Brillouin Amplifier Noise Limiting With a Semiconductor Optical Amplifier for Coherent Communication Applications

Suppression of Brillouin amplifier intensity noise is demonstrated by use of a semiconductor optical amplifier (SOA) at its output to enhance application of its narrowband gain for boosting the carrier to noise ratio (CNR) of spectral lines in coherent communications. This utilizes the fast recovery time of the SOA gain in saturation to limit distortions induced when Brillouin gain is reduced from its maximum by detuning of the input frequency or state of polarization relative to the backward propagating pump. Although the SOA carrier dynamics associated with the varying gain also converts intensity noise to phase distortion, the similarly low bandwidth in the tens of megahertz range as for Brillouin gain permits its removal in communication applications by the same digital signal processing used to compensate the similarly slow phase error from coherent detection with a local oscillator laser. When applied to a spectral line with initial poor CNR of 19.5 dB/0.1 nm that is then boosted by Brillouin gain but at detuned input frequency of ∼±10 MHz from optimum (or 67% of a 30 MHz gain bandwidth), the SOA improves use of the SBA output as a carrier of 72 Gb/s 64-QAM signals with up to 2 dB higher Q ² -factor at the receiver. Overall, SBA's benefit for noise suppression in combination with the SOA is extended to more flexible and wider ranging input parameters.