Automatic configuration and wavelength locking of coupled micro ring resonators in presence of thermal cross-talk.

Thermal cross-talk can impair the efficiency of tuning algorithms employed for the control, calibration and reconfiguration of photonic integrated circuits (PICs). For example, in coupled microring resonator (MRRs) architectures, thermal crosstalk is responsible for an unwanted coupling among the round trip phases of the resonators, thus affecting their resonance frequencies. Here we propose a novel approach, named Transform Coordinate method (TCM), enabling thermal cross-talk cancellation in PICs. In the TCM, instead of controlling the phase shift of each photonic element individually, the eigensolutions of the thermally coupled system are calculated and employed as control variables. The effectiveness of the TCM is demonstrated by implementing a feedback control system providing automatic resonance tuning of 3rd order coupled MRR filters. Numerical simulations, confirmed by experimental results achieved on a high-index-contrast silicon oxynitride (SiON) platform, demonstrate that the TCM enables a tuning process that is faster, more accurate and more robust with respect to conventional methods based on individual tuning of each MRR. Further, the TCM can be used as a wavelength locking algorithm to maintain the tuned condition in the presence of temperature drift as well as random fluctuations of the wavelength and of the power of the input signal. Finally, the TCM can be applied to generic PIC architectures based on arbitrary combinations of MRRs and other integrated interferometric devices.