Direct method for monitoring two-beam coupling in photorefractive materials

A method for directly monitoring two-beam coupling (2BC) in photorefractive materials is presented. This method allows accurate measurement of the gain and loss in the probe and pump beams, respectively, even in situations where substantial power is diffracted into higher order beams (i.e., for large grating spacings). Modulation of the pump beam at a frequency significantly greater than the grating decay rate, coupled with synchronous detection of the modulated probe beam signal, provides this information, automatically correcting for diffractive losses. The probe beam gain and pump beam loss are obtained without further correction. 2BC measurements are made on dye-doped photorefractive polymer-supported liquid crystal films to demonstrate the advantages of this method. The measurements are made for small grating spacings, in which little high-order diffraction is observed, and for large grating spacings for which there are several diffraction orders. (C) 2002 American Institute of Physics.