The Effect of Source Phase Noise on Stimulated Brillouin Amplification

The impact of laser phase noise on the Brillouin gain is studied. It is shown that insufficient coherence of the laser sources used in stimulated Brillouin amplification setups not only leads to reduced gain but also to source-induced gain fluctuations (noise), which may affect their performance. Under commonly encountered conditions, even a relatively coherent source, having a linewidth of a few megahertz, has enough phase noise to become the dominant contributor to the observed Brillouin gain fluctuations. For equally phase-noised pump and signal (also called probe), their (separately) induced gain and gain fluctuations are different: 1) the gain penalty is higher for phase-noised pump; 2) the signal output power is noisier for phase-noised signal, resulting in a larger standard deviation; and 3) the probability distribution function of the signal output power is quite symmetric for the phase-noised pump and distinctively asymmetric for the phase-noised signal. The results for continuous pump and signal waves, obtained numerically from the governing differential equations and validated experimentally, may contribute to the judicial choice of laser sources in practical Brillouin-based setups.