Research On Underwater Laser Proximity Circumferential Scanning Detection Technology

The underwater optical communication,imaging and marine laser radar have been deeply researched since the discovery of blue-green "light window" in seawater.Underwater blue-green laser detection has advantages of good direction,high precision and strong anti-interference ability.It can be applied in proximity target detection system which will achieve better positioning and imaging accuracy than underwater acoustic detection.This paper carried out the research on underwater pulsed laser short-range circumferentially scanning detection,which can identify distance and orientation of the underwater target and provide the information to the torpedo equipped with directional blasting warhead.Then the delay and direction of warhead initiation is precisely controlled to make full use of the explosive power of the warhead.The research has important significance to improve damage efficiency of the torpedo and realize coordination between fuze and warhead.This paper focused on the subject of underwater laser proximity fuze,and the underwater pulsed laser circumferentially scanning detection system was designed,which was suitable to seawater environment and torpedo carrier.The Monte Carlo simulation method of underwater pulsed laser echo was proposed,which contained the photon reflection process of true target surface.The optical path of non-coaxial laser detection system was optimized based on best signal-to-noise ratio(SNR).The adaptive underwater laser echo filtering method was proposed to extract the target echo signal,and then the ranging accuracy of pulsed laser detecting in seawater is analyzed.A new method of target azimuth recognition was designed according to the pulse laser scanning detection methods.The influences of scanning frequency and pulse frequency on scanning detection were studied.Underwater laser proximity circumferentially scanning detection system was finally manufactured,and then the test experiments were carried out in laboratory basin and seawater.The Monte Carlo simulation method of pulsed laser transmission and target echo was proposed based on seawater optical properties and target reflection characteristics.The intrinsic optical properties of the each component in seawater were studied and then the absorption and scattering coefficients of 532 nm green laser were calculated.The appropriate scattering phase function was selected by analyzing the difference among Mie scattering phase function,fitting phase functions and experimental data.The method for sampling direction of the reflected photons was proposed based on bidirectional reflectance function(BRDF).The simulation program was developed to simulate underwater target echo under the conditions of different seawater optical properties and detection system parameters.The optical path of the system was optimized for underwater laser non-coaxial scanning optics system based on detection area distribution and the system signal to noise ratio(SNR).Blind distance formula,echo power equation of underwater target and sea backscatter were derived to analyze the relationship between detection area,system SNR and optical parameters.The optical parameters were optimally calculated based on particle swarm optimization(PSO)under different water quality,and then the best SNR of system and corresponding optical path parameters were obtained.To prove the correctness of the theoretical model,optical simulation system of underwater laser detection was designed to conduct experiment.The influence of the optical fairing on the optical path of the system was analyzed,and the optical fairing was designed to meet the requirement of the seawater environment and the scanning angle.An algorithm for eliminating the seawater backscatter noise based on variable step size minimum mean square(VSS-LMS)adaptive filtering was designed,and then the accuracy of pulsed laser ranging in seawater is analyzed.Through the design and adjustment of the adaptive filter,the backscattering signal of the seawater in the input signal was eliminated,and then target echo was extracted.The time domain waveform of target echo was deduced based on the shock response function of seawater channel and target reflection,and the simulation method of underwater laser ranging data distribution was proposed by combining the echo time identification method and time domain waveform of target echo.The accuracy of underwater laser ranging under the conditions of peak position discriminator(PPD)and constant-fraction discriminator(CFD)was simulated.The influences of pulse width,optical properties of seawater and target characteristics on the laser ranging accuracy were deeply studied.The simulation model of target capture rate for underwater laser scanning detection system was established.The underwater target azimuth recognition scheme was designed.The minimum scanning frequency and minimum pulse frequency that system could exactly detect the target was calculated based on encounter condition of detection system and target.On the basis of that,the mathematical model of capture rate of the underwater small scale moving target was established.The maximum capture rate,the corresponding laser scanning frequency and pulse frequency were obtained when the target was at different distance in different seawater.The prototype of underwater pulsed laser proximity circumferentially scanning detection system was designed and manufactured,and the performance of prototype was tested in both laboratory basin and seawater.The experiment results verified the correctness of the proposed theoretical models and methods in this paper,such as target echo Monte Carlo simulation method,backscatter filtering algorithm and simulation method of underwater laser ranging data distribution.The detection range of the prototype in seawater can reach 4 attenuation lengths,the accuracy of ranging is better than ± 0.3m,and the target orientation recognition accuracy is better than ±5°.