Doubly Leaky Cladding Modes and Short-Wavelength Resonances of Bragg Gratings in Silica-on-Silicon Waveguides

摘要

Modern wavelength-division multiplexing fiber networks require highly wavelength-selective components such as band stop and add-drop filters. Strong interest grows in realizing these components using UV-written Bragg gratings in photosensitive planar waveguide, because of the advantages of mechanical stability and the possibility of mass production and of integrating complex functions. However, the cascadability required for realizing complex functions is limited by short-wavelength transmission losses observed by many authors. Especially, pronounced spectral peaks in the transmission losses have been reported in silica-on-silicon waveguide gratings and have been attributed to coupling to cladding modes similar to the observations made for cladding-mode coupling in fibers However, until now the existence, and the nature of guidance of cladding modes in planar waveguide structures have not been clarified. There is a particular difference between optical fibers and planar waveguide structure: Cladding modes in fibers are guided by the closed cladding/air interface In contrast to this, planar waveguide structures (Fig. 1) are effectively open in the horizontal direction (+-x directions in Fig. 1). Any power radiated into these directions would not be reflected and would thus not build up any resonances.