Embodiments herein describe a photonic chip which includes a coupling interface for evanescently coupling the chip to a waveguide on an external substrate. In one embodiment, the photonic chip includes a tapered waveguide that aligns with a tapered waveguide on the external substrate. The respective tapers of the two waveguides are inverted such that as the width of the waveguide in the photonic chip decreases, the width of the waveguide on the external substrate increases. In one embodiment, these two waveguides form an adiabatic structure where the optical signal transfers between the waveguides with minimal or no coupling of the optical signal to other non-intended modes. Using the two waveguides, optical signals can be transmitted between the photonic chip and the external substrate.