Hollow Fiber Coupler Sensor.

We present a bi-conical optical directional coupler composed of solid and hollow core fibers. Through an evanescent wave coupling mechanism, the detection of liquid refractive index and its temperature are demonstrated. The experimental results illustrated that the sensor offers a sensitivity of 4.03 ± 0.50 volts per refractive index units (V/RIU) for refractive indices ranging from 1.331 ± 0.003 to 1.403 ± 0.003 with a resolution of 3.5 × 10 - 3 refractive index units (RIU).