Design and Fabrication of High-Efficiency Binary-Phase Diffraction Gratings for Spectroscopic and Beam Splitting Applications

Binary-phase diffraction gratings are optical components that distribute an incident light beam to diffraction-order directions, due to the periodic modulation of the refractive index within the grating volume. Gratings are essential components in fields like acousto-optics, holography, spectroscopy, and are typically fabricated using lithography. High-efficiency first-order gratings are particularly important in spectroscopy, since first-order spectral diffraction spatially separates the incident wavelengths to be measured. Designing a linear grating consists of iterations of numerical simulations for a given phase profile, to determine grating diffraction efficiencies. The inverse design process, specifying the efficiencies desired and obtaining the phase profile, is very challenging and can lead to unstable solutions. In this research effort, a two-step design process is used. The general parameter ranges are determined first based on design goals, and then more rigorous simulations are performed to “map-out” the diffraction efficiency as a function of a limited solution parameter-space search. The phase profile that matches the design goals is then chosen.
The first lithographic step in grating fabrication is to create a mask for the grating’s features on a photoresist and develop the device profile. Etching the features into the substrate with a reactive-ion plasma process results in a permanent optical component. The study addresses certain fabrication challenges in binary grating fabrication, associated with areal scaling from a 25×25 mm2 surface area to a much larger 101×101 mm2 desired component size. The greatest challenge is to achieve the proper etch depth for the device function, which is mitigated by multiple masking and etching steps.
The Littrow-mount configuration is commonly employed to enhance a grating’s first-order efficiency performance, but can cause ghost images due to light recombination, a problem often controlled with antireflective treatments. Part of the research effort presented here uses random antireflective surface structures (rARSS), which are randomly distributed conical nano-features, etched into dielectric surfaces to minimize their Fresnel reflectivity. These structures were fabricated and tested on cylindrical lenses and freeform elements, showing significant transmission enhancement in the visible spectrum, minimal wide-angle scattering losses, and no notable wavefront distortion. rARSS were then applied to proof-of-concept (POC) reactive-ion plasma-etched (RIPLE) gratings for the VIRUS2 spectrograph, which was designed for Littrow-mount configuration. The rARSS-treated gratings successfully suppressed the undesirable reflection from the zeroth-diffraction order, enhanced the transmitted first-order, and reduced Littrow ghost intensities to four orders of magnitude lower than the transmitted spectrum baseline.
In parallel, a grating beam splitting device composed of two alternating crossed-cell tile first-order diffraction gratings, oriented orthogonal to each-other, was fabricated to function as both a two-way and three-way beam splitter at oblique light incidence. The tiling spatially separates the first-diffraction orders of each grating cell group,
while it overlaps the undeflected zeroth-diffraction order, creating three light-splitting pathways in orthogonal directions in three-dimensions. Each grating type was optimized to separate light in the 1st− and 0th−diffraction order, in ratios 96:1 and 2:1 for the two-way and three-way beam splitter respectively. The result was the projection of three equal intensity spots in space for the three-way beam splitter, and two spots off the axial direction for the two-way a beam splitter.