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That article is really low on details and mixes up a lot of things. It compares microleds to traditional WDM fiber transmission systems with edge emitting DFB l

That article is really low on details and mixes up a lot of things. It compares microleds to traditional WDM fiber transmission systems with edge emitting DFB lasers and ECLs, but in datacentre interconnects there's plenty of optical links already and they use VCSELs (vertical cavity surface emitting lasers), which are much cheaper to manufacture. People also have been putting these into arrays and coupling to multi-core fiber. The difficulty here is almost always packaging, i.e. coupling the laser. I'm not sure why microleds would be better. Also transmitting 10 Gb/s with a led seems challenging. The bandwidth of an incoherent led is large, so are they doing significant DSP (which costs money and energy and introduces latency) or are they restricting themselves to very short (10s of m) links?

tehjoker 1y ago
short links it’s in the article
- cycomanic 1y ago
  Ah I missed the 10m reference there. I'm not sure it makes more sense though. Typical intra-datacenter connections are 10s-100s of meters and use VCSELs, so introducing microleds just for the very short links instead of just parallelising the VCSEL connections (which is being done already)? If they could actually replace the VCSEL I would sort of see the point.
  - jauntywundrkind 1y ago
    There's been a constant drum-beat that even intra-rack is trying to make its way to optical as fast as it can, that copper is more and more complex and expensive to scale faster. If we have a relatively affordable short range optical system that doesn't require heavy computational work to do, that sounds like a godsend, like a way to increase bits per joule while reducing expensive cabling cost. Sure yes, optical might use expensive longer range optical today! But using that framing to assess new technologies & what help the could be may be folly.
qwezxcrty 1y ago
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.
rajnathani 1y ago
The article is about chip interconnects. Think like replacing PCIe, NVLink, or HBM/DDR RAM buses, with optical communication.
BardiaPezeshki 1y ago
In datacenters people use optics for longer distances (10m to 2km). Within a rack it is almost always copper. The reason is that for short distances lasers are too expensive, unreliable, and consume too much power. We think microLED based links might replace copper at short distances (sub 10m). MicroLEDs into relatively thick fiber cores (50um) are much easier to package than standard single mode laser based optics. on the distance - exactly right. The real bottleneck now in AI clusters is the interconnect within a rack or sub 10m. So that is the market we are addressing. On your second point - exactly! Normally people think LEDs are slow and suck. That is the real innovation. At Avicena, we've figured out how to make LEDs blink on and off at 10Gb/s. This is really surprising and amazing! So with simple on-off modulation, there is no DSP or excess energy use. The article says TSMC is developing arrays of detectors, based on their camera process, that also receive signals at 10Gb/s. Turns out this is pretty easy for a camera with a small number of pixels (~1000). We use blue light, which is easily absorbed in silicon. BTW, feel free to reach out to Avicena, and happy to answer questions.
- throwaway48476 1y ago
  So, toslink?
- LargoLasskhyfv 1y ago
  So has Gb/s a new meaning now? Giga blinks per second?
- mmmBacon 1y ago
  It’s not correct to say that lasers are unreliable. Last year more than 20M transceivers shipped. Your statement is not at all supported by real field failure data. The reliability of micro LEDs. and specifically GaN based micro LEDs is however an open question. In the absence of any dislocation failure mechanisms it will depend on the current density and thermal dissipation. And just like any other material, it will have to survive in a non-hermetic environment and in the presence of corrosive gasses (an issue in data centers). To get the 10G, it’s probably kind of like a VCSEL without the grating and so current density is probably high. How well you’re able to heat sink it is going to determine how reliable it will be. Overall I like the idea. It looks like the beachfront could work. I’d spend more time talking about and how the electrical connection works and what kind of interface to a chip would be needed. I’d also be careful before throwing shade on laser reliability because it could backfire on you (for all reasons above).
adrian_b 1y ago
The article mentions lengths of up to 10 m, so this technology is restricted to links inside a cabinet or between closely located cabinets. The claimed advantage is a very high aggregate throughput and much less energy per bit than with either copper links or traditional laser-based optical links. For greater distances, lasers cannot be replaced by anything else.

tehjoker 1y ago

short links it’s in the article

cycomanic 1y ago
Ah I missed the 10m reference there. I'm not sure it makes more sense though. Typical intra-datacenter connections are 10s-100s of meters and use VCSELs, so introducing microleds just for the very short links instead of just parallelising the VCSEL connections (which is being done already)? If they could actually replace the VCSEL I would sort of see the point.
- jauntywundrkind 1y ago
  There's been a constant drum-beat that even intra-rack is trying to make its way to optical as fast as it can, that copper is more and more complex and expensive to scale faster. If we have a relatively affordable short range optical system that doesn't require heavy computational work to do, that sounds like a godsend, like a way to increase bits per joule while reducing expensive cabling cost. Sure yes, optical might use expensive longer range optical today! But using that framing to assess new technologies & what help the could be may be folly.

qwezxcrty 1y ago

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.

rajnathani 1y ago

The article is about chip interconnects. Think like replacing PCIe, NVLink, or HBM/DDR RAM buses, with optical communication.

BardiaPezeshki 1y ago

In datacenters people use optics for longer distances (10m to 2km). Within a rack it is almost always copper. The reason is that for short distances lasers are too expensive, unreliable, and consume too much power. We think microLED based links might replace copper at short distances (sub 10m). MicroLEDs into relatively thick fiber cores (50um) are much easier to package than standard single mode laser based optics. on the distance - exactly right. The real bottleneck now in AI clusters is the interconnect within a rack or sub 10m. So that is the market we are addressing. On your second point - exactly! Normally people think LEDs are slow and suck. That is the real innovation. At Avicena, we've figured out how to make LEDs blink on and off at 10Gb/s. This is really surprising and amazing! So with simple on-off modulation, there is no DSP or excess energy use. The article says TSMC is developing arrays of detectors, based on their camera process, that also receive signals at 10Gb/s. Turns out this is pretty easy for a camera with a small number of pixels (~1000). We use blue light, which is easily absorbed in silicon. BTW, feel free to reach out to Avicena, and happy to answer questions.

throwaway48476 1y ago
So, toslink?
LargoLasskhyfv 1y ago
So has Gb/s a new meaning now? Giga blinks per second?
mmmBacon 1y ago
It’s not correct to say that lasers are unreliable. Last year more than 20M transceivers shipped. Your statement is not at all supported by real field failure data. The reliability of micro LEDs. and specifically GaN based micro LEDs is however an open question. In the absence of any dislocation failure mechanisms it will depend on the current density and thermal dissipation. And just like any other material, it will have to survive in a non-hermetic environment and in the presence of corrosive gasses (an issue in data centers). To get the 10G, it’s probably kind of like a VCSEL without the grating and so current density is probably high. How well you’re able to heat sink it is going to determine how reliable it will be. Overall I like the idea. It looks like the beachfront could work. I’d spend more time talking about and how the electrical connection works and what kind of interface to a chip would be needed. I’d also be careful before throwing shade on laser reliability because it could backfire on you (for all reasons above).

adrian_b 1y ago

The article mentions lengths of up to 10 m, so this technology is restricted to links inside a cabinet or between closely located cabinets. The claimed advantage is a very high aggregate throughput and much less energy per bit than with either copper links or traditional laser-based optical links. For greater distances, lasers cannot be replaced by anything else.