Laser beam shape converter using spatially variable waveplate made by nanogratings inscription in fused silica

In this work we present a beam shaping technique based on a spatially variable phase retardation plate inscribed inside bulk of fused silica glass by femtosecond laser pulses. Formation of self-assembled periodic nanostructures was exploited to fabricate the converter. During the fabrication process we control induced nanogratings orientation and retardance. Combination of a spatially variable waveplate and a polarizer acts as a spatially variable transmission filter. With a converter fabricated to transform an initially Gaussian beam to a flat-top beam we preserve more than 50% of initial laser power. Theoretically, the efficiency of the proposed converter could be up to 70%. The proposed converter with no absorbing elements possesses resistance to optical damage similar to that of fused silica. Additionally, the already-fabricated converter allows for on-the-fly adjustment of the beam shape from flat-top to a shape with a dip in the middle. The shaped beam was tested in a high power picosecond pulse amplifier.