High sensitivity internal refractive index sensor based on a photonic crystal fiber long period grating

A high sensitivity method is reported for the measurement of the internal refractive index using a photonic crystal fiber long period grating. Long period gratings of different lengths were inscribed in photonic crystal fibers, and the air holes of the fiber had varying refractive indices. A side lobe appeared near the resonant dip through the analysis of the characteristics of transmission spectra showing variation in the refractive index. The resonant dip and its side lobe provided varying sensitivities to the internal refractive index values. The sensitivity of the side lobe was as high as 2343 nm/refractive index unit (RIU), which exceeded the value for the resonant dip (2047 nm/RIU) for refractive indices from 1.3333–1.3792. Due to the high resolution of 8.5 × 10−6 RIU, this method offers promising applications for biological and chemical analysis in which high-precision refractive index measurements are required.