Abstract:

Ultra-sensitive refractive index sensors that can detect a tiny change in refractive index are highly sought in many biochemical and biomedical applications. The multistage fiber Fabry-Perot etalons are attractive for such sensors. The performance of these sensors is measured in terms of sensitivity and detection limit, i.e., how much a small change in refractive index is possibly detected. The exceptional point in multistage fiber Fabry-Perot structures, having inherent parity-time symmetry, has enabled ultra-small refractive index sensing with very high sensitivity. Previously, researchers have used the techniques of mode-splitting and wavelength shift in the exceptional point sensing in whispering gallery mode microcavities, ring resonators, and gyroscope. The mode splitting and wavelength shift result from the joint coupling strength change and is independent of gain or loss tuning in the cavities. We cannot use these sensing techniques in optical set-ups where a cavity is used as a sensing platform. Our work presents a novel method in which we show that the mode FWHM change due to an ultra-small refractive index change near an exceptional point can be used as a sensing probe in two-cavity fiber Fabry-Perot etalon.

Moreover, we also compare the performance of the two-cavity fiber Fabry-Perot sensor with and without the exceptional point. We show that the proposed exceptional point sensor outperforms the conventional one for refractive index sensing applications. In the proposed technique, we determine the exceptional point by adjusting middle reflector reflectivity after balancing gain and loss in adjacent cavities. We use the cavity with loss as a sensing element. Using the finite element method, we introduce the balanced loss in the sensing cavity through the tapered fiber of length 1 cm and 1 μm diameter. After achieving the exceptional point, we change the absorption loss in the sensing cavity by changing the refractive index of the liquid in which the tapered fiber is immersed. The simulation results show the maximum sensitivity of 2.25 x 10⁵ Hz/RIU for an ultra-small change in the refractive index change of order 10^-9 RIU. Our proposed ultra-sensitive refractive index sensor based on two-cavity fiber Fabry-Perot etalon is highly cost-effective and straightforward to fabricate. Significantly, the sensing technique based on the FWHM change is generalizable to ring resonators and other microcavity structures for addressing a wide range of liquid phase applications.