Sensitivity degradation of an optical {2×2} and a {3×3} phase diversity ASK receiver due to gain imbalance and nonideal phase relations of the optical hybrid

The sensitivity penalty due to phase and gain imbalance for a {2×2} and a {3×3} optical homodyne phase diversity amplitude-shift keying (ASK) receiver is investigated for the shot-noise-limited case. It is shown that both types of imbalance lead to a time varying character of the bit error rate (BER) types of imbalance and the phase mismatch. The calculation of the BER is performed as a function of the threshold value. It is shown that for an optimized threshold value, the phase diversity ASK receivers can tolerate rather large amounts of gain imbalance, namely approximately ±10%, and a phase mismatch of approximately ±10° for the {2×2} receiver and approximately ≠15° for the {3×3} receiver without an excessive sensitivity degradation (<2 dB)