We study in general the property of dielectric optical waveguides that are translationally invariant along the field propagation direction. The core and the cladding can be made of either homogeneous or two-dimensional (2D) composite material (or metamaterial). When the metamaterial involved exhibits 2D periodicity, the waveguides are so-called photonic crystal fibers (PCFs). Guidance varieties in such waveguides are studied by examining separately the optical properties of the bulk core and cladding materials as well as by explicit numerical calculation of the overall waveguide structure. Our analysis reveals several neglected modal characteristics of such waveguides. For example, we show that different modes can copropagate in some PCFs with different guidance mechanisms, i.e., index guidance and photonic bandgap guidance. Such a feature can offer additional flexibility in light-wave manipulation. We also show that there exists a cutoff wavelength for the fundamental index-guided mode in some PCFs, which breaks the convention in traditional step-index fibers where such a cutoff does not exist. (c) 2006 Optical Society of America.
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