Analysis of optical signal impairment induced by different types of optical filters in 40 Gbps DQPSK and 100 Gbps PM-DQPSK systems

Abstract

Optical filters are crucial elements in optical communication networks. However, they severely affect the signal quality due to their non-ideal transfer functions. In order to minimize filter-induced signal impairment, we study and simulate signal impairment caused by four types of filters: Butterworth, Bessel, Fiber Bragg Grating and Fabry–Perot, and the center frequency tolerances of cascaded optical filters are also discussed. Signal impairment is analyzed by the eye opening penalty (EOP) and Q-penalty parameters. Simulation results obtained in single-channel systems show that the Butterworth filter has the best performance. When the center frequencies of all filters are well aligned, 18 Butterworth filters can be cascaded in a 40Gbps DQPSK single-channel system and 12 in a 100Gbps PM-DQPSK single-channel system, with 1-dB EOP. In the case of center frequencies of all filters misaligned with ±5GHz, the permitted value is reduced to 12 and 9 for the 40Gbps DQPSK and 100Gbps PM-DQPSK single-channel systems, respectively. In the 100Gbps PM-DQPSK-based multi-channel systems, even if 0.7±0.3dB more Q-penalty is introduced in comparison with the case of a single-channel, the Butterworth filter also shows the best performance. In addition, research into the Butterworth filter with different orders indicates that the second-order filter has the best performance in single-channel systems, whereas it has a similar performance as the third-order filter in multi-channel systems.