Measurement of Atmospheric CH4 by 1.6 μm High Resolution Non-Modulated Laser Heterodyne Radiometer (NM-LHR)

Laser heterodyne spectroscopy detection technology boasts exceptional advantages such as high spectral resolution and high signal-to-noise ratio (SNR). It excels at capturing spectral line broadening information of upper atmospheric molecules. This presents substantial research value in the realms of greenhouse gas profile measurement and the assessment of laser propagation effects in the atmosphere. This paper delves into the investigation of the processing method for heterodyne signals, adopting a non-modulated signal processing approach to construct a near-infrared non-modulated fiber laser heterodyne radiometer. This innovative design significantly enhances the device's response speed and SNR. The radiometer achieves a spectral resolution of 0.006 cm^-1 and an SNR of 300. This facilitates the acquisition of vertical profile distribution and column concentration of CH₄ by measuring the absorption line of atmospheric CH₄. Comparative tests reveal compelling advantages of the non-modulated device, with the modulated device collecting data 6 times in 6 minutes, yielding an SNR of 58. In contrast, the non-modulated device demonstrates superior efficiency by collecting data 6000 times in 2 minutes, resulting in a remarkable SNR of 103. The inversion results of CH₄ column concentration from the laser heterodyne radiometer were compared with those from the Fourier transform spectrometer (EM27/SUN), with average concentrations of 1.88×10^-6 and 1.93×10^-6, exhibiting an overall deviation of approximately 2.6%. The non-modulated laser heterodyne radiometer provides a new reference for the rapid, accurate and high spectral resolution measurement of greenhouse gas concentration.