Research On Modulated-pulse Lidar For Ocean Exploration Based On Frequency Filtering

The principal task of the lidar system design for ocean exploration is to restrain the backscattering. By investigating the light transmission character in seawater, the backscattering rate and the expression of random distribution sources of backscattering are acquired, and the seawater channel model of the lidar system based on backscattering model and target reflection model is formed. This model is similar to the actual optic characters no matter on time domain or frequency domain. It is an important tool of theoretical research in this paper.As known from the analysis of the channel characters, reflection signal from the target can retain the frequency character of the detection signal. Due to the fact that the seawater backscattering comes from the reflection of the random distributed scattering sources, a large amount of random reflection signals overlap to each other so that the backscattering signal shows a low frequency character. Based on such frequency character, a method of using frequency filtering to restrain the backscattering signal is introduced in this paper. This is a method to modulate the detection signal to a high frequency at the transmitter, do frequency detection at the receiver, and use bandpass filter to cease the low frequency signal. Based on this algorithm, the fulfilling scheme of the modulated-pulse lidar system is proposed. Furthermore, by using theoretical deduction and simulating calculation, the signal transmission process in seawater channel and the signal processing ways of the system are fully analyzed. In the end, the validity of this method is illustrated by comparing the target contrast before and after the frequency filtering.In order to optimize the design of the modulated-pulse lidar system, the relation between system parameters and the performance are investigated, and the principle of system parameter design is introduced. System parameters such as transmitting pulse power, range of modulating frequency, modulation index, bandwidth of filter and height of fight are included. In the designing process, the influence of parameters to the system performance must be taken into consideration, and also practical designing parameters will be raised according to nowadays technique level or the developing trend, so that it can give reference opinions to the entity research or the technique developing direction. Besides, a conclusion that the modulated-pulse lidar system is suitable for shallow water exploration in ordinary seawater condition is made based on study of the relation between environment and system performance.Instruments and techniques of modulated-pulse lidar system such as modulation methods of laser pulse, laser detector with fast response time and high sensitivity, filtering and processing module of microwave signal are different compared with previous lidar system. In this paper several suitable instruments, techniques and design scheme for modulated-pulse lidar system are proposed according to these differences and the feasibility of this scheme is validated.The modulated-pulse lidar system has reduced backscattering signals by 18dB and enhanced the target contrast over 10dB in the shallow water in computer simulation, which shows that this system has better detection effect than ordinary algorithms in shallow water with common water quality parameters. The simulation result shows that this system can be applied in underwater imaging other than underwater detection. The frequency filtering method can also be applied to the system design for target identify in other channels with serious backscattering and multi-scattering.