Fabrication And Characteristics Of Fibonacci Quasi-periodic Superturcture Fiber Grating

Fiber gratings are fiber devices in which the effective refractive index of the fiber core is periodically modulated. They are now widly used in optical communications and optical sensors because of the advantages such as: compact structure, low insertion loss and low cost. Since K. O. Hill's first fabrication of UV side writing fiber Bragg grating, the fabrication and application have attracted much attentions.With the development of fabrication technique of fiber gratings and the increasing demand in the field of optical communination, fiber sensors and high power fiber lasers, superstructure fiber gratings with special characteristics have become the focus in the research of these fields. Various superstructure fiber gratings have been designed and used as EDFA gain-flattener, DWDM filters and multi-characters fiber sensors.The propagation of optical waves in Fibonacci multilayer has been extensively studied theoretically and experimentally in the last two decades. Constructed by two kinds of layers which are stacked according to the one-dimensional (1D) Fibonacci sequence, the quasi-periodic structure exhibits a spectrum that has a rich self-similar structure including scaling. The transmission spectrum contains narrow resonances separated by numerous pseudo band gaps similar to the band gaps of a photonic crystal. A strongly reduced group velocity, associated with large delay times, is also observed at the band edge of the Fibonacci structure.However, up to now, the quasi-periodic structure has not been applied to the optical fiber devices due to the limitation of writing technique. In the experimental works reported, the central wavelength of the Fibonacci multilayer located at 700nm or 2000nm, which is outside of the transmission window of optical communication. The input and output of the mulilayer devices must be coupled to fiber during utilization thus have a high insertion loass and the structure is not compact.In this thesis, we design a new type of Fibonacci quasi-periodic superstructure fiber Bragg grating (SFBG). A phase-shifted section corresponding to A and a uniform section of fiber Bragg grating (FBG) corresponding to B is arranged along the fiber axis according to the Fibonacci generation scheme: Sj=Sj-1Sj-2 (j≧2), with S0=B and S1=A.Organization of the thesis as follows:1. In chapter 2, transfer matrices of the fiber grating section and phase shift section were derived from the coupling mode equations. The reflection and transmission spectra of the SFBG with different Fibonacci sequence order were simulated by using the transfer matrix method. For the case ofλ=λ0 andφ=(2m+1)π, I is maximum and the quasiperiodicity is most effective. In addition, the case above has the special feature that the dynamical map of equation Mj has a six-cycle orbit which means Mj=Mj+6 for any j. This implies that the reflectivity of the SFBG has six-cycle property Rf→0→Rf→Rf→4Rf2?(Rf4+1)→Rf→Rf for Sj→Sj+6, where Rf=tanh2(kL1) is the reflectivity of a uniform FBG with length L1.Under such condition, there exists a scale factor which determines the scaling behavior of the reflectivity.2. In chapter 3, we describe the experimental setup and fabrication procedure of the designed SFBG. The quasi-periodic SFBGs were fabricated in a photosensitive single mode fiber by using a 244nm laser beam. The length and central wavelength of the uniform FBG section were 800μm and 1541.86nm respectively. At the specified positions,π-phase shifts were introduced between FBG sections by moving the phase mask with a computer controlled. The SFBGs from S0 (only 1 FBG section) to S10 (comprising 34 FBG and 55π-phase shift sections) were written into the fiber according to the arrangement of Fibonacci sequence. Both theoretical and experimental results show that the reflectivity of the Fibonacci structure exhibits a six-cycle property when the phase of the phase-shifted section at the central wavelength of FBG is (2m+1)πand the spectra are self-similar including scaling.3. In chapter 4, the spectral variation of the SFBG with the influence of change of temperature and axial strain was also discussed and the application of the Fibonacce SFBG as distributed fiber grating sensors was proposed. The results show that the reflective spectra of the SFBG have different response to the variation of temperature and axial strain and can be used as distributed fiber sensor for measuring temperature and strain simultaneously. By varing the substitution rules of Fibonacc sequences, we construct a new generalized Fibonacci-class quasi-periodic SFBG, which can be used as DWDM filters in optical communications.Compared with Fibonacci multilayer, the fabricated SFBG has the advantage of low insertion loss and polarization insensitivity.The complex structure of high order Fibonacci SFBG makes it a good candidate for the cavity that provides feedback for random laser. The Fibonacci quasi-periodic SFBGs can also serve as C-band optical filters with high wavelength selectivity and optical devices for multi-frequency operation.