High-resolution, high-efficiency narrowband spectroscopy with an s-p-phased holographic grating in double pass

¹ Département d’Astronomie de l’Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
² School of Mathematical and Physical Sciences, Macquarie University, Balaclava Road, North Ryde, NSW 2109, Australia
³ Wasatch Photonics, 1305 North 1000 West, Suite 120, Logan, UT 84321, USA

^★ Corresponding author: casper.farret@unige.ch

Received: 21 February 2025
Accepted: 15 May 2025

Abstract

Context. High-resolution spectroscopy (R>50 000) in astronomy is typically performed with echelle-type spectrographs. The science for which these instruments have proven very effective is the detection of exoplanets through the radial velocity method, and the characterization of their atmospheres. For atmospheric characterization, it has been proven tedious to detect these signals, however mostly due to sensitivity constraints. While echelle-type spectrographs provide the necessary broad bandwidth for radial velocity measurements, they compromise total throughput. Additionally, the need for spectral order sorting complicates the optical design and reduces throughput further. A high spectral resolution and a limited bandpass are required to measure the exoplanet atmospheric absorption from the ground. Therefore, we propose a new method to for achieving a very high spectral resolution with significantly higher throughput within a limited bandpass that is focused on a specific spectral line or set of spectral lines of interest.

Aims. We describe and test a novel method for reaching a high spectral resolution with very high unpolarized diffraction efficiency in first-order employing a tuned high fringe-density volume-phase holographic (VPH) grating in double pass. We also provide laboratory tests that highlight the potential of this setup.

Methods. We used a wavelength-tunable laser to measure the dispersion and diffraction efficiency of a tuned VPH grating. We compared a single- and double-pass setup to verify the expected results. We also imaged the resulting spectrum to assess the optical quality.

Results. The VPH grating we tested can reach a diffraction-limited resolving power of >140 000 in double pass, with a peak double-pass diffraction efficiency of 79% for unpolarized light. We tested the grating at a more modest resolution of 38 000 given the sampling constraints. Based on current manufacturing abilities, we estimate that double-pass diffraction efficiencies over 50% with diffraction-limited resolving powers >200 000 should be within reach from the visible to the near-infrared, where the bandwidth is limited by the detector size.

Conclusions. For specific science cases where a relatively narrow wavelength regime at (ultra-)high spectral resolution is required, a double-pass VPH setup can prove to be very efficient. As the grating operates in first order, there is no need for a cross-dispersion, which allows very high total system throughputs and less complicated optics overall. This might enable ground-breaking science with smaller-class telescopes, with relatively compact instruments, and it might be of special interest for exoplanet atmospheric characterization because these observations typically require a large amount of observing time, a high signal-to-noise ratio, and a high spectral resolution.