Photothermal Gas Detection With a Dithered Low-Finesse Fiber-Optic Fabry-Pérot Interferometer

Pump-probe photothermal spectroscopy has been recently proposed for sensitive gas detection. The probe laser wavelength was usually locked at the quadrature point of an interferometer to provide a linear output of the phase signal. However, this detection scheme brings unwanted fluctuations to the interferometer. Here we report a novel phase demodulation method of the probe laser for photothermal spectroscopy to enhance the stability. We demonstrate a proof-of-concept experiment by performing photothermal detection of methane (CH
₄
) inside a hollow-core fiber, which is sandwiched by two pieces of solid-core fibers to form a low-finesse fiber-optic Fabry-Pérot interferometer (FPI). Besides the wavelength modulation of the 1.65-
μ
m pump laser, we employed a dither to modulate the injection current of the probe diode laser, which was frequency-locked to the FP cavity resonance. The photothermal signal was sensitively measured from the error signal of the feedback loop by demodulating the FPI output at the dither frequency. A normalized noise equivalent absorption (NNEA) coefficient of 7.5 × 10
⁻⁹
cm
⁻¹
WHz
^−1/2
was achieved in this work. A comparison to the conventional quadrature point locking technique was performed to reveal the remarkable sensor stability with immunity to optical disturbance.