Research On The Optical Fiber Humidity Sensor Based On F-P Structure

Humidity is of great significance in many fields such as electronics,biopharmaceuticals,agricultural production,and medical care.Compared with traditional electronic humidity sensors,fiber optic humidity sensors have attracted more and more attention due to their advantages of small size,light weight,high sensitivity,corrosion resistance,and electromagnetic interference resistance.In recent years,many new structures and new humidity sensitive materials have been applied to optical fiber humidity sensors.The main research content of this paper is an optical fiber humidity sensor based on Fabry-Perot interference structure,and innovative research is mainly carried out on the structure and humidity sensitive materials to obtain better sensing performance.About the humidity-sensitive material,two kinds of humidity-sensitive materials,which are polymer sodium hyaluronic acid（HA）and polyvinyl alcohol（PVA）,were mixed according to a certain proportion,and then coated on the F-P interferometer to test their humidity sensitive characteristics,this is a new attempt.In terms of structure,the effect of the hole perforated on the F-P cavity by femtosecond laser and vernier effect on the performance of F-P interferometer is explored through experiments.1.Four humidity sensors of HA/PVA composite thin film were prepared according to different mixing ratios,namely pure PVA,HA:PVA=1:2,HA:PVA=1:3 and HA:PVA=1:4.And their humidity characteristics were measured,including humidity sensitivity,cross sensitivity to temperature,stability and response time.It is found through experiments that the humidity sensitivity of adding HA to PVA is higher than that of pure PVA,indicating that mixing HA and PVA in an appropriate ratio can improve the humidity sensitivity of the sensor.Among them,the sensor with a mixed ratio of HA:PVA=1:2 has the best overall performance,high humidity sensitivity,good linearity,low temperature cross-sensitivity,good stability and fast response time.2.The Fabry-Perot cavity was punched by femtosecond laser,and its effect on the humidity performance of the sensor was discussed.In order to obtain the sensor with the best performance,five sensors of HA/PVA films with different mixing ratios and with different aperture sizes were prepared for comparison.And their effects on humidity changes,including humidity sensitivity,stability,and response time,were investigated separately.Experiments have proved that opening holes on the hollow core capillary can significantly increase the contact area between water molecules and humidity-sensitive materials,thereby making the response time faster of the sensor.The experiments also discuss the effect of film thickness on the humidity characteristics of the sensor.Experiments show that when other parameters are constant,the humidity sensitivity increases with the thickening of the film,and the response time becomes slower with the thickening of the film.On the contrary,it is the opposite.However,if the film is too thin,it is easy to cause the film to break.Therefore,for the consideration of comprehensive performance,the thickness of the film should be moderate.3.The vernier effect has an amplifying effect and can improve humidity sensitivity.The single-mode fiber and the hollow-core fiber are spliced,and then the barium titanate microspheres is inserted into the hollow-core fiber,and finally the end face of the hollow-core fiber is coated with the film of PVA.In the experiment,the single-mode fiber to the outer surface of the microsphere of Ba Ti O₃is used as the reference cavity,and the single-mode fiber to the outer surface of the PVA film is used as the sensing cavity.These two cavities with very close FSRs form a vernier effect.The experiment also applied a method combining the vernier effect and bandpass filtering.Using this method,the spectrum of the reference cavity,the sensing cavity,the amplified spectrum of the vernier effect and the spectrum of the envelope are separated to obtain their respective spectrums,which not only improves the sensitivity but also facilitates further analysis.And the method of sensitivity matrix is used to measure relative humidity and temperature at the same time,so as to achieve accurate measurement of RH and temperature.Then the humidity characteristics were measured,including humidity sensitivity,stability and response time.The experimental results show that the sensor has high humidity sensitivity,is amplified,and can measure humidity and temperature at the same time.