Research On Water Temperature And Salinity Measurement Method Based On Frequency Comb

Water temperature and salinity are essential parameters to evaluate the ocean environment,which have a significant meaning to many areas such as research on ocean physical and chemical nature,ocean environment monitor,fishery,sailing and military.Variations in water temperature and salinity will lead to the changes in water refractive index,from which these parameters can be obtained.For the purpose of water temperature and salinity measurement,frequency comb is adopted for an absolute measurement scheme of these physical parameters based on high-precision water refractive index.Compared with the traditional method,this method has clear traceability,high accuracy and fast non-invasive response,which is of great significance for marine standard metrology research.To the beginning,based on the non-linear polarization rotary mode-locked principle,a frequency comb laser source is designed.Mode-lock method,repetition frequency lock procedure and the passive frequency stabilization means are studied in this part.By these means,we acquire mode-locked pulses whose center wavelength is1030nm,repetition frequency is 103.6MHz±1MHz.Besides,we realize 5-hour-long repetition frequency lock,the frequency deviation is only 0.2m Hz.The laser source we study here reserves for the subsequent measurement.In addition,we propose a method for water refractive index measurement which is based on spectral interference.The principle for this method indicates that our scheme will not be limited by the coherent length.Besides,a completely-immersed method has been put forward to compensate the optical distance difference caused by thermal expansion of water tank,where a Michelson interferometer is placed.The experiment results show that the measurement uncertainty for our method is only at3.5×10^-5,compared with the values calculated by Harvey empirical formula.This method can be reserved for the subsequent measurement.Finally,a simple Michelson interferometer system has been designed to measure the water refractive index in time domain.By using two Michelson measurement arms,the thermal expansion of water tank can be eliminated.Based on this method,we put forward schemes for water temperature and salinity measurement,in addition,the corresponding experiments and data processing algorithms are well devised.Compared with the reference values,our results show the differences of below 0.12 C for temperature measurements;differences of 0.06‰for salinity measurements in0?16‰,differences of 0.01‰and sample deviation of 0.02‰in ocean average salinity 35‰.The obtained results indicate that the goal of temperature and salinity measurement has been achieved,which has also verified the accuracy and feasibility of our system and scheme.