Theoretical Analysis Of Transmission Characteristics Of Fiber Bragg Gratings And Their Experimental Study

The applications of fiber gratings in WDM high-speed all-optical communication system and sensing system are firstly introduced in this dissertation. Then the current fabrication methods of fiber gratings and the theories to analyse their transmission characteristics are briefly summarized. Based upon the coupled-mode theory and experiments, the spectra of fiber Bragg gratings (FBGs) are analysed theoretically and the fabrication and application of FBGs are explored experimentally here.The coupled-mode theory is usually used to analyse mode's coupling in waveguides. There exists analytical solution to the coupled-mode equations for the case of a uniform FBG. Combined with piecewise-uniform approach, it can be used to analyse any kind of nonuniform FBG. By this method, the spectral characteristics of a Fabry-Perot (F-P) cavity composed of two single-mode uniform sinusoidal FBGs are analysed. The resonances of this type of F-P cavity only exist inside the reflection band of its FBGs. Due to the phase factor of the reflective coefficient, there is always more than one resonance inside the reflection band between the first zeros on either side of the maximum reflectivity. When the resonance space of the F-P cavity is larger than the FWHM bandwidth of its FBGs reflection, the cavity operates on the state with single resonance. Knowing the characteristics of the F-P cavity and its FBGs, we present how to design a F-P cavity with the single resonance at the FBG's Bragg wavelength, which provides theoretical guidance for the design of fiber lasers with linear cavity.The FBG F-P cavity with single resonance is fabricated in one fiber, which makes the whole resonant spectrum shift linearly with respect to temperature and strain. That the bandwidth of the resonance is much smaller than that of FBG leads to the application of narrow band-pass filters (NBPF). Using this type of NBPF as an intra-cavity frequency-selective component, stable single frequency Er-doped fiber ring laser can be achieved. Because of no limitation of gain medium length, this type of fiber ring laser has advantages of potentially large output power and wavelength tunability.Multimode fiber Bragg gratrings (MMFBGs) are attracting more and more attentions. On one hand, multimode fiber is easier to couple with laser sources for its large core size, and its relatively small dispersion makes it suitable for optical fiber communication system, especially for the access network. On the other hand, the complicated MMFBG spectrum provides more selections for the fiber sensing system. We analyze to our knowledge for the first time the transmission characteristics of a MMFBG theoretically by using the coupled-mode theory and considering two modes coupling. The reflection spectra are simulated here. The results of the calculation show that MMFBGs have multiple reflection peaks due to the coupling between the same modes in counter-propagating direction and the coupling between the adjacent modes incounter-propagating direction, and the spectra depend on excitation conditions of the bounded modes, such as mode power and mode number.We also explore other theoretical methods to analyze the propagation of light in MMFBGs and present a three-dimensional scalar FDTD method for the waveguide with circular symmetry since it's difficult to solve the coupled-mode equations when there are many modes coupled in MMFBGs. Light propagation in a fiber is simulated by using this scalar FDTD method. However, this method needs to be improved to be able to separate the reflection field from the excitation field in order to obtain the transmission and reflection characteristics.To do further experimental research, we study systematically the fabrication and encapsulation of FBGs, and build a set of scanning system of fabrication with phase mask method. The UV source is a double-frequency Ar+ laser system with output power of 100 mW (244 nm). To guide the fabrication of FBGs, an assistant converse program facing directly the process of fabrication is presented. This program needs the parameters of two FBGs fabricated by the system as the standard parameters of the system, then provides the fabrication conditions to fabricate a FBG which we want. In many applications, the Bragg wavelengthes of FBGs need to be insensitive to the environments. A new type of encapsulation of fiber gratings with single-end compensation for temperature effect with simple structure and low cost is thus proposed. Also we present a new method for the fabrication of the apodised FBGs without self-induced chirp. There is no abrupt change of refractive index between the grating and fiber core for the apodised FBG fabricated by this method. Relating to the random factors during the fabrication of FBGs, we analyze the influence of the random fluctuation and transverse asymmetry of the refractive index modulation.In the last chapter, dynamic grating formed in a two-photon absorbing organic dye solution and its induced backward stimulated Rayleigh Bragg scattering are studied here. Due to the enhancement of the two-photon resonant absorption, the medium has large nonlinear coefficient of refractive index. Based upon these studies, we proposed to form a dynamic grating in a fiber with two-photon resonant absorption as a feedback component of lasers to achieve a new type of all-optical wavelength convertor.