Highly Sensitive Flexible Surface Plasmon Resonance Sensor Based on Side‐Polishing Helical‐Core Fiber: Theoretical Analysis and Experimental Demonstration

The prism‐based surface plasmon resonance (SPR) sensing system provides a flexible way to modulate the resonance wavelength by changing the incident angle. Unfortunately, it is difficult to implement such a type of modulation for optical fiber SPR sensor. Herein, a novel and flexible SPR sensor based on a side‐polishing helical‐core fiber (HCF), which is fabricated by twisting an eccentric core fiber, is presented. For the first time to the best of knowledge, the sensing effect of such a sensor theoretically and experimentally is demonstrated. Two SPR excitation methods based on the cladding whispering gallery modes and the core mode are introduced into studying the sensor. The results show that the resonant wavelength of the HCF SPR sensor can be effectively adjusted by changing the twist pitch of HCF. An average sensitivity of 7400 nm/RIU is achieved for the refractive index sensing from 1.375 to 1.385 with a twist pitch of 1.9 mm. This sensitivity can be further improved by using a shorter pitch of HCF. Those characteristics enable the fabrication of sensor arrays in one helical single‐core or multicore fiber with several different twist pitches for multiparameter measurement and may also find wide application in chemo/biosensing.