Surrounding refractive index sensing characteristics of the few-mode cladding-reduced long period fiber grating

Abstract In order to improve the sensitivity of the few-mode long period fiber grating (FM-LPFG) in the surrounding refractive index (SRI) measurement, this paper proposes a few-mode cladding-reduced long period fiber grating (FM-CRLPFG) sensor which is fabricated by using CO2 laser. When the cladding radius decreases, a three-layer dielectric fiber model is vital to solve the effective refractive indices of the core modes (LP01 and LP11) in few-mode fiber (FMF). Through analyzing the influences of cladding radius on the resonant wavelengths of core modes LP01 and LP11, it is found that the smaller the radius is, the higher the SRI sensitivity is. In the experiment, the cladding radius was etched by HF solution, and in the meantime the transmission spectrum in corrosion process was observed constantly. Furthermore, by comparing the sensing characteristics of the resonant peak before and after etching, it is found that the SRI sensitivity can be improved effectively by reducing the cladding radius. The SRI sensitivity after etching is about − 850.80 nm/RIU, which is 1.64 times higher than that before etching. The research on the SRI sensing characteristics of FM-CRLPFG can provide guidance for designing the high sensitivity sensor.