Design Of Fiber Optic FPI Humidity Sensor Based On Polyimide Film And Its Sensing Performance

Humidity is closely related to production and life.It is one of the most basic and important physical quantities affecting industries such as industry and agriculture,construction industry and advanced instrument research and development.Therefore,accurate sensing and detection of humidity is of great significance.Compared with the traditional electronic humidity sensor,optical fiber humidity sensor has the advantages of high security degree,wide application range and so on,and has great application potential,so it has been widely concerned by researchers.In this paper,aiming at the problems of complex preparation process and low sensitivity of optical fiber humidity sensor,fabry-Perot（FPI）optical fiber humidity sensor based on polyimide film coating was proposed and prepared,parallel and cascaded double cavity structures were designed,the humidity sensitivity of the sensor is greatly improved by using the doublecavity vernier effect.The specific research contents are as follows:（1）Based on the analysis of the optical properties of polyimide film and its sensing mechanism with FPI interferometer,the solid gas thermodynamic system formed by polyimide film after absorbing water is described,the adsorption model in dynamic equilibrium and the calculation method of refractive index of polyimide film are summarized;Through the measuring principle of vernier caliper,the optical vernier effect is introduced into the optical fiber humidity sensor to improve the sensitivity（2）A single cavity humidity sensor based on polyimide film coating was designed and fabricated.A layer of polyimide film is coated on the end of the structure of single-mode fiber（SMF）-silica tube（st）,so that the smf-st structure and polyimide film form an F-P cavity.Humidity sensing is realized by using the characteristics of moisture absorption and expansion of polyimide film to change the length of air cavity,resulting in the translation of interference spectrum of F-P cavity.The experimental results show that the interference spectrum of F-P cavity shifts blue with the increase of humidity;In the humidity range of 10% RH-90% RH,the wavelength changes linearly with humidity,and its sensitivity is-0.157 nm /% RH.（3）A parallel two-chamber humidity sensor based on polyimide film coating was proposed and fabricated.The sensor is prepared by the F-P cavity of SMF-ST structure as the sensing cavity and the F-P cavity of SMF-ST-SMF structure as the reference cavity,because the free spectral range of the two F-P cavities is similar but not equal,the vernier effect is produced and the sensitivity of the sensor is improved.After the film absorbs water,the length of the air cavity of the sensing cavity will be changed,resulting in the shift of the two-cavity interference spectrum envelope.The experimental results show that,with the increase of humidity,the parallel two-cavity interference spectrum envelope blue shift,the sensitivity of-2.017 nm/%RH,its sensitivity is 12.8 times that of the single-cavity polyimide coated F-P humidity sensor.This magnification is consistent with the theoretical calculation of the vernier effect magnification,and the humidity sensitivity of the sensor is successfully increased by an order of magnitude.At the same time,because the reference chamber is not coated with polyimide film,it is not sensitive to environmental humidity,and there is no need for isolation treatment,which increases the application potential of the sensor.（4）A cascaded dual cavity humidity sensor based on polyimide coating is proposed and fabricated.The sensor is prepared by coating a layer of polyimide film on the end of SMF-ST-SMF structure,ST in SMF-ST-SMF structure forms an air cavity,composite cavity composed of polyimide film and terminal SMF,moreover,the free spectral range of the composite cavity and the air cavity is close,resulting in cursor effect,which simplifies the sensor structure and improves the sensitivity.The sensing experimental results show that with the increase of humidity,the envelope of the cascaded dual-cavity interference spectrum blue shifts,and its linear sensitivity is-0.980 nm / % RH,which is 6.2 times that of the F-P humidity sensor coated with singlecavity coating polyimide.