MINIATURE OPTICAL FIBRE REFRACTOMETER FOR MEASURING LOCAL CONCENTRATION OF SOLUTION
In this report a new measurement technique of refractive index of liquids using the miniature optical fibre sensor is proposed. The sensor consists of three multimode optical fibres of which the tips are ground and polished. The active points of probe are designed and glued together in order that the incident light reflect back, depending upon the refractive indices of the surrounding liquid material according to Snell's law. The refractive indices of distilled water are measured for temperatures ranging from 5 to 55°C.
It is found that the reproducibility is fairly good and the refractive index can be determined with an accuracy to fourth decimals under the present conditions, corresponding to the accuracy of temerature ±2.5 K. The probe is calibrated with the saline solution. The salt finger phenomenon is observed by turning the cell filled with hot salty water on the cell filled with cold fresh water and contacting abruptly each other's surfaces of liquid. Onset of the double diffusive convection is investigated experimentally using the optical fibre sensor.