Long-path free-space photothermal spectroscopy based on coherent light Fabry-Perot demodulator

We present a long-range free-space photothermal spectroscopy (PTS) system based on a coherent light Fabry-Pérot (FP) demodulator employing a broadband mode-locked pulse laser with high coherence. By combining fast Fourier transform and Buneman frequency estimation, the PTS system achieves a sub-nanometer optical path difference of the FP interferometer with a refresh rate of 17 kHz, which can enable real-time sensing of photothermal-induced refractive index variations over an extended optical path of 1.8 m. The high-resolution demodulation of the FP interferometer makes it possible to detect trace acetylene with a minimum detection limit (MDL) of 3.6 ppm, corresponding to a normalized noise-equivalent absorption (NNEA) coefficient of 4.1 × 10^-7cm^-1·W·Hz^-1/2. The proposed approach offers significant sensitivity improvement compared with conventional white-light demodulators and provides a promising platform for high-precision, long-path PTS in trace sensing.