Dispersion properties of photonic crystal coupled-cavity waveguides

Coupled-cavity waveguides (CCWs) formed by a chain of evanescently coupled cavities are characterized by extremely flat and linear dispersion, which makes them a promising candidate for optical delay lines and dispersionless slow-light devices. The slow-down capability of CCWs in a photonic crystal environment was recently investigated by Notomi et al. [1], who showed that a concatenation of ultrahigh-Q cavities (Q = 1.2×10
⁶
) can effectively delay optical pulses (time delay of 125 ps for initial pulse bandwidth of 1 nm and 150 coupled cavities), while keeping the transmission loss at a low level (1.45 dB/mm). However, when the CCW design is based on ultrahigh-Q cavities, the resulting transmission band is necessarily discontinuous, consisting of sharp, separated peaks, which results in undesired pulse echo and hinders the noise-free propagation.