High Sensitivity Fiber-Optic Humidity Sensor

Humidity sensors has been widely used in air conditioning,pharmaceutical processing,food processing,architecture safety and therapeutic process monitoring.The optical fiber humidity sensors has received much attention due to its good performance such as miniature,easy and convenient manufacture,fast response,and anti-environmental disturbance compared with traditional electronic humidity sensors.Based on existing fiber-optic sensing method,we proposed several new fabrication method of sensors through researching the sensor theory.Moreover,we made fiber-optic sensors and test the performance of the sensors.The experiment result shows that the sensitivity,response speed and reproducibility get great improvement which has great impact on the development of the fiber-optic humidity sensing.Aiming at the low sensitivity of the Mach–Zehnder fiber-optic sensor,a structure based on directly using moisture-sensitive materials to make the sensing arm of the Mach–Zehnder fiber-optic sensor is proposed.Two different fabrication methods are used to make the sensors and test the performance of them.The sensitivity of the sensors made by different methods is improved and the sensors made by different method have different characteristics.The spectrum curve of the offset structure is smoother and the offset structure is compact.The offset structure is easy to make.The smoothness of spectrum curve is hard to be broken in the welding process.Aiming at the low sensitivity and low response speed of the Fabry-Perot fiber-optic humidity sensor,a single cavity Fabry-Perot fiber-optic humidity sensor which directly uses agarose to form the cavity is proposed.The sensitivity of the sensor is improved by shortening the cavity and making the simultaneous change of both cavity and refractive index.Through researching the references and actual test,we chose suitable concentration of the solution which ensured the mechanical strength of the agarose film strong as well as limited the length of the agarose film which improved the speed of the absorbing and releasing water molecule.Short length of the agarose film gave the sensor high sensitivity and fast response speed.Aiming at the low reproducibility and uncontrollable of the Fabry-Perot fiber-optic humidity sensor,this dissertation presents a sensor based on two cavity to measure humidity.We can control the length of the moisture-sensitive material by defining the capillarity time.By this way,we can control the sensitivity of the sensor.We can make the same length of the cavity under same capillarity time which will make the sensors have similar performance.