Abstract

The Helmholtz-free energy surface (F = U − TS) is derived for the photorefractive effect and compared in the hopping and Kukhtarev models. Two contributions for the internal energy U are obtained: one from buildup of the space charge field and a second contribution, the hopping potential, originating from the spatial variation of light intensity. For noninteracting donors, the entropy of grating formation S is obtained in configurational form. Equilibrium grating properties are obtained by free energy minimization and are in agreement with previous results obtained by other methods. However, the thermodynamic approach has the added advantage of giving the spontaneous (thermal) fluctuations about the equilibrium state. Fluctuations in the space charge field are obtained from the curvature of the free energy function at its minimum value. These fluctuations in turn produce fluctuations in the medium refractive index through the usual electrooptic effect, which give rise to scattered light noise. The SNR is obtained and compared with our previous determination of the light scattering noise limits to signal processing in Kerr and artificial Kerr media.

© 1988 Optical Society of America