A High-Sensitivity SPR Sensor Based on MMF-Tapered HCF-MMF Fiber Structure for Refractive Index Sensing

In this article, a high-sensitivity surface plasmon resonance (SPR) sensor based on multimode fiber (MMF)-tapered hollow core fiber (THCF)-MMF structure is fabricated and proposed for refractive index (RI) sensing. As the measurement RI increases, the SPR peak right shifts, a multimode interference (MMI) spectrum will also appear in the visible wavelength in front of the SPR peak, and the MMI resonance wavelength left shifts with the RI increases. The RI changes can be detected by monitoring the resonance wavelength difference between the MMI wavelength and the SPR peak, which is a new method used to measure the RI. The RI sensing performance for the proposed sensor probes with different taper ratios and different HCF core diameters is explored. The experimental results show that the sensitivity could be significantly improved at large RI for the MMF-THCF-MMF probe with a large taper ratio. For the probe with an HCF core diameter of $30 ~\mu \text{m}$ and a taper ratio of 3.3, a high RI sensitivity of 7592.25 nm/RIU is obtained at the RI of 1.40. The proposed SPR sensor is easily fabricated and has a simple structure, which has potential applications in biochemical sensing fields.