Characterization of a fiber optic liquid refractive index sensor

The intensity of light signal transmitted through an optical fiber, whose cladding over a finite length is removed, is used as a sensor of refractive index of liquids, in which the fiber is immersed. The transmitted light intensity is measured as a function of liquid refractive index for different lengths of the unclad section of the fiber and at each unclad length its sensitivity to change in refractive index of liquid is monitored. The liquid refractive index studied ranges from below the refractive index of the cladding to above that of the core. A plot of normalized light intensity as a function of liquid refractive index for a short unclad length shows a local maximum at a liquid refractive index equal to the refractive index of the fiber core. This maximum becomes less prominent as length of unclad region increases and finally disappears. These observations are interpreted in terms of frustrated total internal reflection of light rays. Sensitivity to refractive index differential, of the sensor response, increases with increase in refractive index of the liquid pair, below that of the fiber core. In this range of refractive index, we observe a maximum in this sensitivity at an intermediate unclad length of the fiber. When liquid refractive index equals core refractive index, at the rising edge of the local maximum observed in sensors of shorter unclad length, the sensitivity maximum disappears. The sensitivity to refractive index differential diminishes as liquid refractive index exceeds fiber core refractive index. Measurements have been performed with five different unclad sensor lengths and a core diameter of 400 and 800 μm. The results for the two core diameters show similar qualitative features.