Abstract: Substantial improvements in photorefractive device lifetimes are provided by control of electron migration which results in the decay of gratings in photorefractive materials due to diffusion and other effects. A new class of photorefractive devices using compensating electronic and ionic gratings having relatively low efficiency but nonetheless usable gratings is provided by arranging the gratings to be reflective in a wavelength band outside the photo-excitation band of the photorefractive material, as by using an infrared operating wavelength. Longer lifetimes in high efficiency gratings are achieved by constant or periodic illumination of photorefractive materials to assure uniform charged distribution of electrons and maintenance of the ionic backbone grating.

Abstract: A laser utilizes feedback from a volume holographic grating used as a wavelength standard to lock the laser output wavelength to its desired value. This feedback can be non-optical, wherein the holographic filter output is used to actively control the wavelength through an external control mechanism. This feedback can also be optical, wherein a volume hologram reflection grating is used to generate optical feedback into the laser gain.

Abstract: An X-Y recorder for high speed plotting on a plain web, i.e., a web without registration holes or other special means to maintain alignment of the web during plotting. Two embodiments of these plotters are disclosed with one degree of motion being achieved by moving the web and the second degree of motion being obtained by moving a pen assembly substantially perpendicularly to the movement of the web. To enhance the repeatability of the plotting on the web as it is moved back and forth, a random rough surfaced drive wheel assembly is included with that wheel surface making indentations along one surface of the driven edge of the web as it is first passed through the drive wheel assembly. Then as the web is reversed, these indentations substantially remesh with the random rough surface of the drive wheels.