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Optics Letters

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

Yaakov Glick, Shmuel Sternklar

Published Jun 15, 1992• 11 citations
Publisher

Abstract

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 priori knowledge 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.

DOI
10.1364/ol.17.000862

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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 priori knowledge 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.
To the best of our knowledge, a novel self-phase-conjugation technique for laser radiation is proposed, which is based on the self-filtration of the phase-conjugate component in a nonlinear absorbing medium, bleached by a pump wave with the intricate spatial structure in the region of location of its speckles. Similar to the stimulated Brillouin scattering (SBS) case, the mechanism of the phase-conjugate wave discrimination against the noise background is based on the coincidence of the speckles in the pump and conjugate waves throughout the bulk of the medium. To compensate for a drop in the signal power due to total absorption, the amplification in conventional laser gain cells is used. The feasibility of the phase conjugation with reflectivities from unity to tens of percent is demonstrated by analytical estimates and numerical calculations. The technique can find application in the middle and far IR spectral regions, where the implementation of phase conjugation via SBS is either hindered or impossible at all.
Phase conjugation (PC) of extremely weak signals using a phase-conjugate mirror based on Mandelstam-Brillouin-enhanced four-wave mixing, coupled with an optical quantum amplifier, is considered. PC of a pulse with energy 4*10/sup -17/ J is achieved. PC conditions for amplified superluminescence noise of the optical quantum amplifier are specified and the limiting parameters are estimated. It is shown experimentally that if the signal-to-noise ratio (S/N) is equal to unity, the energy of an input signal for which phase conjugation is possible corresponds to 30 photons per transverse mode.>
The principal problem of light amplifiers is to find a means to amplify a coherent amplitude without decreasing the SNR. Consider a signal with amplitude squeezed quantum fluctuations, that is, less noise in the amplitude quadrature than the phase quadrature. The objective is to find a phase sensitive amplifier, which, while amplifying the coherent amplitude, adds quantum noise preferentially to the phase quadrature. We take as a concrete model for the amplifier an intracavity parametric amplifier. We go beyond the usual linear analysis and include the nonlinear regime where pump depletion is important. We find that in the linear region the quantum noise added by the amplification process may be reduced by squeezing the idler mode, while in the nonlinear regime the quantum fluctuations from the pump become more important. These results are of direct application to practical light amplifiers.