Influences of hybrid phase-shift error on the performance of optical coherent receivers

The influences of phase-shift error induced by a 90° space-optical hybrid on the performance of homodyne coherent receivers are analyzed. Phase-shift errors bring a fixed phase difference between the signal and local oscillation (LO) light. For an optical phase-locked loop OPLL, it becomes the third source of phase noise, further increasing loop phase noise. Under the joint action of phase difference and increasing phase noise, the sensitivity of the receiver deteriorates considerably. For a bit error rate (BER) of 10−9, in order to make the additional received power penalty less than 0.5 dB, the phase shift error of each branch must be less than 2°. Further, if the BER is 10−10, the limiting value becomes 1°. This presents a constraint for the design of the 90° space-optical hybrid.