# Water Temperature and Salinity Measurement Using Frequency Comb  
Haoyun Zhang  
School of Marine Science and Technology, TianJin University, TianJin 300072, China; [EMAIL_ADDRESS] (H.Z. ); [EMAIL_ADDRESS] (X.X. ); [EMAIL_ADDRESS] (H.Z. ); [EMAIL_ADDRESS] (F.D. ); [EMAIL_ADDRESS] (Z.Q.) [PERSON]ADDRESS] (Z.Q.) [PERSON]ADDRESS] (Z.Q. ); [EMAIL_ADDRESS] (Z.  
electrical sensors, such as the thermocouple, resistance temperature device (RTD), and thermistor. The thermocouple [1; 6] has a good stability, but large thermal inertia, low sensitivity and poor accuracy. The thermistor [7] is very sensitive to temperature and has a short response time, but its stability is poor. The RTD [6; 8] represented by platinum resistance is most commonly used in water temperature measurement, and it can be strictly calibrated before leaving the factory. Nevertheless, the contact measurement principle will accelerate the metal corrosion, which will influence the experiment results. For salinity measurement, the conductivity meters [9; 10] are the most widely used and two electrodes are exploited to detect the water conductivity, related to the concentrations of the dissolved salt. However, this method can easily cause water polarization [11]. Besides, when the electrodes are covered by a dirty substance, the length and area of the electrodes will change. Hence, the electrodes should be kept clean. Additionally, the invasive scheme can lead to the oxidation and corrosion of the electrodes, and the changes in electrode geometry will influence sensor performance. Therefore, regular recalibration is required. There is a great deal of indirect methods for salinity and temperature, which can be classified as the water density method, acoustic method, and optical method. As is known, atmospheric pressure, temperature and salinity are three fundamental physical parameters related to water density [12]. When atmospheric pressure is constant, a densimeter can be used to measure the results, which can be converted to the temperature or salinity by the empirical formula. However, when salinity and temperature change together, it is hard to distinguish the source. Acoustic method is also used in temperature and salinity measurement, because sound speed is related to temperature and salinity [13; 14; 15]. In general, the sound speed can be measured by the pulse flight time method [13; 14]. However, the roughly measured distance value and the multi-path effect will result in measurement error. The means of planer laser induced fluorescence [16; 17] has realized the real-time observation of water temperature. The fluorescence dye serves as an indicator, which can be excited by laser source and has a high sensitivity to water temperature. However, the premise of the experiment is that the dye concentration and laser intensity should be constant and stable, which is hard to satisfy. In recent years, optical fibers [18; 19; 20; 21; 22] are widely used in temperature and salinity sensing. However, this type of fiber is not appropriate to be applied in harsh circumstances. Besides, it is hard to eliminate the cross-sensitivity influence among strain, temperature and salinity. The water refractive index can also be a reflection in the essential optical feature of water, including salinity and temperature. The instruments for water refractive index measurement can be mainly divided as refracometer and interferometer. Based on the principle of light refraction, the refracometer can measure the refractive index of water by recording the beam deviations angle [23; 24]. Nevertheless, the beam deviation can be easily affected by the plankton and suspended sediment [25]. In order to solve this problem, the abbe refracometer based on the Snell law has been used. The critical angle of total reflection and the refractive index can be converted to salinity by an empirical formula [25].