A dual-comb spectrometer for open-path monitoring of greenhouse gases concentrations above an urban area 

Remote sensing of trace gases in the atmosphere can be performed with numerous spectrometers relying on different sources of light. Incoherent sources such as those found with typical Fourier transform spectrometers provide broad spectral coverage, thus allowing to measure spectral signatures from multiple species simultaneously. However, this comes at the cost of limited sensitivity and spectral resolution. On the other hand, coherent sources such as lasers offer high spectral brightness and resolution, resulting in high sensitivity and selectivity at the cost of limited spectral coverage. Developed since the advent of the optical frequency comb (OFC) 25 years ago, state-of-the-art spectrometers operating with OFCs as probing light sources combine high sensitivity, high spectral resolution and broad spectral bandwidth. Among all comb-based spectroscopic techniques, dual-comb spectroscopy (DCS) does not require any dispersive or moving optical component to record a spectrum, allowing for relatively small footprints and mechanically robust instruments. This makes dual-comb spectrometers particularly suited for remote sensing [1] and field-deployed operation outside of the optical laboratory [2].Here, we present the recent technical developments of a near-infrared dual-comb spectrometer for open-path monitoring of greenhouse gases above the city of Heidelberg. The instrument is located at the top of the Institute of Environmental Physics in Heidelberg University campus. The light from two fibered OFCs, spanning 1.58-1.7 µm, is coupled into free space with a telescope, and propagates along a 1.5 km path to a retroreflector array. The reflected signal is picked up by the telescope and coupled back into fiber for detection and data acquisition. We discuss performance of the instrument and the results of our upcoming measurement campaign.[1] G. B. Rieker et al., "Frequency-comb-based remote sensing of greenhouse gases over kilometer air paths," Optica 1, 290-298 (2014), DOI: 10.1364/OPTICA.1.000290.[2] S. Coburn et al., "Regional trace-gas source attribution using a field-deployed dual frequency comb spectrometer," Optica 5, 320-327 (2018), DOI: 10.1364/OPTICA.5.000320.