Methods and devices for using photorefractive materials at infrared wavelengths

摘要

A method of writing plane holographic gratings Bragg-matched for reflection in the infrared in a photorefractive material using shorter wavelength light through a face perpendicular to the grating planes. The writing beam wavelength is selected to be within the photorefractive sensitivity range of the crystal and the angles are chosen relative to the wavelength to define a reflection grating with a period such that counter-propagating reflection occurs at the desired IR wavelength. For reflection gratings at different wavelengths, either the transmission or the reflection mode geometry may be used. In the transmission mode, the writing beams are incident on the same side face while in the reflection made the writing is on opposite faces in an off-axis (non-counter-propagating) configuration. Anti-reflection coatings of the appropriate wavelengths are used on the crystal surfaces to reduce reflection losses and improve the diffraction efficiency of the grating.