A simple intrinsic optical fibre refractometer

A refractometer using low-cost components is described. A multimode optical fibre is illuminated by a collimated beam at a variable oblique incidence angle. The value of the refractive index of a liquid is deduced from the measurement of an angle and does not depend on fluctuations of the light intensity. The use of an optical fibre allows remote sensing to be performed. As predicted by a simple theoretical analysis based on geometrical optics, the accuracy of the refractometer is about 1 × 10−4 in refractive index units. Owing to its good sensitivity, this set-up can be used as a chemical sensor.