Coherent beam combining of fiber lasers by volume Bragg gratings

Beam combining is considered as the most promising approach for creation of high power 100 kW-class portable electric lasers. Passive coherent beam combining is based on mixing of output fields of a multichannel laser device, selection of proper combination of transverse and longitudinal modes for feedback to all channels, and some nonlinear mechanisms of phase locking of radiation in different channels. An approach proposed in Phase 1 is based on the unique properties of volume Bragg gratings (VBGs) recorded in a photo-thermo-refractive (PTR) glass which are widely used for spectral and angular locking of different types of lasers, spectral beam combining, and agile beam steering. These elements show both unique spectral and angular selectivity combined with unprecedented for diffractive optical elements tolerance to high power laser radiation. Study of thick VBGs with different tilts of grating vector in respect to the normal to the surface of PTR glass slabs (including corner grating where a diffracted beam crosses a lateral surface) enabled a demonstration of coherent radiation from two fiber lasers coupled by means of a VBG which provides radiation exchange between channels. The proposed Phase II project will deliver a mathematical modeling of a multichannel system (up to 16 channels) with passive coherent combining by volume VBGs, multichannel volume Bragg beam combiners with a number of combined beams up to 16, and multichannel laser systems providing high efficiency coherent beam combining. Year one demonstration of 4-channel combining will be produced at laser laboratory in OptiGrate. Year two demonstration of 16-channel combining will be produced at the fiber laser testbed at AFRL. BENEFIT: Demonstration of high beam quality Coherent Beam Combining fiber laser system with 16+ channels and its modeled scalability to > 100 kW will have the following potential applications / benefits: • passive or active laser beam combining, beam steering to be used in air-borne and free-space weapon systems • applicable to high-power spectral beam combining • exhibits advanced spectral and thermal management • advanced filters can be used for remote angular and/or spectral sensing