Study On The Key Techniques Of An Acquisition Tracking And Pointing System In Wireless Laser Communication

The Acquisition, Tracking and Pointing (ATP) system is the key system that makesthe two terminals in a wireless laser communication system aim each other accuratelyand it is one of the most important systems to build the wireless laser communicationlink. The bandwidth and precision of the control system are the most importantparameters to determine the ATP system performance. In the paper, some keytechniques are researched to improve the two key parameters.In this paper, firstly we researched the relationships of the communicating distanceand the channel characteristic, the effect of the multiple scattering to the laserpropagation, the requirement of the laser communication to the ATP system and theway to determine the angular dispersion and receive field of view. It is proved that inlight haze weather, letting the input and output laser's wavelength are diferrent witheach other, after passing by the filter, it can avoid backward scattering effectually andfor the communication within 15km, the influence of the multiple scattering inatmosphere can be ignored.The character of the Acousto-Optic Deflector (AOD) is important to the bandwidthand precision of an ATP system. In the paper, we research the character and model ofAOD. After detailedly researching the beam dispersion angle and the Number ofResolvable Points (NPR) for an AOD, some effective methods are got to restrain thebeam dispersion, beam spot expansion and transformation, which improve the spatialresolution significantly.By researching the acquisition system, a step scan method for laser acquisition isputforward. For the same requirements to acquisition precision and speed, with the stepscan method, the laser communication system need lower refresh rate of the positionsensitive device than the continuous scan method and the cost of the system is decreased.Then with none or little initial information, we researched the relationship of the wholeacquisition probability with searching and scanning mode, speed and the angle of thereceiving filed of view and the laser beam divergence angle and give the arithmetic tocalculate the whole acquisition probability. After analyzing the relationship of the beamscan speed, the receive filed of view and the minimum acquisition time, we get therelationship that restricted the time completing the acquisition and the bandwidth of theacquisition system.For the Acquisition and Pointing in ATP, an ATP system is separated to three parts,such as acquisition, tracking and signal receiving loops. A novel deflector, AOD, isused to be the laser beam deflector, and basing on the character of AOD, a method oftracking the signal spot with reference laser is put forward, with this method, the outputlaser beam can track the input laser beam immediately, which expands the bandwidth ofthe tracking and pointing system.Quadrant Device (QD) is the key device to determine the precision of tracking and pointing system. In this paper, the effect of spot size, back noise, dark current on a QDis analysed and gives methods to build the QD model and its compensation model withthe Radial Base Function Neural Network and approach the QD position output functionwith high orders function. With studying periodicity, the ATP system can update thecompensation network and the QD position output function by itself and it improve theQD location precision and the adaptive ability of ATP system.