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Optical PCIe 7.0 connection hits 128 GT/s

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204 points WithinReason | 2 comments | 17 Jun 24 11:42 UTC | HN request time: 0s | source
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yjftsjthsd-h ◴[18 Jun 24 00:02 UTC] No.40712649[source]▶
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.
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AceJohnny2 ◴[18 Jun 24 01:17 UTC] No.40713112[source]▶
> 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.

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hughesjj ◴[18 Jun 24 03:27 UTC] No.40713801[source]▶
> 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)

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cycomanic ◴[18 Jun 24 03:52 UTC] No.40713907[source]▶
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.

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aborsy ◴[18 Jun 24 04:43 UTC] No.40714147[source]▶
Photonic wavelengths are shorter than electronic wavelengths. Why are photonics waveguides bigger then?
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murkt ◴[18 Jun 24 05:04 UTC] No.40714222[source]▶
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.
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1. aborsy ◴[18 Jun 24 08:09 UTC] No.40715255[source]▶
Optical fiber communication operates at 1.55 micro meters or 193THz. Electronics operates in the electromagnetic spectrum or at GHz. There might be no fixed size or wavelength in either case, but the shortest wavelength for radio signals that can be transmitted with acceptable loss with today’s technology is mili meters.

Which brings the question: why operating wavelengths are smaller but “waveguides” are bigger in optical fiber communication. In fact, fiber itself is a waveguide and its diameter is tens of micro meters.

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2. silizium ◴[18 Jun 24 08:50 UTC] No.40715452[source]▶
The full optical wave is contained in the dielectric conductor. This conductor needs it's minimum cross section such that the wave can propagate. If it is too small then the wave can not propagate. Also there is a maximum cross section if you want single mode operation.

You get to this result if you take the electromagntic wave equation - a partial differential equation - and solve that for your transmission line configuration.

The proper analogy in the realm of electrical waveguides is the hollow waveguide. The hollow waveguide supports TE- and TM-modes but not TEM modes just like a dielectric conductor. The size is also a function of the dielectric constant ε.

What we mostly use are TEM waveguides like microstrips or coaxial cables. The difference between electrical waveguides that supports TEM modes and waveguides that supports TE/TM modes is that the former has two independent potential planes and the latter only one. Also TEM waveguides do not have a lower cutoff frequency. A TEM wave with any frequency can propagate on any microstrip configuration.

This is not true for TE/TM waves.

What's important to understand is that for microstrips/coaxial cables the power isn't transferred in the metal but in the space (dielectric) around the metal - see Poynting vector. So what happens if you have a second conductor in that space? You get crosstalk! So TEM transmission lines do not contain the wave like hollow waveguides or optical fibers (edit: ok coaxial cables do, microstrips don't)

Now the question, how big is the microstrip? Is it just the width of the signal conductor? No, it is not.

Edit: The width of the metal lines in a chip is given by the current it must carry - current density requirement, electro-migration issues. Power lines are wide because they have to supply power to the circuit but logic traces in CMOS technology only carry negligible amount of current. In circuits like RF power amplifiers with bipolar transistors the trace width is much larger because it has to carry a much larger current. But again, microstrip lines do not have a lower cutoff frequency.