Compact and fault-tolerant photonic crystal add-drop filter

We propose an add-drop filter consisting of two adjacent waveguides carved into a two-dimensional photonic crystal that is etched through a standard guiding structure. This filter is based on distributed energy transfer via the frequency-selective intermediate conversion of the fundamental guided mode to a high-order low-group-velocity mode. This geometry circumvents the fabrication sensitivity on the single-hole scale of previous cavity-based designs. Combining distributed energy transfer and reduced group velocity preserves compactness. The design is analytically optimized with a coupled-mode approach.