Surface plasmon resonance (SPR) represents a collective oscillation phenomenon generated by free electrons under electromagnetic wave excitation. This distinctive optical phenomenon transcends the diffraction limit of traditional optics and enhances light-matter interaction, garnering significant attention. These characteristics have catalyzed advancements in optoelectronics and nanotechnology. Conventional SPR sensors employ Kretschmann prism coupling or grating coupling for surface plasmon excitation. However, their substantial size and integration challenges fail to meet modern sensing technology requirements for miniaturization, high sensitivity, and multi-parameter detection. The photonic crystal fiber (PCF)-SPR sensor integrates photonic crystal light field regulation capabilities with SPR refractive index sensitivity characteristics, potentially overcoming traditional SPR sensor limitations. Despite the PCF-SPR sensor having notable advantages in refractive index sensing, it faces technical limitations regarding insufficient sensitivity in detecting low refractive index media (refractive index n<1.33), constraining its practical application in crucial areas such as biological solution analysis. We present a three-open-loop channel D-type PCF-SPR refractive index sensor designed for low refractive index sensing. The open-channel configuration enhances sensing capabilities by expanding the effective sensing area and increasing fiber core exposure to the target environment. This design innovation augments detection sensitivity while preserving mechanical stability.

The PCF incorporates three distinct pore sizes arranged in a double-layer rhombus configuration. The analyte microfluidic channels were constructed through the introduction of three open rings on the D-shaped polishing plane, with gold plating films applied to the open rings’ inner surfaces. Au was selected as the plasma material due to its established stability in chemical interactions. The open channel design reduces the distance between the fiber core and metal layer, facilitating faster detection response and generating enhanced SPR effects at the metal-dielectric interface. This study employs finite element analysis to evaluate how system structural parameters, including pore size, pore spacing, metal thickness, and fiber core-to-polishing plane distance, influence sensor performance.

The proposed three-open-loop channel D-type PCF-SPR refractive index sensor demonstrates superior sensing performance through optimization of pore size, pore spacing, metal thickness, and fiber core-to-polishing plane distance. Initially, we examine the influence of pore diameters (d₁, d₂, and d₃) on the fundamental mode loss spectrum for analyte refractive indices n_a of 1.26 and 1.27. In Fig. 3, distinct sharp SPR formant peaks emerge at pore diameters of d₁=1.2 μm, d₂=1.5 μm, and d₃=1.8 μm. In Fig. 4, with fixed pore diameters (d₁=1.2 μm, d₂=1.5 μm, and d₃=1.8 μm), we analyze the effects of pore spacing Λ, gold film thickness t_Au, open-loop radius r_s and distance from fiber core to polishing plane h on the fundamental mode loss spectrum. Pronounced SPR formant peaks appear at structure parameters of Λ=3 μm, t_Au=50 nm, r_s=1 μm, and h=6 μm. In Fig. 5, we examine the fundamental mode loss spectrum distribution across analyte RI n_a values from 1.15 to 1.29. The sensor achieves a maximum wavelength sensitivity of 14500 nm/RIU with a corresponding refractive index resolution of 6.90×10^-6 RIU and a figure of merit (FOM) of 114.94 RIU^-1. For enhanced measurement accuracy, the sensing range is subdivided into 1.15?1.22 and 1.23?1.29. Linear fitting achieves a correlation coefficient of 0.98299 for the lower range, while third-order polynomial fitting yields a correlation coefficient of 0.99663 for the higher range.

We present a high-sensitivity triple open-loop channel D-type PCF-SPR sensor. The finite element method was employed to systematically analyze the influence of structural parameters, including aperture size, pore spacing, distance from the fiber core to the polishing plane, and metal film thickness on sensor performance. The findings demonstrate that the sensor effectively detects RI within the range of 1.15 to 1.29. The sensor achieves a maximum wavelength sensitivity of 14500 nm/RIU, with a corresponding resolution of 6.90×10^-6 RIU in the short-wave infrared band of 2?3 μm. Furthermore, the FOM reaches 114.94 RIU^-1. The sensor’s superior sensitivity and capability for low refractive index detection make it particularly suitable for precise measurements of biochemical solutions, including liquid carbon dioxide, liquid medical oxygen, sevoflurane, and biological samples.