We experimentally and numerically study silicon nitride-based laterally waveguide-coupled microresonator optical filters for wavelength division multiplexing (WDM) optical communications. We investigate two types of filters based on different microresonator shapes and waveguide coupling configurations: (i) circular microring/microdisk resonator cross-connect filters, and (ii) hexagonal microresonator add-drop filters....[ Read more ]

We experimentally and numerically study silicon nitride-based laterally waveguide-coupled microresonator optical filters for wavelength division multiplexing (WDM) optical communications. We investigate two types of filters based on different microresonator shapes and waveguide coupling configurations: (i) circular microring/microdisk resonator cross-connect filters, and (ii) hexagonal microresonator add-drop filters.

For the microring cross-connect filters, we experimentally study the effects of the rib waveguide dimensions, including the waveguide width, rib height, and slab height. Our measurements reveal that the Q-factor depends near linearly on the waveguide dimensions, whereas the extinction ratio is optimized at certain dimensions. We model the waveguide dimensions dependence in two steps: (i) numerical beam-propagation method to estimate the microring loss coefficient and the waveguide-microring coupling coefficient, and (ii) Fabry-Perot-like model to approximately calculate the Q-factors and extinction ratios as a function of the microring loss coefficient and waveguide-microresonator coupling coefficient. Our model suggests that the effects of the loss and coupling coefficients on the Q-factor are cooperative, yet on the extinction ratio are competitive. We find qualitative agreement between our model results and the experimental observations.

Furthermore, we experimentally study the effects of a silica upper-cladding on the microring cross-connect filters. Compared with an air-cladding, our measurements show that the silica-cladding eases the filter insertion loss and reduces the off-resonance crosstalk at the add-port. Moreover, the silica-cladding enhances the free spectral range (FSR) by a few %, and shifts the resonance wavelengths. Our two-dimensional (2-D) finite-difference time-domain (FDTD) simulations also suggest similar effects upon increasing the cladding refractive index. Our simulations further show that the high-index cladding induces a resonance wavelength redshift. According to our heuristic ray/wave-optics model, the cladding-induced FSR expansion is due to a few-% drop in the microring ray-orbit round-trip path length, whereas the resonance redshift is largely given by reduced total internal reflection phase shift at the sidewalls.

For the hexagonal microresonator add-drop filters, we study two ways to tune the filter response: (i) rounding the microcavity corners, and (ii) varying the microcavity flat sidewall interaction lengths. By rounding the microcavity corners, we observe general improvements in the Q-factors and extinction ratios. By increasing the sidewall interaction lengths, we also observe general improvements in the extinction ratios, yet the Q-factors are degraded. At a certain round-corner radius, both the Q-factor and extinction ratio reach local minima. Interestingly, our FDTD simulations suggest that at that particular hexagonal cavity shape, the cavity mode-field pattern is a superposition of the 6-bounce ray-orbit mode and the Whispering-Gallery-like mode.