Enhanced axial confinement in a monolithically integrated self-rolled-up SiN_x vertical microring photonic coupler

We report an efficient method to introduce enhanced axial confinement in the self-rolled-up SiN_x vertical microtube coupler by depositing a thin layer of high refractive index material strip within the coupling section and effectively forming a vertical microring. Three times wider mode spacing is observed in such a vertical microring coupler monolithically integrated with a silicon nitride ridge waveguide as compared to the one without such axial confinement. More importantly, single mode operation within the telecomm C-band and S-band is achieved.