High-sensitivity photonic crystal fiber long-period grating methane sensor with cryptophane-A-6Me absorbed on a PAA-CNTs/PAH nanofilm

A high-sensitivity photonic crystal fiber long-period grating (PCF-LPG) methane sensor with cryptophane-A-6Me absorbed on a poly(acrylic acid)-carbon nanotubes/ polypropylene amine hydrochloride (PAA-CNTs/PAH) nanofilm was investigated. The sensing film was coated onto the internal surface of a photonic crystal fiber cladding air holes by an electrostatic self-assembly technique. Based on a finite element method and the coupled local-mode theory, the effects of the sensing film's refractive index (RI) and thickness on the resonant wavelength were theoretically and numerically analyzed. When the sensing film RI decreases from 1.55 to 1.53, and the thickness increases from 100 nm to 200 nm, the resonant wavelength has a blue shift. A higher RI sensitivity with 1.075 × 10³ nm RIU^-1 is observed for the film thickness of 200 nm. The PCF-LPG methane sensor was fabricated by a pressurized injection method. The sensing experimental result shows that the resonant wavelength of the transmission spectra has a blue shift when the methane concentration increases from 0.0% to 3.5% by volume. The sensor has a good sensitivity of 1.078 nm%^-1 and a low detection limit of 0.18% for a film thickness of 210 nm.