An Ag-graphene layers-coated H-shaped photonic crystal fiber (PCF) surface plasmon resonance (SPR) sensor with a U-shaped grooves open structure for refractive index (RI) sensing is proposed and numerically simulated by the finite element method (FEM). The designed sensor could solve the problems of air-holes material coating and analyte filling in PCF. Two big air-holes in the x-axis produce a birefringence phenomenon leading to the confinement loss and sensitivity of x-polarized light being much stronger than y-polarized. Graphene is deposited on the layer of silver in the grooves; its high surface to volume ratio and rich π conjugation make it a suitable dielectric layer for sensing. The effect of structure parameters such as air-holes size, U-shaped grooves depth, thickness of the silver layer and number of graphene layers on the sensing performance of the proposed sensor are numerical simulated. A large analyte RI range from 1.33 to 1.41 is calculated and the highest wavelength sensitivity is 12,600 nm/RIU. In the linear RI sensing region of 1.33 to 1.36; the average wavelength sensitivity we obtained can reach 2770 nm/RIU with a resolution of 3.61 × 10⁻⁵ RIU. This work provides a reference for developing a high-sensitivity; multi-parameter measurement sensor potentially useful for water pollution monitoring and biosensing in the future.

Due to their flexible structure and excellent optical characteristics hollow-core photonic crystal fibers (HC-PCFs) are used in many fields, such as active optical devices, communications, and optical fiber sensing. In this paper, to analyze the characteristics of HC-PCFs, we carried out finite element analysis and analyzed the design for the band gap cladding structure of HC-PCFs. First, the characteristics of HC19-1550 and HC-1550-02 in the C-band were simulated. Subsequently, the structural optimization of the seven-cell HC-1550-02 and variations in characteristics of the optimized HC-1550-02 in the wavelength range 1250–1850 nm were investigated. The simulation results revealed that the optimal number of cladding layers is eight, the optimal core radius is 1.8 times the spacing of adjacent air holes, and the optimal-relative thickness of the core quartz-ring is 2.0. In addition, the low confinement loss bandwidth of the optimized structure is 225 nm. Under the transmission bandwidth of the optimized structure, the core optical power is above 98%, the confinement loss is below 9.0 × 10⁻³ dB/m, the variation range of the effective mode field area does not exceed 10 μm², and the relative sensitivity is above 0.9570. The designed sensor exhibits an ultra-high relative sensitivity and almost zero confinement loss, making it highly suitable for high-sensitivity gas or liquid sensing.

We propose a sensitivity enhanced surface plasmon resonance (SPR) sensor with two-dimensional (2D) hydrogen doped molybdenum oxide (HMO) nanodisks integrated onto an Au-coated optical fiber. Through ionic doping, the molybdenum oxide exhibits tunable plasmonic resonance in the visible to the near-infrared wavelength region, which in turn enhances the light-matter interaction. A resonant peak redshift of the Au-coated optical fiber is observed due to the deposition of the HMO. Within a certain thickness, the refractive index (RI) sensitivity has a positive correlation to the ion doping concentration of HMO, with the average sensitivity showing an enhancement of 84.5%, and the highest sensitivity to be 4566 nm/RIU. Optimized modification of HMO with four cycles is experimentally verified with a sensitivity of 2178 nm/RIU (1.334-1.362) and a figure of merit (FOM) of 29.73 RIU ⁻¹ . This optical fiber SPR sensor can be readily expanded to the application fields like clinical diagnosis and environmental monitoring.