Tunable low dispersion optical delay line using three coupled micro-resonators

There is a large demand for integrated photonic devices capable of producing controllable pulse delays of optical signals. Normally, the achievable delay is proportional to the length of the optical path making it difficult to achieve significant delays in chip based devices. We propose a tunable optical delay line consisting of a waveguide coupled in an all pass geometry to a serial array of three micro-ring resonators. The coupling between the whispering gallery modes of the rings leads to a triplet of absorption resonances in between which the group delay is very large while pulse dispersion and attenuation is very small. Moreover, the delay and bandwidth are widely tunable by the coupling between the resonators with delays and bandwidths up to several hundred picoseconds and several tens of GHz, respectively. Although attenuation between the resonances is small, we show that including a modest amount of gain in the second and third resonators eliminates the attenuation regardless of how large the losses are in the first resonator. (paper)