Gain and noise characteristics of a Brillouin amplifier

Amplification of a weak input signal (~0.5 pJ) in a high-gain (G = 109) Brillouin amplifier has resulted in an output signal-to-noise ratio of 85:1, within a factor of 4 of the theoretical estimate. For a near-diffraction-limited Gaussian pump beam this noise increases exponentially with pump intensity with the same gain coefficient as for an input signal. When the pump beam is aberrated, the effective gain coefficient for the conjugate is enhanced by as much as a factor of 3, depending on the nature of the aberration. The enhancement factor J is found to vary significantly when the laser beam passes through different regions of the aberrator.