The Research On Homodyne Coherent Receiver And Transmission Channel For Coherent Laser Communcation

Free-space optical communication(FSOC),especially the coherent laser communication,is gradually becoming effective in solving microwave bottlenecks,establishing the high-speed space-ground integrated broadband network,realizing the real-time data transmission.As for the FSOC,the optical equality is largely degenerated and unstable as the signal light is transmitted through long reach(several to hundred kilometers).Therefore,the received signal light is not only weak in power,but also suffers from high-range,fast Doppler frequency shifting,pointing errors,atmospheric turbulence effects,which frustrate receiver design,channel modelling,etc.Among different receiver schemes,the Costas optical phase-locked loop(Costas OPLL)is very attractive from technique view point.However,the Costas coherent receiver is faced with problems in narrow locking bandwidth,weak tracking ability for Doppler frequency shifting,low receiver sensitivity,etc.In order to solve those technique challenges,many efforts have been put in this thesis.First,a novel Costas OPLL with aided frequency capture ability is designed,which has realized the digital OPLL,the real-time compensation of Doppler frequency shifting under big frequency difference.In order to improve receiver sensitivity,experiments on optimum signal power splitting ratio for 90⁰optical hybrid and homodyne coherent demodulation for RZ-BPSK signal have been conducted.After that,the combined effects of atmospheric turbulence and pointing error have been modeled,within which the probability density function(PDF)of the combined channel is derived.With this new channel PDF,it is further applied in link analysis for BPSK homodyne link.In the end,the effects of diversity reception and adaptive optical compensation onto suppressing atmospheric turbulence and pointing error are both also investigated.The key innovations in this thesis are listed as follows:1.Digital optical phase and frequency locking and real-time compensation of Doppler frequency shifting based on aided frequency captureDue to fast satellite movement or variation of relative positions,severe Doppler frequency shifting exists between the signal light and the local oscillator laser,i.e.,frequency drifting(±7 GHz)and variation(10 MH/s).Due to problems of narrow and weak frequency acquisition bandwidth for the Costas loop,the Costas OPLL is always frustrated by losing lock.Therefore,a novel delay-line based frequency discriminator and aided frequency capture(AFC)is introduced.Thus,the Costas OPLL is capability of monitoring the frequency variation and OPLL states,controlling the local laser.In this way,the ability of frequency capture and tracking is largely increased.Based on the AFC,a novel Costas homodyne coherent receiver is designed,fast and digital OPLL is realized under the frequency difference of±12 GHz within 4.5 s or 7.3 s.Meanwhile,real-time compensation of Doppler frequency shifting(frequency difference:±12 GHz,frequency shifting rate:600MHz/s)is realized.As for the sensitivity,it approaches to-39.5d Bm@BER=1x10^-9for 10Gbit/s BPSK,which satisfies the requirements for satellite coherent communication.2.Investigation and experiment onto the optimum signal power splitting ratioAs for homodyne demodulation of the BPSK signal,the in-phase arm is used for data demodulation and the quadrature arm is for phase error detection.Once the signal power distribution between the two arms is optimized,i.e.,splitting more signal power into the in-phase arm once the accuracy of phase locking is proved,the receiver sensitivity could be largely improved.So as to solve this problem,the continuous adjusting of signal power splitting ratio for the 90⁰ optical hybrid is realized by changing polarization state of the signal light.Furthermore,the impacts of signal power splitting ratio onto the receiver sensitivity,residual phase error and requirements of laser linewidth are investigated.In this case,the optimum signal power splitting ratio is set around Ks=0.05,which permits a sensitivity improvement of 2.65 d B under the residual phase error less than 10⁰.Meanwhile,the requirement of laser linewidth is 5.26-fold reduced based on the dynamic optimization of OPLL bandwidth.3.Homodyne coherent demodulation of RZ-BPSKIn order to further improve receiver sensitivity,the homodyne demodulation of RZ-BPSK is analyzed and realized.First,the requirement of time alignment for the RZ-BPSK at the transmitter end is verified,i.e.,±10%of the data rate.Second,the phase-locking state for both the NRZ-BPSK signal and the RZ-BPSK signal is compared.Compared to conventional NRZ-BPSK signal,the receiver demonstrates a sensitivity improvement of 1.4d B for 10 Gbit/s RZ-BPSK signal and 2.5 d B for 5 Gbit/s RZ-BPSK signal.4.Modeling the combined channel due to atmosphere turbulence and pointing error and link analysisWhen the signal light is transmitted through turbulence,it is necessary to investigate the combined effects of turbulence(Modified Rician distribution)and pointing errors onto the coherent communication,which also provides theoretical foundations onto exploring techniques for reducing effects of turbulence,pointing errors,etc.For the first time,the combined effect of the turbulence-induced intensity fluctuation(turbulence-induced amplitude fluctuation,turbulence-induced phase distortion)and the pointing error is considered completely.In this way,the novel channel PDF is derived in the closed-form,which enables link analysis under arbitrary pointing errors.Meanwhile,the novel channel PDF is applied in deriving BER in closed form and in high-SNR approximation for single input signal output(SISO)link.More importantly,the effects of adaptive optical(AO)compensation onto reducing both the turbulence and the pointing error is analyzed for the first time.In the end,closed-form of channel capacity and outage probability are also derived.5.The effect of diversity reception onto the coherent communication under the combined channelDiversity reception is promising to reduce the effects of turbulence-induced amplitude fluctuation and turbulence-induced phase distortion by processing different signals from uncorrelated turbulence channels.Based on the combined channel PDF,the single input multiple output link with maximum ratio combing(MRC)and equal gain combing(EGC)is analyzed by introducing the moment-generating function and the characteristic function,respectively.Then,the BER in closed form and in the high-SNR approximation is derived,which avoids high-level numerical integration and greatly simplify link analysis for both MRC and EGC.Through simulation,it has been found that:Under negligible-to-weak pointing error,the BER performance is gradually improved with larger diversity branch numbers.Also,under strong-to-severe pointing error,the diversity gain is adversely reduced with larger diversity branch numbers.In this way,the adaptive optical compensation is needed to reduce the impacts of pointing error.