# Development of a Spatial Heterodyne Terahertz Raman Spectrometer with Echelle Gratings
###### Abstract  
This paper introduces an echelle grating spatial heterodyne terahertz Raman spectrometer (E-SHTRS) that combines echelle gratings with spatial heterodyne terahertz Raman spectroscopy technology by replacing the gratings on the interference arms with 36 gr/mm echelle gratings. Echelle gratings are characterized by high diffraction levels and multi-level simultaneous diffraction capability, giving the E-SHTRS higher spectral resolution and a wider detection band range than the conventional spectrometer. The system's resolution can reach 1.37 cm\({}^{-1}\). The spectral detection range of a single level of the proposed system is 701.61 cm\({}^{-1}\). A total of nine levels are used in the system, giving a total spectral detection range of 6314 cm\({}^{-1}\). Using this system, terahertz Raman spectroscopy of organic acid samples was performed, some food additives and medicines were measured, and a salicylic acid aqueous solution was measured with a minimum measurable concentration of 0.01 mol/L. In addition, the samples were detected over a wide band (10-5131 cm\({}^{-1}\)) to acquire more complete spectral information. These experiments verify that the E-SHTRS offers good detection performance and has a wide range of possible applications, including a theoretical support role in food safety, biomedicine, environmental protection, and other fields.  
terahertz Raman; echelle grating; spatial heterodyne spectrometer +
Footnote †: journal: Applied Sciences  
**Citation:** [PERSON]\({}_{2}\) [PERSON]; [PERSON]; [PERSON]\({}_{2}\) [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON], [PERSON] Development of a Spatial Heterodyne Terahertz Raman Spectrometer with Echelle Gratings.  
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shapes, higher diffraction levels, high dispersion, high resolution, and high diffraction efficiency when compared with traditional gratings. [PERSON] designed the echelle grating spectrometer by using the echelle grating as the main dispersion element. The resolution of this spectrometer was twice as high as that of the concave grating spectrometer that offered the highest resolution at that time, and the spectral detection speed was five times higher [7]. [PERSON] et al. replaced the diffraction grating in the common optical path structure of a spatial heterodyne spectrometer (SHS) with an 18 gr/mm echelle grating. The spectral resolution of the resulting system was 2.4 cm\({}^{-1}\), and the spectral detection width was four times greater than that of the original system. [PERSON] et al. [8] designed a wide-band, high-resolution Mark 1 SHS system that could realize a spectral detection range of five times the number of waves and a spectral resolution power of approximately 700,000. Their system used a double echelle grating mode. [PERSON] et al. proposed the use of a combination of echelle grating and plane mirror in spatial heterodyne Raman spectroscopy (EMSHRS) to detect organic liquids, inorganic solids, and mixtures. Ultimately, the spectral detection range of a single level can reach approximately 1100 cm\({}^{-1}\), with a spectral resolution of 1.051 cm\({}^{-1}\)[9].  
We combined the echelle gratings with spatial heterodyne terahertz Raman spectroscopic technology. Because of the high diffraction level of the echelle grating, the system has a strong spectral resolution ability. In addition, the echelle grating has a large number of diffraction levels, which means that the system's spectral detection band is wide. Therefore, the echelle grating spatial heterodyne terahertz Raman spectrometer (E-SHTRS) will have both high spectral resolution and wide spectral detection band characteristics.