Performance of optically-preamplified PDM-BPSK-M-ary PPM and PDM-QPSK-M-ary PPM systems with finite extinction ratio

In this paper, we evaluate the performance of optically preamplified homodyne receivers for combined modulation schemes. The combined modulation schemes include polarization-division-multiplexed quadrature phase-shift keying (PDM-QPSK) with M-ary pulse position modulation (M-ary PPM). In addition, we investigate the performance of polarization-division-multiplexed binary phase-shift keying (PDM-BPSK)-M-ary PPM. The performance of the two modulation schemes was evaluated under the finite extinction ratio (ER) of optical transmitters. Simulation results indicate that PDM-BPSK M-ary PPM systems are more sensitive to finite ER than PDM-QPSK M-ary PPM systems. The results also indicate that for M ≥ 32, for both modulation schemes, a severe ER penalty (more than 5 dB) exists for ER ≤ 23 dB.