Research On Humidity Sensing Of Modal Interference Based Optical Fiber Interferometer

Humidity measurement plays an important role in many fields,such as agriculture,medical and health,biological engineering,chemical engineering,environmental monitoring,food and drug processing,and cultural relic protection.Traditional electrical humidity sensors have wide range of applications,but they suffer low response speed and couldn't work in extreme environments.Optical fiber humidity sensors have advantages of anti-electromagnetic interference,fast response,small size,light weight,corrosion resistance,etc.,which have important research significance and application prospects.Compared with the traditional optical fiber interferometers,the modal interference based optical fiber interferometer reduces the complexity and enhances the stability of the system,which has high research value in the field of humidity sensing.In this thesis,four interferometers for humidity sensing have been proposed and fabricated,and their response characteristics to humidity and temperature have been experimentally studied.The main works are summarized as follows:(1)An optical fiber Michelson interferometer based on modal interference by large coreoffset fusion is proposed and demonstrated.The experimental results show that the humidity sensitivities of interferometers with core-offset of 25 ?m and 50 ?m are-0.058 d B/%RH and-0.093 d B/%RH,in the humidity range of 35 %RH-95 %RH,the linearities are 0.993 and 0.991,respectively.The temperature sensitivities of interferometers with a core-offset of 25 ?m and 50?m are 0.014 nm/? and 0.009 nm/?,in the temperature range of 20 ?-180 ?and the linearities are 0.992 and 0.996,respectively.The humidity measurement error caused by temperature is 0.01 %RH/?.The interferometer has advantages of simple manufacturing,low cost,simultaneous measurement of temperature and humidity.An optical fiber Michelson interferometer based on a single mode fiber(SMF)-thin-taperphotonic crystal fiber(PCF)structure is fabricated by using fiber fusion taper technology.The experimental results show that the humidity sensitivity of the interferometer is-0.166 d B/%RH in the humidity range of 30 %RH-90 %RH,and the linearity is 0.991.The humidity measurement error caused by temperature is 0.09 %RH/?.The standard measurement error of the interferometer is 0.25 %RH in the stability experiment.In the human breathing test,the humidity response time and recovery time of the interferometer are 400 and 200 ms,respectively.The interferometer has high sensitivity,good linearity,fast response speed,and can realize temperature-insensitive environmental humidity measurement.(2)An optical fiber Michelson interferometer based on a PCF is proposed.The experimental results show that the humidity sensitivity of the interferometer is-0.095 d B/%RH with a linearity of 0.998 in the humidity range of 30 %RH-90 %RH.The temperature sensitivity of the interferometer is 0.008 nm/? with a linearity of 0.997 in the temperature range of 20-100 ?,and the humidity measurement error caused by temperature is 0.09 %RH/?.The standard measurement error of the interferometer is 0.25 %RH in the stability experiment.The response time of the interferometer is 190 ms in the human breathing test.The interferometer has high humidity sensitivity,good stability,fast response characteristics,and can realize temperatureinsensitive humidity measurement.(3)An optical fiber Mach-Zehnder humidity interferometer coated black phosphorene based on modal interference is proposed.The interferometer is composed of a thin core fiber(TCF)sandwiched in SMF with two tapers.The experimental results show that the interferometer sensitivity are 0.018 nm/%RH and 0.003 nm/%RH without coating sensitive material in the humidity range of 40 %RH-60 %RH and 60 %RH-70 %RH,respectively.After coating the black phosphorusene-polyvinyl alcohol film,the humidity response of the interferometer is significantly improved,the humidity sensitivity of the interferometer are 0.269 nm/%RH with a linearity of 0.992 and 0.173 nm/%RH with a linearity of 0.994,respectively,in the humidity range of 40%RH-60%RH and 60%RH-70%RH.The humidity measurement error caused by temperature is 0.36 %RH/?.Temperature sensing experiments show that the spectrum of the interferometer is not significantly sensitive to temperature,so it can perform temperatureinsensitive humidity measurement.