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Photonic Integrated Circuits using Scattered Waveguide elements in an Adaptive, Reconfigurable Mesh.

Periodic Reporting for period 2 - PhotonICSWARM (Photonic Integrated Circuits using Scattered Waveguide elements in an Adaptive, Reconfigurable Mesh.)

Reporting period: 2018-10-01 to 2020-03-31

In PhotonicSWARM, we are building general-purpose photonic chips, where light can be controlled in software to for a diversity of functions, from fiber-optic communications over sensors to radio-frequency filters for 5G networks.
Photonic integrated circuits, or PICs, manipulate light on the surface of a chip through waveguide. Today, PICs are mostly used in optical communication, either in long-haul fiber networks or to connect servers inside a datacenter. But new applications are being developed that use the same technology to create extremely accurate sensors, spectrometers, and processors for high-speed microwave signals. Photonic chips are also one of the key technologies to develop artificial neural networks or optical quantum computers.
Today, photonic chip technology is increasingly making use of the same technology to make electronic circuits: silicon photonic makes it possible to pack thousands of optical building blocks together on a chip, and make these chips in very large volumes.
Of course, this only makes sense if there is a market for large volumes of chips. Today, the need for photonic chips is orders of magnitude lower than electronic chips, and this makes it costly to develop new photonic chips. On top of that, today’s photonic chips are all developed for a particular purpose; they are so-called application-specific photonic integrated circuits (ASPIC). Developing a new ASPIC takes more than a year and is very costly, just like with a new custom electronic chip.
The programmable PICs being developed in PhotonicSWARM will change this. A generic programmable PIC is not designed for a specific function, but can be configured on the fly to using control electronics and software. In this, they resemble programmable electronics, such as microprocessors, digital signal processors (DSP) or field-programmable gate arrays (FPGA). Today, these electronics can be purchased off-the-shelf and programmed directly, shortening the development time from more than a year to weeks. PhotonicSWARM wants to lay the foundations for a similar ecosystem for photonics.
Programmable PICs consists of a mesh of optical waveguides which are coupled together using tunable couplers that can be electrically controlled. Our initial demonstration, the first programmable PIC in silicon photonics, was a simple 4x4 beam coupler that could map 4 inputs onto 4 outputs in any arbitrary linear combination. Such ‘forward-only’ circuits are especially useful in applications that require accelerated matrix operations, such as neural networks and quantum computing. But they are limited when it comes to more advanced functions such as the construction of optical wavelength filters.
Therefore we switched to ‘recirculating’ meshes, where the light can be routed in loops. We studied different connectivity schemes for such meshes, and how accurately they need to be controlled to avoid unwanted effects due to unwanted waveguide paths. To guarantee that the light can be directed anywhere in the mesh, we devised a tolerant circuit for the most critical component in the mesh: the tunable coupler. We then applied a similar technique to compensate the imperfect behaviour of fast silicon modulators. Because all elements on the photonic chip need to be electrically controlled, we developed an electrical layer based on commercial FPGAs that allows us to digitally control a large number of the on-chip heaters which are used to tune the couplers. Our custom-designed heaters in IMEC’s silicon Photonics Platform allow us to use matrix addressing, drastically reducing the number of electrical connections needed to drive over 300 tuners on our second-generation demonstration chip.
This second demonstration chip consists of a hexagonal waveguide mesh of 49 ‘cores’, and is capable of handling up to 16 optical fibers as either inputs or outputs, but it also sports 4 high-speed microwave inputs and 8 microwave outputs. This chip now serves as a platform to develop the higher levels of control circuits and configuration algorithms which we have developed, such as routing multiple light paths simultaneously, or distribution trees.
With the second-generation chip, we intend to show that it is indeed possible to program such a circuit for multiple applications: it can function as a spectrometer or switch, and with its built-in modulators and detectors it can function as an optical transmitter or receiver circuit, or even a microwave signal processor.
In parallel, we are looking to alternative architectures and tuning mechanisms that will allow us to eliminate the heaters we currently use for the tunable couplers and phase shifters. These are very power-hungry. One promising alternative approach is the integration of liquid crystals on the surface of the silicon chip.
Even when we are only starting to use our second-generation chip, we are already planning the design and fabrication of the next generation chip, where we intend to scale up the complexity and look into new circuit architectures. These architectures implemented in silicon photonics, with power-efficient tuners, electronic drivers, control algorithms and programming strategies, form complete technology stack. This way, PhotonicSWARM is laying the foundation for an ecosystem that will enable a future with off-the-shelf multifunctional programmable PICs.