Towards an optical FPGA – Programmable silicon photonic circuits [pdf]

We have optical chips in operation everywhere for decades, that's not something new. An programmable optical chip has been demonstrated by different research groups, and, for some limited applications, I do believe that we will have it in the marked in 7 to 10 years.

We are also working on different topologies besides the one mentioned in the paper.

When the light propagates in a waveguide as the one used in my circuit, the E and H components of the EM wave are not fully confined to the waveguide, but part of it stays outside the waveguide. If you put two waveguides close to each other and makes the light travel to the first waveguide, part of the EM field of the light will also see the second waveguide.it makes part of the ligh couple to the second waveguide. As the wave travels, more and more light couples to the second waveguide. If you engineer it well, at some point 50% of the light will be confined in each waveguide. At this point you separate both WG and you have a 50:50 coupler.

Their hope is that by building a chip with lots of generally useful elements, many different industries will find it useful because they can just program the chip to utilize the components they need. (Fig. 6) Instead of computing with this chip, industry and researchers will be more interested in making things like optical transceivers. Having one common, reconfigurable chip that everyone uses would mean it could be mass produced at a lower cost, making research and production of silicon photonic systems more accessible.

At least that's what I've gathered from the paper - I'm definitely not an expert here. In theory I guess you could put optically switched gates in there too to make something more like an electrical FPGA - I'm sure that's long-term goal for the authors.

What we do is using interference to make a switch. More specifically, we use an apparatus called Mach-Zehnder interferometer (combined with a phase shifter) to manipulate the light.

What we do with photonics is to improve areas where the electronics is not efficient.

Case and point: interconnections. If you want to move data from point A to point B (being A and B either two different chips in your board our two datacenters), we can do it using electric signals, it works just fine, but at some cost. Electric signals dissipate power when they travel through a conductor, no matter how good the conductor is.

If instead we use optical signals instead of electrical, we have advantage in a number of points, specially power efficiency.

Photonics chips and programable photonics are not a way to substitute conventional electronics, but a replacement/complement for some areas of the field where electronics is not efficient.

Photonics ICs are all around. You can find it in many different applications, but mostly in datacom and telecom.

A very simple example: anywhere you have a optical fiber you also need a photonic IC to, at least, convert the light into electrical signal to interface your electronics (and vice-versa).

What we are doing with photonics is adding more functionality in the optical part (filtering signal, multiplexing, modulation, etc) once, in mostly cases, it's more efficient doing that in the optical domain instead of using electronics.

The point here is that our current implementation has a limited IO capability (our linear operator has 4 inputs and 4 outputs), but increasing the number of IOs leads to a linear increase in the size of the device.