Fiber-enhanced Fourier-transform photothermal spectroscopy for multi-component gas sensing

We present Fourier transform photothermal spectroscopy (FT-PTS) with a hollow-core fiber (HCF) absorption cell for gas sensing. A broadband pump beam, covering the absorption lines of target gas species, passes through a scanning two-beam interferometer, which modulates the light intensity at different radiofrequencies for different wavenumbers (wavelengths). The modulated pump is absorbed by the gas molecules inside the HCF, which generates heat via photothermal (PT) effect and modulates the refractive index of the gas medium and hence the phase of a probe beam propagating through the HCF. The PT phase modulation contains a spectrum of radiofrequency (RF) components, corresponding to the absorbed pump wavelengths. The PT phase modulation is detected with a Fabry-Perot interferometer to obtain an interferogram, which is Fourier-transformed to obtain the RF spectrum of the PT phase modulation, from which the gas components and concentration are determined. Preliminary experiments with a 10-cm-long HCF gas cell demonstrate the detection of acetylene (C₂H₂), carbon dioxide (CO₂), and a mixture of C₂H₂ and CO₂, with the measured absorption spectra matched well to the calculated spectra based on the HITRAN database. The proposed FT-PTS system would enable multicomponent gas sensing with a single light source and a single HCF gas cell.