Minimising nonlinear Raman crosstalk in future network overlays on legacy passive optical networks

There is a desire to overlay future optical access networks onto legacy passive optical networks (PONs) to provide increasingly advanced services by filling empty wavelength bands of legacy gigabit rate PONs. Nonlinear Raman crosstalk from new wavelengths onto legacy RF video services (1550-1560 nm band) and onto the legacy digital downstream at 1490 nm, however, can limit the number and the launch power of new wavelengths. As an example, straightforward physical-layer adjustments at the optical line terminal that increase the number of new, 10 Gbit/s channels launched in the 1575-1580 nm band by a factor of 16 without increasing the Raman penalty on the video signal are illustrated. A physical-layer (RF) filter modifies the on-off-keyed signal feeding each 10 Gbit/s transmitter, suppressing the RF Raman crosstalk on the video signal by 9 dB while incurring a power penalty on each 10 Gbit/s link of <;0.5 (2.0) dB with (without) forward error correction. The previous Raman mitigation work used non-standard line-coding to shape the 10 Gbit/s electrical spectrum. In addition, polarisation-interleaving of new wavelengths lowers the worst-case RF DC crosstalk by ~3 dB in fibres and it limits and stabilises DC crosstalk in low polarisation mode dispersion fibre links.