2023, Volume 44, Issue 6
As an essential tool for generating attosecond lasers and exploring the microcosmic world of materials, the precise measurement of the temporal profile of ultra-short laser pulses is significant. Several methods for the generation of few-cycle laser pulses and widely employed characterization techniques were reviewed. The characterization techniques were generally classified into two major categories: frequency domain and time domain measurement. In the frequency domain, the envelope and phase of ultra-short pulses were reconstructed through the measurement of the spectral information generated by nonlinear processes. In the time domain, the temporal profile of the pulse was obtained by directly sampling the optical field information using the ultrafast gate. These two types of techniques have distinct emphases in different application scenarios. The frequency domain measurement is widely employed in fast characterization experiments due to its simplicity and convenience, while the time domain sampling is commonly used in ultrafast physics experiments that are directly associated with the optical field, as it allows for the direct acquisition of photoelectric field information.
Performance of high energy laser (HEL) directly determines the damage ability and strike range of laser directional energy, and the United States has launched a series of plans to develop high-power, high-efficiency, compact, lightweight and robust lasers for battlefield. In 2019, the high energy laser scaling initiative (HELSI) was proposed, and the latest national laser amplification route was formulated. Firstly, the background, research contents and implementation stages of this HELSI were introduced. Secondly, the research progress of 300 kW in the first stage of HELSI was analyzed. Finally, the subsequent influences of HELSI progress was reviewed. Comprehensive analyses show that the Lockheed has won the first stage by its spectrally combined fiber laser technology, not only obtaining production orders of 300 kW HEL of US Army, but also entering the second stage of HELSI to get development contract of 500 kW HEL firstly. The progress of HELSI has made missile defense agency (MDA) pay more attention to laser weapons used to intercept boost-phase ballistic missiles. HELSI supports four technical approaches of HEL for competitive development. However, which one will eventually be deployed on the battlefield in the future needs to be strengthened the early research and break through the technical fog.
High temperature resistant ceramics, as high melting point materials, have excellent high-temperature ablation resistance and may meet the needs of future laser protection. In order to understand the laser protection performance of ZrB2 ceramic coating, a high-power solid-state laser was used as the test light source, and a laser ablation experimental platform as well as laser coupling characteristic measurement system were established. The focus was on conducting laser ablation experiments and coating reflectivity tests on ZrB2 ceramic coating. The laser ablation resistance of ZrB2 coatings under different laser parameter conditions, and the influences of doped phases (SiC, MoSi2) were studied by experiment. The results show that compared to the undoped ZrB2 coating, the doped ZrB2 coating shows a significant decrease in laser ablation resistance. The conclusion analysis suggests that the doped phase can improve the oxidation resistance of ZrB2 coating, but it is not conducive to the high reflection and insulation effects of the oxidation product ZrO2, resulting in a decrease in the threshold of laser damage resistance. The reflectivity test results of the coating before and after laser damage also confirms that the high reflectivity of ZrO2 is the key to enhancing the laser damage resistance threshold of ZrB2 coating. Meanwhile, a temperature calculation model for ZrB2 ceramic coating under continuous laser ablation was established by using finite element software, and the typical laser ablation threshold parameters of ceramic coatings were simulated based on the criterion of substrate melting.
In recent years, with the demand and development of high-power and high-energy laser systems, the anti-laser capability of optical films has become one of the bottlenecks restricting the development of high-quality laser systems. Based on the different preparation methods and process parameters, the damage threshold of the film will have an important impact. The methods for improving the damage threshold of the film before, during and after plating were briefly introduced, including the film material selection, membrane system design, solvent cleaning, ion beam cleaning, preparation method, ion beam post-processing, laser pretreatment, and etc.
High power solid-state laser has been widely used in energy, advanced manufacturing, information, medical and military defense fields. As a new generation of solid laser materials, the laser ceramic materials are characterized by uniform and controllable doping concentration, good optical uniformity, strong resistance to laser thermal damage, easy to prepare large size and composite structure,which have great potential in high power laser generation. In this paper, the research progress of high power ceramic slab laser in recent years was reported. The Nd:YAG ceramic slab laser was developed. It could be realized that the maximum output power was 4.35 kW and the laser pulse width was 160 μs at a repetition rate of 400 Hz. Moreover, the Yb:YAG ceramic slab laser was developed with the maximum output power of 9.8 kW under the pulse width of 560 μs and repetition frequency of 160 Hz. The optical conversion efficiency was 60% relative to the absorbed pumped power. These studies have explored new technical ways for the further development of high beam quality and high efficiency solid-state laser with higher output power.
Diamond crystals not only have excellent optical properties, but also have extremely high thermal conductivity and low thermal expansion coefficient, which makes diamond laser an important path to achieve high-power laser output without heat. However, with the further increase of laser power, thermal effects that cannot be ignored in diamond Raman lasers (DRLs), which poses a challenge to the performance improvement of diamond lasers. The thermal effect of the DRLs under high power operation was studied theoretically. Based on the thermal conduction equation and finite element analysis method, the temperature, thermal stress and thermal deformation distribution of diamond were simulated, and the effects of pump and crystal parameters on the above factors were analyzed. Moreover, a novel heat sink structure for diamond was designed based on the transverse thermal conductivity characteristics of graphite sheets. Compared with traditional heat dissipation methods of single copper-sheet, under the pump power of 800 W and waist radius of 40 μm, the center temperature of the diamond was reduced by 10.16 K, the average stress on the lower surface was reduced by 19.857 MPa, and the average deformation of the end face was reduced by 0.055 μm. The numerical simulation results show that this method has important guiding significance for mitigating the thermal effect of diamond laser, further enhancing the output power of DRLs and achieving high beam quality laser output.
Fiber transmission is a common mode of laser transmission, with the improvement of optical fiber preparation technology, energy transfer fiber with large core diameter and large numerical aperture is widely used in multi-mode laser transmission. Efficient laser coupling is the premise of stable transmission of optical fiber, in order to achieve laser coupling with wide band and high power density, according to the transmission transformation characteristics of mixed-mode Gaussian-like beam and the initial parameters, combined with aberration analysis, an aspherical lens coupling device between large-core four-in-one bunched fiber and single-core fiber was designed, and the maximum fiber coupling efficiency of 60.6% was obtained through mechanical installation experiments. Coupled with a self-developed laser resonance ionization time of flight mass spectrometer, the laser saturation power density corresponding to the three-color, three-step photoionization path of neodymium isotope was measured, and it was verified that the device could meet the spectral experiment requirements of specific isotope resonance excitation ionization induced by complex lasers.
The solar-pumped laser is a device that directly converts the sunlight into the laser. It has the advantages of less energy conversion, high efficiency, simple and reliable structure, stable performance and no pollution. An all-solid-state solar-pumped laser for the space environment (4 K) was modeled, and the thermal effect of the pumped solar collection phase was analyzed. The steady-state thermal analysis was carried out by using ANSYS Workbench@ thermal analysis software, considering two mounting bases and four solar deflection conditions. According to the feasibility verification of the first case, the suggestions for further improvement and optimization were put forward, and the actual model building and testing were preliminarily envisaged. In view of the second infeasible case, several possible improvement measures were put forward, and the future work was prospected. This study provides a new feasible scheme of temperature control system with all-solid-state conduction cooling for the practical application of solar pumped laser in space environment, and provides research data for the practical application of space solar pumped laser in the future.
The damage threshold measurement device is an important technical indicator of high-power laser technology, which is mainly used for the development and testing of high-power laser optical components. The synchronous trigger module serves as a controller for controlling the timing between modules, and is one of the key technologies in the development of damage threshold devices. A synchronous trigger module and method for laser damage threshold measurement device were introduced. A hardware solution based on field programmable gate array (FPGA) as the main control chip was designed. By setting synchronous trigger parameters through the upper computer control software, the width of each output synchronization signal and the timing between each signal were controlled, which could greatly improve the accuracy and efficiency of synchronous trigger. Through experimental verification, the adjustment accuracy between synchronous pulse signals is 2 ns, and the minimum width of synchronous pulse signals is 10 ns, which meets the needs of laser damage threshold measurement devices.
In the analysis and design of Nd:YAG Q-switched lasers, researchers usually choose to ignore the influence of laser lower level lifetime on pulse waveform. When the laser pulse width is much larger than the laser lower level lifetime, this approximation generally does not bring much deviation. When the pulse width reaches the nanosecond level, the influence of the lower level lifetime of the Nd:YAG crystal about 30 nanoseconds on the pulse waveform will become very serious. The theoretical analysis model of the influence of Nd:YAG lower level lifetime on the output pulse waveform was established, and the output waveform of the narrow pulse width Q-switched Nd:YAG laser was simulated. The results show that in the case of narrow pulse width laser output, the lifetime of the lower energy level of the laser will lead to the tail peak of the Q-switched pulse after the main peak, and the tail peak energy can reach more than 100 % of the main peak energy. At the same time, the experimental system of Nd:YAG acousto-optic Q-switched laser was established. The Q-switched pulse waveform was measured under the condition of approximate simulation calculation, and the tail peak phenomenon consistent with the simulation results was observed. The correctness of the theoretical model was verified by experiments.
Aiming at the features of nonuniform distribution and irregular shape for far-field laser spots, a measurement method based on improved Zernike moments was proposed to identify the far-field laser spot center. Firstly, the image was denoised by bilateral filtering. Then, based on traditional Zernike moment subpixel edge detection, a new logistic edge detection model and step threshold adaptive extraction method were used to improve the accuracy of actual edge recognition,as well as reduce the manual misjudgment of step threshold. Finally,the least squares ellipse fitting was used to obtain the high-precision laser spot center. This method has a single frame error of about 0.5 pixel in far-field laser center detection, and a continuous multi-frame center deviation fluctuation of less than 1 pixel, which has high accuracy and reliable stability.
In order to solve the problem that the current beam quality analyzer can only be used to evaluate the beam quality of small caliber and low power lasers, an experimental study on the attenuation and compression technique of high power laser beam quality measurement was carried out. A device for measuring wave aberration and beam quality of the compressed beam laser was built, and an experiment was carried out to determine the influence of the assembly error on beam quality. The experimental results show that, with the increase of the field of view, the measurement deviation of M2 of 1/3 compressed beam assembly at 1.2° field of view is less than 5%. A polarization splitting device was built, and the influence of attenuated components on beam stability was studied. The experimental results show that the stability of the quasi-single mode laser with random polarization is more susceptible to the depolarization of the attenuated components than that of the multi-mode laser. A high power laser beam quality measuring device with high reflection type and wedge-plate type was built to measure the beam quality of 1 kW quasi-single mode laser. The experimental results show that in the high reflection measuring device, the depolarization occurs when the beam passes through the high reflector, which leads to a smaller beam quality measurement result, and the results of the wedge-plate measuring device can more accurately reflect the beam quality.
The laser warning system performs warning positioning by extracting the relevant parameters of incoming laser, and its angle positioning accuracy is directly related to the battlefield survivability. In order to effectively improve the angle positioning accuracy of the laser warning system, an algorithm for high-precision measurement of incoming laser parameters was proposed based on the principle of grating diffraction laser warning. Firstly, the laser warning system was calibrated, and the 0-level spot center of the diffraction spot calibration image was accurately extracted by Gaussian fitting. The laser angle of the future attack was fitted with the spot center of the corresponding angle. According to the fitting results, the angle parameters of the incoming laser corresponding to the diffraction image of the spot at any angle were determined. The experimental results show that the azimuth measurement error is better than 0.29°, and the pitch measurement error is better than 0.38°. The algorithm effectively improves the measurement accuracy of the laser warning system.
To solve the problem of beam pointing jitter in laser applications, a laser beam pointing stability optimization scheme based on Gaussian process regression was proposed. The composition and principle of the laser beam pointing stability system device were introduced, and the principle of Gaussian process regression method and its advantages as the fast stabilization control algorithm for laser beams were discussed. After optimization by this method, the pointing jitter reaches 2.3 μrad in the horizontal direction and 3.3 μrad in the vertical direction, which improves the pointing stability of the laser system by more than an order of magnitude. The pointing jitter is 20% of the existing linear feedback system, and has a significant effect on high-frequency noise optimization. This study is of great significance for precision experiments and precision machining that are sensitive to the directionality of laser beams.
Compound refractive lens (CRL) is used as the X-ray beam focusing device of hard X-ray free electron laser (XFEL) because of its compact structure, tunable focal length and convenient collimation. In order to meet the requirements of XFEL beam focusing with high precision, the performance parameters of CRL made of different materials were analyzed based on the geometric structure design principle of CRL, and it was proved that single crystal diamond material was the preferred material for XFEL beam focusing devices. At the same time, considering the difficulty of the machining characteristics of superhard diamond materials, and the practical problem that low repetition rate and low power titanium sapphire femtosecond laser could not realize efficient machining of large thickness materials, the feasibility of efficient machining of large thickness single crystal diamond materials by high repetition rate and high power fiber femtosecond laser was explored. The research results show that the femtosecond laser precision machining technology with high repetition rate is an effective method to fabricate diamond CRL devices.
This article presents the design and implementation of a system for laser automatic welding bead offset tracking and identification during laser processing. The system consists of a structured light sensor, a horizontal offset recognition algorithm and a real-time data smoothing algorithm. It can automatically extracts weld bead feature point data of various shapes in real time under different working conditions. By calculating and processing the feature point data, the weld bead is obtained. The real-time offset data is filtered by a real-time smoothing algorithm and then output to the laser automatic welding device, ultimately achieving automatic control of horizontal tracking. In actual on-site welding tests, the system can collect weld bead offset data in real time and accurately guide the welding laser output head to horizontally track the weld bead, laying the foundation for the realization of automated laser welding.
An inverted 3-FOV(field of view) Schmidt telescope based on 3-facet mirror was developed, and its key technical problems were studied. Based on the geometric symmetry of regular triangular pyramid, the relationship between the visual axis angle of 3 FOVs and the angle of facet mirror was derived, and a 3-facet mirror was designed to realize the multi-field observation function. The influence of the gravity deformation of primary mirror of inverted Schmidt telescope on the image quality was analyzed by finite element method. The influence of the key parameters of testing optical path on the processing error of Schmidt corrector was expounded. The stray light of optical system was studied by Monte Carlo method. Finally,the whole optical system was tested experimentally, and a 3-facet mirror was actually developed. Results show that the included angle between the mirrors is 133.08°, which can observe 3 FOVs perpendicular to each other at the same time. The test result of the telescope optical system is PV=0.614 λ , RMS=0.105 λ (λ=632.8 nm). The system can be used to measure the earth attitude in space, which expands the application of Schmidt telescope.
In order to solve the problems of high cost, large volume, vertical color difference and distortion correction for most existing ultra short focal projection equipment,an ultra-short focal micro projection lens based on DMD (digital micro-mirror device) chip was designed, and its tolerance analysis and thermal analysis were carried out. The system consists of 7 lenses, greatly simplifying the structure and reducing the cost. The F number of the system is 1.7, the focal length is 1.65 mm, the optical length is 79.39 mm, the projection ratio is 0.16, and the system has the characteristics of large aperture, large field of view and miniaturization. The design results show that the MTF (modulation transfer function) of the full field of view is larger than 0.65 at the space cutoff frequency of 93 lp/mm, which is close to the diffraction limit. The vertical color difference of the system is controlled within 1.2 μm, the maximum distortion is −2.17%, and the full-field relative illuminance is greater than 0.95, which satisfy the design requirements.
In recent years, with the rapid development of ADVB avionics digital bus, it has gradually replaced the original airborne video transmission methods such as DVI, VGA, LVDS, and etc. It has become an important data carrier in the avionics systems of new test aircraft such as advanced fighter jets, civilian aircraft, helicopters, and trainer aircraft. The existing test methods are no longer sufficient for the new video bus. Starting from the basic protocol research,in combination with the ICD structure feature information of airborne avionics system,the self-adaptive processing of high-speed and diversified video formatsand were completed through the technical approaches such as adaptive identification, resampling, timing reconstruction,and etc., and the generalized test of multi-type machines was realized. By building an experimental platform, the acquisition and recording of the unconventional videos such as 1 680×1 050@60 fps and the high-resolution videos such as 2 560×1 024@30 fps were successfully realized, and the playback video data was complete and clear, which could verify the method effectiveness. The designed test system is compatible with the existing video test status, which can effectively reduce the cost of flight test and improve the efficiency of flight test.
Continuous zoom system is a kind of photoelectric imaging device that can switch the field of view, detect and identify the target, continuously. It is fast and stable. Aiming at its control requirements for high accuracy and stability , a design method of fractional-order PID controller based on internal mode control was proposed. The proposed controller has only 3 tuning parameters obtained from desired gain crossover frequency and phase margin. This method can greatly simplifies the design and improves the implementation of controller.The control effect was compared with the traditional integer order PID on Matlab.Simulation results show that the fractional order PID controller increases the steady-state error from 0.1 mm to 0 mm,it has the characteristics of strong anti-interference and robustness. In addition, there is no overshoot and no static error after digital realization. Finally, the digital fractional-order PID was applied to actual continuous zoom system, and the system can obtain clear and stable images, which verifies the effectiveness of the control strategy.
Since the fiber Bragg grating (FBG) sensor networks have been used more and more widely, the requirements for their multiplexing are increasing;however, the FBG multiplexing may lead to spectral overlap. Aiming at the problem that the overlapping spectra could affect the demodulation accuracy of FBG sensor network, an overlapping spectrum demodulation method based on differential marine predator optimization algorithm was proposed. This method adopted the differential evolution algorithm to optimize the individual updating position strategy, which could enhance the ability to jump out of the local optimal solution. 2~4 FBG overlapping spectra were simulated, and 2 FBG overlapping spectrum demodulations were experimentally verified. Furthermore,the performance of the proposed algorithm was compared with the particle swarm optimization algorithm and the marine predator algorithm.Results show that the proposed algorithm can effectively reduce the probability of falling into local optimum, and improve the stability and reliability of the algorithm.
The segmentation of adjacent obstacles is a technical difficulty in the field of driverless vehicles. The low-line light detection and ranging (LIDAR) point clouds are sparse, and they can't cluster long-distance objects. However, the more LIDAR wire beams, the more expensive. In order to realize low-cost clustering segmentation of adjacent obstacles, the adjacent people and vehicles of common traffic scene objects were selected as the experimental scenes, and a adjacent-obstacle segmentation method based on multi-frame fusion was proposed. Based on the inertial measurement unit (IMU) and LIDAR fusion of multi-frame point cloud, it could solve the problem that low-line LIDAR is unable to cluster long-distance adjacent pedestrians due to its low resolution. The improved Euclidean clustering was proposed, and two new segmentation criteria of adaptive threshold and vector angle constraint were applied into this algorithm to improve the segmentation effect of adjacent obstacles. The experimental results show that this method has the characteristics of low cost and accurate clustering. Compared with the single-frame Euclidean cluster, the accuracy of this method for segmenting adjacent obstacles improves by about 30.7%, which has certain reference significance for the clustering and detection of low-line LIDAR in obstacles.
The performance of object tracking algorithm is usually related to the quality of initial bounding box. In UAV (unmannd aerial vehicle) ground reconnaissance tasks, due to limited response time, it is often difficult for operators to draw accurate initial bounding box, resulting in poor object tracking results. Current bounding box initialization method has some limitations and cannot meet the needs of UAV ground reconnaissance tasks. To meet the demands of actual system,a semi-automatic initialization and optimization strategy was proposed in combination with human subjective choice and visual cognition, which could give example of adaptive optimization algorithm based on visual saliency and salient region segmentation. The strategy was divided into 3 stages: coarse election, adaptive optimization and fine selection. The effectiveness of tracking box optimization algorithm was verified on 2 benchmark datasets. On VisDrone2018-SOT-test-dev dataset, in comparison with before optimization, the average success rate is increased by 0.138, and the highest is increased by 0.262. The average accuracy is increased by 0.135, and the highest is increased by 0.165. On UAVDT (unmanned aerial vehicle detection and tracking) dataset, in comparison with before optimization, the average success rate is increased by 0.093, and the highest is increased by 0.147. The average accuracy is increased by 0.082, and the highest is increased by 0.177. When processing 200×200 pixels image slices, theoretical parallel speed can reach 10 frame/s, which basically meets the real-time requirements. The proposed strategy can be combined with any tracking algorithm and has portability in embedded devices. The main contribution is the discussion of tracking initialization problem and a strategy to improve the accuracy of initial tracking box, rather than algorithmic innovation.
Aiming at the problems of the insufficient feature information extraction and the blurring of the reconstructed image details in current image super-resolution reconstruction algorithm, a multi-scale two-stage network was proposed to realize image super-resolution reconstruction. First of all, considering the phenomenon of insufficient feature information extraction in single-scale convolution layer, a network model was designed based on the general framework of multi-scale convolution layer.Secondly, considering the effect of the reconstructed image, the whole network was divided into two stages: the first stage was to extract and reconstruct the feature information according to the input low-resolution image, and the second stage was to further refine the features of the reconstructed image, so as to improve the visual effect of the reconstructed image. Jump connection and attention module were also introduced in the overall network to enhance the effective transmission of feature information. Finally, the data sets Set5, Set14, Urban100, BSDS100 and Manga109 were used as the test sets of the experiment, and the peak signal-to-noise ratio and the structural similarity were used as the evaluation indicators of image quality. The experiment shows that the values of both are improved and the visual effect of reconstructed image is good. Therefore, the algorithm has achieved good results in both objective evaluation and subjective vision.
In order to solve the problem that the quantity value of visibility parameters at home and abroad cannot be absolutely traceable, the quantity value reclaiming method tracing to the geometry amount was proposed by using working principle, structural characteristics, and environmental conditions. A high transmission visibility instrument as the main standard device was designed to calibrate the transmission visibility meter, which promoted the development of visibility value to trace the origin system. The main standard device used precision processing technique, which followed the absolute traceability method of geometric parameter measurement, and could greatly reduce the measurement uncertainty of the transmission ratio. The traceability chain combined with measurement of geometry and optical transmittance parameter solved the problem that could not reproduce and accurately measure the high transmission ratio in existing technologies. In addition, the use of cone occulter could avoid the introduction of other sources of uncertainty except rotating factors, which greatly improved the calibration capability of the visibility meter resolution.
In order to overcome the molding technology of high-steep spherical optical parts, a sub-aperture grinding method was proposed, taking the hemispherical and hyper-hemispherical infrared optical domes as the research object. The traditional generating method grinding forming theory is expanded, the spherical surface is discretized into a series of sub-aperture rings, the grinding wheel "steps" along the ring, and a complete spherical surface is obtained by splicing and forming. In this paper, the transformation relationship between the forming spherical surface and the position coordinates of the three-axis machine tool was analyzed, the machining motion trajectory was simulated, and the radius error compensation verification experiment and the variable feed parameter optimization experiment were conducted. A method of variable radius grinding was proposed to solve the problem of over-cutting of hyper-hemisphere machining materials. The forming process test was carried out on the hot-pressed zinc sulfide and magnesium-aluminum spinel spherical dome with the aspect ratios of 0.5 (hemisphere) and 0.55 (hyper-hemisphere) respectively, the sag height difference of each point on the processing surface < 4 μm, and the surface roughness Ra<1.5 μm. The results show that the method is feasible and can provide an effective solution for deep high gradient spherical processing.