Research On Scanning Imaging Ladar System

For its quality of concealment and advantage of rich-information (such as distance, intensity and distance-angle, etc.),3D laser imaging has a broad application prospect in target identification, topographic mapping, machine vision, and many other fields. In this thesis, the theoretical research is taken on the single-pixel scanning3D imaging system based on TOF of the laser pulse. And according to the development of prototype,3D image could be acquired finally. The main research works of this thesis are as follows:In the beginning of this thesis, it is studied that several factors involved power detection ability and related theories within ladar system, including atmospheric transmission of laser signal, the reflection of target, interference of background light radiation and photo-electricity detection. A theoretical method has been proposed in thesis for researching additional power attenuation about pulse reflection, basing on the spatial-temporal characteristic of laser time-dependent cross section. According to this method, the reflection ladar range equation is re-given, which has a unified expression for both pulsed laser reflection and continuous one. After discussing of the generation process about output signal and noise in receive photodetector, the best reverse bias of APD based on minimal SNR is calculated.Basing on Q-switched laser pulse generation theory, the numerical simulation of output laser pulse waveform is given, using a simplified two-level laser model. Through the analysis of gain frequency response of transimpedance amplifier, the bandwidth design is given for pulse photoelectric conversion. The methods for improving the accuracy of time measurement are studied, and then the experimental test are taken for two kind of time intervals measuring devices on the test platform we set up. According to number of experiments about time delay solutions and effect of time-walk reducing, a constant fraction discrimination circuit is designed and implemented.A method for a non-coaxial optical structure of the bistatic ladar system is proposed, which placed away from the focal plane and has a non-scanning large FOV. The len parameters and energy loss could be calculated consequently. A non-coaxial synchronous scanning structure designed for orthogonal dual-galvanometer scanning system is proposed, as well as the constraints of instantaneous receiver FOV under the influence of bistatic station spacing. Through mathematical modeling for the dual galvanometer system, theoretical studies is taken on the generation process of projection error, and the practical methods of distortion correction are explored subsequently.Time error induced by laser pulse waveform distortion in the reflection process is studied. For this purpose, analytical solution of two reflected pulse waveforms, whose original forms are Gaussian model and Gamma model, from tilt plane are derived, basing on time-dependent laser scattering cross section. The study on single pixel time error duel to reflection stretching gives an interpretation to generation of distortion in detection3D image. By analyzing the main factor of distortion, a compensation algorithm is proposed based on the slope of the point-cloud image probed. The results of numerical simulation show that, the height distribution could be restored to the image consistent with the actual value after three compensated loops.According to the research on coordinate model for system scanning mechanism, the coordinate transformation for3D model reconstruction is established. Basing on computer graphics, the mathematical transformation and realization of operation of3D model are studied, and the visual display and operation of model are achieved ultimately.Integrating all the findings above, one prototype of single pixel scanning3D laser imaging system have been development successfully, which have no domestic report yet. Current system gets the furthest detect distance of36m without focus lens, the distance error of less than0.2meters, the imaging FOV of±5°, the standard deviation of time measurement of less than340ps, the distance resolution of7cm and the lateral resolution of1.5mrad. This prototype has already been used in testing work on the target reflective properties.