Laser heterodyne spectrum measurement technology has the characteristics of high spectral resolution, high detection sensitivity and short sampling time. This technology can not only obtain the high-resolution spectral information of the whole layer of atmospheric molecules, but also facilitate the observation of gas concentration in different scenarios due to its small size and easy integration. Therefore, it has been widely concerned by researchers. At present, domestic research on laser heterodyne spectrum measurement system mainly focuses on solar tracking, spectral resolution and inversion algorithm. Due to the low power of the detection signal received by the heterodyne system, the signal-to-noise ratio (SNR) of the heterodyne system is low in actual operation. Therefore, based on the principle of Kepler telescope, a set of sunlight beam shaping structure is designed to improve the sunlight power, and the system SNR is improved by matching the size of the two beams.

In this paper, a 3.93 μm distributed feedback interband cascade laser (DFB-ICL) is used as the local oscillator light source to build a laser heterodyne spectrum measurement system, and sunlight is used as the signal light. The coupling of sunlight and laser is designed and simulated by Zemax optical simulation software. In the experiment, considering the distance between the shaping lens and the photosensitive surface of the detector, the coupling efficiency and the performance of the detector, the incident sunlight diameter is set to 4.5 mm, the plane-convex lens 1 with a focal length of 750 mm and the plane-convex lens 2 with a focal length of 500 mm are selected to form the Kepler telescope structure for shaping the sunlight. The absorption spectrum of N₂O in the range of 2542.9-2545.0 cm^-1 is measured by studying the beam reduction rule of the sunlight and the SNR of the system. The optimal estimation method is used to inverse the measured spectra, and the N₂O column concentration is obtained. Finally, the inversion results of the laser heterodyne spectrum measurement system and the commercial Fourier transform spectrometer are compared and analyzed.

After the laser is emitted from the collimator, the spot diameter is 3.0 mm. Zemax simulation software is used to simulate the structure for sunlight beam reduction. After the simulation and optimization of Kepler telescope structure, the input parameters are as follows: the distance between the two lenses is 1320.155 mm, and the radius of sunlight after 1.5 times beam reduction is 1499.76 μm (Fig. 7). These meet the required spot size requirements. The diameter of the sunlight spot before beam shaping is 4.5 mm, and the SNR of the system is 80.6 when compared with the laser beat frequency result of 3.0 mm diameter sunlight spot. In this case, the size of the two spots can be matched while improving the sunlight power. The optical power of the two beams at the beat frequency is fully utilized, and the heterodyne coupling efficiency is the best. Therefore, the SNR of the system can reach the highest, which is 162.1 [Fig. 9(a)]. According to the best SNRs of the system with the diameter of 2.2, 2.5, 3.0, 3.4 and 4.1 mm after the sunlight beam is reduced through the lenses, the more matched the diameters of the two facula, the higher the SNR of the system (Fig. 8). The N₂O absorption spectrum in the range of 2542.9-2545.0 cm^-1 was measured before and after the beam shaping structure was added. There are two N₂O absorption spectral lines in this band [Fig. 9(b)]. The measurement results show that the amplitude of the spectral signal after beam shaping is significantly improved when only the size of the sunlight spot is changed, which can provide more accurate spectral data for the subsequent inversion of N₂O concentration profile and column concentration.

Comparing the measured spectrum with the inversion fitting spectrum, the residual error of the two curves is within ±0.08 V (Fig. 10). The N₂O column concentration results obtained by the laser heterodyne spectrum measurement system are compared with the measured results of commercial Fourier transform spectrometer EM27/SUN. The variation trend of N₂O concentration measured by the two methods is relatively consistent, and the measurement results obtained using the two methods show a correlation coefficient of 0.856 (Fig. 12).

In this paper, a set of high-resolution laser heterodyne system is built with a 3.93 μm laser as the local oscillator light source, the sunlight is taken as the signal light, and the Kepler telescope structure and Zemax optical simulation software are used to shape the sunlight, so that the size and focus angle of the light spot incident on the photosensitive surface of the detector are smaller than the effective receiving area and field of view of the detector, respectively. The beam reduction of the sunlight in the free space and the size matching of the two spots on the photosensitive surface are realized. The experimental results show that the single-pass SNR of the system is up to 162.1 after the sunlight is shaped and matched with the laser beam, which is twice as high as that of the system without beam shaping. At the same time, the absorption spectrum of N₂O was measured, the optimal estimation method was used to realize the inversion of N₂O column concentration, and the inversion results were compared with those measured by the Fourier transform spectrometer EM27/SUN. The variation trend of N₂O column concentration obtained by the two methods is relatively consistent, and the measurement results obtained using the two methods show a correlation coefficient of 0.856. Through the research on the 3.93 μm laser heterodyne spectrum measurement system, the main factors affecting the SNR of the heterodyne optical path are grasped. The follow-up research will be carried out on the system signal processing and instrument linear function optimization to further improve the SNR and provide favorable conditions for the subsequent high-sensitivity detection of greenhouse gases in the atmosphere.