Pattern dependency comparison of four different 40-Gb/s optical duobinary transmitters using low-bandwidth modulators

We investigate the pattern dependency of the 40-Gb/s optical duobinary signals generated by a 10-Gb/s-rated Mach-Zehnder modulator with four different configurations. These configurations correspond to all possible combinations of a 15-GHz-bandwidth low-pass filter and a 0.24-nm bandwidth optical filter used to shape up the duobinary signals. We compare through experiment the back-to-back sensitivities and transmission performances of the duobinary signals for pseudorandom bit sequence lengths of 2⁷-1 and 2³¹-1. The results show that the combination of the 10-Gb/s-rated modulator and the 0.24-nm optical filter exhibits the best performance in terms of both the receiver sensitivity and pattern dependency owing to the good frequency response of the optical filter.