# A High-Resolution MIR Echelle Grating Spectrometer with a Three-Mirror Anastigmatic System
###### Abstract  
With the emergence of high-performance infrared detectors and the latest progress in grating manufacturing technology, high-resolution and high-sensitivity infrared spectrometers provide new methods for application to many fields, including astronomy and remote sensing detection. Spectral detection has attracted considerable attention due to its advantages of noncontact and stability. To obtain the detailed features of the missile's tail flame spectrum, traditional plane reflection gratings are used as the main dispersive element; however, the instrument's volume will increase with increasing resolution, which is not conducive to remote sensing detection from airborne platforms. Such spectrometers cannot meet high-resolution spectroscopy requirements. To address this problem, this paper proposes an immersion echelle spectrometer combined with a three-mirror astigmatism optical system. High resolution and compact size were achieved. In this paper, a small high-resolution infrared echelle spectrometer optical system was created by combining an off-axis three-mirror anti-astigmatism system, a Littrow structure, and a concave grating Wadsworth imaging device. The optical system operated in the 3.7-4.8 \(\upmu\)m band; the echelle grating worked under quasi-Littrow conditions, while the concave grating was used for auxiliary dispersion to separate overlapping orders. The resolution of the optical system in the entire working band was 23,000-45,000. The optical plane size of the spectrometer was around 360 mm \(\times\) 165 mm. The results show that the Mid-IR echelle spectrometer achieved high spectral resolution, better than 0.25 cm\({}^{-1}\), meeting missile tail flame detection requirements. This device has the potential for real-time long-range target detection when warheads are destroyed. While this study focuses on the mid-wave infrared band, its approach can also be extended to other infrared bands. high resolution; silicon immersion grating; echelle spectrometer; concave grating; optical design +
Footnote †: journal: Applied
dispersive elements operate in cross directions, resulting in a two-dimensional image on the detection plane [1]. Although cross-dispersion enables spectrometers to achieve a wide wavelength range and high resolution in a compact space, the resulting optical structure becomes quite complex; in addition, many researchers have suggested using cylindrical lenses or annular mirrors as compensation devices to correct for astigmatism, further increasing the system's complexity. Additionally, increasing the surface area of the optical element will reduce the light transmission efficiency of the optical system. Echelle spectrometers, with echelle gratings as dispersive elements, have the advantages of high resolution, high dispersion rate, high diffraction frequency, full-spectrum transient direct reading, and low detection limit. However, because the grating works in an advanced state, the spectral order overlap phenomenon occurs as a result. In general, a secondary dispersion element must be applied to eliminate interference between spectral orders. Therefore, suitable auxiliary dispersive components have also become an important part of echelle spectrometer design. To achieve broadband spectral detection, such as for missile tail flames, the choice of a spectrometer is critical. Traditional Fourier transform spectrometers are the typical instrument of choice in infrared field applications due to their potential for high resolution, high signal-to-noise ratios, and broad spectral coverage. However, considering the working environment factors, their performance is limited; specifically, component movement may cause interference, thereby affecting the device's resolution, while the signal-to-noise ratio is often low when mounted on an airborne platform. If a typical plane grating spectrometer is chosen, achieving high resolution requires a large-area grating, increasing the instrument's overall volume.