Mid-Infrared Laser Heterodyne Radiometer for Simultaneous Sensing of Methane and Water Vapor*

Laser Heterodyne Radiometry (LHR) is a suitable technique for remote sensing of chemical profiles in the Earth's atmospheric column. The technique involves mixing a modulated radiative intensity from the sunlight with a local oscillator (LO) at a fixed frequency. The mixed signal can further be demodulated with spectral signatures of sunlight (with LO mixed modulated intensity from absorption) at the absorption wavelength from the reference LO. This makes this technique unique for Volatile emissions for environmental and atmospheric applications. Although conventional spectrometers can be used for gas sensing, they often lack the spectral and spatial resolution and sensitivity required for accurate spectral analysis under real-world atmospheric conditions. The LHR overcomes these limitations by providing high spectral resolution and sensitivity in a compact, remote-sensing configuration. In this project, a narrowband distributed feedback (DFB) laser operating near 3.27 µm is used as the local oscillator for methane detection. The local oscillator is mixed with sunlight that has undergone absorption in the atmosphere, generating an intermediate-frequency beat signal that encodes the high-resolution spectral information. Due to its fine spectral precision, the instrument is also capable of retrieving vertical concentration profiles of methane through radiative transfer inversion of the measured rotational vibrational transitions and resulting absorption lineshapes