3 ms·

I'm curious about the relative latency of photonics vs. electronics, when connecting e.g. two chiplets in the same package. My (possibly wrong) understanding i

I'm curious about the relative latency of photonics vs. electronics, when connecting e.g. two chiplets in the same package. My (possibly wrong) understanding is that the E-O and O-E conversion adds a non-trivial amount of latency in a situation like that.

adgjlsfhk1 6y ago
Has there been much research on making cpus that are fully light based? It seems like if that were possible, it would make a lot of this much easier.
- moistbar 6y ago
  I remember hearing about one back in the early 2000's, but I'm unable to find the article I read way back then. I recall it making some extremely lofty claims regarding clock speed, somewhere on the order of 300 GHz IIRC, but it never saw the inside of a production machine as far as I know.
- amelius 6y ago
  I guess the main problem is that Maxwell's equations are linear, and thus you need some optical non-linear effect to make a logic gate. And you need some way to prevent attenuation of your signals.
- N1H1L 6y ago
  I think one of the problems would be miniaturization. Visible light is hundreds of nanometers, and X-ray photonics are still in it's infancy
  - entropicdrifter 6y ago
    You aren't limited by the wavelength in terms of pure transmission and reception. Bass frequency sound waves are multiple feet long, yet earbuds transfer those frequencies just fine.
    - derefr 6y ago
      A wave of sound is a wave of pressure — i.e. density. Sound waves are one-dimensional. You can have a material one atom thick in a vacuum, and it’ll transmit sound just fine, as long as that material has some elasticity to it. Light waves, on the other hand, are two- [linearly-polarized] or three- [circularly-polarized] -dimensional. They’re heading in one direction, and also wiggling side-to-side as they go. If light didn’t have a “thickness” — that is, if light could “sneak through” gaps narrower than its “diameter” — then 1. “Optical” fibre would be capable of acting as a waveguide for all frequencies, not just optical ones; and 2. the Faraday cage around a microwave wouldn’t prevent the microwaves from escaping. (You might think we’d also get universal antennas out of that, but no, antennas are a quantum thing—it’s not about squeezing through a gap, but about exciting an AC electric field in the material, so letting the whole wavelength hit the antenna actually causes the induced field to destructively interfere with itself. Think of an antenna as absorbing the wave “head-on”, rather than “side-on.” Your antenna needs to catch half the wave — node to anode to node — to get the highest-power signal out. But you don’t miss the wave entirely if you catch less. You just get a less powerful signal — it’s still possible to do useful work [like power an optical receiver, or decode an AM radio signal] with just a 1/Nth slice of each cycle of the wave.)
      - entropicdrifter 6y ago
        TIL, thanks!
- CoastalCoder 6y ago
  Lightmatter [0] is doing something along those lines. But IIUC they're only attempting optical computing for certain mathematical ops (FMA and/or convolution?) [0] https://lightmatter.co/ https://lightmatter.co/
  - mmmBacon 6y ago
    Matrix multiplication. They use a cascaded Mach-Zhender MEMs array as their systolic matrix multiplier. These types of schemes are limited to the amount of loss that they have. Additionally since these are analog components they have a certain physical size which is difficult to reduce. In my view this type of architecture does not scale very well.
- magicalhippo 6y ago
  AFAIK yes. Of course you need some actual non-light hardware to guide the light and such though. https://en.wikipedia.org/wiki/Optical_computing https://en.wikipedia.org/wiki/Optical_computing https://www.eetimes.com/how-does-optical-computing-work/ https://www.eetimes.com/how-does-optical-computing-work/
- musingsole 6y ago
  Most projects I've read focus on specific components (mainly ALUs). I'm aware of a photonics flip-flop equivalent design, but I'm unaware of equivalents for dense memory. I imagine DRAM would remain mundane electronics even if the CPU were fully migrated to the tech.
jl2718 6y ago
You can calculate it as the RC constant of the PN junction. The issue is that the photon wavelength makes a fairly large area. This may be solvable with plasmonics.
mmmBacon 6y ago
The latency at this scale of the device is largely irrelevant as it would be dominated by FEC. The PN junctions themselves have BW of about 25GHz, well above anything compute is doing.

adgjlsfhk1 6y ago

Has there been much research on making cpus that are fully light based? It seems like if that were possible, it would make a lot of this much easier.

moistbar 6y ago
I remember hearing about one back in the early 2000's, but I'm unable to find the article I read way back then. I recall it making some extremely lofty claims regarding clock speed, somewhere on the order of 300 GHz IIRC, but it never saw the inside of a production machine as far as I know.
amelius 6y ago
I guess the main problem is that Maxwell's equations are linear, and thus you need some optical non-linear effect to make a logic gate. And you need some way to prevent attenuation of your signals.
N1H1L 6y ago
I think one of the problems would be miniaturization. Visible light is hundreds of nanometers, and X-ray photonics are still in it's infancy
- entropicdrifter 6y ago
  You aren't limited by the wavelength in terms of pure transmission and reception. Bass frequency sound waves are multiple feet long, yet earbuds transfer those frequencies just fine.
  - derefr 6y ago
    A wave of sound is a wave of pressure — i.e. density. Sound waves are one-dimensional. You can have a material one atom thick in a vacuum, and it’ll transmit sound just fine, as long as that material has some elasticity to it. Light waves, on the other hand, are two- [linearly-polarized] or three- [circularly-polarized] -dimensional. They’re heading in one direction, and also wiggling side-to-side as they go. If light didn’t have a “thickness” — that is, if light could “sneak through” gaps narrower than its “diameter” — then 1. “Optical” fibre would be capable of acting as a waveguide for all frequencies, not just optical ones; and 2. the Faraday cage around a microwave wouldn’t prevent the microwaves from escaping. (You might think we’d also get universal antennas out of that, but no, antennas are a quantum thing—it’s not about squeezing through a gap, but about exciting an AC electric field in the material, so letting the whole wavelength hit the antenna actually causes the induced field to destructively interfere with itself. Think of an antenna as absorbing the wave “head-on”, rather than “side-on.” Your antenna needs to catch half the wave — node to anode to node — to get the highest-power signal out. But you don’t miss the wave entirely if you catch less. You just get a less powerful signal — it’s still possible to do useful work [like power an optical receiver, or decode an AM radio signal] with just a 1/Nth slice of each cycle of the wave.)
    - entropicdrifter 6y ago
      TIL, thanks!
CoastalCoder 6y ago
Lightmatter [0] is doing something along those lines. But IIUC they're only attempting optical computing for certain mathematical ops (FMA and/or convolution?) [0] https://lightmatter.co/ https://lightmatter.co/
- mmmBacon 6y ago
  Matrix multiplication. They use a cascaded Mach-Zhender MEMs array as their systolic matrix multiplier. These types of schemes are limited to the amount of loss that they have. Additionally since these are analog components they have a certain physical size which is difficult to reduce. In my view this type of architecture does not scale very well.
magicalhippo 6y ago
AFAIK yes. Of course you need some actual non-light hardware to guide the light and such though. https://en.wikipedia.org/wiki/Optical_computing https://en.wikipedia.org/wiki/Optical_computing https://www.eetimes.com/how-does-optical-computing-work/ https://www.eetimes.com/how-does-optical-computing-work/
musingsole 6y ago
Most projects I've read focus on specific components (mainly ALUs). I'm aware of a photonics flip-flop equivalent design, but I'm unaware of equivalents for dense memory. I imagine DRAM would remain mundane electronics even if the CPU were fully migrated to the tech.

jl2718 6y ago

You can calculate it as the RC constant of the PN junction. The issue is that the photon wavelength makes a fairly large area. This may be solvable with plasmonics.

mmmBacon 6y ago

The latency at this scale of the device is largely irrelevant as it would be dominated by FEC. The PN junctions themselves have BW of about 25GHz, well above anything compute is doing.