Study On Laser Communication Technology Based On Acousto-optic Modulation PPM Coding

Optical communication is a kind of technique which takes advantage of a certain modulation method(internal modulation or external modulation)loading the signal to the light wave to transmit information.It is divided into wireless and wired by the transmission medium.Wired optical communication uses optical fiber as the medium to communicate,while wireless optical communication(optical wireless)refers to the laser in space transmission as the carrier to send information between a number of terminal which need to communication.Wireless optical communication as a kind of high-speed,confidential communications which not only have kind characters of optical broadband,high rate and good confidentiality but also the deployment of flexible,convenient and suitable for many special scenes(e.g.between satellites,stars,ships,terrain complex Alpine buildings,etc.).We focus on short-range atmospheric laser communications.In this paper,a semiconductor laser with wavelength of 637 nm as the light source,an acousto-optic modulator based on the Bragg diffraction model as the external modulation device,the photomultiplier tube with high sensitivity and broad spectral response as the light receiving device and the 16-PPM codec Program are chosen.Under the condition of above and by analyzing the characteristics of the atmospheric channel transmission and the channel model of the laser,a complete laser communication system is designed.There exist three problems to accomplish the PPM modulator.First problem is the slot frequency,frame frequency,superframe frequency generation and control.Second problem is the precise control of the pulse position,and the last problem is the pulse width control.Using FPGA development board,we designed the system clock module,counter,comparator and pulse generator which can solve the above problems.There exist two problems to accomplish PPM demodulator design: one is how to do slot synchronization and the other is how to achieve the frame synchronization.Using pulse edge detection method first problem is solved,and using four groups of Barker codes the second problem is solved.In order to improve the communication speed between the host computer and the FPGA,we designed the USB control module with USB2.0 technology,and used 512 bytes as a data packet for data transmission.The maximum communication capacity is 480 Mbps,which can realize bidirectional communication.On the basis of the full testing of the main devices,we built a one-way laser communication system,carried out the text information transceiver experiment,video image acquisition and display experiment.The frame frequency of the text message transmission and reception experiment is 200 KHz / s,and the transmission rate is 800 Kbit / s which make the system work for a long time.