A Two-Tier Inverse Taper based Vertical Coupler for Photonic Integrated Circuits

A low-loss, compact, two-tier spot size-converter (SSC) has been designed and demonstrated to couple light between silicon (Si) and silicon nitride (SiNx) platforms. The high index contrast between the materials is accomplished by matching the optical mode impedance of Si and SiNx. This allows the achievement of record low coupling loss of 0.058 ± 0.01 dB per transition at 1525 nm. The geometric parameters of the vertically integrated SSC were optimized to ensure the adiabatic transition of the optical mode from Si-to-SiNx and vice-versa. The SSC converter shows excellent broadband coupling over 100 nm bandwidth at telecom wavelengths.