Passband Narrowing and Crosstalk Impairments in ROADM-Enabled 100G DWDM Networks

100G dense wavelength-division multiplexing networks with reconfigurable optical add-drop multiplexers (ROADMs) enable dynamically reconfigurable networks and are therefore part of the solution needed to meet increasing bandwidth and routing flexibility requirements for transport networks. ROADMs, in particular cascaded ROADMs, also incur penalties including those induced by passband narrowing, frequency drift, and imperfect isolation. As networks migrate toward denser wavelength assignments in search of higher capacity, these issues will become more challenging. Here, passband and in-band crosstalk effects for long-haul ROADM-enabled 112 Gb/s polarization division multiplexing quadrature phase shift keying systems are examined. We investigate the pulse format dependence for passband narrowing/frequency detuning. We also demonstrate a spectral weighting method to evaluate crosstalk penalties for widely varying crosstalk spectral content. Our experimental results demonstrate that the conventional crosstalk metric is insufficient. Finally, we demonstrate experimentally and in simulation a nonlinearity-enhanced crosstalk penalty that results from the nonlinear parametric interaction between the primary signal and crosstalk. We show that this penalty is more limiting than the nonlinear interactions between signal and amplified spontaneous emission noise.