Their paper “Hybrid integration of silicon photonics circuits and InP lasers by photonic wire bonding” published in the Optica journal detail how starting from readily processed III–V lasers and photonic circuits assembled side by side on a common heat-sinking carrier substrate, they could connect the dies’ optical paths through 3D lithography-designed photonic wire bonds, manufactured in-situ to accommodate for any misalignments.
This hybrid integration allows photonic chip designers to integrate the best characteristics of native III–V light sources together with silicon-based modulators, only using pre-tested chips and conventional pick-and-place equipment, yet achieving very low insertion losses (down to 0.4 dB) compared to 2.3dB optical coupling losses as reported so far in literature.
As the authors explain, the photonic wire bonding (PWB) is fabricated in-situ relying on the additive nanofabrication of freeform polymer waveguides between the pre-positioned photonic chips. That means the optical paths can be freely configured in 3D between the chips for the best fit while relaxing die-placement accuracy.
As a demonstration, the researchers connected indium phosphide (InP)-based horizontal-cavity surface-emitting lasers (HCSEL) to passive silicon photonic circuits. But using the same photonic wire bonding strategy, they had already successfully demonstrated direct SiP chip to SiP chip bonding and connected the individual cores of a multi-core fibre to an array of planar SOI waveguides.
The photonic wire bonds are fabricated through two-photon polymerization by exposing a negative-tone photoresist drop-cast onto the multi-chip photonics module. The photoresist had to cover both the HCSEL emission window and the coupling region of the targeted Si nanowire, and the respective coupling interfaces were optically detected in the volume of the resist prior to exposure. Tapered sections minimize mode-field mismatch at both interfaces, the optical wires being designed with a 2.0×1.4μm rectangular cross section.
In the experiments, writing speed was not optimized, note the authors, and each photonic wire bond took about 3mn to expose, but they are confident this could be largely improved, promoting photonic wire bonding as a universal integration platform for hybrid photonic multi-chip assemblies. Karlsruhe Institute of Technology – www.kit.edu