3 ms·

I guess they are doing direct modulated IMDD for each link so the DSP burden is not related to the coherence of diodes? Also indeed very short reach in the arti

I guess they are doing direct modulated IMDD for each link so the DSP burden is not related to the coherence of diodes? Also indeed very short reach in the article.

cycomanic 1y ago
The problem with both leds and imaging fibres is that modal dispersion is massive and completely destroys your signal after only a few meters of propagation. So unless you do MMSE (which I assume would be cost prohibitive), you really can only go a few meters. IMDD doesn't really make a difference here.
- adgjlsfhk1 1y ago
  I think this is intended for short distances (e.g. a few cm). cpu to GPU and network card to network card still will be lasers, the question is whether you can do core to core or CPU to ram with optics
  - cycomanic 1y ago
    But why are they talking about multicore fibres then? I would have expected ribbons. You might be right though.
    - topspin 1y ago
      > I would have expected ribbons. The cable is just 2D parallel optical bus. With a bundle like this, you can wrap it with a nice, thick PVC (or whatever) jacket and employ a small, square connector that matches the physical scheme of the 2D planar microled array. It's a brute force, simple minded approach enabled by high speed, low cost microled arrays. Pretty cool I think. The ribbon concept could be applicable to PCBs though.
      - cycomanic 1y ago
        You might be right and they are talking about fibre bundles, but that that's something different to a multicore fibre (and much larger as well, which could pose significant problems especially if we are talking cm links). What isn't addressed is that leds are quite spatially incoherent and beam divergence is strong, so the fibres they must use are pretty large, coupling via just a connector might not be easy especially if we want to avoid crosstalk. What I'm getting at is, that I don't see any advantage over vcsel arrays. I'm not convinced that the price point is that different.
        topspin 1y ago
        > You might be right and they are talking about fibre bundles The caption of the image of the cable and connector reads: "CMOS ASIC with microLEDs sending data with blue light into a fiberbundle." So yes, fibre bundles. > I don't see any advantage over vcsel arrays They claim the following advantages: 1. Low energy use 2. Low "computational overhead" 3. Scalability All of these at least pass the smell test. LEDs are indeed quite efficient relative to lasers. They cite about an order of magnitude "pJ/bit" advantage for the system over laser based optics, and I presume they're privy to vcsels. When you're trying to wheedle nuclear reactor restarts to run your enormous AI clusters, saving power is nice. The system has a parallel "conductor" design that likely employs high speed parallel CMOS latches, so the "computational overhead" claim could make sense: all you're doing is latching bits to/from PCB traces or IC pins so all the SerDes and multiplexing cost is gone. They claim that it can easily be scaled to more pixels/lines. Sure, I guess: low power makes that easier. There you are. All pretty simple. I think there is use case for this outside data centers. We're at the point where copper transmission lines are a real problem for consumers. Fiber can solve the signal integrity problem for such use cases, however--despite several famous runs at it (Thunderbolt, Firewire)--the cost has always precluded widespread adoption outside niche, professional, or high-end applications. Maybe LED based optics can make fiber cost competitive with copper for such applications: one imagines a very small, very low power microLED based transceiver costing only slightly more than a USB connector on each end of such a cable with maybe 4-8 parallel fibers. Just spit-balling here
dtgriscom 1y ago
[flagged]

I guess they are doing direct modulated IMDD for each link so the DSP burden is not related to the coherence of diodes? Also indeed very short reach in the article.

cycomanic 1y ago

The problem with both leds and imaging fibres is that modal dispersion is massive and completely destroys your signal after only a few meters of propagation. So unless you do MMSE (which I assume would be cost prohibitive), you really can only go a few meters. IMDD doesn't really make a difference here.

adgjlsfhk1 1y ago
I think this is intended for short distances (e.g. a few cm). cpu to GPU and network card to network card still will be lasers, the question is whether you can do core to core or CPU to ram with optics
- cycomanic 1y ago
  But why are they talking about multicore fibres then? I would have expected ribbons. You might be right though.
  - topspin 1y ago
    > I would have expected ribbons. The cable is just 2D parallel optical bus. With a bundle like this, you can wrap it with a nice, thick PVC (or whatever) jacket and employ a small, square connector that matches the physical scheme of the 2D planar microled array. It's a brute force, simple minded approach enabled by high speed, low cost microled arrays. Pretty cool I think. The ribbon concept could be applicable to PCBs though.
    - cycomanic 1y ago
      You might be right and they are talking about fibre bundles, but that that's something different to a multicore fibre (and much larger as well, which could pose significant problems especially if we are talking cm links). What isn't addressed is that leds are quite spatially incoherent and beam divergence is strong, so the fibres they must use are pretty large, coupling via just a connector might not be easy especially if we want to avoid crosstalk. What I'm getting at is, that I don't see any advantage over vcsel arrays. I'm not convinced that the price point is that different.
      - topspin 1y ago
        > You might be right and they are talking about fibre bundles The caption of the image of the cable and connector reads: "CMOS ASIC with microLEDs sending data with blue light into a fiberbundle." So yes, fibre bundles. > I don't see any advantage over vcsel arrays They claim the following advantages: 1. Low energy use 2. Low "computational overhead" 3. Scalability All of these at least pass the smell test. LEDs are indeed quite efficient relative to lasers. They cite about an order of magnitude "pJ/bit" advantage for the system over laser based optics, and I presume they're privy to vcsels. When you're trying to wheedle nuclear reactor restarts to run your enormous AI clusters, saving power is nice. The system has a parallel "conductor" design that likely employs high speed parallel CMOS latches, so the "computational overhead" claim could make sense: all you're doing is latching bits to/from PCB traces or IC pins so all the SerDes and multiplexing cost is gone. They claim that it can easily be scaled to more pixels/lines. Sure, I guess: low power makes that easier. There you are. All pretty simple. I think there is use case for this outside data centers. We're at the point where copper transmission lines are a real problem for consumers. Fiber can solve the signal integrity problem for such use cases, however--despite several famous runs at it (Thunderbolt, Firewire)--the cost has always precluded widespread adoption outside niche, professional, or high-end applications. Maybe LED based optics can make fiber cost competitive with copper for such applications: one imagines a very small, very low power microLED based transceiver costing only slightly more than a USB connector on each end of such a cable with maybe 4-8 parallel fibers. Just spit-balling here

dtgriscom 1y ago

[flagged]