Optical Intensity-Type Refractometer for Remote Measurements Via Fiber-Optic Cables

We present a fiber-optic refractometer with intensity-type transducer which can operate remotely via long fiber-optic cables. This version of the refractometer is intended for accessing the concentration of aqueous solutions of sodium chloride in the range from zero to about 145⁰/₀₀ (1.327 <; n <; 1.353 at λ = 940 nm and 25^°C), which is of significant practical interest for industrial users. We address the relationship between the span and linearity of the intensity-type optical transducer with spherical working surface which we employ in the refractometer. We optimized the transducer for the specified measurement range and achieved good sensitivity and non-linearity as small as 0.05. We significantly reduced the negative effect of the unstable optical loss of the fiber-optic cables, variation of the optical source power and photo-detector response on the readings of the refractometer by using an optical bridge configuration followed by an analogue optoelectronic signal processing unit. The refractometer has a parasitic variation of its output as low as 1.5% under a total variation of the optical source power and a fiber-optic cable loss of 20 dB. Potential applications of this refractometer include the in-situ characterization of various liquids such as the sodium chloride salt-brine solutions in food industry.