Volume index gratings in the intermediate and form-birefringence regimes.

We present a study of volume index gratings operating in the form-birefringence regime and what we call the intermediate regime, defined as the regime between form birefringence and the onset of diffraction. The operating regime is characterized by the ratio between the measurement wavelength and the grating period: form birefringence regime below 10, diffraction around 3, and intermediate regime between 10 and 3. The behavior of the gratings in a given regime is studied by plotting the induced retardation as a function of the incidence angle. The shape of the curve fully characterizes the regime. We report on theoretical calculations of retardation by use of a rigorous theory and its experimental validation by measuring retardation of recorded gratings in photopolymers.