A Novel Fiber Optic FPI Sensor Composed Of Capillary Quartz Tubes

With the rapid development of science and technology,optical fiber sensing technology has gradually been integrated into human life.Optical fiber sensors,widely used in aerospace,food hygiene,building structure testing,geological exploration,and other fields,have many advantages,such as light weight,small size,simple structure,easy manufacturing,and strong electromagnetic interference resistance.Based on the Fabry Perot interferometer,this thesis introduces three different structures of optical fiber sensors based on quartz capillary tubes,focusing on the relevant structures of single mode optical fibers and quartz capillary tubes,starting from three aspects: basic sensing principles,fabrication process of optical fiber structures,and experimental exploration.The main work of this thesis is as follows:(1)A new type of gas pressure and temperature sensor was designed and experimentally verified.The sensor is composed of a Fabry-Perot(F-P)cavity and is prepared by splicing a section of a quartz capillary tube filled with polydimethylsiloxane(PDMS)to the end of a single mode optical fiber(SMF).It utilizes the elastic deformation properties and temperature sensitivity characteristics of PDMS materials to achieve high gas pressure sensitivity and temperature sensitivity,with small measurement errors,good stability,and repeatability.(2)A novel high sensitivity relative humidity(RH)sensor based on cascaded F-P cavities has been studied and experimentally demonstrated.This structure uses an improved vernier effect spectral interference theory,which is composed of two F-P cavities with a similar free spectral range(FSR)and has an amplification effect.Splice a section of capillary quartz tube with a single mode optical fiber,and then immerse the capillary quartz tube in a polyimide(PI)solution to form an extremely thin PI film.Utilizing the humidity sensitive property of the PI solution,a vernier effect humidity sensor is formed,which can amplify the low sensitivity of a single sensor and improve the amplification coefficient.The experimental results show that the relative humidity sensitivity of the sensor is 0.3444 nm/% RH,which is 3.77 times the relative humidity of a single sensor.The sensor has small error,good stability,repeatability,and short response time.(3)A high sensitivity relative humidity sensor based on two parallel Fabry Perot interferometers(FPIs)was proposed and experimentally verified.FPI1 and FPI2 are sensor arms and reference arms,respectively.Their structures are similar and both contain a series connected sandwich structure,namely,single mode optical fiber-capillary quartz tube-single mode optical fiber.However,the surface of the capillary quartz tube of the sensing arm FPI1 is coated with a multilayer polyimide(PI)humidity sensitive film for measuring relative humidity.When FPI1 and FPI2 have a small cavity length difference,their spectra overlap to form a vernier effect envelope spectrum.By tracking the drift of the envelope spectrum,the sensitivity of the sensor is improved.The experimental results show that the sensitivity of the sensor is as high as 586.6 pm/% RH,17.3 times that of a single FPI1 sensor,and 31.3 times that measured using high-frequency interference fringes;The temperature cross sensitivity is about 0.75%/ ℃,which means that temperature has little impact on humidity measurement.The advantage of this sensor is that it is easy to manufacture,has good stability,and can be repeatedly manufactured.