Study Of Shot-noise-limited Sensitivity Coherent Receiver For FSO System Applications

With the free-space communication requirement increasing,there is a need to switch to higher carrier frequencies.Thus,free-space optical(FSO)communication is an easily deployable,cost-effective,and easy-to-install alternative to microwave links due to the fact that the frequency of operation of laser links is about five orders of magnitude higher than microwave links.As the various advantages over microwave system in terms of greater bandwidth,narrow spatial beam width and smaller antennae size,the FSO communications have attracted attention as an efficient solution for a wide range of applications.In order to achieve long-distance communication,it is very important to improve the sensitivity of optical receiver,which is the key component of the FSO communication systems,to shot-noise-limit.Since the conventional optical receivers based on APD,based on SOA and based on coherent detection can not achieve the shot-noise-limited sensivitiveiy,the schemes of the shot-noise-limited sensivitivity optical coherent receivers have been studied and designed according to the theory of the sensitivity of the optical receiver,which is studied and analyzed in this thesis.On one hand,the optical systems of the receiving terminal are designed to low optical loss of the optical signal and enhance alignment accuracy of the receiver for ensuring that the optical signal is received entirely by the photodetector in optics.On the other hand,the coherent receivers are improved and studied to amplify signal,decrease noise and increase quantum efficiency for ensuring that the coherent receivers achieve the best signal-to-noise ratio(SNR)in electronics.Finally,the shot-noise-limited sensitivity of the coherent receiver can be realized.The main content and innovation points of the thesis include:(1)A low-loss polarization-independent receiver is proposed.By controlling the phase or frequency of the two components of the LO with orthogonal states of polarization(SoP),the ultimate output IF signal from the photodiode of the heterodyne receiver consists of two high frequency components and polarization diversity can be achieved in frequency domain.The polarization-independence can be achieved by just using filters and the loss of the signal optical power can be avoided.(2)A novel dual-PSD is proposed to integrate with the PD of the optical receiver in the same condition.When the light spot is shot to the dual-PSD,the position information of the light spot is expressed by the polar coordinate(r,?),which can be got by the dual-spiral PSD.The diameter of the light spot is the key for this optical alignment based on dual-spiral PSD,and can be also deduced and adjusted by the response of the dual-spiral PSD.When the light spot is moved to the center of the receiver,the receiver can receive the signal light spot accurately and entirely without the impact of the aberration of the optical system.(3)A coherent receiver based on InAs e-APD is designed for the high speed FSO communications.As the gain to weak signal provided by both the coherent gain from the LO power and the internal gain of the InAs e-APD,the proposed receiver can operate in shot-noise-limit with a low LO-power,so it can get a higher signal gain and a better sensitivity improvement than the traditional coherent receiver based on PIN photodiodes due to the low RIN from the low LO power.(4)A coherent receiver based on solid-state impact-ionization multiplier(SIM)is proposed.By this way,only the differential current of the BPD is amplified,so the effect of the excess noise of the SIM can be suppressed and the SNR can be further improved.On the other hand,the proposed receiver can avoid the saturation of the BPD and achieve very great signal gain,so it can be suitable for free-space quantum communications.In addition,in order to solve the problem of the low bandwidth of the traditional SIM,a high-speed,high-saturated SIM structure is designed,which can improve the bandwith of the proposed receiver to exceed GHz.(5)Experiment research on fabrication and measurement of the PIN photodiode with partly depleted absorption layer is made.It is first found that when the photocurrent density is high,the responsitivity of the PIN photodiode with partly depleted absorption layer can be improved due to the heat effect of the photodiode.And according to this phenomenon,a large-active-area photodiode with photon-trapping microstructures is designed for achieving high speed,high quantum efficiency and high photocurrent density.