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Journal of the Optical Society of America B

Increase of light-beam coherence by two-wave mixing in photorefractive crystals

Nickolai V. Bogodaev, N. M. Polozkov, Vladimir V. Shkunov, L. I. Ivleva, A. S. Korshunov

Published Dec 1, 1993• 17 citations
Publisher

Abstract

We demonstrate that two-wave mixing of partially coherent light beams inside a medium with an inertial nonlinear response allows one to increase the coherence substantially. An increase of the coherence function from 0.24 to 0.94 is demonstrated experimentally in SBN:Ce crystal.
DOI
10.1364/josab.10.002287

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Two-wave mixing allows one to change effectively Intensities, phases, space profiles, polarizations and time evolution of interacting beams [1,21. We will show, that two-wave mixing of partially coherent beams may be used to increase degree of coherence of these beams. The physics of the effect under discussion is as follows: two partially coherent beams write a refraction index grating in a nonlinear medium whose position and profile are practically stationary. Scattering of strong beam on this grating in the direction of the weak one means that this weak beam acquires an addition, that is coherent with the strong beam, thereby increasing degree of coherence of the beams. Similar effects were discussed in connection with scattering of different frequency components at the common running grating [3] and with Interaction of different polarization components [4,5].
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Two-wave mixing in photorefractive crystals has been shown to combine coherently a modulated optical signal beam with a strong pump optical field.’ For a phase-modulated signal beam, the induced index of refraction phase grating inside the crystal yields a coupling of the optical fields of the pump and signal so that subsequent intensity detection results in apparent intensity modulation of the transmitted signal or pump beams. This provides an easy way to detect phase differences between two optical beams. Some of the advantages the above scheme offers over commonly used coherent optical phase detection schemes are: no loss of 3 dB in the signal beam intensity at the 50/50 beam splitter (used to combine signal and local oscillator fields); automatic alignment of the index of refraction grating with the two incident optical wavefronts; insensitivity to slow random changes in the phases of both beams. Experimental results are given, and a new definition for the resulting mixing gain for the phase-modulated beam is introduced, through which optimal conditions for input beam intensity ratios can be found. Applications to coherent homodyne detection are discussed.
The effect of partial spatiotemporal coherence in photorefractive two-beam coupling is studied theoretically and experimentally. Coupled-wave equations and a beam-propagation method are used to study the influence of coupling on the statistical properties of the beams. The spatial coherence is found to improve for amplified and to deteriorate for deamplified beams. A two-beam coupling experiment with light of reduced spatial coherence was performed with photorefractive barium titanate. Qualitative agreement between theory and experiment was obtained.