Mid-infrared dual-comb absorption and dispersion spectroscopy and temperature measurement in a plasma

Abstract

Traditional Fourier transform spectrometers (FTSs) in an asymmetric configuration can provide both absorption and dispersion spectra of the molecules simultaneously. However, they require a long measurement time, mainly due to the limitations of the signal-to-noise-ration (SNR) as well as maximum speed and travel distance of the retro-reflector mirrors in the FTS. These constraints make FTS inefficient for time-dependent multispecies detection with high SNR in certain specific applications, like plasma and combustion systems. These limitations can be resolved by mid-infrared dual-comb spectroscoy (DCS), which provides unique features of broad spectral bandwidth, high spectral resolution, high detection sensitivity, and simultaneous measurement of absorption and dispersion spectra in the fingerprint region within a short acquisition time. Here, we report time-resolved absorption and dispersion spectroscopy of methane and ethane diluted in helium under static and dynamic electrical discharge plasma, covering a spectral bandwidth of 2850-3150 cm -1 , with a spectral resolution of 0.16 cm -1 [1] . We used an asymmetric configuration for the DCS to measure the absorption and dispersion spectra in a 50-cm-long discharge tube. In this configuration, the gas sample is placed in the beam path of the signal mid-infrared comb, and the transmitted beam interferes with the local mid-infrared comb on a beam splitter. Thus, the molecular absorption and dispersion information is probed by the signal comb, and the local comb is used to retrieve the molecular information through multi-heterodyne down-conversion. The absorption and dispersion spectra retrieved from DCS portray a complete picture of electromagnetic filed interaction with gas phase molecules, which can provide the broadband complex refractive index of the gas samples under discharge.