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Comment by Liftyee | Hacker News Reader

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.

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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.

Phase variations will not introduce any issues here, they most certainly are talking about intensity modulation. You can't really (easily) do coherent modulation using incoherent light sources like leds.

SNR is obviously an issue for any communication system, however fiber attenuation is orders of magnitude lower than coax.

The bigger issues in this case would be mode-dispersion, considering that they are going through "imaging" fibres, i.e. different spatial components of the light walking off to each other causing temporal spread of the pulses until they overlap and you can't distinguish 1's and 0's.

Mode dispersion is frequency dependent phase changes.

That's chromatic dispersion, mode dispersion is spatial "path" dependent phase changes. Vibration is actually somewhat more relevant because if it wasn't for that we could theoretically undo mode dispersion (we would need phase information though).

That said all of that is irrelevant to what the previous speaker said, vibration induced phase variation as an impairment. Thats just not an issue, vibrations are way too slow to impair optical comms signals.

How do the gravity wave optical paths solve the vibration issues? Couldn't TSMC do something similar?

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!

The energy densities required for photon-photon interactions are so far beyond anything we need to worry about that it's a non-issue. Photons also aren't going to just ignore local potential barriers and tunnel at the energy levels and scales involved in foreseeable chip designs either.

P-p interactions are not an issue but we do have high enough field intensities in high bandwidth fibers to run into p-f nonlinearity issues.

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.

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

This is the main mechanism of interference anyhow, called inter-symbol-interference.

luckily photons are boson (if we ever pushes things to this level of extreme)

This comment appears insightful but I have no idea what it means. Can someone elaborate?

Electrons are fermions which means that two electrons can't occupy the same quantum state (Pauli exclusion principle). Bosons don't have the limit so I believe that implies that you can have stronger signals at the low end since you can have multiple photons conveying or storing the same information.

Also less chance for external interference.

What the previous poster is implying is that electrons interact much more strongly than photons. Hence electrons are very good for processing (e.g. building a transistor), while photons are very good for information transfer. This is also a reason why much of the traditional "optical computer" research was fundamentally flawed, just from first principles one could estimate that power requirements are prohibitive.

> This is also a reason why much of the traditional "optical computer" research was fundamentally flawed

presumably also because photons at wavelengths we can work with are BIG

Fermions can “hit each other” whereas bosons “pass through each other”.

(Strong emphasis on the looseness of the scare quotes.)

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