Analysis On The Performance Evaluation Of Adaptive Optics For Free Space Optical Communication

Free-Space-Optics(FSO)is becoming the core of Next generation high-speed communication technology.Comparing to the RF communication,FSO has many advantages including the high bandwidth,the concentrate of energy and the anti-interference performance.Comparing to the optical fiber communication FSO also has advantages as the low-cost of laying,the flexibility of networking and so on.The main distraction of FSO is the attenuation of signal power because of atmospheric turbulence.The atmospheric turbulence may make the drift of laser beam and the distortion of laser wavefront,and the influence can only be weakened by the real-time compensation of Adaptive Optics(AO)system.AO system has been applied to astronomical observation.The design parameters and evaluation indicators are established by the quality of imaging which indicated by the Strelh Ratio.But FSO is an energy harvesting system and the explicit indicator of communication is Bit-Error-Ratio(BER).In this paper we present an evaluation criterion and a design index of AO for FSO based on Power-in-backet(PIB),and based on the above analysis we have designed AO system for FSO,which possesses the basic ability of high speed communication under turbulence.In this paper we present the evaluation of FSO energy efficiency based on PIB which solves the problem to precisely calculate the energy efficiency under different size of target surface.First we model the PIB and derive a specific formula of PIB,then combining turbulence theory the average PIB has been derived as well as the average BER,which provides a basis for the design of FSO-AO.According to the theory the receiving energy are related to the radius of receiving surface and the strength of turbulenc.Only when the surface radius is 0.6 of radius of Airy discthe energy can be fitted by Strelh ratio.Under middle turbulence with D/r0=4,the PIB with R of 5 times of Airy disc,the difference between PIB and Strelh ratio is 0.9,which means a great deviation of Strehl ratio.Then in the simulation,the random wavefronts have been generated by computer to calculate the average PIB.The results show an error of 1% between simulated PIB and the theory,which verify the effectiveness of PIB theory.At last the experiment of PIB has been done and the error between experimental data and the results calculated by PIB theory is 3.5%,and the error between PIB and Strehl Ratio is 29%,which shows a higher accuracy in the calculation of energy efficiency.We first analysis the probability of AO in the correction of turbulence and its influence on the BER of FSO,which provides the theoretical basis for the design of FSO-AO.The influence of corrected Zernike terms number on average PIB has been analyzed,then based on the probability theory the probability distribution of each Zernike term has been derived.Combining with PIB,the missing probability of laser beam has been derived.The tip-tilt correction value is with the exponentiation of 5/6 with turbulence strength.Under the probability of 10-5 and the strength of turbulence of D/r0=4,the max tip-tilt correction value is 10 rad.Also the correcting value of AO tip-tilt and the orders of Zernike terms have been revealed.The standard of AO frequency is analyzed by the turbulence power spectrum.The energy efficiency would be 80% under the correction frequency of 1.5-2 Greenwood frequency.Based on the theory above,we have designed the FSO-AO system,which has been tested in the lab.The atmospheric turbulence was generated by a turbulence simulator.First we test the correction of statical turbulence.For the aberration of 0.2-0.4?(?=808nm)the residual aberration is less than 0.1? and the coupling efficiency has been promoted to 85%.For the atmospheric coherent length of 1cm and the Greenwood frequency of 40 Hz,after correction the average coupling efficiency has been promoted by 20%.By using the bit-error tester a 2.5GHz pseudorandom signal passed though the laser path.After correction the BER has been reduced from 10-5 to 10-7,which is an improvement by two orders of magnitude.An experiment of 9km horizontal path correction has been realized in China.We chose two stations in Dalian,Miaoxi station and Paotaishan station.The two stations are 9km apart and are separated by the sea.In the night of 2017.11.8 we first experimented the 2.5GHz IM/DD communication.The aberration before correction is about 0.2? and reduced to 0.08?,and the BER from 10-6 to 10-10,which revealed necessity of AO system.This paper is a pioneering work on adaptive optics of free space optics which analyzed on the design and evaluation of AO system applied to laser communication system.The first time in China we experimented the 9km horizontal path crossing the sea,the datum of atmospheric turbulence and AO correction will provide a theoretical basis and experimental experience on the development of adaptive optics.