Study On Spectral Characteristics Of Fiber Gratings And Their Applications In WDM All-Optical Networks

Optical fiber grating is a kind of passive photonic device formed by ultraviolet light into the core of a photosensitive fiber. The advantages of fiber gratings include low insertion loss, insensitive to polarization, and simply spliced into fiber systems. Therefore, fiber gratings have numerous applications in almost all the realms of optical fiber, especially in wavelength-division multiplexing (WDM) all-optical networks. This dissertation presents a detailed study of the following three types of fiber gratings: fiber Bragg grating fabricated with cladding mode suppression fiber, linearly chirped moiré fiber grating, and long-period fiber grating with rectangular index modulation. It is expected to provide some solid basis and useful designs for the new fiber devices based on fiber grating, thus motivate the development of WDM all-optical network.In Chapter 1, the development of optical fiber communication, WDM all-optical network and its essential devices have been reviewed first, and followed by a systematic summary on the writing technology and applications of fiber gratings.In Chapter 2, we present the theoretical basis of the whole dissertation. We begin with the Maxwell-field equations, deduce the wave-propagation equations, and then analyze the characteristics of guided modes in optical fiber. Based on these, we derive the coupled-mode equations that describe the interaction between the modes. Furthermore, we analyze fiber Bragg grating of a constant refractive index modulation and period, found the coupled-mode equations that describe interactions between the forward-propagating guided core mode and the backward-propagating guided core mode itself.In Chapter 3, fiber Bragg grating fabricated with cladding mode suppression fiber is studied. First, we use a simple three-layer-model to gain the expressions of the electric field of cladding mode and its dispersion equation, at the same time, we present the expressions of the electric field of the core mode and its dispersion equation. Then we deduce the couple-mode equations that describe the mode coupling behaviors in this kind of gratings. Based on this, we study the photosensitive inner cladding's influence on the core-mode-core-mode coupling and the core-mode-cladding-mode coupling in this kind of grating. A fiber Bragg grating was experimentally fabricated with this kind of photosensitive fiber. The experimental result is in good agreement with the theoretical analysis.In Chapter 4, the spectral characteristics and its application in optical add-drop multiplexer (OADM) and in multi-channel dispersion compensator of linearly chirped Moiré fiber grating are studied. According to the coupled-mode theory and considering the construction features, we derive the couple-mode equations that describe the mode interaction in this kind of grating. The spectral and group delay characteristics are studied and the potential uses of this type of grating for wavelength selection and dispersion compensation are discussed. A novel scheme of OADM configuration based on linearly chirped Moiré fiber gratings is proposed. We have designed a LCMFG to compensate dispersions for eight channels at the same time. In Chapter 5, a long-period fiber grating with rectangular index modulation is studied by the coupled-mode theory. The couple-mode equations that describe the mode coupling in this kind of grating are presented. Furthermore, the method and steps for numerical simulation are presented. We numerically calculate the transmission spectrum of a long-period grating written in Corning FlexcorTM 1060 fiber. The calculation results agree with the experiment results reported in literature.