Dependence of Optical Refractive Index of Seawater on Temperature, Salinity, Pressure, and Wavelength in Near-Infrared Waveband for Warm and Shallow Sea

Dependence of optical refractive index (RI) of seawater on temperature, salinity, and pressure is crucial for optical fiber sensors operated in seawater. Expressions of RI of seawater on temperature, salinity, pressure, and wavelength applicable to near-infrared waveband are developed based on a great deal of RI data measured by a semi-encapsulated microfiber Mach-Zehnder interferometer sensor. To verify the accuracy of these expressions, RI measured by infrared refractometer, theoretical calculated sensitivity and experimental measured sensitivity are used, by which the most accurate expression is determined. Based on this expression, a variable sensitivity matrix is established to demodulate temperature, salinity and pressure simultaneously with errors of 10.91 %, 6.09 % and 18.80 %, respectively. The expression proposed here will provide valuable references for theoretical and experimental study on optical fiber sensors operated in infrared waveband and push the applications of fiber optical sensors in ocean investigations.