4 ms·

So I guess what this makes me wonder is: Why are we using electrical signals to connect the data lanes between components and computers these days, rather than

So I guess what this makes me wonder is: Why are we using electrical signals to connect the data lanes between components and computers these days, rather than moving everything to optical for data movement (obviously power would stay electrical, but that's already on separate lines)? I assume there's an element of cost, and once the photons get where they're going they have to be turned back into electrical signals to actually be used until such time as we get around to getting pure light based computers working (someday but not yet...), but that must not overwhelm the advantages or we wouldn't be looking at this being developed.

cma 2y ago
Since thunderbolt is related to PCIe, there's this that goes into copper vs optical there: https://en.wikipedia.org/wiki/Thunderbolt_(interface)#Copper_vs._optical https://en.wikipedia.org/wiki/Thunderbolt_(interface)#Copper...
- watersb 2y ago
  Intel's product name foe Thunderbolt was initially was "Light Peak".
  - throwaway48476 2y ago
    I believe it was originally supposed to be optical.
asadhaider 2y ago
So we're moving towards crystals from Stargate? Neat
AceJohnny2 2y ago
> I assume there's an element of cost, and once the photons get where they're going they have to be turned back into electrical signals to actually be used until such time as we get around to getting pure light based computers working (someday but not yet...) You got it. We can't make optical transceivers as good as electrical ones. Not as small or power-efficient. They require significantly different fabrication processes, and we don't know how to fab them into the same chip as electrical ones. I mean: you can either have photonics, or performant digital (or analog) electronics. We've gotten really, really good at making small electronics, per the latest tech coming out of Intel & TSMC. We are... not that good at making photonics.
- hughesjj 2y ago
  > Not as small or power-efficient. I wonder what the latency for switching medium is these days too (for the super small transceivers). To my understanding optical is better for attenuation than electric (less noise, and thus easier to shove more frequencies and higher frequencies on the same pipe), and can be faster (both medium dependent, neither yet approaching the upper bound of c). I'm imaging the latency incurred by the transceiver is eventually offset from the gains in the signal path (for signal paths relevant to circuit boards and ICs)
  - cycomanic 2y ago
    Depending how you do the actual modulation, optical modulation does not add any significant latency (there can be no processing involved and the extra transmission length you'd need for the modulation (i.e. Electroopotic conversion, rf amplifiers...) is negligible. The big issue is really 1. Photonic waveguides are much larger than electronic ones (due to the wavelength) 2. You loose dynamic range and in EO conversion (shot noise is significant at optical frequencies) 3. Co integration of optics and photonics components is nontrivial due to the different materials and processes. 4. Power efficiency of EO conversion is also not that great. Where photonics shines is for transmission of high frequencies (i.e. a lot of data) over long distances and being immune to EM interference. So there is certainly a tradeoff for at what transmission distances to go optical and as data rates keep going up the tradeoff length has become shorter and shorter. Intel, Nvidia, AMD et al. All do research into optical interconnect.
    - aborsy 2y ago
      Photonic wavelengths are shorter than electronic wavelengths. Why are photonics waveguides bigger then?
      - murkt 2y ago
        What’s an electronic wavelength in this context? What’s its size? Photonic ones I assume are in near-IR, on the order of a micrometer.
lazide 2y ago
Changing from light to electricity (and vice versa) is relatively slow, expensive, and cumbersome. Additionally, we don’t have a decent way of transferring significant power over fiber optics. So since everything has to have copper power fed to it anyway, unless there is some compelling reason (like distance) to make optical/fibers disadvantages worth it, copper only is usually simpler and better. At least for now.
- hinkley 2y ago
  If memory serves the original plan for Light Peak was power wires and shielding braided around a fiber optic core.
  - lazide 2y ago
    Hence why we ended up with USB-C cables. Less fragile, similar data rate, simpler BOM, less sensitive to dirt and debris, plenty of power.
    - hinkley 2y ago
      What's the minimum bend radius for plastic fiber these days?
      - pezezin 2y ago
        Around 10 mm for OS2 cables.
      - lazide 2y ago
        Larger than ‘getting kinked in a desk drawer’. Though smaller than when the market made the decision of course. You still need the copper wires to do power delivery, so either you end up with an even thicker cable, or multi-purpose the copper cables for signaling too.
Aurornis 2y ago
> I assume there's an element of cost, This assumption is very correct. Optical interconnects are extraordinarily expensive relative to copper. We have the art of manufacturing copper PCBs and connectors mastered. Putting optical interconnects into a system requires that the signal go through transceivers at either end as well as external optical cables, which are not integrated into the PCB. It’s extra components and complexity everywhere. The reason optical interconnects are being explored here is that next gen PCIe is so extremely fast that the signals cannot travel very far in PCBs without significant losses. PCBs built for these speeds require special, expensive materials on the layers with those signals. They might require retimer chips to amplify the signal past a certain distance. These limitations may not apply to consumer motherboards with a single GPU very near to the CPU, but datacenter motherboards might need to connect many GPUs across a large chassis. The distances involved could require multiple retimer chips along the way and very expensive PCBs. Going to optical interconnects could introduce much more flexibility into where the GPUs or other add in cards are located.
- chx 2y ago
  As a side note: while the O in OCuLink does stand for optical that variant is not in use. Every OCuLink connector and cable are just ordinary copper interconnection. Meanwhile Samtec has PCIe active optical cables, they have had them since 2012, it's a very niche application currently.
ryukoposting 2y ago
We're a lot closer to the limits of copper than I realized. Apparently motherboard designers have to make the length of clock/data traces for DDR5 memory as close to equal as possible, otherwise the entire bus just doesn't work right (maybe this isn't news to other folks).
- bangaladore 2y ago
  You have to do that for all high-speed multipin interfaces. Generally, once you get past roughly 100 MHZ, it is good practice to match the lengths as well as possible, even though it probably doesn't matter until a couple of gigahertz (it depends on many factors, but generally, signals move at 15 cm per nanosecond on PCBs).
hinkley 2y ago
I know when silicon photonics were brand new, they had a big limitation because they only had designs that fired out of the plane of the chip, not across it. That limits you to interconnects and not cross-chip signaling. And since for a CPU you need a heat sink on top that means you have to fire down, toward the motherboard. Also it turns out the speed of light in glass is not that impressive. So encoding and decoding at the ends eats up the speed advantage. That’s my impression as to why a lot of high profile articles on optical logic came out shortly thereafter. What if we just keep it as light for longer?

cma 2y ago

Since thunderbolt is related to PCIe, there's this that goes into copper vs optical there: https://en.wikipedia.org/wiki/Thunderbolt_(interface)#Copper_vs._optical https://en.wikipedia.org/wiki/Thunderbolt_(interface)#Copper...

watersb 2y ago
Intel's product name foe Thunderbolt was initially was "Light Peak".
- throwaway48476 2y ago
  I believe it was originally supposed to be optical.

asadhaider 2y ago

So we're moving towards crystals from Stargate? Neat

AceJohnny2 2y ago

> I assume there's an element of cost, and once the photons get where they're going they have to be turned back into electrical signals to actually be used until such time as we get around to getting pure light based computers working (someday but not yet...) You got it. We can't make optical transceivers as good as electrical ones. Not as small or power-efficient. They require significantly different fabrication processes, and we don't know how to fab them into the same chip as electrical ones. I mean: you can either have photonics, or performant digital (or analog) electronics. We've gotten really, really good at making small electronics, per the latest tech coming out of Intel & TSMC. We are... not that good at making photonics.

hughesjj 2y ago
> Not as small or power-efficient. I wonder what the latency for switching medium is these days too (for the super small transceivers). To my understanding optical is better for attenuation than electric (less noise, and thus easier to shove more frequencies and higher frequencies on the same pipe), and can be faster (both medium dependent, neither yet approaching the upper bound of c). I'm imaging the latency incurred by the transceiver is eventually offset from the gains in the signal path (for signal paths relevant to circuit boards and ICs)
- cycomanic 2y ago
  Depending how you do the actual modulation, optical modulation does not add any significant latency (there can be no processing involved and the extra transmission length you'd need for the modulation (i.e. Electroopotic conversion, rf amplifiers...) is negligible. The big issue is really 1. Photonic waveguides are much larger than electronic ones (due to the wavelength) 2. You loose dynamic range and in EO conversion (shot noise is significant at optical frequencies) 3. Co integration of optics and photonics components is nontrivial due to the different materials and processes. 4. Power efficiency of EO conversion is also not that great. Where photonics shines is for transmission of high frequencies (i.e. a lot of data) over long distances and being immune to EM interference. So there is certainly a tradeoff for at what transmission distances to go optical and as data rates keep going up the tradeoff length has become shorter and shorter. Intel, Nvidia, AMD et al. All do research into optical interconnect.
  - aborsy 2y ago
    Photonic wavelengths are shorter than electronic wavelengths. Why are photonics waveguides bigger then?
    - murkt 2y ago
      What’s an electronic wavelength in this context? What’s its size? Photonic ones I assume are in near-IR, on the order of a micrometer.

lazide 2y ago

Changing from light to electricity (and vice versa) is relatively slow, expensive, and cumbersome. Additionally, we don’t have a decent way of transferring significant power over fiber optics. So since everything has to have copper power fed to it anyway, unless there is some compelling reason (like distance) to make optical/fibers disadvantages worth it, copper only is usually simpler and better. At least for now.

hinkley 2y ago
If memory serves the original plan for Light Peak was power wires and shielding braided around a fiber optic core.
- lazide 2y ago
  Hence why we ended up with USB-C cables. Less fragile, similar data rate, simpler BOM, less sensitive to dirt and debris, plenty of power.
  - hinkley 2y ago
    What's the minimum bend radius for plastic fiber these days?
    - pezezin 2y ago
      Around 10 mm for OS2 cables.
    - lazide 2y ago
      Larger than ‘getting kinked in a desk drawer’. Though smaller than when the market made the decision of course. You still need the copper wires to do power delivery, so either you end up with an even thicker cable, or multi-purpose the copper cables for signaling too.

Aurornis 2y ago

> I assume there's an element of cost, This assumption is very correct. Optical interconnects are extraordinarily expensive relative to copper. We have the art of manufacturing copper PCBs and connectors mastered. Putting optical interconnects into a system requires that the signal go through transceivers at either end as well as external optical cables, which are not integrated into the PCB. It’s extra components and complexity everywhere. The reason optical interconnects are being explored here is that next gen PCIe is so extremely fast that the signals cannot travel very far in PCBs without significant losses. PCBs built for these speeds require special, expensive materials on the layers with those signals. They might require retimer chips to amplify the signal past a certain distance. These limitations may not apply to consumer motherboards with a single GPU very near to the CPU, but datacenter motherboards might need to connect many GPUs across a large chassis. The distances involved could require multiple retimer chips along the way and very expensive PCBs. Going to optical interconnects could introduce much more flexibility into where the GPUs or other add in cards are located.

chx 2y ago
As a side note: while the O in OCuLink does stand for optical that variant is not in use. Every OCuLink connector and cable are just ordinary copper interconnection. Meanwhile Samtec has PCIe active optical cables, they have had them since 2012, it's a very niche application currently.

ryukoposting 2y ago

We're a lot closer to the limits of copper than I realized. Apparently motherboard designers have to make the length of clock/data traces for DDR5 memory as close to equal as possible, otherwise the entire bus just doesn't work right (maybe this isn't news to other folks).

bangaladore 2y ago
You have to do that for all high-speed multipin interfaces. Generally, once you get past roughly 100 MHZ, it is good practice to match the lengths as well as possible, even though it probably doesn't matter until a couple of gigahertz (it depends on many factors, but generally, signals move at 15 cm per nanosecond on PCBs).

hinkley 2y ago

I know when silicon photonics were brand new, they had a big limitation because they only had designs that fired out of the plane of the chip, not across it. That limits you to interconnects and not cross-chip signaling. And since for a CPU you need a heat sink on top that means you have to fire down, toward the motherboard. Also it turns out the speed of light in glass is not that impressive. So encoding and decoding at the ends eats up the speed advantage. That’s my impression as to why a lot of high profile articles on optical logic came out shortly thereafter. What if we just keep it as light for longer?