Design of highly efficient transmission gratings with deep etched triangular grooves.

The design of highly efficient fused silica transmission gratings with deep-etched triangular-shaped grooves in the -1st order diffraction is realized by the rigorous coupled wave analysis (RCWA). The antireflective effect of a subwavelength triangular-groove grating with gradient effective refractive index results in the higher diffraction efficiency (>99.9%). The performance of the presented gratings is clearly better than traditional rectangular and blazed ones. The gratings are designed under Littrow mounting at a wavelength of 1064 nm to be used in high-power laser systems. A detailed fabrication tolerance, covering not only the errors in height but also the errors in the lateral dimension, is demonstrated. The physical process of the diffraction characteristics for such a triangular-groove grating can be well explained by the simplified modal method based on two-beam interference of the first two propagating modes excited by the incident wave. Based on the fact that the transmittance derived from the modal method is in good agreement with that calculated by the RCWA, the simplified modal method can be effectively utilized as an easily designed tool of the triangular-shaped gratings.