LFMTS-assisted reflection interference elimination method for a coherent same-wavelength bidirectional optical communication system

We propose a novel reflection interference (RI) elimination method using a linear frequency modulated training sequence (LFMTS). Three LFMTSs and the signal are mixed in the electrical domain before being modulated onto an external cavity laser, and then utilized in reconstructing the RI. In the digital signal processing of the receiver, two LFMTSs identify the time mismatch between the signal and the RI, while the third is for carrier recovery of the RI. Simulations were carried out to investigate the impact of critical factors on the RI elimination, including the time mismatch, the bandwidth limitation, the laser linewidth and the frequency difference between lasers. Experiments were carried out on a 21-GBaud QPSK system to prove that the RI can be eliminated. Both simulations and experiments have proved the effectiveness of the proposed method in respect of RI elimination.