Beam cleanup using photorefractive two-wave mixing

Abstract

The purposes of this paper are to point out the absence of phase crosstalk in photorefractive two-wave mixing 1,2 and to report an experimental observation using strontium barium niobate (SBN). Specifically, using an argon-ion laser at 5145 A, we have demonstrated the amplification of a clean beam with a pump whose phase is either spatially or temporally aberrated. A microscope slide etched with hydrofluoric acid is used as a spatial phase aberrator. With the pump and the signal beam intensities of the order of 400 and 8 mW/cm 2 , respectively, the signal gain, defined as the ratio of the signal output intensities with and without the pump beam, is a factor of 7 when the spatial phase of the pump beam is strongly aberrated. The amplified signal bears no traces of the pump aberration. The temporal phase modulation of the pump beam is introduced by the Doppler shift from a moving mirror. For different combinations of the signal and pump beam intensities, ranging from a few mW/cm 2 to a few W/cm 2 , the gain varies from 10, for the case of unmodulated pump, to almost unity with a pump modulation frequency of a few hertz. This is related to the response time of the photorefractive sample and furnishes an indirect means for determining the time constant (typically of the order of a fraction of a second) associated with hologram decay. Our results indicate that the time constant decreases linearly with the logarithm of the total beam intensity and is relatively insensitive to the intensity ratio of the two interacting beams.