Low‐loss and polarisation‐insensitive interlayer coupler on three‐dimensional SiO x ‐waveguide platform

A low‐loss and polarisation‐insensitive vertical directional coupler that connects vertically stacked SiO x waveguides is reported. The fabrication process features the low‐temperature formation of an SiO x film to prevent thermal damage to the lower layer waveguides. Experimental results showed interlayer coupling losses of <0.1 dB in both transverse‐electric and transverse‐magnetic modes.