Fabrication And Sensing Characteristics Of The Polymer Interferometric Microstructures On The Fiber End

In recent years,the novel optical fiber sensing structures based on combination of the interferometric microstructure on the fiber end and polymer materials have attracted extensive attention.The interferometric microstructure on the fiber end has the advantages of small size,simple fabrication,compact structure and easy integration.The polymer materials have the characteristics of high coefficient of thermal expansion and low Young’s modulus,and they can react reversibly with external environmental parameters.Therefore,the combination of interferometric microstructure on the fiber end and polymer materials will help to greatly improve the performance of the sensor and expand the types of detection parameter of the sensor.In this paper,the optical fiber sensor based on the polymer interference micro-structure on the end of the optical fiber and its integrated and fabricated technology are investigated.The performance,preparation efficiency and cost of the optical fiber sensor are all improved.The industrialization of the optical fiber sensor can be further promoted.The main work is as follows:By using drawing-curing method and UV exposure technology,a compact polymer micro-rod was fabricated on the end-face of fiber.The humidity and temperature characteristics of the sensing structure were investagted.To avoid the impact on quality of spectrum and make the sensor have the ability of rapid response to humidity,the mode field radius of the light field distribution emitted from fiber end was calculated theoretically.According to the calcaulated results,a polymer-rod structure with diameter of 9.2 μm and height of 11.3 μm was fabricated on the fiber end.In order to solve the problem of cross sensitivity between humidity and temperature,the sensing structure based on polymer-rod and Bragg grating were connected together.The humidity characteristics of the sensing structure were investagted.Experimental results show that the rise time and recover time of the sensor have been improved to 291 ms and 886 ms when the relative humidity of the environment changes between 61% and 76%.The proposed polymer-rod structure on the fiber end can realize rapid humidity sensing.By coupling the UV-light into the multimode fiber,the light emitted from the optical fiber end was used to illuminate the polymer,the polymer-tubules were fabricated on the substrate.A polymer-tubule was integrated into the fiber end,and then the sensing structure based on the polymer-tubule was achieved for the contact force sensing.The light field emitted from the optical fiber end was exposed to the polymer,and the polymer-tubule was obtaned.Hence,the electric field intensity distributions at the end-face of MMF were simulated.We analyzed the influence of inner and outer diameters of the polymer-tubule on the sensitivity of the sensor by using the method of finite element analysis in static mechanics.A contact force sensor based on polymer-tubule structure was fabricated.The contact force sensing characteristics of the structure were investigated experimentally.Experimental results show the contact force sensitivity of the sensor reaches as high as 1.08 nm /μN,and the detection resolution of the sensor is 0.2 μN.On this foundation,a section of SMF was fixed on the end of the contact force structure to obtain the tilt angle sensor.We simulated the deformations of the polymer column when the sensor tilts at different angles by using the method of finite element analysis in static mechanics.Experimental results show that the proposed sensor is capable of distinguishing tilt angle and its direction in large angle-range from-90 deg to+90 deg.The tilt sensitivity in the range from-45 deg to 45 deg is-92.7pm/deg.By using the method of fiber applying adhesive,a polymer-cap was fabricated on the fiber end.A cavitation bubble was generated in the polymercap on the fiber end by using femtosecond laser.The structure based on cascaded cavities was then obtained and used for temperature sensing.The spectra of the structure based on cascaded cavities are superposed,as a result,the output spectrum exhibits dominant interference dip with high distinguish ability.In order to make the highly recognizable main peak always exist in the sepcrtrum,we simulated the spectra of the structure and evaluated the significance of the main peak.The length matching condition was obtained.We also analyzed the influence of the focus position and average power of the laser beam on the position of the bubble and diameter of bubble,respectively.According to the simulated results,we fabricated a cascade cavities structure based on a cavitation bubble.The temperature sensing characteristics of the sensor were investigated.Experimental results show that the sensitivity of the sensor reaches 258 pm/℃ within the temperature range from –10 to 70 ℃.Compared with the capped structure without bubble,the wavelength demodulation range of the proposed structure is increased from 13 nm to 50nm。By using the multi-step irrigation method,the liquid polymer was filled into the microcavity and the Vernier effect was generated,the sturctrue was then obtained and used for temperature sensing.The working mechanisms of traditional Vernier effect reduced Vernier effect and enhanced Vernier effect were analyzed and compared theoretically.The effects of the optical path of the cascaded interference cavity on the amplification and the shift direction of the envelope were analyzed.By using the multi-step irrigation method,the sensor based on enhanced Vernier effect was fabricated,and its temperature sensing characteristics were investigated expeimentally.The relationship between the length of liquid-cavity and times of irrigation was obtained.The actual temperature sensitivity of the proposed sensor around 35 ℃ reaches as high as39.21 nm/℃ and the error is only 0.02 ℃,The fabricated sensor can realize tmperature sensing with high sensitivity and high precision.