Fabrication And Sensing Properties Of Fiber Gratings Induced By Femtosecond Laser

In recent years, Fiber grating has been widely used in the fields of modern fibercommunication, fiber sensors and compact fiber lasers for its small size, high detectsensitivity, capability of quasi distributed measurement and so on. On the other side,infrared femtosecond laser, emerging as a new tool of micromachining, has attracted moreand more scientific attention because of its high precision, good quality and real3D-manufacture, especially the heat effect can be ignored during the micromachining.Combined with technology of fiber grating and femtosecond laser, this dissertation mainlyinvestigates the fiber Bragg gratings inscribed by the infared femtosecond laser. Based onthe classical couple mode theory, we study the fabrication technique and sensing propertiesof fiber Bragg gratings inscribed in the active Er-doped fiber, solid-core Bragg fiber andall-solid photonic bandgap fiber respectively. The main contents are listed as follows:(1) The basic theory and numerical simulation about fiber grating have beeninvestigated theoretically. Based on the couple mode theory, the detailed couplingmechanism of fiber Bragg grating has been demonstrated and two reflection spectrum ofBragg gratings with different inscription parameters have been calculated. Besides, theprinciple of refractive index modulation caused by femtosecond radiation has beendiscussed and the widespread applications of femtosecond-induced gratings have also beenintroduced.(2) The fabrication technique based on femtosecond laser and phase mask has beeninvestigated. An experiment system with high precision has been created for the gratinginscription, including a femtosecond pulse output system, a dynamic grating fabricationsystem, a spectrum recording system and an annealing test system. Bragg gratings werefabricated using different inscription parameters and their spectral properties have been alsostudied.(3) The spectral characteristics and sensing properties of fiber Bragg grating inscribedin the active Er-doped fiber have been investigated. A Type I-IR grating and a Type II-IRgrating were written into the hydrogen-free and hydrogen-loaded Er-doped fibersrespectively and their mechanisms of refractive index modulation have been studied. The temperature response, strain response and decay of grating reflectivity under ultrahightemperature have been analyzed and compared between two types of gratings. Besides, theshift of Bragg wavelength with increasing temperaturen and strain has been analyzed andthe corresponding mechanism has been discussed.(4) The spectral characteristics and sensing properties of fiber Bragg grating inscribedin the solid-core Bragg fiber have been investigated. Four deep resonant modes withdifferent temperature sensitivities and similar strain sensitivity have been observed in thetransmission spectum. Results show that the resonant wavelengths of these modes exhibitnearly zero-sensitivity but the resonant strengths exhibit direction-sensitivity for the bendsensing. Besides the cross-sensitivity between temperature and strain of such inducedgrating has been studied by the matrix inversion method and the correspondingmeasurement error has been calculated. An effective pre-annealing method was proposed toiliminate the evident wavelength hysteresis during the high temperature annealing.(5) The temperature and strain responses of fiber Bragg gratings inscribed in all-solidphotonic bandgap fibers have been investigated. The mode coupling mechanism for thismultimode resonances have been studied based on the finite element method. Especially,experiments show that the multimode Bragg grating induced in the fiber with highGe-doping can keep a high reflectivity when the temperature is as high as1100oC. Thepossible reasons for its high thermal stability have been discussed.Our work has paved the way to investigate fiber Bragg gratings inscribed by infraredfemtosecond laser. However, there are still lots of works to do in the future, for example,the mechanism of refractive index modulation, the improved couple mode theory, theenhanced grating fabrication technique and the potential applications for these proposedBragg gratings.