Brillouin Amplifier Noise Characterization by a Coherent Receiver and Digital Signal Processing

Brillouin amplification of a CW laser is evaluated by a coherent receiver and offline digital signal processing (DSP) to assess electric field distortions relevant to noise sensitive applications, and in particularly, optical communications. In this approach, the Brillouin amplifier output is heterodyne detected at the receiver so that the proportional electric field restored by DSP can be resampled into a virtual quadrature phase shift keyed (QPSK) signal. From this, an effective Q² -factor distortion and amplitude and phase error are quantified. Characterization by the approach of Brillouin gain in a 4.46 km standard signal mode fiber shows amplitude error as the dominant noise feature while impact from phase error is generally negligible. Large gain ≈≥26 dB with minimal Q² factor degradation <1 dB is observed achievable but with constraints on 1) input and 2) pump powers, and alignment of 3) frequency, and 4) state of polarization (SOP) for peak gain. Detuning of the SOP for minimum gain is observed to excite low frequency amplitude pulsations that grow in intensity and frequency with increasing pump power. For frequency detuning, signature amplitude and phase distortion patterns are uniquely visualized by the virtual QPSK constellations. Spectral analysis by DSP indicate an output carrier to noise ratio as being capped to ≈40 dB at 30 dB gain for the particular test fiber and noise as highly polarized. The characterized low noise and operating constraints of the Brillouin amplifier demonstrated by the approach aids design for noise sensitive applications.