Remote methane sensing system based on heterodyne enhanced chirped laser dispersion spectroscopy
Abstract
We report a remote methane sensing system capable of heterodyne enhancement (HE) of weak optical signals and range-resolved optical path-length determination for methane leak detection. This is particularly useful in long distance path-integrated trace gas sensing, as we can achieve simultaneous acquisition of ranging information and spectroscopic information at lower return powers. The system is based on chirped laser dispersion spectroscopy (CLaDS), which probes the presence of methane molecules through optical dispersion using a multi-frequency beam. The spectroscopic information can be extracted by frequency demodulating the beatnote signal through optical heterodyne detection at the photodetector. The CLaDS signal is therefore unaffected by optical intensity fluctuations. Additionally, the phase-sensitive method has been demonstrated to preserve linearity for optically thick gas samples, making it especially suitable for remote sensing applications. The HE-CLaDS remote sensing system consists of a transmitter that launches a laser beam with dual sidebands generated by an electro-optic modulator (EOM) into free space and a receiver that collects the return optical signal from reflective or scattering surface using a telescope. We show that for return optical power below 10 µW, the system can be configured to use a higher-power optical local oscillator to improve the sensitivity through heterodyne enhancement. We further present that by superimposing a linear chirp on the driving RF input signal to the EOM, range measurement is enabled based on the principle of frequency-modulated continuous wave radar. We demonstrate that the HE-CLaDS remote sensing system achieves below 2 ppm-m/√Hz sensitivity for methane detection, 0.04 m/√Hz ranging precision, and 10 to 20 dB enhancement of carrier-to-noise ratio (CNR) of the heterodyne beatnote in free space testing. Instrument design and test results will be discussed.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.A43R3433C
- Keywords:
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- 0345 Pollution: urban and regional;
- ATMOSPHERIC COMPOSITION AND STRUCTUREDE: 3394 Instruments and techniques;
- ATMOSPHERIC PROCESSESDE: 0428 Carbon cycling;
- BIOGEOSCIENCESDE: 0478 Pollution: urban;
- regional and global;
- BIOGEOSCIENCES