Volume Bragg gratings for spectral and coherent beam combining in MWIR spectral region

ABSTRACT: The purpose of this proposal is to develop a new photosensitive optical material with working range from 3 to 5 um, which enables recording of high efficiency volume holograms with tolerance to high power laser radiation, and to demonstrate the use of those holograms for beam control of mid IR lasers. Three directions will be pursued in the material part of the project. The first one is to develop optical glass transparent up to 5 um that would possess ability for photo-thermo-induced crystallization and, therefore, for refractive index change in the exposed areas. The second approach is to find proper compositions and conditions of irradiation of chalcogenide glasses that would enable direct writing of holograms by laser radiation. Chalcogenide glasses are transparent up to 12 um. This technology is well known for chalcogenide films but it is a challenging approach for recording of bulk holograms. The third direction will be the study of iron doped lithium niobate which is transparent up to 5 um and shows refractive index change resulted from electro-optical phenomenon caused by spatial separation of charges. We will use revealed photoinduced phenomena tor record holographic gratings for spectral beam combining and spectral locking of MIR lasers. BENEFIT: One of the hot topics in military photonics development is a robust emitter in 3 to 5 um spectral region for countermeasures against IR self-guided missiles. The most portable and efficient light sources are semiconductor lasers. However, semiconductor lasers operating in mid IR spectral region have not enough power and brightness for efficient countermeasures. A possible solution of this problem is to develop a method for spectral or coherent combining of NIR semiconductor lasers. This combining is routinely produced for near IR region by volume Bragg gratings recorded in a photo-thermo-refractive glass. However, this glass is not transparent in mid IR. The proposed research is directed to development of a phase photosensitive material that would be transparent up to 5 um. This material would enable fabrication of volume Bragg gratings that will be used for spectral and coherent locking of mid IR lasers and for spectral combining of multiple laser sources. Such approach with robust passive elements will provide dramatic increase of brightness of mid IR laser sources and will be used in a great number of military laser systems. Additionally these gratings could be used for narrow band spectral filtering in multiple night vision systems.