Study On The Transmission Characteristics Of Pulse Laser For Near-field Fixed-distance In The Rain

For the laser fixed-distance system possesses so many excellences such as strong anti-interference ability, high angular and ranging resolution, great monochromaticity and so on, that it has always been particularly recognized and widely used in the military warfare. Laser detection as a superduper fixed-distance measure must conquer the climate factors and atmospheric influences when they are used in the conventional ammunitions. As laser transmits through the rain, the interaction of light with raindrops such as absorption and scattering will take place, then the receiving laser energy is attenuated and the facular shape is changed. When these instances are serious that lead to the decline or invalidation of the system performance. As a consequence, the impact of rainfall on laser transmission characteristics is an important foundation for studying the laser propagation rules and determining the parameters of laser fixed-distance system. Under the background of laser fixed-distance system in the front of conventional ammunition, the impact of laser transmission characteristics on laser fixed-distance performance in the near-surface rainfall field at different wavelengths is deeply investigated.Based on the working principium of laser fixed-distance system and the capability of target detection, the laser ranging equation and its influencing factors are analyzed. Then the rainfall physical characteristics are analyzed, which are related to the laser transmission in the rain, such as raindrop size, shape, raindrop size distribution, water refractive index coefficient and so on. The corresponding raindrop shape model and the raindrop size distribution model are founded that adapt for rainfall distribution in the region of our country.According to the model of raindrop shape, based on Fraunhofer diffraction and geometrical optics scattering theory, the light scattering models for a spherical and a nearly ellipsoidal raindrops on the blue-green and near-infrared bands are investigated and established. The calculation formula of scattering attenuation cross section for different raindrop sizes and wavelengths is developed. The light scattering energy distribution of raindrops is obtained by numerical calculation and the factors that affect the scattering properties are analyzed.According to the practical engineering applications, the forward scattering of raindrop attenuation cross section is corrected. The absorption efficiency factors of raindrops on the blue-green and near infrared bands are analyzed, and with the raindrop scattering attenuation cross section, the rain attenuation calculation model is educed. Then the attenuation coefficients for typical wavelengths on the blue-green and near infrared bands under different rainfall rate are computed. By the calculation results, the influences of laser beam divergence angle and transmission distance to the energy attenuation are analyzed. At last, based on the raindrop attenuation model, the back scattering features are analyzed, and with a particular type system as an example, the SNR of laser backscattering at different rainfall rates and distances is calculated.Based on the Monte Carlo simulation method, the transmission model of pulse laser in the rain is founded. And the computer simulation of photon trajectory as well as the statistical process is presented. Then the impact of rainfall rate, transmission distance, emission pulse width and other factors on the laser pulse delay are analyzed, as well as the ranging error brought by the pulse delay. According to the simulation results, the impact of various parameters in the rainfall on the laser spot extending is analyzed.According to the theoretical study of laser attenuation characteristics of raindrops and rainfall, the simulation and experimental methods for laser transmission characteristics are presented. By combination with the Bouguer-Lambert attenuation law, the experimental principle and testing programs are instituted. Using ZEMAX optical software to simulate the light scattering characteristics of532nm laser through the spherical water droplets, and the simulation data are compared with the theoretical results. The experimental raindrops are obtained from water droplet generating device, and then the the energy attenuation of laser beam at532nm and1064nm wavelengths through a single or multiple raindrops are tested. On the other hand, an outdoor rainfall field to simulate real rainfall is set up to test the laser transmission characteristics at different rainfall conditions.In order to achieve the reliable detection of the target in the rain for laser fixed-distance system, the optimization methods to improve the performance of laser fixed-distance are developed. First of all, the paper analyzes the laser pulse transmission characteristics and influencing factors of the laser ranging equation, then based on the existing pulsed laser transmitter and receiver components, the programs to improve the accuracy of fixed-distance are given, that is, the method to collimate laser beam by using elliptic and hyperbolic cylindrical lens as well as non-spherical and deformed prism group, the technique to improve the sensitivity of receiving system by using avalanche diode (APD) and its amplifier circuit, the means to enhance the transmitting power by using high DC voltage driving laser diode to emit high-power pulse.The findings of this disquisition can be directly applied to design and develop the laser fixed-distance system, and provide theoretical foundation and technical support for laser fixed-distance system accurately ranging in the near-surface rainfall field environment. The researches are the key to improve the transmission efficiency of laser fixed-distance system and to accommodate the laser fixed-distance system to the rainfall environment.