Flexible bandwidth control of the single-mode guided band in a 2D photonic crystal coupled-cavity waveguide

We proposed flexible bandwidth control for a two-dimensional (2D) photonic crystal coupled-cavity waveguide. The 2D waveguide is designed to operate in single-mode. The bandwidth not only determines the operating frequency range of the waveguide, but also affects the group velocity of the guided modes much. Researches in enlargement and precise controlling of bandwidth are of great importance for waveguide structure design based on photonic crystals. Moreover, to keep the signal pulse shape along a single-mode waveguide, minimized group velocity within a wide bandwidth is required for the design. In our previous studies we have demonstrated controlling the upper and lower cut off frequencies of the guided band both independently and simultaneously. In this work, large bandwidth-tuning for a single-mode guided band with fixed center frequency is realized by changing two configuration parameters, namely defect radius and defect width. Plane wave expansion method is utilized for calculation. The largest bandwidth tuning range up to 50.7% of photonic bandgap (PBG) is achieved for normalized center frequency at 0.377. Furthermore, for different bandwidths, we investigate the relations of group velocities and wave vectors, which are crucial to engineer the group velocity dispersion in the waveguide. Our results demonstrate the possibility of large bandwidth tuning while single-mode operation is maintained, which could be extended to photonic crystal slab waveguide with some modifications. We believe this work will contribute to the design of integrated optical devices based on photonic crystal waveguides, such as multiplexers and de-multiplexers which can make use of the flexible bandwidth control capabilities.