Tapered Optical Fiber Humidity Sensor Based On Silk Fibroin Composite Film And Its Characteristics

Humidity is an important physical parameter of environmental measurement.In recent years,researchers have proposed a variety of humidity sensors for monitoring humidity in different environments.Compared with traditional humidity sensors,optical fiber humid it y sensors have attracted attention from researchers because of its advantages such as strong antielectromagnetic interference,small size and light weight.Due to the poor hydrophilicity of quartz fiber,the optical fiber is insensitive to environmental humidity.Therefore,optical fiber humidity sensors often need to introduce moisture-sensitive materials to improve the response to humidity.At present,researchers have developed many optical fiber humidity sensors based on moisture-sensitive materials.However,most moisture-sensitive materials are synthes ized artificially,expensive,non-degradable,and environmentally harmful.Silk fibroin(SF)is a natural protein polymer,which has the characteristics of biocompatible,biodegradable and environmentally friendly.Therefore,SF has been widely used in the biomedical field.As an environmentally friendly and renewable material,SF can be processed into thin films.SF film has water absorption properties.In a humid environment,the hydrogen bonds in the SF film can interact with water molecules in the environment.However,the water absorption of SF film is not strong.Doping Li Br can improve the water absorption properties of SF films.The experimental results show when the Li Br doping ratio reaches 18.91 wt%,the SF composite film has the best water absorption property.This thesis proposes a tapered optical fiber humidity sensor based on SF composite film.As the moisture-sensitive coating of the sensor,the SF composite film is coated in the waist taper region of the tapered optical fiber for humidity sensing.The evanescent field generated in the waist taper region can detect the change of external refractive index.When the external humidity changes,the refractive index of SF composite film can change,which leads to the effective refractive index difference of participating in the mode interference to change,causing the shift of the transmission spectrum.Therefore,the humidity of the environment can be monitored by measuring the wavelength shift of the transmission spectrum.This thesis first analyzes and deduces the mode theory of tapered optical fibers,the propagation theory of evanescent waves and the mode interference principle of sensors.Then the spectrum of the sensor is simulated on the basis of theoretical derivation,which optimizes the performance parameters of the sensor.The optimized parameters are used for sensor preparation and performance test.The test results show that the sensor has an average humidity sensitivity of-1.3 nm/%RH in the 29%～99% humidity range.The sensitivity of the sensor does not exceed90 pm/%RH in the temperature range of 20 ℃～80 ℃.The response time is 900 ms,and the sensor has good repeatability and low hysteresis error.In this thesis,SF composite film is combined with tapered optical fiber technology by coating method,which broadens the types of optical fiber humidity sensor manufactur ing processes and provides the possibility for large-scale production of optical fiber humid it y sensors.In addition,the tapered fiber humidity sensor has many advantages such as simple structure,low cost,wide measuring range,easy production,high sensitivity,and good repeatability,which has potential application prospects in the fields of biomedical and biosensor.