Research On The Optical Homodyne Coherent Detection Technology

Recently,coherent optical homodyne receivers have been widely used in free-space optical communication,mainly because of their high sensitivity,less vulnerability to stray or background light,and compatibility with complex modulation format.In addition,one main advantage of the homodyne detection lies in the fact that the baseband signal can be directly obtained,in contrast to heterodyne receivers which usually requires a rather high intermediate frequency.Therein,the design of the optical phase-locked loop?OPLL?is the core technology of the homodyne receiver.Among all homodyne coherent receiver architectures,such as balanced loop,Costas loop and dither loop,Costas loop is one of the most popular approach thanks to the robust.And homodyne receiver based on Costas loop outperforms in terms of frequency selectivity,sensitivity and phase noise suppression.Therefore,the dissertation focuses on the design of the 10-Gbit/s BPSK homodyne receiver based on Costas loop.However,the OPLL bandwidth was usually limited by the large loop delay,which makes it difficult to maintain system stability when lasers had large phase noise and frequency drift.Therefore,only using a conventional homodyne Costas OPLL,without an auxiliary acquisition and locking circuitry,the OPLL faces the challenge in maintaining stable.In addition,Doppler effect owing to relative movement between satellites must be taken into consideration in inter-satellite laser communication links.The frequency offset caused by Doppler effect in the free-space optical communication link is approximately+/-7 GHz with maximum rate of 10.2MHz/s,which is much larger than the typical lock range of the OPLL.In order to overcome this problem,Doppler shift compensation function is added to the homodyne receiver,and finally the Doppler shift is able to be compensated efficiently in real time.In this paper,we report on a 10-Gbit/s BPSK homodyne receiver using a Costas OPLL.The basic theory and design process are introduced in details.Differing from the conventional configuration,the demonstration in this paper with enhanced dynamic performance by using an auxiliary acquisition and locking circuitry is able to achieve fast phase locking to signal laser within 10 s at an initial frequency offset as large as+/-12 GHz.The proposed receiver can also compensate for Doppler shifts of±8 GHz with max rates of 600 MHz/s in real-time.10 Gbit/s Binary Phase Shift Keying?BPSK?data has been demodulated with sensitivity of-39.5 dBm at Bit Error Rate?BER?of 1×10^-9.