Cost-effective ethanol sensor utilising inherent mode-transition in photonic crystal fiber

In this paper, we analyse a novel photonic sensor utilising mode-transition in hexagonal photonic crystal fiber (HPCF) to monitor the ethanol content in an ethanol-gasoline blend. Using the finite-element method, the mode-transition from LP₀₂ cladding-mode to LP₀₁ core-mode is accomplished by raising the refractive index (RI) of the analyte layer, which removes the necessity of an additional high RI layer deposition at the fiber surface. We have rigorously optimized the air-filling fraction of the HPCF cladding such that the analyte RI range for the mode-transition would correspond to 0–25% v/v of ethanol in the blend, which is within its commercial range of ethanol-gasoline blend. With increasing analyte RI, we have observed the occurrence of a minimum in the total modal power carried by the sensor. We determine the sensitivity through this modal power variation by dividing it (about the power minimum) into two RI dynamic ranges of 1.400–1.410 (i.e., 25–11% ethanol) and 1.410–1.418 (i.e., 11–0% ethanol), respectively. The maximum calculated sensitivity of the sensor within the linear regime of the modal power variation is 0.46 dBm/% v/v and 0.40 dBm/% v/v, respectively, which are twice as high as the sensitivity offered by the FBG and LPG based sensors over the same dynamical range. In addition to the high sensitivity, the proposed sensor does not require any high-RI layer coating, making its design simpler and easier to implement.