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In this work we present the development of an optical probe which can be used to measure the absolute value of the real and imaginary parts of the refractive index of transparent, opaque or turbid liquids indistinctly. The device can work as a portable, immersion-type, critical-angle refractometer or as a high resolution optical sensor to monitor physical or chemical processes in liquids. The instrument is based on scanning laser reflectometry and measures the reflectance angular-profile around the critical angle. It also measures directly the angle-differential profile of the reflectivity by dynamic reflectometry. For sensing variations of the RI, one can monitor in time either the reflectance or its angular derivative in the vicinity of the critical angle. The uncertainty of the instrument in measuring the real part of the RI can be 10-6. The sensing resolution can be 10-7 when monitoring the differential reflectivity. When the RI is harmonically modulated in time (of RI), it is possible to achieve a resolution as high as 10-10. The applicability of the technique to turbid media consisting in suspension of particles is briefly discussed. We also show that the RI of liquid sample can be monitored through a physical or chemical process if the liquid is vigorously stirred. Regarding the instrument design, we use a novel mechanism to control the angle of incidence which allows to keep all components fixed (laser, detector, semi-cylindrical lens, angle modulator, and cables), except for a mirror and a collimating lens. It only requires linear displacements, for which a conventional micrometer is enough. This design is a considerable improvement over the typical laboratory arrangement used by several authors, and permits to have a portable, compact instrument with all the capabilities of the laboratory technique. The design offers a wide measurement range as to cover most water solutions. We believe the device offers an acceptable balance between size and stability. We give results obtained with an experimental prototype of the device. The aim of this work is a general overview of the foundations of the technique, the development of the sensor and some applications.
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