A method is disclosed for producing a superimposed fast transient grating on a permanent holographic grating which provides high scattering ability, fast modulation response times, and the capability of writing and erasing signals therein. A glass host is doped with trivalent rare earth ions, and is...http://www.google.ca/patents/US4728165?utm_source=gb-gplus-sharePatent US4728165 - Superimposed fast transient and permanent holographic gratings
