| Free space optical communication uses light as communication carrier,and its transmission channel is atmosphere channel.It has many advantages,such as high speed,rich spectrum resources and convenient communication equipment.In free space optical communication,balanced detection is a novel and important way.It has better sensitivity and lower noise.To further improve the sensitivity of balanced detector in free space optical communication,the sensitivity characteristics of balanced detector are studied from the characteristics of atmospheric channel,the differences of different modulation modes and the balance detector itself.Firstly,to research the modulation way with low bit error rate in atmospheric channel,the combined model of atmospheric effects is deduced with the refractive index fluctuation model and the theoretical basis of atmosphere.The average bit error rate of three modulation systems,OOK(On-Off Keying),QPSK(Quadrature Phase Shift Keying),and Pol SK(Polarization Shift Keying),is calculated and compared by numerical simulation.The results show that the average bit error rate of Pol SK is the lowest,because the influence of atmospheric channel on polarization state is lower than that on phase and intensity of the light.The bit error rate of QPSK is higher than Pol SK and lower than that of OOK.DP-QPSK(Dual Polarization-Quadrature Phase Shift Keying)communication system is chosen for the next research by comprehensive consideration.Secondly,to research the influence of the balanced detector structure on the sensitivity,the expression formulas of the signal-to-noise ratio and sensitivity of the balance detector are deduced and further numerical simulation is carried out.The results show that when the consistency parameter is 0,the sensitivity reaches the extreme point when the beam splitting ratio is 0.5.When the consistency parameter is greater than 0 or less than 0,the minimum value of sensitivity increases,and the extreme point moves to the right and left respectively.This shows that the extreme point can be obtained by changing the value of beam splitting ratio when the diode is unbalanced,but the performance of the detector will be deteriorated.On the other hand,about the trade-off between the balance,the average value of quantum efficiency and the extreme value of quantum efficiency,the numerical simulation shows that if the average value of quantum efficiency has little difference,the balance is more important outside the compensation interval of beam splitting ratio.In the compensation range of beam splitting ratio,the extreme value of quantum efficiency is more important.If the average value of quantum efficiency is quite different,the average value of quantum efficiency has the greatest influence in most situations.Finally,about the relationship between sensitivity and local oscillator power,numerical simulation shows that when the beam splitting ratio is 0.5,the greater the local oscillator power,the lower the sensitivity and the higher the signal-to-noise ratio.When the beam splitting ratio is not 0.5,there is a tolerable maximum local oscillator power.With the splitting ratio getting closer to 0.5,the tolerable maximum local oscillator power gets greater.Finally,to research the sensitivity characteristics of balanced detection in the actual communication system,a DP-QPSK-balanced detection communication system is designed,and it is proved theoretically that it can realize the preset function.The simulation software is used to build the communication system,and several groups of experiments are carried out.The relationship between sensitivity and beam splitting ratio,consistency parameters and local oscillator optical power is analyzed through the fitting curve,and the results are compared with the aforementioned theoretical results.The results show that the system can realize high sensitivity,and the simulation results are in good agreement with the theoretical analysis results. |
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