Study On Sensitivity Of Curved Surface Liquid Refractometer

The salinity and density are important marine environemntal parameters of sea water,and they are closely related by the equation of the state of the sea water.The measurement of salinity and density of sea water is of great significance for the navigation of underwater vehicle,the forecast of hydrometeor,study of marine ecological environment,and exploration of seabed resources.At present,the conductivity,temperature and pressure of seawater are measured by the CTD sensor(conductivity temperature depth)to calculate the practical salinity and density of seawater according to the equation of state of seawater.However,the conductivity of sea water is only related to the content of electrolyzable compounds in sea water,ignoring the non-ionic substances dissolved in sea water.The optical refractive index of seawater,which correlates with all the substances dissolved in the water,is a better proxy for the salinity and density of seawater.Therefore,the measurement of optical refractive index of sea water is considered to be an effective method to realize the in-situ measurement of absolute salinity and density of sea water.Among various liquid refractive index measurement methods,the V-groove refractometer has been widely recognized for its advantages of simple structure.low cost.simple signal processing,easy integration and corrosion resistance.However,the resolution and measurement range of this instrument are limited by the distance between the V-groove and the PSD(Position Sensitivity Device)as well as the size and resolution of the PSD.Its best reported resolution is about±4x10-6.However,this result still need to be improved to meet the demand of marine exploration.Therefore,it is necessary to study how to further increase the sensitivity of the refractometer.In view of the above challenges,the main contents of this thesis are as follows:(1)Analyze the principle of V-groove refractometer and the key factors related to its sensitivity.(2)The optical path of the instrument is modeled by MATLAB,and the meta-surface which can amplify the sensitivity is obtained through iterative calculation,the meta-surface also ensures that the position of the beam on the PSD at 20cm varies linearly In this thesis,the sensitivity and the dependence on incident angle of the designed refractometer with integrated meta surface are analyzed in detail.(3)The gain of the sensitivity of the refractometer is achieved by using a discreted aspheric lens instead of an integrated meta-surface.The effects of increasing sensitivity and non-linearity caused by aspheric lens are studied theoretically and demonstrated experimentally.In addition,the effects of the curvature,aspheric coefficient and position of the aspheric lens on the measurement results are also analyzedThe novelty ofthis thesis is as follow:(1)To improve the sensitivity of the traditional V-shaped refractometer,an integrated meta-surface structural is proposed and designed in this thesis.The meta-surface structure could greatly improves the sensitivity of the liquid refractometer without increasing the size of the instrument,and makes the beam position on the receiving surface change linearly with the liquid refractive index.(2)A tunable gain module based on the V-groove refractometer is proposed to meet the varying requirement for resolution and range by using discreted aspherical lens.(3)Digital filtering is deployed on the position information by using Kalman filter,and relatively stable experimental results are obtainedThe analysis results show that:compared with the traditional flat exit surfacein the V-shapered refractometer,the optimized meta-surface could achieve a refractive index sensitivity gain of 4.8 times,and achieves the refractive index sensitivity of 5.527× 107?m/RIU.As for the discreted aspherical lens,the experiment verified that the lens with a focal length of-25mm can achieve a refractive index sensitivity gain of 2.9 times,and the sensitivity increases from 8.97×105?m/RIU to 2.60× 106?m/RIU at an incident angle of 650 and a working distance of 20cm.The most widely used NKE NOSS refractorometer has a sensitivity of about 7.5×105?m/RIU at a working distance of 20cm Therefore,the use of the gain module could greatly increase the resolution of the V-groove refractorometer.