Research On Optical Fiber Interferometric Sensors For Flowrate And Liquid Level Sensing

Gas flow rate and Liquid level sensing is of great importance in various fields.Mostly,traditional sensors based on mechanical and electrical techniques cannot meet the requirements of gas flow rate and liquid level measurement in harsh environment.Optical fiber sensors have numerous advantages,such as immune to electromagnetic,resistance to chemical erosion,high sensitivity and compact structure with micron size.Various sensing systems have been proposed based on diverse principles,as long period grating(LPG)and fiber Bragg grating(FBG).However,such a sensor exploits special and complicated fabrication process and the sensor structure is also fragile because of treatment for enhancing the sensitivity.In this paper,we firstly introduced the development of fiber sensors,especially the flow rate and liquid level sensors of which we explained their advantages and disadvantages.Through analyzing the theories of Fabry-Perot interferometer and Hollow core Bragg fiber,we proposed two kinds of fiber sensors.The first one is an optical fiber based microfluidic gas flowmeter which can be used at high throughout conditions.The flowmeter is a Fabry-Perot interferometer formed by two SMFs with a microfluidic gas channel running across the two end faces.Because of the pressure changes when gas flow passes through the microfluidic channel,the cavity length changes resulting the peak wavelengths of reflective spectrum of the F-P cavity shifting.By monitoring these shifts,we can get the information of flow rate.Experiment was carried out and the sensing capability of the sensor was demonstrated.The other is a liquid level sensor.We optimized parameters of Bragg bi-layers and redesigned a hollow core Bragg fiber(HCBF)which combing the Bragg reflection and anti-resonant reflection to improve the external surrounding response.Some theoretical calculation and actual experiment of the fiber's spectrum have been done.We proposed a simple optical fiber liquid level sensor based on such HCBF which can be commercial produced and the fabrication of the sensor only requires the fusion splicing of several centimeters of HCBF and single mode fibers.When some parts of the HCBF immerged into the liquid,the transmission spectrum will change and we can get the value of liquid level which related to the immerging depth.The sensitivity of our liquid level system is 1.05 dB/mm.