| Fiber gratings,as important fiber optic devices,are used widely in many fields such as fiber optic sensing,fiber optic communication and microwave photonics,because of their significant advantages such as resistance to electromagnetic interference,low transmission loss,as well as their small size,ease of multiplexing and ease of integration.Carrying out research on the preparation method and characteristics of fiber grating not only helps to accelerate the solution of grating preparation process problems,but also to further promote the maturation of fiber grating technology in more areas of application,scale and industrialization to provide strong support and effective protection.The chirped fiber Bragg grating is a fiber grating device which has a wide range of applications,and plays an important role in the fields of fiber optic sensors,fiber filters,fiber lasers and optical communications.The common preparation methods for chirped fiber Bragg gratings are phase mask method and femtosecond laser direct writing method.Phase mask method is stable,repeatable and suitable for mass production,but the phase mask plate is expensive,easy to damage（high-energy laser processing）,and the prepared grating wavelength is single;femtosecond laser direct-write technology has the advantage of flexibility and adjustability,and the parameters such as wavelength,period and chirp rate of chirped fiber Bragg grating can be set flexibly.In this thesis,a chirped fiber Bragg grating preparation method by femtosecond laser direct writing is proposed,and chirped fiber Bragg gratings with different dispersion characteristics are obtained;research on highly sensitive curvature,temperature and strain fiber sensors,filters and tunable multi-wavelength lasers based on chirped fiber Bragg grating structures is carried out.The specific research contents and results are as follows:1.The spectral properties of chirped fiber Bragg gratings were simulated and analyzed using coupled mode theory.The theoretical model of uniform fiber Bragg grating is analyzed by coupled mode theory,the transmission characteristics of phase-shifted fiber grating and chirped fiber Bragg grating are analyzed by using the transmission matrix method,and the spectra of the above gratings are simulated by MATLAB simulation software.The mechanism of interaction between transparent materials and femtosecond laser is investigated,the model of femtosecond laser-induced refractive index change is outlined,and finally,the focusing characteristics of Gaussian beams are analyzed and methods to optimize the focusing quality are proposed.2.A femtosecond laser preparation method for chirped fiber Bragg gratings is proposed.Chirped fiber Bragg gratings are prepared within the single-mode fiber using a femtosecond laser point-by-point direct writing method by controlling a three-dimensional displacement stage with uniformly accelerated motion.Chirped fiber Bragg gratings with lengths of about 17.4 mm and bandwidths of 2 nm,5 nm and 12 nm were prepared,and the spectral properties of chirped fiber Bragg gratings were investigated and tested for sensing and time-delay characteristics.The temperature sensitivity of the gratings was about 14.91 pm/°C,and the strain sensitivity was about 1.21 pm/με;and the dispersion of the chirped fiber Bragg gratings with three different bandwidths was about 77.3 ps/nm,34.3 ps/nm and 14.7ps/nm for three different bandwidths of chirped fiber Bragg gratings,respectively.The reflectance spectra and group time delay curves of the chirped fiber Bragg gratings are calculated and plotted by MATLAB simulation software,and the simulation results are in agreement with the experimental data.3.Sensors based on chirped fiber grating Fabry-Perot interferometer were designed and prepared.Pairs of chirped fiber gratings were prepared to form a Fabry-Perot interferometer by femtosecond laser point-by-point direct writing technique using front-back stagger left-right parallelism on both sides in a single-mode fiber core.Applying the interferometer to bending sensing,a bending sensitivity of about 4.435 d B/m-1 was measured over a curvature range of-1.5 m-1 to1.5 m-1（where the minus sign represents the opposite bending direction）,with extremely low temperature cross-sensitivity.Highly sensitive temperature and strain sensors were prepared by cascading two chirped fiber grating Fabry-Perot interferometers combined with the Vernier effect.In strain sensing,three Vernier effect sensors were prepared and tested with strain sensitivities of-79.54 pm/με,-95.68 pm/με,and-123.8 pm/με,respectively,which are 58,70,and 91 times more sensitive with respect to a single Fabry-Perot interferometer,which is in agreement with the theoretical results.The temperature cross-sensitivities of the three vernier effect systems were measured to be-0.19με/°C,-0.15με/°C,and-0.14με/°C,respectively,demonstrating that the temperature effect on the strain sensors is small.4.A method for the preparation of filters based on phase-shifted chirped fiber gratings is proposed.Femtosecond laser direct-write preparation of phase-shifted chirped fiber Bragg gratings is achieved by introducing one or more phase shifts by introducing the blank refractive index modulation regions within the chirped fiber Bragg grating.The transmission characteristics of phase-shifted chirped fiber Bragg gratings were analyzed by the transmission matrix method,and the phase-shifted fiber grating spectra with different phase shifts,different phase-shift point positions and different numbers of phase-shift points were simulated by MATLAB simulation software.A chirped fiber Bragg grating with an approximateπ-phase shift（a phase-shifted point with a length of about 1.1μm is introduced within the chirped fiber Bragg grating）was obtained,and the half-peak width of the phase-shifted peak in the transmission spectrum of this phase-shifted grating was about 85 pm,with a signal-to-noise ratio of about 17.989 d B.When the width of the phase-shifted points was changed to 0.85μm and 1.35μm,the narrow passband filtering characteristics of the phase-shifted peaks decreased significantly,the half-peak widths became wider,and the signal-to-noise ratio decreased.After that,a chirped fiber Bragg grating with two phase shifts was successfully prepared by inserting a blank refractive index modulation region of 1.1μm width into one-third and two-thirds positions of the chirped fiber Bragg grating as two phase-shifted points,respectively.Two distinct phase-shifted peaks appeared in its transmission spectrum with half-peak widths of130 pm and 120 pm and signal-to-noise ratios of 15.371 d B and 14.813 d B,respectively.5.A wavelength-tunable multi-wavelength fiber laser based on chirped fiber gratings was designed and prepared.A linear multi-wavelength erbium-doped fiber laser was designed with a chirped fiber Bragg grating and a Fabry-Perot interferometer based on chirped fiber Bragg gratings as the two mirrors of the resonant cavity,and a pump source wavelength of 980 nm.The number of laser wavelengths output from the multi-wavelength laser is controlled by adjusting the polarization controller to provide a Polarization hole burning effect,resulting in single,dual,triple,quadruple,quintuple,and hexa-wavelength laser outputs and tuning.The stability of the laser output was measured over a 30-minute period,and the experimental results show that the maximum wavelength shift of 1～6 wavelength laser output is 0.012 nm,and the maximum energy intensity fluctuation is 1.378 d B.The center wavelength and energy of the output laser maintain a high stability.In summary,this thesis proposes a new method to prepare chirped fiber gratings by femtosecond laser direct-writing technique,and high-quality chirped fiber Bragg gratings were prepared within silica fibers.By designing grating structures,chirped fiber Bragg gratings are applied to dispersion compensation,curvature sensing,high sensitivity temperature and strain sensing,multiwavelength fiber lasers,and fiber filters,expanding the preparation methods and application scenarios of chirped fiber Bragg gratings. |
