Demonstration and evaluation of an optimized RFS comb for terabit flexible optical networks

We experimentally demonstrate and evaluate an optimization strategy of a recirculating frequency shifting (RFS) optical comb for terabit flexible optical networks. We achieve an increased optical signal-to-noise ratio (OSNR) with good stability (no system outage) by reducing erbium-doped-fiber amplifier gain in the shifting loop and deploying an in-loop noise suppression filter. We demonstrate that this source can support 20 × 200 Gb/s dual polarization Nyquist-16QAM transmission. With optimization, the RFS comb has greater and more uniform OSNR per channel. Flexible optical networks with software-defined networking are particularly suited to this enhanced RFS due to 1) programmable frequency spacing, 2) dense, stable spacing enabling very high spectral efficiency, 3) uniform performance across channels, and 4) sufficient OSNR for high-order modulation. The RFS can be used in short links when using low overhead forward error correction (FEC). Distances as great as 1150 km are achieved when using a 20% FEC overhead. Long-distance tests at 4 Tb/s result in a post-FEC net rate of 3.3 Tb/s and 6.3 bit/s/Hz of spectral efficiency.