Sensitivity enhancement of a lossy mode resonance based tapered fiber optic sensor with an optimum taper profile

In this study, we have presented the design considerations of lossy mode resonance (LMR) based tapered fiber optic sensors for sensitivity enhancement. The study includes the effect of the taper ratio and the taper profile, i.e. exponential-linear, linear, parabolic, gaussian and quadratic, on the sensitivity. Also, we have analyzed the effect of various coating materials on the sensitivity. It is revealed from the analysis that with an increase in the taper ratio, the sensitivity increases. The effect of the taper profile shows that among five profiles considered for comparison, the exponential-linear profile has the maximum sensitivity value whereas the quadratic profile has the lowest value. For indium tin oxide (ITO), for the exponential-linear taper profile, the sensitivity corresponding to the first and second lossy modes is 12.005 mu m/RIU (refractive index unit) and 0.8 mu m/RIU, respectively, at the taper ratio 1.7; whereas for aluminum doped zinc oxide (AZO), the corresponding values are 0.515 mu m/RIU and 0.235 mu m/RIU, respectively, at the taper ratio 2.0. For surface plasmon resonance (SPR) based tapered fiber optic sensors, the maximum sensitivity value for the exponential-linear taper profile for platinum is 6.39 mu m/RIU at the taper ratio 2.0, whereas for copper its value is 3.665 mu m/RIU. At the taper ratio 1.0, i.e. the uniform core, the sensitivity of the LMR based tapered fiber optic sensors using an ITO coating is two times greater than the platinum-coated SPR based fiber optic sensor. Increasing the taper ratio further enhances the sensitivity around 40% when compared to the uniform core fiber optic sensor. Therefore, LMR based tapered fiber optic sensors with the considered optimum design parameters demonstrate around a 2.5-fold enhancement in sensitivity compared with conventionally used SPR based fiber optic sensors, giving an optimum structure for sensitivity enhancement. We believe that this study will find suitable applications in research/industry for those working in the area of chemical and biological sensing.