Balancing filter distortion and crosstalk impairments in optical networks

The design of optical communication networks with network switching elements operating in the optical domain requires careful system analysis and potentially stringent component requirements. We consider here network elements such as transparent optical cross-connects that demultiplex WDM signals, optically switch individual channels, and then multiplex the wavelengths together again before transmission into the next span. Network element optical impairments that can degrade signal quality are in-band (same wavelength) crosstalk, out-of-band crosstalk, and signal distortion from filter concatenation effects. These impairments can limit the transmission distance of a signal before regeneration is required. We examine the trade-offs between crosstalk and filter distortion in the context of the optical filters used in the optical multiplexers and demultiplexers in the network elements. We demonstrate the balance that must be struck between these impairment types in the design of the filters and network system. We study a 10 Gb/s network with 50 GHz channel spacing, examining both NRZ and RZ modulation formats. In both cases, we find optimal filter bandwidths that minimize the total signal degradation measured in terms of Q penalty. The total penalty suffered by RZ signals is higher than that of NRZ signals and must be considered when estimating system reach.