
Ultra-dense optical data transmission over standard fibre with a single chip - paulstovell
https://www.nature.com/articles/s41467-020-16265-x
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praveen9920
Wow. Impressive.

> 25% increase every year

Is there anything like Moore's law for optical cable bandwidth?

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Zenst
More so when you work out how much data is inflight, maybe we see fiber reels
used as storage/memory. Kinda like what we had in the early days of wire
memory. May well see that come back into play with speeds like this for some
uses cases.

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jdsully
The speed of light works against you in that case. Even at the incredible
14Thz line rate achieved here, 1km of fibre can have ~146kbits bits in flight
at a time.

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Zenst
Yes, though in this instance they pulled of 44terrabits rate per second, so
with that you would be looking at 156 megabits inflight over a 1km cable if my
napkin maths are correct. Thats 18 megabytes, sure error rate etc and
correction would limit that a bit as nothing works as ideal, but sure does
seem like may have some uses come into play.

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jdsully
46 megabits at 3.34µs per kilometer[1], however error correction brings that
way down. Regardless 1KM of fibre is quite substantial, we're talking
densities far lower than even 70s technology. Worse if you try to up the cable
length for larger storage latency will increase with it.

1: [https://www.m2optics.com/blog/bid/70587/Calculating-
Optical-...](https://www.m2optics.com/blog/bid/70587/Calculating-Optical-
Fiber-Latency)

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sfink
What kind of wacky unit is the spectral efficiency of bits/sec/Hz? Isn't that
the same as 'bits'?

I mean, I'm guessing it's just saying the bit rate depends on the signal
frequency, but it's still funny to see canceling units.

Related: a "spacing" measured in GHz.

Again, it makes sense, but it struck me that someone could write a total spoof
paper with nonsense units and I wouldn't be able to tell the difference!

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namibj
It's the normal unit/measurement of spectral efficiency. You have X Hz
bandwidth, and you multiply it by the spectral efficiency to get the data
rate.

The fact that the time-unit cancels out just shows that spectral efficiency is
independent of any sense of time one might have.

Measuring "spacing" in GHz makes sense if you consider the way heterodyne
mixing shifts the signal around, without affecting the spacing of sub-
carriers.

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Zenst
Imagine having your CPU linked to memory or data-bus over a single fiber
connection instead of complicated multi traced connections. One day perhaps.

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amalcon
Don't the electronic:optical and optical:electronic transitions introduce
quite a bit of latency? I've seen estimates of around 500 microseconds per
conversion. That's not enough to matter for networking, but RAM chips have
internal latencies measured in nanoseconds.

Of course, "one day perhaps" this could change.

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wmf
The laser and detector only introduce ~1 ns of latency and the serdes might
introduce a few more ns (e.g. OpenCAPI Open Memory Interface claims 4 ns).

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willis936
Impressive stuff. I think the BER is appallingly high. Current applications
want pre-FEC BER on the order of 10^-12, not 10^-2. TANSTAAFL.

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perlgeek
10^-12 _before_ FEC strikes me as outlandishly low.

Why would applications even care about the BER before FEC?.

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willis936
FEC does not perform miracles. It only trades off some throughput for a few
orders of magnitude in BER. FEC can also be defeated by common types of
distortions. If you get a spike of alien crosstalk then you aren’t going to
just get one bit error, so you really do need that high fidelity connection if
you don’t want your link to shit the bed in the real world.

