Optimization of continuous-profile blazed gratings using rigorous diffraction theory

Abstract

Surface-relief elements with a high diffraction efficiency play an important role in the industrialization of diffractive optics. Optimal methods for the design, analysis, and fabrication of diffracting structures are essential for producing high quality applications. With direct laser beam writing it is possible to produce deep continuous-profile diffractive lenses and gratings with periods on the order of few micrometers. The finite size of the writing beam cannot be neglected as the period becomes comparable to the wavelength of the laser. We present optimization results for deep continuous-profile blazed gratings with periods between 4 and 6 μm designed to work in the 3rd, 4th, or 5th diffraction order in the visible range. These gratings possess large diffraction angles, and thus they appear in the outer zones of diffractive lenses with a high numerical aperture (NA). Optimization of the profiles is essential for practical applications, since the diffraction efficiency drops significantly because of smoothing caused by the finite size of the writing beam. The proposed optimization scheme utilizes the simulated annealing method and models the diffraction with the rigorous coupled wave analysis. Improvements of several tens of percents in the diffraction efficiency are achieved. The sensitivity of the optimized structures to fabrication errors is analyzed.