Grating-assisted-cylindrical-resonant-cavities interlayer coupler.

A grating-assisted-cylindrical-resonant-cavities (GARC) interlayer coupler made of Si/SiO2 is designed and simulated to achieve efficient and broadband interlayer coupling. This coupler consists of three cylindrical resonant cavities: two waveguide cavities in the horizontal direction and one cylindrical via cavity in the vertical direction. The resonant strengths of the two cylindrical waveguide cavities are enhanced by circular Bessel-function-defined gratings and distributed Bragg reflectors. The interlayer coupling efficiency of this Si/SiO2 GARC coupler is simulated as ηc =68%(-1.7 dB) for transverse electric polarization at 1.55 μm wavelength, which is generally higher than those of conventional rectangular silicon-on-insulator gratings with additional features such as reflectors, overlayers, chirped periods, dual gratings, etc. The GARC couplers are predicted to have favorable attributes compared to previous couplers, including wider operational bandwidth (δλ,1 dB =270  nm), larger tolerance to inplane misalignment (±2  μm for 1 dB extra loss), easier grating patterning (wider grating ridges), smaller footprint (20 μm in diameter), and more flexible choices of interlayer distances (2-5 μm). A sensitivity analysis is also provided as a guide in fabrication. In general, it is found that the vertical dimensions of the GARC couplers need to be carefully controlled while the horizontal dimensions are less critical.