High-sensitivity photonic crystal fiber methane sensor with a ring core based on surface plasmon resonance and orbital angular momentum theory

With the advent of optical fiber sensing technology, photonic crystal fiber (PCF) sensors based on surface plasmon resonance (SPR) have become a research hotspot. Herein, a PCF-SPR methane sensor with a ring-core structure is designed based on the orbital angular momentum (OAM) theory. To our best knowledge, this type of sensor has not been reported yet. Through the finite element method, the PCF structure is designed and the appropriate filling material is selected. The sensor consists of a three-layer clad air hole and an extra-large central gas channel, forming a ring core for beam transmission. By adding high refractive index material Amethyst and methane sensitive material to the central gas channel, the mode field is improved and the methane concentration is detected efficiently. Numerical results show that the sensor can stably transmit OAM modes and excite SPR with the OAM2,1 mode, namely the HE3,1 eigenmode. With the help of methane sensitive film, changes in methane concentration can greatly affect the refractive index distribution in PCF and alter the excitation characteristics of SPR. In the methane concentration range of 0–3.5 %, the resonance wavelength shifts from 4.52 µm to 4.58 µm. The average wavelength sensitivity reaches 17.07 nm/%, and the corresponding detection limit is 117 ppm. The results reveal that the designed PCF-SPR methane concentration sensor has excellent performance, which can stably transmit OAM mode while achieving SPR effect. It has great innovation and commercial potential in the petroleum, biomedical, chemical, and other industry.