Programmable PICs consists of a mesh of optical waveguides which are coupled together using waveguide couplers that can be electrically controlled. Our initial demonstration, the first programmable PIC in silicon photonics, was a simple 4x4 beam coupler that mapped 4 inputs onto 4 outputs in any arbitrary combination. Such ‘forward-only’ circuits are useful in applications that require matrix operations, such as neural networks and quantum computing. But they are limited when it comes to more advanced functions such as optical wavelength filters.
Therefore we switched to ‘recirculating’ meshes, where the light is routed in loops. We studied different connectivity schemes for such meshes, and how accurately they need to be controlled to avoid unwanted parasitic waveguide paths. Because all elements on the photonic chip need controlled, we developed an electronics layer based on commercial FPGAs that allows us to digitally control a large number of the on-chip tuners.
We built multiple iterations of these new programmable chips, using different waveguide connectivity, actuation mechanisms (heaters, MEMS, liquid crystals) and control schemes. However, not everything worked right out of the box. While these chips are conceived to shorten development time and eventually bypass the need for ASPICs, their own design, fabrication and testing still goes through the traditional slow cycle, with the risk that the chip does not work as expected after it returns from the fab. As these are very complex circuits, we experienced different failure mechanisms on multiple prototypes. In the first 3 generations we encountered a combination of optical, electrical and thermal issues that limited the functionality of the chip, and a 4th-generation chip is still in fabrication.
We managed to demonstrate different core technologies for these programmable photonic circuits, ranging from low-power actuators, tolerant building blocks and subcircuits (e.g. for the tunable coupler gate that is used throughout our circuit), and control routines to calibrate and configure routing and filtering functions in the waveguide mesh. This also included the use of high-speed modulators, where we added programmability to make them reconfigurable and optimize their characteristics, so they can be used to process high-speed microwave signals. We demonstrated these smaller results as part of dedicated application-specific circuits, but their use in a fully-programmable photonic circuit still needs to be demonstrated.
Even though the hardware side is not yet fully functional, we have developed a software framework to design, simulate, control and configure programmable photonic circuits. This framework already allows us to experiment with new algorithms to configure and control the circuit, such as powerful graph-based routing or filter synthesis routines. The simulation tools also allow new researchers and developers to already assess if programmable photonic circuits could be suitable for their applications.
We have published our results in multiple peer-reviewed journal publications and at scientific conferences, and our introductory videos on Youtube have proven very popular. We have also looked into different application spaces where these programmable photonic chips could bring the most value, and we are now exploring avenues to bring this technology to the market in the ERC proof-of-concept grant LIQUORICE.
