  Automatic resonance alignment tuning is performed in high-order series
coupled microring filters using a feedback system. By inputting only a
reference wavelength, a filter is tuned such that passband ripples are
dramatically reduced compared to the initial detuned state and the passband
becomes centered at the reference. The method is tested on 5th order microring
filters fabricated in a standard silicon photonics foundry process. Repeatable
tuning is demonstrated for filters on multiple dies from the wafer and for
arbitrary reference wavelengths within the free spectral range of the
microrings.
