Some Trials to Suppress Unnecessary Resonance in Photonic Crystal Cavity Structure

Propagation and filtering characteristics of two dimensional pillar-type photonic crystal waveguide with triangular lattice were measured and improved experimentally. Measurement was done by using a scale model in microwave frequency with lattice period P=26.5 mm, diameter of pillar .=7.5mm and its dielectric constant ε_r=36.0 around 4 GHz. First, symmetric Y-branch waveguide was set up by using line-defect in array of pillars. After confirming symmetric branching operation in frequency range between 3.6 and 4.2 GHz, a pair of pillars which have length of 3.0P and 2.5P respectively, were situated in each output waveguide as cavities. These cavities showed resonant characteristics at frequencies corresponding to λ_g/2 in the waveguide. Also, good extinction ratios of over 20 dB between output ports were observed at these resonant frequencies. Next, cascading the Y-branch by two stages and situating four cavities with different lengths at each output port, filtering characteristics were observed at each output in frequency range of 3.6 to 4.2 GHz. The extinction ratios at each resonance frequency were over 17 dB. Though the structure showed good filtering characteristics, unnecessary resonances in frequency domain were observed near main peaks of filtered frequency. To suppress these unwanted peaks, stabs with length of λ_g/4 were inserted after the cavities, where λ_g is wavelength of the unnecessary frequency. Due to the stabs, peaks of the unnecessary resonances were suppressed over 20 dB compared with simple cavities. This structure is applicable as a basic filter component in wavelength division multiplexing (WDM) systems in photonic crystal optical circuits.