Fabry–Perot Interferometric C₂H₂ Gas Sensor Based on Spatially Coupled Hollow-Core Photonic Crystal Fiber

In this paper, a Fabry Perot interferometric cavity (F-P cavity) is formed by spatial coupling between single-mode fiber and hollow-core photonic crystal fiber. Based on this cavity, a photothermal spectrum detection system for trace acetylene (C2H2) is constructed. The actual effective length of the F-P cavity and the stability of the sensor's operating point are analyzed theoretically and experimentally. Combined with the wavelength modulation, the optimal modulation depth of the detection system corresponding to the sinusoidal amplitude, the optimal pump power and the optimal detection gas chamber pressure were determined to be 0.06 V, 2.732 mW and 2.75 atm, respectively. Under the optimal experimental conditions, detection experiments were carried out on several concentrations of C2H2. The results show that there is a good linear relationship between the net second harmonic signal amplitude and C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>H<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentration, the goodness of fit is 0.999. Based on Allan-Werle deviation analysis, the detection limits of 0.175 ppm for C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>H<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> at the integration time of 255 s were achieved, corresponding to the normalized noise equivalent absorption (NNEA) coefficient of 8.45 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>WHz<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1/2</sup>. The results provide ideas and references for the establishment of trace gas HC-PCF detection.