Journal of Electrical & Electronic Systems

The requirement for large-scale heterogeneous integration is being driven by the ongoing trend of exponential development in data communications and processing. We are seeing an increasing tendency in the development of 3D photonic integrated circuits (PICs), in addition to that of 2D PICs, which is similar to the pattern we have seen in the development of electronic integrated circuits. For creating 3D PICs, there are primarily two techniques. The first technique allows for freeform shaping of waveguides in any shapes and configurations thanks to ultrafast laser inscription (ULI). The second technique, multilayer stacking and coupling of planar PICs, makes use of relatively advanced 2D PIC fabrication procedures that are successively applied to each layer. There are benefits and drawbacks to both ways of fabricating 3D PICs, thus some applications might favour one over the other. The combination of 2D and 3D PICs, however, enables the creation of integrated microsystems with novel capabilities like non-mechanical beam steering, space-division multiplexing (SDM), programmable arbitrary beam shaping, and photonic signal processing. Examples of 3D PICs and 2D/3D integrated PICs are discussed in relation to two applications: Optical beam steering with optical phased arrays and SDM employing orbital-angular-momentum (OAM) multiplexing/demultiplexing. A 2D PIC has difficulties in supporting both polarizations even though it can act as an OAM multiplexer or demultiplexer on its own. A 3D PIC made by ULI, on the other hand, can effortlessly accommodate both polarizations with little propagation loss.