High-Efficiency Polarization-Insensitive Diffraction Grating for All-Glass Monolithic SBC Fiber Laser System

Spectral beam combination (SBC) of fiber lasers is the most practical approach to DEW-class HELs using diode-pumped fiber lasers. Use of conventional diffraction gratings to combine wavelengths is limited in that metal is inherently absorbing. Regardless of the metal layer thickness, optical power is absorbed in the grating, altering the spectral and efficiency properties of the grating and inducing optical aberrations that reduce beam quality. Designed to overcome the peak-power damage limitations for use in petawatt (short-pulse) lasers, multi-layer dielectric (MLD) diffraction gratings are essentially transmission gratings paired with a dielectric mirror stack. Although the lack of metallic surfaces exist eliminates absorption issues, MLD gratings by definition rely on interference within the stack, resulting in much higher intensities within the stack and leading to optical damage at reduced power levels. We propose to use an all-glass diffraction grating whose high efficiency is yielded by inhibiting transmitted orders, and can therefore be made of a single material with no coatings or layers. Used in an immersion topology, this grating allows for a fully monolithic SBC fiber laser system to be realized, with no free-space components. The grating and the monolithic SBC sytem re-design are the focus of the Phase I proposal.