Design of a hybrid fiber-capillary optofluidic coupler with multiple dispersion turning points for ultra-sensitive refractive index sensing

Optofluidics, which combines optics and microfluidics, is emerging as a powerful technique for biochemical sensing. Here, we propose and demonstrate a hybrid fiber-capillary optofluidic coupler sensor with three dispersion turning points for ultrasensitive refractive index (RI) sensing. The device comprises a tapered microfiber and a tapered microcapillary fused in parallel. The microfiber acts as a light guide, while the microcapillary acts as both a light guide and a fluidic channel for the liquid sample. We systematically investigated the effective RI, the power occupancy ratio within the microchannel, the RI sensitivity, and the spectral responses to internal RIs of optofluidic couplers with different parameters. An ultra-high sensitivity of −110015.1 nm/RIU was finally achieved. The proposed sensor shows potential for use in lab-in-a-fiber applications and tunable fiber optic devices.