Abstract

In the variable grating mode (VGM) liquid crystal device (LCD), a birefringent optical phase grating is generated by a dc bias voltage. The spatial frequency of the grating is observed to vary linearly with the applied voltage. The VGM LCD exhibits quite unusual polarization effects in the various diffraction orders.¹ Although the device is fairly thin (~6 µm), the strong natural birefringence of the liquid crystal medium makes anisotropic diffraction effects significant. For the VGM LCD the odd and even diffraction orders exhibit strikingly different angular alignment sensitivity characteristics as a function of the incident light beam angle. The observed results suggest that the even diffraction orders can be interpreted in terms of a simple isotropic Raman-Nath diffraction model, whereas the behavior of the odd diffraction orders is characteristic of Bragg regime anisotropic diffraction. An analytic model of the liquid crystal molecular orientation as a function of spatial coordinates, the applied electric field, and the surface constraints has been developed. A numerical method for the analysis of anisotropic optical beam propagation through volume grating media, described in an accompanying paper, is used to simulate optical propagation through a thin VGM device and to predict the resultant optical diffraction characteristics.

© 1985 Optical Society of America