A fibre optic Fresnel ratio meter for measurements of solute concentration and refractive index change in fluids

A new and simple normalization technique that greatly enhances the measurement resolution of conventional fibre-optic reflectometry based on Fresnel reflection from the tip of a fibre is used for demonstrating the feasibility of measuring solute concentrations and index changes in fluids to very high precision. The amplitude of pulses originating from reflection from the fibre–fluid interface is compared in real time with the amplitude of reference pulses from a fibre–air interface such that errors caused by pulse amplitude fluctuations and slightly varying detector responses are corrected. Using solutions of sodium chloride and water, it is demonstrated that the technique is capable of measuring index changes of 2 × 10⁻⁵ corresponding to a NaCl concentration of 0.02%.