Analysis of giant waveguide tapers with funnel geometry: multi-mode Interference regime to single mode
In this study, we are interested in giant tapers operating in a totally Multi-Mode Interference (MMI) regime capable of producing still an adequate single-mode field at the output. The idea is therefore to provide an answer on a possible giant acceptable limit of such MMI tapers still causing a single-mode output and then determining which opto-geometric parameters on this simple mathematical object and geometry act on the behavior. To this end, we have defined energy criteria per volume at the output. We consider families of tapers with a constant height corresponding to the output rib waveguide and a triangle shape. Each taper is defined by its input size, its length and its output size. The objective is to determine the minimum length of the taper to get enough energy or the desired guided mode(s) in the output waveguide. Two various approaches have been investigated and compared: numerical simulations by a finite element method (COMSOL) and a pure mathematical and geometrical study in conditions of total reflection on the walls of the taper plus a specific plane transformation. When the length of the taper increases, the energy increases in the core up to a limit value and decreases in the cladding. The size of the output guide is fixed to ws = 2 microns and the input size of the taper varies from typically 6 microns to `giant' 400 microns. Four kinds of behavior were identified. For small lengths all the energy is reflected by the walls of the taper. For first intermediate lengths a part of the light is difused in the cladding and the other part is guided to the output guide. For following intermediate lengths the major part of the light is guided to the output guide with multi-modes propagation. Lastly for greater lengths only the fundamental guided mode appears in the output guide.
Univ. de Rennes, CNRS, IETR (France)
Bruno Bêche is Professor of Physics at the University of Rennes 1 (IETR of CNRS). He is Honorary Member of the 'Institut Universitaire de France' - IUF Paris. His teaching activity took place in several universities as well as in engineering schools, holding lectures in fundamental and applied physics at all student levels of education. His research career started on the development of optical components as non-linear optical devices, and then he worked in the development of III-V semiconductors as components for the wavelength division, multiplexing and lasers. At the University of Rennes 1, he started a new field of research, working since a few years in the development of various micro-resonators based on hybrid technologies which combine the use of polymers and plasma treatments, soft matter with fluidic and biology concepts. His research work covers both the theoretical description of the physical aspects of these photonic devices and technologies.
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