The linear and nonlinear characteristics of waveguiding structures made of direct-coupled Fabry-Perot or ring resonators are investigated. It is shown that the slowing down of the optical propagation in these structures may be advantageously exploited to enhance wave interactions.
Directional Waveguide Couplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Optical slow-wave structures such as coupled photonic crystal cavities, coupled microrings, coupled quantum wells, etc. are promising candidates for slow light devices because of the nature of their dispersion relationship which follows the tight-binding model well-known in solid-state physics. In a perfectly uniform structure, the slope of the dispersion curve, which defines the group velocity, is zero at the edges of the waveguide band, leading to slow light and enhanced light-matter interaction. Although the effects of group velocity dispersion (and higher-order dispersion) can limit the performance of slow light structures [1-4], the effects of dispersion can, in principle, be compensated by a variety ofmechanisms, including some ofwhich are used commonly in optical fiber communications.
The possibility of dynamically controlling the speed of light can be extremely useful in all the optical applications where a variable delay is needed, such as bit and frame synchronization and optical storage. To this aim, coupled resonator slow-wave structures (SWSs) represent a promising approach to slow down the propagation of optical signals with as large bandwidths as required by optical communication systems. We show that by suitably setting the rings' resonances, SWSs can act efficiently as tunable delay lines, with the overall time delay being continuously adjustable from zero to a maximum value, typically limited by the effects of loss and dispersion. Every ring of the structure can be used as an optical gate and optical pulses can also be trapped and released by the SWS, so as to realize an optical memory or buffer. A time delay of 300 ps and a slowing ratio of 7.5 over a 3.3 GHz bandwidth are experimentally demonstrated in SWSs realized in silicon oxynitride technology