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This paper presents an all-fiber, miniature Fabry-Perot gas sensor based on photothermal absorption spectroscopy with a custom-made and cost-efficient interrogation system. The sensing gas microcell has a diameter of 125 μm, a length of 1 mm, and allows for free gas exchange within the optical resonator through micromachined slits. High light intensity and confinement is ensured by delivering the excitation light directly into the gas microcell through a lead-in single-mode fiber. This enhances the photothermal effect and provides a short system response time. The interrogation system utilizes the modulation of an excitation laser diode with a fixed frequency while locking the probe laser onto the gas microcell's quadrature point and observing the variations of the reflected optical power. To show the potential of the proposed system, nitrogen dioxide was measured in dry air. Thereby, a limit of detection of 20 ppm could be achieved for 10 seconds' integration time. Furthermore, the small dimensions of the sensor allow for improved dynamic performance with photothermal modulation frequencies as high as 7 kHz.
Njegovec et al. (Fri,) studied this question.