Photonic crystal fiber methane sensor based on modal interference with an ultraviolet curable fluoro-siloxane nano-film incorporating cryptophane A

A photonic crystal fiber (PCF) methane sensor based on modal interference is presented, which is made by coating an ultraviolet curable fluoro-siloxane nano-film incorporating cryptophane A onto the internal surface of PCF cladding air holes. The methane sensing principle is studied by using modal interference in a PCF. Effects of the refractive index (RI) and thickness of sensing film on the effective RI difference (wavelength shift) between core mode and cladding modes of PCF are analyzed theoretically and numerically. The results show that there is a slow decline of effective RI difference when the film RI decreases from 1.455 to 1.410 and the thickness increases from 100 nm to 300 nm, and the interference fringes have a wavelength blue-shift. For the sensor with thickness of 250 nm, a sudden change of effective RI difference can be observed. Response characteristic of the film RI to methane is demonstrated with a RI in-situ measurement system. As the increase of methane gas concentration, the film RI decrease linearly. The PCF methane senor is fabricated by a capillary dip-coating technique and an offset splicing method. It is shown that the characteristic wavelength of interference spectrum has a blue-shift when the methane concentration is in a range of 0.0–3.5% by volume. The senor has a good sensitivity of 0.514 nm%−1 and a low detection limit of 0.16% for film thickness of 240 nm.