Noise limitations of Brillouin two-beam coupling: Theory and experiment

A theoretical and experimental study of noise in Brillouin two-beam coupling is presented, which includes both the nondepleted and the depleted pump regimes. The signal-to-noise ratio (SNR) is shown to be proportional to the weak signal input power as well as to the pump intensity in the nondepleted regime. Pump depletion causes both the SNR and the gain to degrade significantly, leading to an optimum working point of g_BI_pL ≃ 20. The minimum detectable signal power per mode of noise, predicted and measured to be approximately 5 μW at λ = 1.06 μm, is dependent on frequency and temperature, while the peak gain is also dependent on the Brillouin coupling coefficient gB, interaction length, and cross-sectional area of the amplifier.