We numerically and experimentally study silicon-based laterally waveguide-coupled square microcavity channel add-drop filters for wavelength division multiplexing (WDM) optical communications. Three microcavity designs: square, corner-cut square and curved-sidewall square microcavities are investigated. _{x y}...[ Read more ]

We numerically and experimentally study silicon-based laterally waveguide-coupled square microcavity channel add-drop filters for wavelength division multiplexing (WDM) optical communications. Three microcavity designs: square, corner-cut square and curved-sidewall square microcavities are investigated.

Using two-dimensional finite-difference time-domain (FDTD) numerical simulations, we study the spectral characteristics and mode-field patterns of parallel waveguide-coupled square microcavities. Simulated mode-field patterns are modeled by superposition of two orthogonal sinusoidal standing-wave fields and labeled by pairs of integer mode numbers (m_x , m_y). Vortex mode-field patterns, which are formed by interference of degenerate modes, are observed in high-Q resonances. Fourier Transform technique is applied to verim the modal composition. Cavity modes that have the propagation mode angles closed to the side-coupled waveguide fundamental mode angles are preferentially coupled.

In order to suppress the multimode and standing-wave resonances, we propose and numerically simulate alternative square microcavity design with the four corners cut at 45°. Our simulations reveal nearly single-mode resonances. Four-bounce closed-loop traveling-wave modes are preferentially coupled, whereas standing-wave modes with open orbits are suppressed. Vortex mode-field patterns are observed in dominant resonances. By tuning the corner-cut dimension, we optimize the coupling efficiency to 98.0% and on/off ratio to 22 dB with 1-μm corner-cut in 10-μm square microcavities of n = 2 .

In order to eliminate the open ray orbits walk-off and thereby preferentially coupled with the four-bounce close-loop modes, we propose and numerically study another alternative square microcavity design with curved opposite sidewalls. For 10-μm square microcavities of n = 2, by tuning the radius of curvature of the curved sidewalls to L/R =1.0, we optimize the filter performance obtaining coupling efficiency = 99.7%, on/off ratio = 24dB and Q = 1100. Multimode resonances are suppressed with side-mode suppression ratio = 25dB. Four-bounce Gaussian mode-field patterns are observed in dominant modes.

On the experimental front, we design, fabricate and characterize the first laterally parallel and crossed waveguide-coupled square microcavity channel add-drop filters on silicon nitride and silicon-on-insulator. Throughput of silicon nitride waveguide coupled square microcavities are characterized. The measured FSRs are consistent to the four-bounce closed-loop trajectories. Large coupling efficiency > 99% can be obtained. This can be attributed to the long waveguide-cavity interaction length. Singlemode is observed in waveguide crossing coupled square microcavities. This can be attribute to the preferential coupling of 45° modes, which is the only angle that have same incident angle on both input and output faces.