# Development of a Spatial Heterodyne Terahertz Raman Spectrometer with Echelle Gratings
## 3 Experiment
### Aqueous Solution Measurement Results  
Because terahertz waves are strongly absorbed by water, it is difficult to use traditional terahertz detection methods to measure aqueous solution. In this article, the spatial heterodyne terahertz Raman spectroscopy technique based on the echelle gratings is used to detect the samples of aqueous solution. Figure 6a shows the results of terahertz Raman spectroscopy detection of salicylic acid aqueous solution. It can be seen from the figure that the lowest concentration that can be measured this time can reach 0.01 mol/L, and the location of characteristic peak is consistent with the reference [25]. Figure 6b,c show the measurement results of 0.1 mol/L gallic acid aqueous solution and 0.1 mol/L indomethacin aqueous solution, respectively. The measurement results of both are consistent with the characteristic peak locations of the results measured in this article when the samples were in solid state.  
To verify that the proposed system has wide spectrum detection characteristics, in addition to signal detection within the low frequency range, we also measured the wide band spectra of the samples. Figure 7a,b show the wide band spectra of benzoic acid and sulfur samples, respectively, with measured spectral detection ranges from 10 cm\({}^{-1}\) to 5131 cm\({}^{-1}\) (0.3-153.93 THz). The figures show that when the wide spectrum information of the sample is obtained, the low-wavenumber part of the signal is still obvious, and the locations of the characteristic peaks remain unchanged. We also performed wide  
Figure 6: (**a**) Terahertz Raman spectra of salicylic acid aqueous solutions; (**b**) terahertz Raman spectrum of gallic acid aqueous solution; (**c**) terahertz Raman spectrum of indomethacin aqueous solution 3.6. Wide band terahertz Raman spectroscopy experiment.  
band spectral detection for organic solutions, with results as shown in Figure 7c. Methanol, ethanol, and acetone were measured, and the characteristic peak positions of their respective spectral curves were all consistent with the theory. It is precisely because of the multi-level simultaneous diffraction characteristics of the echelle grating that the wide band spectra of these samples can be measured rapidly. In this case, we used the 7 th to 15 th levels. In fact, wider bands can be measured by simply increasing the number of levels.  
Figure 7: (**a**) Wide band spectrum of benzoic acid; (**b**) wide band spectrogram of sulfur; (**c**) wide band spectra of three organic solutions (methanol, ethanol, and acetone).  
We measured the SNRs of sulfur under various laser power and integration time conditions. As shown in Figure 8, under the same laser power conditions, the SNR value increased rapidly when the integration time ranged from 0 to 5 s, before subsequently entering a slow rising stage. When the integration time reached 13 s, the SNR value gradually stabilized. The SNR curve is a square root function curve, which is consistent with the theoretical SNR formula. When the laser power was 60 mW and the integration time was 31 s, the highest SNR reached 837.