Research On Laser Communication System Based On Acoustic-optic Frequency-shift Technology

Free space optical communication (FSO) system is a system transmitting text,audio, and video signals by using a laser beam as a carrier and the atmosphere as thetransmission channel. In accordance with the transmission beam modulation anddetection ways, FSO communication system can be divided into intensitymodulation\direct detection (IM\DD) communication system and frequency or phasemodulation\coherent demodulation communication system. The laser communicationsystem based on acousto-optic frequency shift described in this article refers to usingacousto-optic modulator to dually modulate intensity of transmission beam. Comparedwith IM\DD system, the system has better communication SNR.Firstly, the transmitter of acousto-optic frequency shift of laser communicationsystem transmitter has been designed and manufactured, the transmitter including thelight source, acousto-optic modulator and optical beat frequency system. Acousto-opticmodulator can modulate intensity and frequency of light beam. ComparingRaman-Jonas diffraction and Bragg diffraction it can be obtained: Bragg diffraction canproduce a stable frequency shift, less series of diffracted beams, and high utilization ofoptical power. So, Bragg diffraction is a good intensity and frequency modulationscheme. Optical beat frequency system can converse optical frequency modulation tointensity modulation. The analysis of principle of optical beat frequency system showedthat the more consistent the polarization direction, the more close the beam’s relativeintensity, the narrower the spectral width of beam, and the smaller the angle of twobeams, the more strong the IF signal of optical beat frequency system. Then on the basisof structure of optical beat frequency system given in the literature, a new structure hasbeen designed. The new structure is simple and convenient for debugging.Then the receiver of acousto-optic frequency shift of laser communication systemtransmitter has been designed and manufactured. The photomultiplier tube detects thesignal, the LC bandpass filter amplifier of the receiver filters and enlarges the receivedsignal, and the D flip-flop down converses the frequency of the carrier. The fieldprogrammable gate arrays (FPGA) complete envelope detecting and signaldemodulation. A variety of optical, electronic devices have been chosen to achieve thetransmitter and the receiver. Transmitter contains helium-neon laser source,acousto-optic modulator, high reflectors, and transflective reflector. The receiver contains photomultiplier tubes, analog LC filter circuit, and FPGA.The test of the system has been carried. For the transmitter, firstly, the waveformand spectrum of the transmitter beam after being transmitted a long way in atmospherichave been got; secondly, the waveform and spectrum of the IM\DD communicationsystem transmitter beam have been got in the same way; finally, the waveform andspectrum of two transmitters have been compared. The compared result shows thesystem has good SNR. For the receiver, tests for each modules and the whole receiverhave been carried, the test results show that each module can complete its own functionand the receiver has low bit error rate.Finally, a summary and analysis of the system has been made, and some ways forthe system improvements have been given.