Femtosecond laser based in-fiber long period grating fabrication for improved solution sensing

This work reports femtosecond laser based fabrication of long period fiber gratings (LPFG). Index modulation in the core of single mode fiber (SMF) is written employing femtosecond pulse filamentation technique. Highly repeatable filamentary voids written in line-by-line femtosecond laser inscription technique enables steady and noise free growth of LPFGs. The sharp transmission valley (with a narrow full width at half maximum of 5 nm) of long period grating offers better resolution for refractive index (RI) measurement of a solution. The LPFGs inscribed by femtosecond laser radiation show RI sensing sensitivity of 29.199 nm/RIU which is three times higher than the sensitivity of LPFGs written by UV radiation (sensitivity: 11.179 nm/RIU). The position of transmission dip of a grating can be tailored relatively easily simply by varying the period of index modulation.