Research On Interferometric Optical Fiber Humidity Sensor And Its Sensitivity Enhancement Technolog

Humidity monitoring has an important role within agriculture,the pharmaceutical industry,environmental monitoring,semiconductor devices,food manufacturing,heritage preservation,and food storage.Traditional electrical humidity sensors have a slow response rate and cannot meet the need for accurate measurements in extremely severe environments.The advantages of fiber optic humidity sensors are more obvious,such as high sensitivity,small size,light weight,anti-electromagnetic interference,high temperature resistance,corrosion resistance,and long-distance real-time monitoring,etc.Therefore,optical fiber humidity sensor has attracted extensive attention and research in recent years.In this dissertation,the interferometer optical fiber humidity sensor and its enhancement technique of sensitization are studied.The main work of the dissertation is as follows.(1)The Mach-Zehnder optical fiber interferometer was fabricated based on smallsize off-set spliced technique,and the effect of different misalignment on fringe contrast of the interferometer was investigated.Experimental results showed that the maximum fringe contrast of the interferometer was 17.6 d B and the insertion loss was 13 d B when the misalignment of the interferometer was 5.5 μm.The interferometer was tapered to improving its sensitivity.The influence of the tapered length on sensor spectrum and humidity sensitivity was investigated.(2)An optical fiber humidity sensor based on Mach-Zehnder interferometer coated with polyethylene glycol diacrylate(PEG-DA)composite film was proposed,and taper technique was applied to sensitize the humidity sensor.The surface of the sensors with same tapered length was coated with different proportions of solution to investigate the effect of different proportions of solution on the humidity sensitivity of the sensor.The sensors with different tapered length were coated with same proportion of solution to investigate the effect of tapered length on humidity sensitivity of sensor.The sensor with a tapered length of 8000 μm was coated with 100:1 proportion of solution,and its humidity characteristics,stability,repeatability and temperature characteristics were investigated.The coated sensor with a tapered length of 8000 μm showed a humidity sensitivity of 465.6 pm/%RH and a linear fit of 0.98 in the humidity range of 80-98 %RH.(3)A Mach-Zehnder optical fiber humidity sensor coated with polyvinyl alcohol(PVA)material was fabricated,and taper technique was applied to sensitize the humidity sensor.The effect of different proportions of PVA solution on sensor sensitivity was investigated by coating different proportion solutions on surface of sensor,such as 0%,5%,10%,15% and 20% proportions of PVA solutions.The sensor with a tapered length of 8000 μm was coated with 15% PVA solution,and its humidity characteristics,stability,repeatability and temperature characteristics were investigated.The humidity sensitivity of the sensor was 2730 pm/%RH with a linear fit of 0.992 in the humidity range of 91-98 %RH.