| Atmospheric optical communications are hot issue in the communication fieldpresently. The random fluctuations of beam amplitude and phase caused by atmosphericturbulence will induce intensity scintillation and wave-front aberration at the receiver,which severely limite the performance of optical communication systems. Adaptiveoptics (AO) is an important technique to alleviate the turbulence and improve thepropagation quality of beam. This dissertation mainly researched the impact ofturbulence and the action of AO in optical communication based on On-Off-Keying(OOK), Intensity Modulate/Directly Demodulate (IM/DD) inherent communication andhomodyne Binary-Phase-Shift-Keying (BPSK) coherent communication systems. Themain contents include:An optical communication system model based on OOK,IM/DD was built withnumerical simulation method to analysis the effect of AO correction in improvment ofinherent communication performance. The results showed that AO correction at both thetransmitter and receiver were useful for improving the performance of opticalcommunications, however, the outcome of AO correction was related with the aperture:with big aperture, receiver AO was more available than transmitter AO; with smallaperture, receiver AO had litter effect, but transmitter AO was available, too.Experiment of AO correction in2.5Gbps optical communication is developed inthis dissertation. When multiple-mode fiber was used as receiver, the bit-error-rate(BER) with AO closed could be improved more than4orders. When single-mode fiberwas used as receiver, AO could restrain the deep fading of received signal effectively.And BER with AO tip-tilt correction would be decreased one order, while there was nobit-error in120seconds when AO was closed. The difference between receiver andtransmitter AO correction was contrasted in experiment: the average BER decreased2~3orders and7orders after receiver and transmitter AO correction respectively.In this dissertation, a numerical simulate model of space homodyne BPSK coherentcommunication was built. The result showed that total piston induced by turbulencewould cause serious error-codes. When the piston was removed, a probability densityfunction (PDF) statistic method of wave-front RMS and PV was proposed to study therelationship between the level of wave-front aberration and BER. In the case ofinfinitely SNR, error code was prone to emerge if wave-front RMS≥0.28λ and PV≥1.02λ; when SNR=8, error code would be emerge if wave-front RMS≥0.16λ andPV≥0.71λ. And the RMS and PV value of wave-front would be strictly required withthe decreasing of SNR.The deformable mirror fitting correction and Zernike mode correction were used toanalysis and discuss the effect of AO in the coherent optical communication system.The result showed that, with weak turbulence, the homodyne mixing efficiency could beraised when tip-tilt aberration was corrected only. Well, higher order correction wasneeded to raise the mixing efficiency when turbulence is strong. When the correctionorders of AO system and atmosphere coherent length r0were fixed, there was an optimalaperture, which could maximize signal-noise-ratio (SNR) gain.180degree optical hybrid and homodyne BPSK coherent optical communicationtest system were built in lab. The effect on BER of coherent optical communication ofatmospheric turbulence was analyzed by transmitting random binary code. The effect ofpiston was the amplitude of received signal was unstable, and the received signal wouldreverse. Strong simulative turbulence would cause fluctant and offset of received signal.It is confirmed through theory, simulation and experiment that, AO technologywere useful for decreasing the BER by improving system SNR and decreasing theintensity and frequency of fading in inherent optical communication system. In BPSKcoherent optical communication system, AO technology could correct wave-frontaberration effectively, and improve the phase matching degree of local beam and signalbeam to decrease system BER. |
PDF Full Text Request