Study Of Liquid-level Sensor And Density Of Liquid Sensor Both Based On Two Fiber Bragg Gratings

Liquid level sensing and density of liquid sensing are very important in both modern industrial production and daily life.liquid-level sensor and density of liquid sensor both based on fiber Bragg grating(FBG)have been extensively investigated and applied because of their advantages such as small size,high accuracy and rapid response,etc.At present,the main response signal of optical fiber sensor based on FBG is the Bragg wavelength.In this paper,we propose a new kind of liquid-level sensor and density of liquid sensor both based on two Fiber Bragg Gratings.Different from the current technology,the tracking signal of this new sensor has been the optical power.Firstly,we theoretically simulate the sensing mechanism of the structure of two fiber Bragg gratings.We use two fiber Bragg gratings and a coupler to make up the sensor structure.Light from a broadband source is coupled into one FBG via a coupler;the reflected light from it is then coupled into the other FBG via the same coupler,then there will be two peaks called “left peak” and “right peak” appearing in the transmission spectrum of the latter.When external factors result in a shift of Bragg wavelength of one FBG,the power of the two peaks will change.We use the Power difference as the tracking signal to measure.This is the sensing mechanism of the structure of two fiber Bragg gratings.At the same time,we also discuss the effect factor of the measurement accuracy.That for the bigger reflectivity of FBGs,the measurement sensitivity will improve.Secondly,we use two FBGs(FBG1,FBG2)with the same reflectivity to set up the liquid level sensing system.Their Bragg wavelength is about 1532 nm.The liquid level affects the axial force acting on FBG1,and results in a shift of Bragg wavelength.Then,the power difference of two peaks appeared in the transmission spectrum of FBG2 changes.Using the power difference as the tracking signal,we realize continuous level measurements for liquids with different densities.Theoretically,we simulate the emergence and the changes of the two peaks in the transmission spectrum,and discuss the influence of the temperature.And we find an interesting phenomenon that we can change the measurement accuracy by changing the bottom area.The results show good linearity and a high sensitivity of 0.92 d Bm/mm is achieved.Finally,we use two cascaded FBGs with the same reflectivity to set up the sensing system of density of liquid.Their Bragg wavelength is about 1540 nm.The density of liquid affects the axial force both acting on two FBGs.The transmission spectrum of one FBG enters into the other one.And then there will be a new peak called “X” peak appearing in the transmission spectrum of the latter.Using the power of “X” peak as the tracking signal,we realize the measurements for the density of liquid.Theoretically,we simulate the emergence and the changes of the “X” peak in the transmission spectrum,Experimentally,we realize the measurement for the density of liquid in the linear range of the variation of “X” peak,and discuss the affect of the temperature and heterogeneous strain.The theory is agree with experiment.A high sensitivity of 42.87 d Bm/(g/cm3)is achieved.