Hybrid multi-chip assembly of optical communication engines by in situ 3D nano-lithography

Optics: 3D nano-lithography greatly simplifies photonic multi-chip assembly Scientists have demonstrated photonic multi-chip modules that rely on 3D-printed waveguides for connecting photonic chips. Current integrated optical systems are often assembled from discrete dies through high-precision alignment techniques, which leads to high fabrication costs and often limits the application range. A team of German researchers led by Christian Koos from Karlsruhe Institute of Technology (KIT) has now demonstrated optical communication engines that exploit 3D-printed freeform waveguides, so-called photonic wire bonds, as chip-chip connections. The photonic wire bonds are fabricated in-situ onto the chips by advanced 3D lithography and can efficiently connect a wide variety of photonic integration platforms. By greatly simplifying the assembly of advanced photonic multi-chip modules, the technique has the potential to transform a variety of applications, ranging from high-speed communications and ultra-fast signal processing to optical sensing and quantum information processing. Three-dimensional (3D) nano-printing of freeform optical waveguides, also referred to as photonic wire bonding, allows for efficient coupling between photonic chips and can greatly simplify optical system assembly. As a key advantage, the shape and the trajectory of photonic wire bonds can be adapted to the mode-field profiles and the positions of the chips, thereby offering an attractive alternative to conventional optical assembly techniques that rely on technically complex and costly high-precision alignment. However, while the fundamental advantages of the photonic wire bonding concept have been shown in proof-of-concept experiments, it has so far been unclear whether the technique can also be leveraged for practically relevant use cases with stringent reproducibility and reliability requirements. In this paper, we demonstrate optical communication engines that rely on photonic wire bonding for connecting arrays of silicon photonic modulators to InP lasers and single-mode fibres. In a first experiment, we show an eight-channel transmitter offering an aggregate line rate of 448 Gbit/s by low-complexity intensity modulation. A second experiment is dedicated to a four-channel coherent transmitter, operating at a net data rate of 732.7 Gbit/s - a record for coherent silicon photonic transmitters with co-packaged lasers. Using dedicated test chips, we further demonstrate automated mass production of photonic wire bonds with insertion losses of (0.7 +/- 0.15) dB, and we show their resilience in environmental-stability tests and at high optical power. These results might form the basis for simplified assembly of advanced photonic multi-chip systems that combine the distinct advantages of different integration platforms.