Studies of long-period grating devices formed in rectangular-core waveguides

Abstract

In recent years, significant efforts have been made on the research and development of long-period fiber gratings. However, the restrictions in geometry and material of the fiber devices limit the functions that long-period fiber gratings can achieve. To circumvent these limitations, long-period waveguide gratings have been proposed, and a number of novel and interesting techniques have been reported for the fabrication of long-period waveguide gratings, in both the 2-D waveguide devices and 3-D waveguide devices. The aim of this project is to carry out a detail investigation on the transmission characteristics of long-period gratings formed in single-mode rectangular-core waveguide. An overview of integrated optics and the characteristics of integrated optical devices are firstly provided, which include the basic theories of the integrated optical waveguide devices. As one of most important elements, gratings, which are fabricated in waveguide devices, can be used as various passive components. The development of gratings and the theoretical explanations of the principles of operation for long-period waveguide gratings are introduced. By introducing periodic modulations of either the refractive index of the film or the thickness of the film, long-period waveguide gratings promote the occurrence of the couplings between the fundamental guided mode and the forwards propagating cladding modes. The well-known coupled-mode theory is a powerful tool to describe the coupling mechanism. The coupled-mode equations are derived from the coupling mode theory associated with the film mode matching method in detail. In waveguide structure, the transmission characteristics of the long-period gratings are sensitive to the various waveguide parameters and the local environments, such as the cladding parameters, the grating parameters, the refractive index of the external medium and the temperature, and so on. Various properties of sensitivity of long-period waveguide gratings is due to the strong dependences of refractive indices of fundamental guided mode and the propagating cladding modes on the structure and the environments, as well as the influences of the structure and the environments on the cross-overlap integrals between the fundamental guided mode and the cladding modes. The transmission characteristics of long-period waveguide gratings with different waveguide parameters and under different local environments have been studied theoretically in details. Most waveguide devices are based on the 3-D structures because of the efficient control of the guided mode by the transverse light confinement with the guided surface in addition to the light confinement along the waveguide depth. Research on long-period gratings formed in 3-D waveguide structure is much more practical than those formed in 2-D waveguide structure. Our studies help to understand the transmission characteristics (the fundamental guided mode coupled to the first-order propagating cladding mode) of long-period gratings, which are formed in a single-mode 3-D waveguide.

Date of Award	3 Oct 2005
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yue Bun Edwin PUN (Supervisor)