Research On Design,fabrication And Sensing Characteristics Of Novel Microstructured Optical Fibers

By virtue of their flexible structures and excellent performances,microstructured optical fibers have been widely concerned by domestic and overseas scientific researchers for decades.With the development and improvement of fabrication and post-processing technologies of microstructured optical fibers,microstructured optical fiber sensing has opened a whole new chapter in the field of optical fiber sensing.The optimal design,complete fabrication and appropriate post-processing of microstructured optical fibers play significant roles in the realization and enhancement of sensing performances.In optical fiber sensing,fiber optical mode characteristics have fundamental impacts on the performances of fiber interferometers and long period gratings.The intrinsic optical characteristics suitable for sensing can be directly realized through structural and material optimization of microstructured optical fibers,and the optical mode characteristics of fibers can be enhanced through structural physical deformation and refractive index distribution changes of microstructured optical fibers.Therefore,this thesis mainly focuses on three aspects of microstructured optical fibers fabrications,post-processing and research on sensing characteristics:the designs and fabrications of three kinds of microstructured optical fibers suitable for sensing,using post-processing methods to enhance the performances of optical fiber sensing,experimental research on sensing characteristics of the specially designed and post-processed microstructured optical fibers.In this thesis,the light guiding principles and classifications of microstructured optical fibers were briefly introduced.Based on the theoretical analysis and the simulation method represented by the all-vector finite element method,two kinds of birefringent microstructured optical fibers were designed by the cladding geometric asymmetry method,and the high birefringence characteristics of the fiber were realized by single-material fabrication.An annular core hollow fiber was designed by annular doping with a single air hole in the center,realizing the effective suppression of inter-mode interference and antiresonant reflecting guidance effects.A kind of cladding defects type all-solid microstructure fiber was designed by cladding periodically absented arrangement,realizing the coexistence of vibrant core mode and cladding mode in the fiber.Microstructured optical fibers with high designed-structural and optic characteristical reproductions were fabricated by using MCVD-based ion doping technology and stack-drawing technology.According to the structures,optical characteristics and sensing applications of microstructured optical fibers,the post-processing technologies represented by carbon dioxide laser processing,liquid filling and acid etching treatment were adopted to enhance the fiber sensing performances.A kind of hollow core long-period grating was fabricated by thermally-induced periodic tapering,and the core management and tuning characteristics of hollow core long-period grating were realized by liquid filling method.A long-period grating suitable for high temperature sensing was fabricated by thermally-induced periodic diffusion of germanium element.Fiber cladding mode evanescent field enhancement which is beneficial to improve refractive index sensing performance was realized by hydrofluoric acid etching treatment method.By setting up the optical fiber sensing experiment platforms,the realization and enhancement of the sensing characteristics by microstructural design and post-processing were studied.In the experiment of torsional sensing and temperature response of birefringent microstructured fibers,both hybrid and slotted microstructured fibers showed favorable torsional sensing characteristics in Sagnac interferometer,in which the slotted microstructured fiber showed temperature non-response characteristics with 1.26×10^-3high birefringence.In the temperature sensing experiment,the liquid-filled hollow core long-period grating realized fiber core mode management and temperature sensitivity enhancement,and the highest linear temperature sensitivity was increased by more than 50times,up to-0.618 nm/℃,and it can worked stably in the low-temperature environment of-10℃.In the high temperature sensing experiment,the thermally-induced element periodic diffusion long-period grating based on high numerical aperture fiber,after annealing treatment,showed stable performance in the repeated heating and cooling processes from room temperature to 700℃,with linear temperature sensitivity up to 0.1 nm/℃.In the refractive index sensing experiment,the acid-etched cladding-defects type all-solid microstructured fiber showed favorable mechanical strength and temperature non-response refractive index sensing characteristics in mach-zehnder interferometer,with the highest linear refractive index sensitivity of 2183.6 nm/RIU.