High-sensitivity parallel Vernier fiber-optic sensor based on tapered Fabry–Perot interferometers

A parallel Vernier fiber-optic sensor based on tapered Fabry–Perot interferometers (FPIs) is proposed and analyzed. Both the sensor and the 3 dB coupler were independently designed to improve sensitivity and measurement accuracy. The structure consists of two tapered FPI cavities arranged in parallel, with one arm serving as a reference and the other as a sensing arm exposed to variations in the surrounding refractive index (RI). Slight differences in cavity lengths produce distinct free spectral ranges (FSRs), creating a Vernier envelope that amplifies wavelength shifts induced by minor RI changes. Numerical simulations indicate that the sensor achieves a sensitivity of approximately 14,000 nm/RIU over the 1100–1400 nm wavelength range, effectively detecting subtle RI variations. The designed coupler ensures equal power distribution between the arms and maintains phase coherence, enabling a stable and enhanced Vernier response. The proposed sensor features simple fabrication, requires no specialized materials, and is suitable for high-precision applications in biological, chemical, and environmental sensing.