We report the first observation to our knowledge of self-pumped Brillouin-induced four-wave mixing in a manner analogous to self-pumped four-wave mixing in BaTiO(3). Using carbon disulfide as a scattering medium, we have observed both double and treble Brillouin-shifted conjugate beams.
Investigations of self-pumped phase conjugation using Brillouin-enhanced four-wave mixing in TiCl4 at 1 μm are presented. We have (1) observed both forward and backward Brillouin scattering into the optical cavity used for self-pumped phase conjugation during a single pump laser pulse and enhanced the forward-scattered component relative to the backward-scattered component by changing the angle of incidence of the pump beam, (2) demonstrated up to 80% coupling of pump radiation into cavity radiation, (3) examined quenching of the radiation in the optical cavity and eliminated both multiple Stokes orders from Brillouin scattering and Raman scattering as possible mechanisms, (4) observed mode beating in the cavity intensity profile coincident with the quenching, and (5) studied ring-resonator geometries, demonstrating both forward and backward scattering, unidirectional oscillation, and bidirectional oscillation leading to a triple-shifted phase conjugate.
We have prepared several phase plates to observe phase conjugation by stimulated Brillouin scattering in carbon disulfide in the usual way. One of the plates was specially made, lightly ground by fine emery. Its spatial Fourier spectrum shows a very bright spot in the center of the speckle pattern, which means that the light beam that passed through this plate consisted of a strong plane wave component and many weak random components. This is different from the phase plate in phase conjugation by stimulated Brillouin scattering given in earlier papers,1 but, using only this special plate, we have successfully obtained phase conjugation.
The results of an experimental investigation of a new geometry for producing phase conjugation by Brillouin-enhanced four-wave mixing are presented. In this geometry, the four-wave mixing medium is carbon disulfide, and the backward-going pump wave is created from the transmitted forward-going pump wave by stimulated Brillouin scattering (SBS) in glycerol. The two pump waves are hence phase conjugates of each other, and the quality of the phase-conjugation process is not degraded even by the use of an aberrated pump wave. The probe wave is created by SBS in carbon disulfide, which has a Brillouin frequency half that of glycerol, and the conjugate wave is therefore generated at the same frequency as the probe. Since the pump and signal waves differ in frequency by the Brillouin frequency of the carbon disulfide four-wave mixing medium, high reflectivities (approximately 2000%) are obtained as a result of Brillouin resonance enhancement.
We report on a theoretical and modeling study of self-pumped four-wave mixing in a Brillouin medium. A theoretical analysis gives expressions for the threshold intensity and for the steady-state reflectivity at small input intensities. Operation over a range of input intensities is also investigated by using a computer model that predicts that, for high input intensities, the conjugate can become unstable and oscillate chaotically in time. Experimentally we observe phase conjugation with an input threshold 25 times less than that required for phase conjugation by stimulated Brillouin scattering.
As it is known, self-conjugation of the IR laser beam via Brillouin radiation stimulated scattering phenomenon is not realised because of some physical limits on radiation intensity which are the principal limitations, to the present time. That is why, one of the promising technique of phase conjugation of infrared radiation is a self-conjugation of a laser beam wavefront via four wave interaction in a nonlinear medium with feed-back loop (FWMF) [1]. Unlike conventional our wave mixing schemes, the FWMF self-conjugation is supposed to be realised without any additional beams pumping a phase conjugation mirror.
It has previously been shown that Brillouin enhanced four-wave mixing can lead to large phase conjugate reflectivities for moderate laser powers.1,2 However, in the geometry considered by previous workers the pumps have equal frequency and hence the conjugate signal is shifted in frequency with respect to the probe. We have conducted an experimental and theoretical study of Brillouin-enhanced four-wave mixing for the different case in which the two pump waves differ in frequency by twice the Brillouin frequency of the four-wave mixing medium, and in which the probe wave frequency is halfway between those of the pump waves. In this case, the phase conjugate wave has the same frequency as the probe. Reflectivities > 100 % have been measured. Experimental results are compared to those of a theory that includes the effects of phase mismatch, pump depletion, and the finite response time of the medium.