Characterization of a symmetric-facet dual-ruled grating for spatial heterodyne spectroscopy

Abstract

Dual-bandpass spatial heterodyne spectrometers (DB-SHS) enable simultaneous high-resolution measurements of widely separated passbands, providing powerful diagnostics of astrophysical and planetary environments. However, DB-SHS instruments require a single incident beam to span two adjacent diffraction gratings with distinct ruling densities and blaze angles, resulting in a large gap between ruled sections that reduces throughput. Dual-ruled gratings solve this problem by integrating multiple ruled panels onto a single substrate, minimizing the dead space between ruled sections. We present experimental validation of a first-generation symmetric-facet dual-ruled grating manufactured by Bach Research, mechanically ruled at $800$ and $2000\mathrm{gr}\mathrm{m}{\mathrm{m}}^{-1}$ with a $13.8^{\circ }$ blaze angle. Using a stabilized deuterium source alongside a Czerny-Turner monochromator, we measured diffraction efficiencies into the $m=0,\pm 1,\pm 2$ orders from $200$ to $700\mathrm{nm}$. We compare these results with theoretical predictions from rigorous coupled-wave analysis (RCWA), inferring a facet asymmetry of $\lesssim 1^{\circ }$ and $\sim 70\%$ facet duty cycle indicative of minor manufacturing defects. This work demonstrates the viability of mechanically ruled, symmetric-facet, dual-ruled gratings and lays the foundation for laboratory validation of the first DB-SHS, ultimately enabling high-resolution spectroscopy of distinct spectral regions relevant to astrophysical and planetary remote sensing.

13 pages, 9 figures. Submitted to SPIE Astronomical Telescopes + Instrumentation 2026, Conference 14154: Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation VII