Polarization Insensitive Diffraction Grating

The intent of this proposal is to design, and later fabricate, a non-polarizing diffractive beam combining device which meets the stringent requirements of a fiber-laser based Laser Weapon System (LWS). The diffractive beam-combiner is a key element in the system in that provides for combining several high power lasers which differ in wavelength by ~ 5nm into a single beam without suffering wavefront distortion due to thermal absorption of the laser. Recent improvements in both fiber lasers and in laser diodes have made compact and power-efficient systems possible, but all depend on a diffractive beam combining device to achieve required power levels. The existing technology of Multilayer Dielectric Reflection Gratings (MLD) developed for high power laser pulse compression will be used to develop the non-polarizing beam combiner gratings. The requirements as outlined in the announcement are stringent and include 1) High power handling capability, 2) High Diffraction Efficiency over a 40 nm bandwidth, 3) Polarization insensitivity, 4) Low absorption. Additionally, PGL has been informed that a lower dispersion device with a larger grating period is preferred for most applications. These are all challenging requirements but they may be best met with the proposed MLD type gratings.