Design of an Efficient Fabry-Perot Biosensor Using High-Contrast Slanted Grating Couplers on a Dual-Core Single-Mode Optical Fiber Tip

A novel and efficient Fabry-Perot biosensor on the tip of a dual-core single-mode optical fiber is proposed. The incident light emerges from one of the cores and is coupled to the second core via high-contrast silicon nitride slanted grating couplers (SGCs), separated by a silicon nitride waveguide segment disposed along the fiber tip. The challenges in designing high-contrast SGCs of very short length were overcome using an efficient analytical design approach augmented by simulations in 2D, resulting in grating designs that produce core-to-core coupling efficiencies of up to 29%. The waveguide segment separating the SGCs acts as a Fabry-Perot sensing cavity producing narrow Fabry-Perot fringes in the transmittance spectrum of the structure. Monitoring these fringes leads to bulk sensitivity and figure of merit of 50 - 100 nm/RIU and 16.67 - 33.34 (RIU $^{-1}$ ), for sensing a large range of biomaterials. The structure operates in transmission, thus eliminating the need to separate the reflected light from the incident light at the input of the fiber, which simplifies the interrogation system. Properly packaged, the fiber tip can be dipped or inserted directly into the sensing medium, which removes the need for microfluidics. The scheme enables robust, flexible, compact, and remotely-interrogated biosensors.