Femtosecond Laser Two-photon Polymerization Printing Micro-fiber Grating And Its Sensing Characteristics

Owing to their robust mechanical properties,large evanescent field and low optical attenuation,optical micro/nano-fibers are excellent host for Bragg-grating structures.Various technologies for fabricating micro/nano-fiber Bragg grating(?-FBG)have been developed in the past decades,including chemical-corrosive machining,femtosecond laser inscription,UV laser inscription,focused ion beam inscription,lithography,and femtosecond laser erosion.The mechanism of these fabrication approaches is to induce periodical index modulation on the body or surface of micro/nano-fibers.A promising application of ?-FBG is fiber sensing.Thanks to its large evanescent field,micro/nano-fibers exhibit a high sensitivity to environment.As a consequence,sensors based upon ?-FBG are very attractive tools for detecting the concentrations of gas or liquid.In this thesis,a novel fabrication technology of ?-FBG,namely femtosecond laser two-photon polymerization(FLTPP),is proposed and comprehensively studied.?-FBGs are successfully fabricated via FLTPP,including spiral-type one.The spectral characters and the index/temperature sensitivities of the ?-FBGs are investigated.The layout of this dissertation is listed below:(1)The introduction of micro/nano-fibers,micro/nano-fiber Bragg gratings,and femtosecond laser two-photon polymerization.The fabrication technologies and applications of ?-FBG are reviewed,in particular the application of measuring the index of liquid.The manner of the interaction between femtosecond laser and matters is introduced.The development of FLTPP is reviewed.The mechanism of FLTPP is elucidated.(2)The introduction of the FLTPP system.The system consists of a Ti:Sapphire femtosecond laser,a microscope-integrated machining platform,3-D translators,and multiple supporting devices(power meter,attenuator,shutter,reflection mirror,etc.).The resolution of the system is 10 nm.The system is capable of fabrication 2-D or 3-D micro-structures.(3)The report on ?-FBG fabricated with the FLTPP system.The output power of the laser is optimized for improving the performance of the ?-FBG.The spectrum of the ?-FBG is found to be of two reflection peaks that locate at 1481.2 nm and 1488.6 nm respectively.The fabricated ?-FBG is employed to index sensing,from which a maximum sensitivity of 207.14nm/RIU is achieved at the index of ~1.466.(4)The report of a ?-FBG of spiral structure that is realized with the FLTPP technology.The spiral ?-FBG is fabricated with an improved photoresist.A femtoLAB system is employed to inscribe the FBG,which is composed of a Yb:KGW femtosecond laser,high-resolution 3-D translators,expert-version control software,and other components.The performance of the spiral ?-FBG is intensely studied and compared with the ordinary ?-FBG.It is found that the spiral ?-FBG is of stronger mechanical resistibility,better structure stability,and higher index sensitivity.In the region of 1.365-1.395,the index sensitivity of the ?-FBG is 229nm/RIU for high order mode and 120nm/RIU for fundamental mode.Moreover,the index sensitivity of the ?-FBG in the high index region(1.422-1.438)is as high as 348.5nm/RIU.The temperature sensitivity of the ?-FBG is also investigated,which is determined as 4.97 pm/?,a value that is comparable to ordinary ?-FBGs.