Research Of Electro-Optical Tracking System For Wireless Laser Communication

Wireless laser communication offers many potential advantages,including small size,light weight,high bandwidth,no need for frequency allocation,and resistance to electromagnetic interference and interception.However,the extreme narrow divergence angle of the laser beam brings main challenge to the establishment of the laser links between moving communication transceivers.Therefore,the high precise electro-optical(EO)tracking control technology is one of the core technologies in laser communication.It's still a major bottleneck restricting the practical use of laser communication.This dissertation studied the intelligent tracking technology based on the genetic algorithm and neural network,and the non-mechanical tracking technology based on the liquid crystal optical phased array(LCOPA).The main contents and the innovative pionts of this dissertation are summed as follows.1.The EO tracking system is difficult to accurately model because of its strong nonlinear characteristics.Nonlinear resistance torque is proposed to describe the system's nonlinear phenomenon and the genetic algorithm is used to identify model parameters.The model's root mean square error was reduced using nonlinear resistance torque by 2.5 times compared to the Stribeck friction model and by 12 times compared to the linear model.Under the identified model,the system's nonlinearity was effectively compensated,which demonstrate the the feasibility and effectiveness of the proposed method for the identification of EO tracking systems.2.To overcome the slow convergence of neural network learning problem in a real system with complex nonlinearity and noisy environment,a novel method of neural network control with nonlinear compensation is proposed.The nonlinear compensation reduces the complexity of the system in order to improve the learning efficiency of the neural network.The algorithm design is in accord with the order of nature.In analogy with the evolution providing effective methods for a kind of typical problem,the genetic algorithm realizes nonlinear model identification and compensation regardless of time overhead.In analogy with the nervous systems dealing with the environment stimulation quickly,the neural network realizes real-time optimization of control effects from learning the target movement information.The proposed intelligent control method was practically used in the EO tracking system of laser communication.Indoor and field experiments were conducted to validate the intelligent tracking system as follows.(1)In the indoor sinusoidal motion tracking experiment,the actual performances of the single neuron,BP neural network and RBF neural network were tested.All the three algorithms can intelligently optimize tracking effect,and the Bode diagram indicated that the tracking performance was better than-27dB with the disturbance frequency of 1Hz and the amplitude of 3 deg.(2)In the ship to shore laser communication experiment at a distance of 4km,the two-axis tracking errors(1?)of the intelligent tracking system are 0.0101 deg and 0.0204 deg,respectively.The intelligent tracking system has reached the mature technological level and can assist to implement 2.5 Gbps laser communication link with a bit error rate of 7.0199×10-10.Compared to the conventional tracking system,the intelligent tracking system has the advantage of automatic optimization of controller parameters from learning the target movement information without human intervention,which is more suitable for complex application environment.3.To satisfy the requirements of miniaturization laser communication systems,the LCOPA based non-mechanical tracking technology was investigated.The system transfer function and the MATLAB simulation model were built.Three demonstrative experiments were executed,including:(1)beam pointing experiment,which simulated a few typical 2-dimensional target motion tracks,such as beeline track,U-turn track,and S-curve track;(2)beam tracking experiment,which indicated that the tracking performance was better than-17dB and-lOdB when the disturbance frequency was 1Hz and 2Hz;respectively;and(3)multi-beam pointing experiment,which realized one-beam-in and multi-beam-out using one single LCOPA device.Simulation and experimental testing demonstrated the feasibility of the non-mechanical tracking technology that employs LCOPA and revealed its broad application prospects for wireless laser communication network.