Performance of silicon immersed gratings: measurement, analysis, and modeling

Abstract

The use of Immersed Gratings offers advantages for both space-and ground-based spectrographs. As diffraction takes place inside the high-index medium, the optical path difference and angular dispersion are boosted proportionally, thereby allowing a smaller grating area and a smaller spectrometer size. Short-wave infrared (SWIR) spectroscopy is used in space-based monitoring of greenhouse and pollution gases in the Earth atmosphere. On the extremely large telescopes currently under development, mid-infrared high-resolution spectrographs will, among other things, be used to characterize exo-planet atmospheres.At infrared wavelengths, Silicon is transparent. This means that production methods used in the semiconductor industry can be applied to the fabrication of immersed gratings. Using such methods, we have designed and built immersed gratings for both space-and ground-based instruments, examples being the TROPOMI instrument for the European Space Agency Sentinel-5 precursor mission, Sentinel-5 (ESA) and the METIS (Mid-infrared E-ELT Imager and Spectrograph) instrument for the European Extremely Large Telescope.Three key parameters govern the performance of such gratings: The efficiency, the level of scattered light and the wavefront error induced. In this paper we describe how we can optimize these parameters during the design and manufacturing phase. We focus on the tools and methods used to measure the actual performance realized and present the results.In this paper, the bread-board model (BBM) immersed grating developed for the SWIR-1 channel of Sentinel-5 is used to illustrate this process. Stringent requirements were specified for this grating for the three performance criteria. We will show that -with some margin-the performance requirements have all been met.

Optical Designs with Curved Detectors for Fiber Bragg Grating Interrogation Monitors

“…The prism rear surface has a reflective coating. This solution is to some extent similar to immersed gratings proposed in Reference [16,17]. The freeform mirror in Design B serves as both the collimator and camera.…”

Spectrographs for astrophotonics

“…Since the spectral resolution is determined by the maximum optical delay ∆ applied within the spectrometer, delaying in a glass of high optical index like silicon (n ∼ 3.4 in NIR) or Germanium (n ∼ 2.4 in NIR) rather than in air reduces the size of the dispersing element by an equivalent factor. Developments on immersion gratings are ongoing for the next generation of spectrographs for the ELTs [51], with already some demonstrators on sky, like IGRINS [52,53]. This concept is of course equally applicable -and applied-to integrated spectrographs.…”

Study on immersion grating used in weak carbon dioxide band detection spectrometer

“…However, the concentration of CO2 in the atmosphere is low. To use this band to detect changes in CO2 concentration, it is necessary to ensure that the detection instrument has a sufficiently high spectral resolution [3][4] . Using the immersion grating as the dispersive element in the spectrometer can greatly reduce the size and weight of the instrument while achieving high spectral resolution.…”