Waveguide mode converter based on two-dimensional photonic crystals.

We propose and numerically analyze a novel mode converter based on two-dimensional photonic crystal waveguides with square arrays of cylindrical dielectric rods in air. The mode converter uses small perturbation defects to decouple various modes in the multimode waveguide, thereby permitting propagation of only one mode at any given frequency, which permits one-to-one mode conversion without exciting unwanted modes. The mode converter can efficiently convert a TM0 mode supported in a single-mode photonic crystal waveguide into a TM2 mode supported in the multimode waveguide that is laterally coupled to the single-mode waveguide section for a wide wavelength range. Influences of different sizes and positions of perturbation rods on the band structure of the multimode waveguide are studied.