Density and Guard Band in Migration Scenarios to Coherent Ultra-Dense WDM

In this work the maximum density is numerically investigated giving the compromise of physical impairments and spectral efficiency on coherent ultra-dense wavelength division multiplexing passive optical networks (UDWDM-PON). The required guard band to legacy 1.25-2.5 Gb/s intensity modulation direct detection (IMDD) systems specified closely to Gigabit-capable PON equipments is also analyzed in order to mitigate linear and nonlinear crosstalk. Using 3 GHz frequency grid guaranteed the best performance of the system (32×1.25 Gb/s quadrature phase-shift keying with coherent detection) with respect to the 10% Error Vector Magnitude limit; 12.5 GHz guard band to legacy IMDD system was sufficient to minimize strong linear and nonlinear crosstalk (mostly four-wave mixing) on both systems.