Tunable Fiber Bragg Grating Research Based On Femtosecond Laser Inscription Technology

Fiber Bragg grating(FBG)is electromagnetic interference resistant,corrosion resistant,compact and naturally compatible with fibre optic systems.Therefore,FBG has broad application prospects in the fields of fiber optic sensors,fibre lasers,optical communication systems,etc.Previous studies have shown that the FBG enables coupling in between forward transmission mode and reverse transmission mode at the resonance wavelength.Currently,there are a number of methods for preparing FBGs,mainly divided into two categories,UV exposure and femtosecond laser inscription.Among these,UV exposure method requires the assistance of phase mask or amplitude mask,and the resonant wavelength of the FBG is determined by the template period,which limites the flexibility of FBG preparation.At the same time,this method also requires not only a photosensitive optical fiber which usually needs previous hydrogen process.Among them,the resonant wavelength of FBG is determined by the template period,which largely affects the flexibility of FBG preparation.At the same time,this method not only requires the optical fiber to be photosensitive,but also requires the fiber to be treated with a hydrogen carrier.In contrast,femtosecond laser inscription is flexible and simple to prepare,and the fiber does not require special treatment before inscription.Meanwhile,the femtosecond laser pulse has very high peak power and ultrashort pulse width,which interacts with the fiber core material to modulate the local microstructure of the material through multiphoton absorption and producing permanent changes in the local refractive index of the fiber core,making the FBG resistant to high temperature.In this thesis,an in-depth and systematic study of the effects of different parameters on the spectral properties of FBGs prepared by the point-by-point method with a femtosecond laser at a wavelength of 520 nm has been done,and a new type of fiber grating filter has been proposed.The main work was summarized as follows.1.A basic experimental exploration of the method and parameters of the inscribed FBG using a femtosecond laser with a wavelength of 520 nm was carried out to investigate the effects of different inscription methods,such as femtosecond laser energy,FBG length,motorized stage moving speed,relative position of the FBG in the core,and FBG order on the spectra.2.A new type of fiber grating filter is innovatively proposed by using femtosecond laser point-by-point inscription of FBGs and combining with fiber taper pulling technique.It consisted of a pair of FBGs with slightly deviated reflection peak wavelengths,which were inscribed in a single-mode fiber after tapered by femtosecond laser point-by-point technique.This cascaded fiber grating structure provided a transmission spectrum similar to that of a conventional phase shifted fiber Bragg grating(PS-FBG),while simultaneous modulation of reflection peak wavelength and bandwidth can be achieved according to strain variation,and this structure can also be used for multi-parameter sensing of temperature and tension.The proposed fiber grating filter can be used in multiparameter sensing and tunable fiber laser applications.