Compact all-pass filters in photonic crystals as the building block for high-capacity optical delay lines

Optical all-pass filters, which generate strong on-resonance optical delay while maintaining a unity transmission coefficient throughout the entire resonant bandwidth, are of great importance for constructing delay lines in optical buffer applications. We provide an analysis of optical delay lines based upon cascading multiple stages of all-pass filter structures. We show that the maximum capacity of such delay lines is determined primarily by the dimensions of each stage. Motivated by this analysis, we describe compact optical all-pass filters in two-dimensional photonic crystals. The accidental degeneracy of the cavity modes introduces a strong group delay and dispersion while maintaining total transmission.