We describe a novel scheme based on evanescent guided-wave coupling for optically interfacing between conventional fiber-optic and planar photonic crystal devices such as waveguides and resonant cavities. By considering the band structure of bulk photonic crystal slabs, we analyze the k space properties of a linear defect waveguide and establish a set of design rules to ensure efficient evanescent coupling with optical fiber tapers. These rules are used to design a waveguide in a square-lattice photonic crystal. The coupling efficiency is calculated with a coupled-mode theory incorporating the finite-difference time-domain-calculated uncoupled modes of the fiber taper and photonic crystal waveguide. On the basis of this coupled-mode theory, 95% power transfer from the fiber taper to the photonic crystal waveguide is possible over a coupling length of 80 lattice periods and with a bandwidth of 1.5% of the center wavelength. The integration of this waveguide with a photonic crystal defect resonant cavity is also presented, thus showing the usefulness of the combined fiber taper and photonic crystal waveguide system for efficient, optical fiber-based probing of optical elements based on planar photonic crystal technologies.
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