It's a really smart idea to try to leverage the inherent scalability of semiconductor photonics. I think the use of a linear optical resonator to amplify a weak optical nonlinearity is quite genius, and something the relatively small nonlinear photonics community has been trying to do forever. That they showed this kind of 'all-optical-ish' nonlinearity on a relatively mature process in a foundry is nothing to scoff at, and likely one of the biggest results in semiconductor photonics in a while. At the single device level I think it makes so much sense, but what concerns me in general is how well this scales from a few perspectives:
1. resonators and device-to-device variance: in general it's pretty hard to get these resonant effects to line up with each other from a production POV, especially with large arrays. Silicon photonics has come far, but I don't think it has approached the level of uniformity as electronics. They have demonstrated some level of electro-optic tunability, which is the traditional solution, but they still need to leverage that for their nonlinear effects too.
2. area and space: the 'minimum' trace size of these planar photonics circuits is still quite large (~200 nm minimum feature size typically for these waveguides). This is essentially due to a minimum size needed to confine light within a waveguide which depends generally on the waveguide's refractive index and target wavelength. These are currently all integrated on a planar manner, so each channel becomes quite large, especially if now you also need a relatively large ring resonator, which in this case is at least ~100 micrometers or so in diameter
3. the combination of 1 and 2: high device-to-device variation, along with a large planar footprint means that these things are quite expensive and difficult to manufacture, without some kind of miniaturization benefit that you would typically get with electronics (at least not yet). This effect appears to be more than the sum of 1 + 2.
The planar footprint can be relatively easy be mitigated by just using a mirror and sending the next layer back to the same plane a few nanometers higher?
It is a solution to increase density, but this introduces significant integration complexity in addition to just increasing the overall cost. I think if this optical analog computer takes off, this kind of routing will likely be necessary
Very interesting to see how work in this space is progressing. Reminds me of a Nature paper a few years back about using physical systems as neural networks:
Deep physical neural networks trained with backpropagation (2022)
This sounds impressive, but this bit stood out to me:
> This process works by sending a tiny bit of the optical signal to a photodiode that measures how much optical power is there.
It seems that the benefit of the approach in general is to keep compute in optics, because crossing the optical to electrical boundary takes too long. But then in the middle of their described process is a boundary transition.
How is this so different to the CMOS/CCD boundary? Is a photodiode that much quicker to activate that it doesn't matter?
I'm sure you'll get a better answer eventually but yes photodiodes are widely available that have sub-nanosecond response time, and the output could potentially be used in its raw analog form do whatever modulation they are describing.
Edit: Turbo encabulator description from the paper linked at the bottom:
>To realize a programmable coherent optical activation function, we developed a resonant electro-optical nonlinearity (Fig. 1(iii)). This device directs a fraction of the incident optical power ∣b∣2 into a photodiode by programming the phase shift θ in an MZI. The photodiode is electrically connected to a p–n-doped resonant microring modulator, and the resultant photocurrent (or photovoltage) detunes the resonance by either injecting (or deplet-ing) carriers from the waveguide.
... and a couple of notes on the observed latency later in the paper
>We experimentally characterized the computational latency of the NOFU in this mode, finding that the response time for carrier injection was shorter than 100 ps and that 75 μA of photocurrent was sufficient to detune the resonator by a linewidth, corresponding to a static power dissipation of 60 μW.
>As our architecture computes entirely in the optical domain and is integrated onto a single photonic circuit, inference latency is limited only by the optical time of flight through the chip
light speed isn't really all that good when you remember that information in your electrical circuit is traveling as fast (if not faster) than light in the medium in this case. Sounds good in marketing, but the key here is bandwidth, not 'speed'
Layman here. Isn't the big speed penalty in digital electronics the clock speed of the transistors changing state? Not the actual signal going down the wire from gate to gate?
certainly -- not necessarily the raw velocity of light, but the ability to augment other properties of light to increase the bandwidth.
Is it correct then to say that augmenting the other properties of light increases overall information density/capacity of light as a medium? whereas with electricity we only have 2D: amplitude & freq?
Information in an electric circuit travels along that circuit at ~0.9c. The physical light pulses that are running through the optical waveguides are more or less traveling at around ~c/n, where n is the refractive index of the material (in this case silicon, so n ~ 3.5).
the actual optical "packet" of information is traveling slower than the electric "packet". The key here is that the electric packet can a few bits, while the photonic packet in theory has a much larger bandwidth.
It's a really smart idea to try to leverage the inherent scalability of semiconductor photonics. I think the use of a linear optical resonator to amplify a weak optical nonlinearity is quite genius, and something the relatively small nonlinear photonics community has been trying to do forever. That they showed this kind of 'all-optical-ish' nonlinearity on a relatively mature process in a foundry is nothing to scoff at, and likely one of the biggest results in semiconductor photonics in a while. At the single device level I think it makes so much sense, but what concerns me in general is how well this scales from a few perspectives:
1. resonators and device-to-device variance: in general it's pretty hard to get these resonant effects to line up with each other from a production POV, especially with large arrays. Silicon photonics has come far, but I don't think it has approached the level of uniformity as electronics. They have demonstrated some level of electro-optic tunability, which is the traditional solution, but they still need to leverage that for their nonlinear effects too.
2. area and space: the 'minimum' trace size of these planar photonics circuits is still quite large (~200 nm minimum feature size typically for these waveguides). This is essentially due to a minimum size needed to confine light within a waveguide which depends generally on the waveguide's refractive index and target wavelength. These are currently all integrated on a planar manner, so each channel becomes quite large, especially if now you also need a relatively large ring resonator, which in this case is at least ~100 micrometers or so in diameter
3. the combination of 1 and 2: high device-to-device variation, along with a large planar footprint means that these things are quite expensive and difficult to manufacture, without some kind of miniaturization benefit that you would typically get with electronics (at least not yet). This effect appears to be more than the sum of 1 + 2.
The planar footprint can be relatively easy be mitigated by just using a mirror and sending the next layer back to the same plane a few nanometers higher?
It is a solution to increase density, but this introduces significant integration complexity in addition to just increasing the overall cost. I think if this optical analog computer takes off, this kind of routing will likely be necessary
The preprint of the research paper:
https://arxiv.org/abs/2208.01623
Interesting that they don't mention event based sensors at all.
https://www.sony-semicon.com/en/technology/industry/evs.html
Very interesting to see how work in this space is progressing. Reminds me of a Nature paper a few years back about using physical systems as neural networks:
Deep physical neural networks trained with backpropagation (2022)
https://www.nature.com/articles/s41586-021-04223-6
This sounds impressive, but this bit stood out to me:
> This process works by sending a tiny bit of the optical signal to a photodiode that measures how much optical power is there.
It seems that the benefit of the approach in general is to keep compute in optics, because crossing the optical to electrical boundary takes too long. But then in the middle of their described process is a boundary transition.
How is this so different to the CMOS/CCD boundary? Is a photodiode that much quicker to activate that it doesn't matter?
I'm sure you'll get a better answer eventually but yes photodiodes are widely available that have sub-nanosecond response time, and the output could potentially be used in its raw analog form do whatever modulation they are describing.
Edit: Turbo encabulator description from the paper linked at the bottom:
>To realize a programmable coherent optical activation function, we developed a resonant electro-optical nonlinearity (Fig. 1(iii)). This device directs a fraction of the incident optical power ∣b∣2 into a photodiode by programming the phase shift θ in an MZI. The photodiode is electrically connected to a p–n-doped resonant microring modulator, and the resultant photocurrent (or photovoltage) detunes the resonance by either injecting (or deplet-ing) carriers from the waveguide.
... and a couple of notes on the observed latency later in the paper
>We experimentally characterized the computational latency of the NOFU in this mode, finding that the response time for carrier injection was shorter than 100 ps and that 75 μA of photocurrent was sufficient to detune the resonator by a linewidth, corresponding to a static power dissipation of 60 μW.
>As our architecture computes entirely in the optical domain and is integrated onto a single photonic circuit, inference latency is limited only by the optical time of flight through the chip
Hmm... NNs will be at light speed..?
Are we still in the "glass era"?
light speed isn't really all that good when you remember that information in your electrical circuit is traveling as fast (if not faster) than light in the medium in this case. Sounds good in marketing, but the key here is bandwidth, not 'speed'
Layman here. Isn't the big speed penalty in digital electronics the clock speed of the transistors changing state? Not the actual signal going down the wire from gate to gate?
Also keep in mind that photonics will generate practically zero heat and use less power overall, which will be great news for data centers.
certainly -- not necessarily the raw velocity of light, but the ability to augment other properties of light to increase the bandwidth.
Is it correct then to say that augmenting the other properties of light increases overall information density/capacity of light as a medium? whereas with electricity we only have 2D: amplitude & freq?
definitely
faster?
Information in an electric circuit travels along that circuit at ~0.9c. The physical light pulses that are running through the optical waveguides are more or less traveling at around ~c/n, where n is the refractive index of the material (in this case silicon, so n ~ 3.5).
the actual optical "packet" of information is traveling slower than the electric "packet". The key here is that the electric packet can a few bits, while the photonic packet in theory has a much larger bandwidth.