Study On The Traceable And Flattening Measurement Method Of Seawater Sound Velocity

The speed of sound in ocean has a lot of important applications,it is just like the speed of light to the atmosphere.It also means a lot for us to understand the ocean.At present,the measurement accuracy of seawater sound velocity generally stays at a level of several centimeters per second,and the error analysis lacks the traceability basis.The main reason is that the traceability measurement of seawater sound velocity is not realized,so it is necessary to achieve the metrology of it.The National Ocean Measurement “Thirteenth Five-Year Plan” also clearly proposed that we should develop the seawater sound velocity measuring devices based on direct method.To achieve the traceable and flattening measurement of seawater sound velocity,the measurement method we used must have at least two characteristics.First,the measurement principle should minimize the error factors that affect the measurement results.Second,the physical parameters related to the experimental result should be measured as accurate as possible to ensure the accuracy of the measurement results.The optical frequency comb appears as a periodic and ultrashort pulse sequence in the time domain,and appears as an equally spaced frequency line in the frequency domain.The repetition frequency can be locked to the microwave frequency standard,and from the change of the physical characteristics of the optical frequency comb,we can get the sonic flight time and flight distance to obtain the seawater sound velocity,this can meet the relevant requirements of the traceable and flattening measurement.Based on the previous research on direct measurement,a new method for direct measurement of seawater sound velocity based on optical frequency comb interferometer combined with acousto-optic effect is proposed in this paper.We also analyzed the reason of the change of the zero-order caused by the acousto-optic diffraction.The optical frequency comb is introduced into the measurement of seawater sound velocity,the flying distance of the acoustic wave in water is measured by optical interference,and the flight time of the ultrasonic pulse is measured by the intensity variation of the zero-order diffracted light caused by the pulsed acousto-optic effect.The problem of difficulty in eliminating errors in the existed direct method based on the piezoelectric effect is effectively improved,the measurement accuracy of the flight distance of the acoustic wave and the corresponding flight time is improved,and the measurement accuracy of the sound velocity of the seawater is improved.Compared with the traditional direct measurement method based on piezoelectric effect,the method of this study is more suitable for the measurement requirements of the flattening of the tracer.Based on this principle,an experimental scheme was designed.A measuring device based on the Mach-Zehnder interferometer was built in the laboratory.The difference between the two arms of the interferometer was used to characterize the acoustic flight distance,and the dents left on the optical comb were used to characterize the acoustic flight time.The experimental results show that the method means a lot to improve the accuracy of measuring seawater sound velocity,reaching 0.03cm/s compared with the commercial sound velocity profiler.This work provides a new idea for the realization of seawater sound velocity measurement.