A reconfigurable architecture for continuously variable optical slow-wave delay lines

A novel reconfigurable architecture based on slow-wave propagation in integrated optical ring resonators is proposed for the realization of variable optical delay lines. A continuously variable delay is achieved by combining a coarse discrete (digital) delay, provided by a coupled resonator slow-wave structure, with a fine continuous (analog) delay given by a cascaded ring- resonator phase-shifter. The reflective configuration of the structure enables a simple, accurate and robust tuning of the delay and provides a footprint reduction by a factor 2 with respect to conventional coupled resonator optical waveguides. Proof-of-concept devices realized in 4.4% silicon oxynitride waveguides and activated by a thermal control are discussed. Experimental results demonstrate, in both spectral and time domain, a continuously variable delay, from zero to 800 ps (2 bit fractional delay), on a 2.5 Gbit/s NRZ signal, with less than 8 dB insertion loss and less than 5 mm2 device footprint.