Enhanced performance salinity sensor based on dual-wavelength laser speckle correlation

Abstract This paper presents a simple signal processing procedure to enhance the performance of a free-space laser speckle-based salinity sensor system. The experimental setup comprises a cylindrical glass vessel containing the liquid sample of interest, which is probed by a light beam formed by merging two laser beams of different wavelengths. The mixed laser beam produces a speckle pattern that provides quantitative information on the sample concentration by correlation coefficient determination. The definition of a multiplicative fusion function, which involves the multiplication of the measured correlation coefficients for each two wavelengths, enhances the sensor’s sensitivity compared to a single-wavelength approach without altering the system noise, thereby increasing the signal-to-noise ratio. The combined product function allows the sensor system to exhibit high linearity at low concentrations, an enlarged output range, a salinity sensitivity of -0.853 /%, which is significantly higher than that reported for fiber optic speckle-based salinity sensors, and a limit of detection comparable to that offered by state-of-the-art, more complex, interferometer-based optical fiber salinity sensors.