Ultra-sensitive and large dynamic range refractive index sensor utilizing annular core photonic crystal fiber

In this work, we demonstrate a refractive index (RI) sensor employing an annular-core photonic crystal fiber (AC-PCF), which exhibits a large dynamic range and high sensitivity. The AC-PCF represents a very recent type of fiber tailored for the transmission of vector/vortex beams. The mode of operation of the proposed sensor is based on the simple and convenient intensity modulation scheme. A numerical study of the effect of varying the refractive index of the air holes (composing the photonic crystal cladding) on the propagation loss of the fundamental guided mode at 488, 980 and 1550 nm wavelength was performed via a full-vector finite-element method simulation with PML boundary conditions. Our results indicate that the fiber loss increases exponentially as the RI of the holey cladding approaches the value of that of the fiber material (i.e. fused silica in this case) due to outer radiation and scattering. Our simulation assumes that the holey cladding can be filled with analytes of RI varying between 1 to 1.4; thus, showing a large dynamic sensing range. In particular, we observed a strong propagation loss (spanning 2 orders of magnitude in dB/m scale) for analytes with RI values ranging from 1.31 to 1.39 that cover many biochemical solutions of interest. The theoretical results show that the sensitivity is as high as 2.65 × 104 (dB/m)/RIU at 1550 nm and 7.83 × 103 (dB/m/)RIU at 980 nm experimentally. The numerical results were validated with experimental demonstrations using a custom-fabricated AC-PCF.