Experimental Study of Surface Plasmon Resonance Refractive Index Sensor Based on Side-Polished Few-Mode Fiber

For surface plasmon resonance (SPR) sensors, sensitivity and full width at half maximum (FWHM) are incompatible parameters, and it is difficult to realize a sensor with both high sensitivity and low FWHM. To weigh two parameters, we propose and demonstrate an SPR sensor based on a few-mode fiber (FMF) structure. The FMF is polished, which can enhance the evanescent wave leakage to excite the SPR effect. Since this FMF can transmit only five modes, the proposed sensor is characterized by high sensitivity and low FWHM. Refractive index sensing experiments were performed on it, and the results showed that the polishing length and depth did not affect the location of the SPR valley but the shape of the valley. In addition, we compared the proposed FMF structure with single-mode fiber (SMF) structure and multimode fiber (MMF) structure in terms of performance. We found that the FMF sensor has higher sensitivity and stronger mechanical strength than the SMF sensor and lower FWHM than the MMF sensor. Therefore, FMF is a compromise between SMF and MMF. The sensitivity of the proposed sensor is 5635 nm/RIU. The lowest FWHM is 127 nm. This study provides a new method for balancing sensitivity and FWHM.