Abstract
We used simulations and experiments to investigate the filtering penalty caused by optical filters such as wavelength-selective switches for a 128-Gbit/s PDM-QPSK signal transmission system based on digital coherent detection technology. In particular, to take account of the center frequency shift of optical filters in practical transmission lines, we define an approach for evaluating the optical filtering penalty. Moreover, we explain that the optical filtering penalty in a digital coherent detection system has characteristics that are distinctly different from those of conventional transmission systems based on direct detection technology when we compare the simulation results for a 43-Gbit/s DQPSK signal transmission system as an example.
