Study On Key Techniques Of Long-range Laser Detection

To meet the needs of surveying,mapping,reconnaissance,and precision guidance,a 3D laser imaging technology based on pulse slice was developed with high performance.Several key aspects of this imaging system were studied including the design of the dual wavelength laser for detection,how the resonator mirror quality affects the beam quality of the high-power solid-state laser,spatial structure of the light emission method based on laser multiple sub-beam interference,the study of the 3D laser imaging system based on pulse slice,the analysis of high-frequency noise in the system and its suppression method,and the test of the 3D laser imaging prototype based on pulse slice.Design of dual wavelength laser based on complementary absorption technology of Nd:YAG/Nd:YVO4:the long-range detection laser source should have the characteristics of high peak power,narrow pulse width,high beam quality,and high temperature stability.However,the traditional Nd:YAG laser is limited by its laser crystal material,so the output power and power stability of the laser cannot meet the requirements of long-range detection under high-power pumping.Therefore,we propose a complementary absorption technology of Nd:YAG/Nd:YVO₄,which combines the good thermal physical properties of Nd:YAG laser crystal with the wide spectrum absorption characteristics of Nd:YVO₄ laser crystal,resulting in a detection laser source with high output power,high beam quality,high polarization characteristics,and high power stability.The experimental results show that the optimal multi-segmented crystals are 40 mm length with 0.2%doping concentration(Nd3+)and 16 mm with 0.2%for the Nd:YAG and Nd:YVO4,respectively,allowing high efficiency,high power,and stability of the laser pulse output.The 532 nm laser output is realized by intracavity frequency doubling technology to complete the design of dual wavelength laser for long-range detection.Resonator mirror quality affects high-power solid-state laser maximum beam quality:beam quality is an important evaluation index of a high-power solid-state laser.The influence of the thermal lens effect of the gain medium,the uniform doping distribution within the gain medium,and the mirror quality of the resonant cavity on the maximum laser beam quality cannot be ignored under high-power output condition.The effects of resonator mirror deformation and mirror flaw on maximum laser beam quality are calculated according to the perturbation theory of quantum mechanics.Micro-deformation and micro-flaws are perturbations to the ideal resonator in terms of the light-field motive equation and relevant quantum mechanics theory.A detailed numerical processing method for the resonant cavity is presented which can be applied to quantitatively analyze the influence of resonator micro-factors on maximum beam quality based on the eigenvalues and eigenfunctions of the light-field equations with perturbation.This method can also be applied to analyze the influence of the thermal lens effect and the uniform doping distribution within the gain medium on the maximum laser beam quality,which can be used as a simulation method for laser basic research.A spatial structure light emission method based on laser multiple sub-beams interference:a spatial structure light emission method based on laser multiple sub-beam interference is proposed.With good coherence of the laser,spatially structured light with characteristics of light and dark phases is formed.The main interference peak intensity of spatially structured light is high,and the divergence angle is small,which can effectively improve the far-field divergence of laser beam during laser transmission.As the size of the laser center spot is reduced,the ability of the laser source to detect a long-range small target is greatly improved.Study of the 3D laser imaging system based on pulse slice:a 3D laser imaging system based on pulse slice is proposed,which contributes to long-range high-precision 3D laser imaging.This method not only has the technical advantage of range-gated 3D laser imaging,but also overcomes the limitation of pulse width on the longitudinal distance resolution for range-gated 3D laser imaging by using a CCD(CMOS)optical sensor with a large-scale pixel array.The opening time of the CCD imaging detector shutter is adjusted with a certain delay step and the laser pulse signal reflected by the target is intercepted in multiple frames to generate a dynamic gray-scale time series on each pixel of the detector.By processing the gray-scale time series,we can determine the return time of the pulse peak position to obtain the relative longitudinal distance of the target surface corresponding to each pixel point,and then realize 3D measurement of the target.The experiment is designed to reconstruct the target plate with a longitudinal distance of 10 cm at 1 km.The results show that the system can achieve long-range high-precision 3D imaging,and the range resolution reaches centimeter magnitude,indicating this method can surpass the previous limitation of the laser pulse width on the longitudinal range resolution of the 3D laser imaging system.The analysis of high-frequency noise and its suppression method in 3D laser imaging system:we determined the effects of mode competition on the high-frequency noise of an output optical pulse according to the expressive mode of the noise.High-frequency noise within the ranges of 0.25-0.75 GHz and 0.75-1.8 GHz is mainly caused by high transverse and multi-longitudinal modes in the laser resonator.This high-frequency noise seriously affects the stability of the laser light field,resulting in many undulating noises in the 3D laser reconstruction results.Through noise reduction of the captured laser pulse,we can obtain a uniform and smooth pulse signal,which greatly reduce the laser pulse burr jitter phenomenon.Using this method,we can get a relatively smooth 3D reconstruction result,which improves the 3D imaging measurement accuracy and imaging quality.Test of 3D laser imaging prototype based on laser slice:a 3D laser imaging prototype with long-range,high-precision was developed by combining a high-efficiency laser with a high-response CCD imaging detector.In a long-range field test,at an action distance 1 km,the longitudinal distance was less than 10 cm with a field depth and a field angle greater than 57 m and 3°,respectively.In a heavy haze environment,good 3D reconstruction results were obtained,indicating that the prototype has good penetration ability,suggesting it can be used for high-precision 3D measurement of targets in dense fog or other obscured environment.The results show that this 3D laser imaging system has technical advantages of long working distance,high detection accuracy,and large imaging field of view.