Compact multimode optical ring resonators for interconnection on silicon chips

Multimode ring resonators made of silicon nitride waveguides with widths of 2.0 and 3.0 μm have been fabricated. The dependence of the optical power on the wavelength between 1.28 and 1.32 μm is measured. For the waveguide width of 2.0 μm, clear subpeaks caused by the higher-order modes are not observed, and the resonance characteristics behave similarly to those of single-mode ring resonators. On the other hand, subpeaks caused by the higher-order modes appear for the waveguide width of 3.0 μm. The reason for these characteristics is discussed. The dependence of the full width at half maximum of the resonance shape on the gap between the input/output waveguides and the ring is examined. The bending loss of the multimode and the single-mode waveguides is simulated to show that the bending radius is reduced by about 25% for the multimode waveguides. The multimode ring resonators are useful as optical filters for interconnection on ultralarge-scale integrated circuits because they can be fabricated more compactly.