Effect of Modulator Bias Control in the Presence of a Finite Extinction Ratio in DQPSK Pre-Equalization Systems

The effect of the finite extinction ratio of a dual-parallel Mach-Zehnder modulator in 43-Gb/s differential quadrature phase-shift keying systems employing pre-equalization is reported. The degradation of the signal quality with 10000-ps/nm dispersion pre-equalization is 2.9 dB at an extinction ratio of 26 dB. We show that bias adjustment can improve the degradation due to a finite extinction ratio. The improvement at an extinction ratio of 26 dB is 2.2 dB. In practical pre-equalized systems, bias control is a key enabler for maintaining high signal quality.