Fiber-Laser-Based Sensor for Measuring Refractive Index and Solute Concentration of Aqueous Solutions

Abstract

We report a novel‐principle fiber‐laser intra‐cavity sensor for measuring refractive index and solute concentration of aqueous solutions. The sensor operation is based on a variation of the laser oscillation relaxation frequency (RF, the measured parameter), sensitive to the intra‐cavity loss. As the loss includes the reflection coefficient on the frontier aqueous solution—silica fiber cleaved cut, being the laser output coupler and simultaneously the sensor head, the RF becomes affected by the changes in refractive index of the aqueous solution; in turn, because the solution index is proportional to the solute concentration in water, the RF becomes also a function of the latter. The sensor capacity is demonstrated on the example of measurements of sugar concentration in water. A modeling of the sensor operation is presented, allowing its performance optimization.