Whispering gallery mode and cladding mode resonance in optical fibers

Optical fibers were originally designed to confine the light in the core minimizing the disturbances from the outer world. In many cases, to meet the need of tunable functionality of the fiber optic devices or to make highly sensitive sensors, active manipulation of the core mode is required. Since the silica glass itself is an inactive material, it is desirable to make the confined light directly interact with an active medium. Unlike the core mode, the cladding bounded modes can see the outer medium through the evanescent wave interaction.; In this dissertation, two different approaches, whispering gallery mode resonance by fiber bending and cladding mode resonance by long-period gratings, are investigated to convert the core mode to the cladding bounded modes for such an interaction. First, bending the optical fibers can induce the core mode to radiate into the cladding. Geometrical perturbations caused by fiber bending make the confined light partially lose its guidance. Some escaping light can recouple to the core mode interfering with each other. This phenomenon having had surprisingly little attention, although discovered almost 30 years ago, is revisited. By removing the fiber jacket and thinning the cladding, the radiating light can be confined by the cladding and propagate along the fiber wall forming 'whispering gallery mode (WGM)' without significant power loss.; The applications of this rediscovered phenomenon are also explored. The cladding bounded WGM enables refractive index sensing through evanescent field and the deep resonance peaks can be utilized for temperature sensing by monitoring the shift of the peaks.; Second, cladding mode resonance is obtained by long-period fiber gratings (LPFGs). Periodic refractive index perturbation with a period longer than the light wavelength can induce the core mode to transition into the cladding modes. Cladding modes in LPFGs have been extensively investigated in recent years. This dissertation focuses on the interactive nature of the cladding modes with the outer medium and actively tunable LPFG filters are investigated as a way of implementing true electro-optic tuning devices. The interaction capability or sensitivity can be greatly enhanced by reducing the cladding thickness. This cladding thinning strategy is used for increasing the sensitivity. The interaction property of the cladding modes in cladding-thinned optical fibers is discussed.; Also, novel characteristics of LPFGs written by an electric arc discharge method are investigated. The non-UV fabrication method proves its usefulness and flexibility in terms of producing LPFGs with unique properties such as very strong coupling bend insensitivity, and thermal stability.