Programmable Multiring Butterworth Filters With Automated Resonance and Coupling Tuning

We demonstrate a bandwidth and wavelength programmable silicon double microring optical filter that can be automatically, accurately, and repeatably set to an optimal Butterworth shape for maximum passband flatness. The algorithm uses a critical coupling condition to set the Butterworth shapes, followed by bandwidth tuning using a calibration dataset for the coupling coefficients. Silicon microrings with independently tunable phase shifts and coupling coefficients were designed for and fabricated at a foundry to demonstrate the method. Fabrication variations and thermal crosstalk are also compensated automatically.