Benefits of quasi-continuous symbol rate tunability in links constrained by ROADM filtering

As modern optical networks continue evolving toward higher symbol rates while supporting tight channel spacing, penalties due to optical filtering incurred by transmission through reconfigurable optical add-drop multiplexers (ROADMs) can become significant within the overall link budget. For this reason, filtering impairments must be realistically assessed and accurately modeled, particularly in terrestrial networks where signals may traverse many ROADMs. Given that modern transceivers can provide flexible modulation options and most ROADMs in use today support flexible grid operation, there remains the challenge of optimizing performance and/or capacity while managing these multiple degrees of freedom. In this work, we use real-time transceivers with a tunable symbol rate up to 72 GBaud and variable modulation up to 64QAM to experimentally investigate the impact of ROADM filtering on the optimum operating condition required to maximize channel capacity or margin. We also present a simplified model for considering the filtering impairment as an additive noise term and use it to predict performance in other scenarios. It is shown that symbol rate tunability in steps below 5 GBd is required to maximize the margin for a given link passband. Finally, we explore the difference in performance for filtering conditions with passband narrowing on both sides of the signal, assuming individual wavelength routing, or only on one side of the signal, assuming multi-carrier routing.