OSNR penalty-free add/drop performance of DSP-enabled ROADMs in coherent systems

The newly proposed digital-signal-processing (DSP)-enabled reconfigurable optical add/drop multiplexer (ROADM) without incorporating optical-electrical-optical conversions, referred to as a soft-ROADM, performs dynamic, flexible, and transparent add/drop operations at wavelength, sub-wavelength, and spectrally overlapped orthogonal sub-band levels. The cost-effective soft-ROADMs offer vital software-defined-networking-based networking functionalities for seamlessly integrating optical and mobile networks to realize elastic cloud access networks. In this paper, detailed theoretical and numerical investigations are undertaken of soft-ROADM add/drop operation performance in coherent systems. It is shown that the soft-ROADM add/drop operation performance is independent of both signal spectral location and fiber transmission distance and also shows excellent robustness to variations under system operating conditions. More importantly, under practically achievable system conditions, the soft-ROADM add/drop operations introduce negligible optical signal-to-noise ratio (OSNR) penalties. Moreover, numerical explorations of the impacts of key physical aspects associated with both the soft-ROADMs and coherent systems are also conducted, based on which optimum soft-ROADM design parameters are identified.