Two-dimensional photonic crystal waveguides with multiple sharp bends

Abstract

We emphasize the importance of optical confinement in the vertical direction for a successful two-dimensional photonic crystal waveguide (2D-PCW) performance by presenting optical properties of a 2D-PCW that has a special feature to provide a strong vertical confinement. The 2D-PCW, which was designed to operate in microwave regime, was composed of two parts: an ordinary 2D-PCW composed of alumina rods in air for lateral optical confinement and a pair of aluminum metal plates forming a metal waveguide in the vertical direction. Cylindrical alumina rods, each being 2 mm in diameter, were arranged in a square-lattice with its spatial period of 9 mm, which according to a simple photonic band calculation produces a photonic bandgap in the frequency range of 14.7~16.5 GHz for TM-polarized light (E-field parallel to the rods). This 2D-PCW was then embedded between two aluminum metal plates. Light propagation loss for a straight waveguide, which was estimated from transmission measurement as a function of waveguide length, is as low as 0.05 dB/cm whereas most of transmission loss could be attributed to the input and output couplings due to mode mismatch. When multiple 90°-bends were incorporated, on the other hand, estimated bending loss was only 0.1 dB/bend, which indicates that high performance 2D-PCWs are indeed possible if an appropriate strong confinement in the vertical direction is provided. Fabry-Perot oscillations seen in transmission spectra, whose oscillation period was observed dictated only by total waveguide length regardless of the number of bends, are another strong evidence for the low propagation and bending losses of our waveguide structure.