Femtosecond-induced spiral micro-structured SMS fiber structure for refractive index measurement.

A single-multi-single mode (SMS) fiber structure with spiral microgroove, fabricated by femtosecond laser inscription has been proposed and successfully employed for refractive index (RI) sensing. The multimode interference in the SMS structure is effectively affected by the external perturbation due to the microgroove, which goes deep into the core of the multimode fiber (MMF). Experimental results show that this femtosecond-induced spiral micro-structured SMS (FISM-SMS) fiber structure exhibits a linear response to eternal liquid refractive index in a large RI range of 1.3373-1.4345. The maximum sensitivity of the structure can reach to 2144 nm/RIU and can be further improved by increasing the depth of the spiral micro-grooves.