Design and fabrication of 1300-line/mm polarization-independent reflection gratings for spectral beam combining

We designed and fabricated polarization-independent multilayer dielectric reflection gratings of 1300 line/mm for spectral beam combining. Because the designed grating has a duty cycle (or fill factor) much greater than 0.5, we used the lift-off process to invert a less-than-0.5 duty cycle photoresist mask to a greater-than-0.5 duty cycle chromium mask. Because the simultaneous polarization-independence and high-angular-dispersion requirements greatly reduce fabrication tolerance, we employed the end-point detection techniques to control accurately the duty cycle and groove depth of the grating profile to reach the design target. The measured polarization-averaged diffraction efficiency of the fabricated gratings was greater than 97% in the wavelength range of 1050 nm–1080 nm.