Research On Wireless Communication Technology Based On Acoustic-optic Modulation

Free Space Opitcal(FSO) Communication，using laser beam as a data carrier，is acommunication technology which can transfer text、image and sound informationdirectly in the space. FSO system has advantages such as low supply power、small size、easy to set-up. FSO system is quite fit for communication across vally、temporarycommunication、internal data links and so on. And，this technology is a better way tosolve “the last mile” problem. In this work，a FSO syetem based on acousto-opticmodulation(AOM) has been designed. This system is also a intensity modulated/directdetection system as a traditional FSO system is. What’s more，the designed system hashigh light-using efficiency、high modulated rate and better noise restraining ability.Firstly，the acoustic-optic effect was investigated. According to the analysis，aHe-Ne laser，which has a wavelengh of632.8nm，was used as syetem light source. Thelaser has a gauss intensity distribution and a small beam divergence angle. And，aAOM，using PbMoO4as acousto-optic material，was selected. This AOM has a lightextinction ratio of1000:1、a acousto frequency of100MHz and light transmittance of95%at632.8nm. Because acousto-optic modulator has excellent temperature stabilityand good optoelectronic quality, and He-Ne laser has good power stability, theconsistency of the beam quality parameter, these are both suitable for use in free-spaceoptical communication systems.Secondarily，combined with work characteristics of the acousto-optic modulator,Frequency-shift keying(2FSK) method was selected as digital modulating scheme.Some system parameters，such as signal bandwidth、frequency shift index and powerspectrum，was investigated. The2FSK modulating and demodulating was implementedby Field Programmable Gate Array(FPGA). In this manner，the ASIC design andproduction process was not needed，that can reduce overall system cost and size，andincrease system reliability.Then，the light signal receiving and filtering circuit was designed. A PMT wasselected as light signal receiver，which has a large detective area and short responsetime and high sensitivity. The voltage signal was obtained by the current to voltageconverter. Then，the signal was shaping and filtering by the main filter and amplyfiler，which has a bandwidth between1.5MHz and2MHz. And the next，a standard CMOSdigital signal，which can be directly dealed with inside FPGA，was obtained by the voltage transformerFinally，the demonstration system was set up and tested. The original informationwas sent to FPGA，inside the DE2，using the serial ports of a PC. The data rate was12.8kbps，and the rate of high frequency carrier was1.6MHz while low frequencycarrier was0.8MHz. The outcome was recorded and analysed. The bit error rate of thesystem was0.8%.The designed FSO system, based on acousto-optic modulation, is demonstrated aseasy to set-up，low coast and good noise restraining ability. The system can be usedwhen conventional communication was failed and a new communication system has tobe set up rapidly.