Triangular-Shaped Coupled Microrings for Robust Wavelength Multi-/Demultiplexing in Si Photonics

We propose and demonstrate triangular-shaped coupled Si microrings for robust wavelength multi-/demultiplexing. Thanks to the geometry, stable interring couplings are obtained at straight waveguides, and heaters for thermal tuning can be isolated maximally with each other so that the thermal crosstalk is suppressed between rings. To suppress the bend loss and enhance the free spectral range (FSR), 2-μm-radius low-loss sharp bends with an optimum waveguide offset are employed. A CMOS-compatible process is used for fabricating second- and fourth-order coupled-microrings designed for the Butterworth and Chebyshev spectral responses, respectively. In the second-order rings, flat-top Box-like drop spectra with fluctuations <; ±0.1 dB, insertion loss of 1.2 dB, out-of-band suppression >30 dB, bandwidth of 2.2 nm, FSR of 19 nm, and thermal tuning up to 8 nm are obtained experimentally. The efficiency of the thermal tuning is better and the thermal crosstalk is lower than those in conventional circular and race-track rings for such wide FSR.