Reducing the noise in Brillouin amplification by mode-selective phase conjugation

We demonstrate techniques for reducing the noise at the output of a Brillouin amplifier. This is accomplished by nonlinear postprocessing of the amplified signal and noise in a phase-conjugate mirror. For a case in which the signal channel direction is known, the signal-to-noise ratio is enhanced from 1 at the amplifier output to 10⁴ after processing. Without a prioriknowledge of the signal channel, an enhancement in the signal-to-noise ratio from 0.8 to 350 is obtained. This method relies on the speckled diverging nature of the noise emitted from a Brillouin amplifer and on the nonlinear reflectivity of stimulated Brillouin backscatter.