Low-noise refractive index sensing with short-length long-period gratings written with femtosecond laser

We present experimental results of refractive index measurements using femtosecond laser-written long-period gratings (LPGs) with periods ranging from 13 μm to 50 μm and a total grating length of only 4 mm. The peaks observed in transmission correspond to cladding modes with alternating L parity, as shown by calculations of EH modes using a 4×4 transfer matrix method, and their spectral positions and sensitivities show good agreement with experimental results. The peaks, which are well contrasted and easy to track with a commercial FBG interrogator, were monitored as the fiber was exposed to different glycerol concentrations in water. Sensitivities of the peak positions versus refractive index were extracted, showing values up to 421 nm/RIU and refractive index resolutions down to 1.1•10^-6 RIU, which demonstrate their great potential for biosensing. We also discuss trade-offs between sensitivity, measurement range, and linearity, offering recommendations for selecting optimal grating periods based on specific application requirements.