Phase Conjugate Mirror With Gain Based On Brillouin Scattering

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ABSTRACT We describe a new type of phase-conjugate mirror based on Brillouin enhanced four-wave mixing that has many attractive features distinguishing it from previous types of phase- conjugate mirrors. Chief among these features are its high reflectivity, ease of alignment, and its insensitivity to aberrations on the pump waves. Both theoretical and experimental results are presented.The production of phase conjugate (PC) beams1 has become one of the most interesting and attractive applications of nonlinear optics. Two popular techniques for producing PC beams are four-wave mixing (FWM) and stimulated Brillouin scattering (SBS). Recently a hybrid of these two techniques called Brillouin enhanced four-wave mixing has been discovered2 and described theoretically. 3 In this paper we present a theoretical and experimental investigation of a new Brillouin enhanced four-wave mixing geometry which combines the desirable features of the FWM and SBS processes and which eliminates the undesirable features of both. Our geometry differs from previous Brillouin enhanced four- wave mixing geometries in that our counter-propagating pump beam is generated using an SBS phase-conjugate mirror which introduces an SBS frequency shift chosen so as to exploit the Brillouin resonance in our FWM cell. The result is that this geometry has many distinct advantages over the SBS or FWM techniques for producing a PC signal. For example, we can achieve high reflectivities (»100J) and the PC signal is not shifted in frequency with respect to the input probe. In addition, the resonance enhancement in our process is achieved through elect rostri cti on and does not rely on a material resonance; hence the enhancement is independent of the pump laser frequency. Also, since our backward pump wave is generated from the transmitted forward pump wave by the SBS process, the two pump waves are phase conjugates of each other.1* It is known that high quality PC by FWM requires that the pump beams be phase conjugates of one another; 5 hence our geometry allows the use of aberrated pump waves for the FWM process, as is verified in the experimental section. This geometry also insures that the two pump waves are automatically counter-propagating, greatly simplifying the alignment of the FWM process.THEORYFor our theoretical analysis we assume the FWM geometry shown in figure 1 . The laser at frequency a) 1 forms the forward pump beam, the backward pump beam is assumed to be at frequency u) 1 -2fl+A2 , and the probe beam enters the FWM region in the direction of the laser and has frequency Wj-fl+Ag. Here ft is the Brillouin frequency of the nonlinear medium and A2 and A3 are the detunings from exact resonance.