  We demonstrate a robust and miniaturized fiber Fabry-Perot cavity-based
sensor for photothermal spectroscopic signal retrieval. The proof-of-concept
experiment involved the use of a near-infrared pump laser to detect methane
molecules on an isolated overtone 2.3 R(4) transition located at 6057.1 cm-1.
The photothermal-related modulation of the gas refractive index was induced at
the center of the interferometer, which was filled with the sample.
Subsequently, the phase change of the resonating probe beam was measured as a
shift in the reflected beam intensity, which was proportional to the methane
concentration. A normalized noise equivalent absorption coefficient of 7.06 x
10-8 cm-1 W Hz-1/2 was achieved, suggesting significant potential for the
design of small and versatile gas detectors with excellent detectivity. We
discuss future improvements of the proposed photothermal gas detection
approach.
