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Connecting Theory and Practice in Optoelectronics
The problem of circuit control can be particularly challenging in high-index-contrast photonic platforms, due to the large sensitivity of the optical parameters of the devices even to tiny geometrical variations occurring during fabrication, mainly inducing random phase errors. This is particularly true for high-order coupled microring resonator filters. The use of these devices cannot be addressed without proper tuning and locking techniques that can be significantly more complex than the approaches required for the control of a single ring, due to the larger number of degrees of freedom, the existence of non-negligible coupling between rings and the risk of trapping in sub-optimal local solutions. Common tuning strategies presented in literature explore iterative approaches to overcome this problem. These sequential tuning algorithms are based on sweeping the resonance of each microring individually (generally through thermo-optic actuators) until reaching an optimal working point for the entire filter. On the other hand, the use of thermo-optic actuators leads almost unavoidably to the problem of handling thermal cross-talk that causes unwanted phase changes in adjacent rings close to the one under control. Thermal cross-talk may hence make sequential methods inefficient in locking schemes. We propose here a novel method (transformed coordinate method) for the tuning and locking of a coupled microring resonator filter. With this technique, the thermal phase controller of all the rings that realizes the filter are tuned simultaneously at each iteration of the algorithm to minimize the target error function. The effectiveness of this technique is demonstrated by experimentally tuning a 3rd order microring-based filter fabricated in SiON platform starting from five different randomly perturbed conditions [Fig. (a)]. Using transformed coordinate method in all of the cases fine-tuned filters were obtained and such status was maintained via locking scheme in presence of perturbations [Fig. (b)]. More details will be presented at the NUSOD 2017 conference in Copenhagen (talk WB1).