A microfiber half coupler for refractive index sensing

We report a reflection mode based microfiber half coupler probe for refractive index (RI) sensing. The probe was made by cleaving a fused 2 $\times$ 2 optical fiber coupler at the center of the tapered region. When used for RI sensing, the reflection spectrum blueshifted as the surrounding RI increased. The sensing capability of the probe was tested using sucrose solutions with different concentrations. The relations between the RI sensitivity and the taper waist diameter as well as the RI range were investigated. The RI sensitivity increased quickly as the taper diameter decreased.