Polarization coherent optical communications with adaptive polarization control over atmospheric turbulence

In wireless optical communication systems, the coherent detection scheme suffers severe performance degradation due to the phase fluctuation and polarization offset. To deal with such challenges, we propose a polarization coherent optical communication scheme with adaptive polarization control (PCPC), where the transmitted continuous-wave light is split into two paths-the binary phase shift keying modulated signal light and the reference light-which are orthogonally polarized and combined before being sent to free space. In the receiving site, the interference between the two lights can effectively suppress the atmospheric turbulence-induced phase fluctuation. Furthermore, we propose an adaptive polarization control scheme for eliminating the polarization offset caused by the transceiver misalignment and the atmospheric turbulence. Performance evaluation of the PCPC scheme with analytical expressions and numerical simulations shows that the PCPC scheme can effectively suppress the phase fluctuation without phase-locked loop and phase tracking algorithm; thus, the PCPC scheme outperforms the traditional coherent modulation in the turbulence channel. An elaborate designed indoor experiment was implemented to verify the scheme performance, where the measured residual phase fluctuation and measured residual polarization offset are both very small, and the experimental bit error rate performance can conform closely to the theory.