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TSMC bets on unorthodox optical tech

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Re: TSMC bets on unorthodox optical tech

#91

Post by Salgat »

wyager wrote: post #49
Liftyee wrote: post #2

As I understand it (from designing high-speed electronics), the major limitations to data/clock rates in copper are signal integrity issues. Unwanted electromagnetic interactions all degrade your signal. Optics is definitely a way around this, but I wonder if/when it will ever hit similar limits.

We already regularly run into optical nonlinearity issues in submarine cables. The instantaneous EM fields generated in high bandwidth fiber are sufficiently strong to cause nonlinear interactions with the fiber medium that we have to correct for.

I don't believe this is a factor for the distances that inter-chip transmission has. From what I can find, this is at most an issue for communication spanning tens to hundreds of meters in a datacenter.

Re: TSMC bets on unorthodox optical tech

#92

Post by nsteel »

topspin wrote: post #89
nsteel wrote: post #78

Earlier quoted context omitted.

I understand that CPO reliability concerns are specifically with the laser drivers. It's very expensive to replace your whole chip when one fails. Even if the cables are a concern (I've no idea), having more reliable drivers would still be preferable to less reliable cables, given how much cheaper/easier replacing cables would be (up to a point, of course).

> I understand that CPO reliability concerns are specifically with the laser drivers. Yes. I've replaced my share of dead transceivers, and I suspect the laser drivers were the failure mode of most of them. That doesn't fill in the blank for me though: how reliable are high speed, dense microLEDs?

And are they going to work out any better than Linear Drive Optics, the more obvious alternative?

Re: TSMC bets on unorthodox optical tech

#93

Post by ziofill »

mmmBacon wrote: post #87
sunray2 wrote: post #81

Somewhat related: there's a relatively big push for optical interconnects and integrated optics in quantum computing. Maybe this article yields insight onto what may happen in future. With quantum computing, one is forced to use lasers. Basically, we can't transmit quantum information with the classical light from LEDs (handwaving-ly: LEDs emit a distribution of possible photon numbers, not single photons, so you los…

Quantum computing is still a technology of the future. When we are still talking about 12 qubits as a breakthrough, there’s a long way to go. Optical interconnects are the least of quantum computing’s problems. However, it’s not correct to say lasers are unreliable. It’s fundamentally false and it’s not supported by field data from today’s pluggable modules. 10’s of millions of lasers are deployed in data centers tod…

Actually optical interconnects are the biggest of (photonic) quantum computing problems. If we had good enough optical interconnects (i.e. with low enough optical loss) we would already have a fault-tolerant quantum computer. See https://www.nature.com/articles/s41586-024-08406-9 (also note that Aurora produces 12 physical qubit modes at each clock cycle)

Re: TSMC bets on unorthodox optical tech

#94

Post by spwa4 »

to11mtm wrote: post #26
notepad0x90 wrote: post #10

Earlier quoted context omitted.

isn't attenuation also an issue with copper? maybe with small electronics it is negligible given the right amps? in other words, with with no interference, electrons will face impedance and start losing information.

Attenuation is going to be an issue for any signal, but in my experience Fiber can go for many miles without a repeater whereas something like Coax you're going one to two orders of magnitude less. [0] [0] - Mind you, some of that for Coax is due to other issues around CTB and/or the challenge that in Coax, you've got many frequencies running through alongside each frequency having different attenuation per 100 foot.…

> Coax is due to other issues around CTB and/or the challenge that in Coax, you've got many frequencies running through alongside each frequency having different attenuation per 100 foot

Actually this is true for fibers as well. In DWDM (all internet links are DWDM, including fiber-to-the-home in most places) you have many frequencies running alongside and each frequency has differences in attenuation (though generally measured per kilometer, not 100 foot)

Optical light are standing electromagnetic waves. Which means they don't disrupt each other. Electrical signals aren't standing waves. They affect each other.

The difference can be put like this: how many X (electrical waves, but essentially everything, protons, ...) fit on the tip of a needle? (or in a cable)

1) electrical waves? Some finite number. Can be large of course, but ...

2) photons (ie. fiber signals)? ALL OF THEM. Literally every photon that exists in the entire universe would happily join every other photon on the tip of a needle nothing would interfere with anything else

Re: TSMC bets on unorthodox optical tech

#95

Post by topspin »

nsteel wrote: post #92
topspin wrote: post #89

Earlier quoted context omitted.

> I understand that CPO reliability concerns are specifically with the laser drivers. Yes. I've replaced my share of dead transceivers, and I suspect the laser drivers were the failure mode of most of them. That doesn't fill in the blank for me though: how reliable are high speed, dense microLEDs?

And are they going to work out any better than Linear Drive Optics, the more obvious alternative?

LDO is just integration. It certainly has value: integration almost always does. So it's clearly the obvious optimization of conventional serial optical communication.

This new TSMC work with parallel incoherent optics is altogether distinct. No DSP. No SerDes. Apples and oranges.

Re: TSMC bets on unorthodox optical tech

#96

Post by BardiaPezeshki »

cycomanic wrote: post #33

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-…

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.

Re: TSMC bets on unorthodox optical tech

#97

Post by BardiaPezeshki »

cycomanic wrote: post #67
topspin wrote: post #63

Earlier quoted context omitted.

> 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 P…

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 especial…

The fiber cables we use are basically 2D arrays of 50um thick fibers that match the LED and detector arrays. We've made connectors and demonstrated very low crosstalk between the fibers. Advantage over VCSELs is much lower power consumption overall, much lower cost (LEDs are dirt cheap and extremely high yield), because we are blue light, the detector arrays are much easier and can be modified camera technology, and most importantly, much better reliability. VCSELs are notorious for bad rel.

Re: TSMC bets on unorthodox optical tech

#98

Post by BardiaPezeshki »

topspin wrote: post #74
nsteel wrote: post #72

Earlier quoted context omitted.

Aren't they also claiming this is more reliable? I'm told laser reliability is a hurdle for CPO. And given the talk about this as a CPO alternative, I was assuming this was for back plane and connections of a few metres, not components on the same PCB.

> Aren't they also claiming this is more reliable? Indeed they do. I overlooked that. I know little about microLED arrays and their reliability, so I won't guess about how credible this is: LED reliability has a lot of factors. The cables involved will probably be less reliable than conventional laser fiber optics due to the much larger number of fibers that have to be precision assembled. Likely to be more fragile a…

we use borosilicate fibers that are used for illumination applications. You might have seen a bundle in a microscope light for example. And they are incredibly robust compared to single mode fibers. Note the very tight bend angle in the picture - that's a 3mm bend radius. Imagine doing that with a single mode fiber!

Re: TSMC bets on unorthodox optical tech

#99

Post by BardiaPezeshki »

lo0dot0 wrote: post #16
Liftyee wrote: post #2

As I understand it (from designing high-speed electronics), the major limitations to data/clock rates in copper are signal integrity issues. Unwanted electromagnetic interactions all degrade your signal. Optics is definitely a way around this, but I wonder if/when it will ever hit similar limits.

Optics also have signal integrity issues. In practice OSNR and SNR limit optics. Cutting the fiber still breaks it. Small vibrations also affect the signal's phase.

aha! that is true with lasers that are coherent. But not with LEDs. We don't care about modes, polarization, or phase. Also, no worry about feedback into the lasers, so no isolators. LEDs are way easier!

Re: TSMC bets on unorthodox optical tech

#100

Post by BardiaPezeshki »

cubefox wrote: post #86

This article is misleading. TSMC doesn't "bet" on the tech by Avicena (the startup in question). Instead, Avicena appears to simply pay TSMC to help them with manufacturing. Here is the linked press release by Avicena: https://www.businesswire.com/news/home/20250422988144/en/Avi... Noting also that there have been multiple articles on IEEE Spectrum about this startup in the past, I really hope the journalists don't o…

TSMC is developing custom detectors for Avicena on their own dime. They almost never do anything like for start-ups. That is why it is a bet on Avicena. They are nurturing this technology because they think it has real potential. See the TSMC quotes in the article.
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