NLC TEB30A and laser dye PM597, respectively. A frequency-doubled Nd: YAG solid-state pulsed laser with a wavelength of 532 nm is used as the pump source. Random lateral laser radiation in the wavelength of 575-600 nm is observed in the standard NLC cell. Whereas, in the SU-8 grating based NLC cells with periods of 100 μm and 8 μm, the spectrum of the multi-wavelength lateral laser radiation is obtained. With the increase of the pump energy, the strongest lateral laser radiation peaks appear at 583-585 nm and 588-592 nm, and the FWHM is about 0.3 nm. According to the theory of the optical waveguide and analysis of the device structure, the introduction of the SU-8 grating into the standard NLC cell enhances the optical waveguide effect of the LC device and induces multi-wavelength laser radiation.
2023, 52(2): 20220350.
doi: 10.3788/IRLA20220350
Al-Li alloy has become a new generation of aerospace application materials with its advantages of light weight, high strength and corrosion resistance. Compared with other Al-Li alloys, 2195-T8 Al-Li alloy has the best welding performance. Based on the welding demand of liquid rocket tank joint, better joint quality can be gotten with laser wire filling double-sided welding. For the fluid flow and temperature change in the molten pool in the welding process, heat-flow coupling mathematical model is established. By using the method of numerical simulation, the welding process of 2195-T8 Al-Li alloy is studied, and then the joint axial tensile strength test is carried out. The effect of welding speed and wire filling velocity of the molten pool on the flow and heat input is illustrated. The maximum joint strength under different welding parameters is obtained. The results show that under four different welding parameters, the fluid flow trend in the molten pool is basically the same between the first welding side and the second welding side, which is mainly clockwise eddy current on the left side and counterclockwise eddy current on the right side. Increasing welding speed or wire filling speed can improve the forming quality of the molten pool, reduce the heat input of the molten pool, refine the grain represented by columnar crystal in the welding fusion zone, and effectively improve the mechanical properties of the joint. By comparing and analyzing the numerical simulation and experimental results of 4 groups of different welding process parameters, the weld with the best forming quality and the smallest heat input is finally obtained. The axial tensile strength of the joint is up to 426.4 MPa, which is 72.6% of the base metal strength. The corresponding welding speed and wire filling speed are 50 cm/min and 1.8 m/min respectively.
