
Clariphy targets 400Gbps with new 16nm DSP silicon - bifrost
http://www.lightwaveonline.com/articles/2016/03/clariphy-targets-400g-with-new-16-nm-dsp-silicon.html
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virtuallynathan
Nifty... 70Tbps per fiber is about 2x - 2.5x the current capacity.

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mmmBacon
A better metric is bits/Hz/km.

64-QAM does not go very far and requires very low linewidth lasers. The OSNR
requirements for 64-QAM at 32GBaud means that you basically have no reach
(distance). So makes a nice announcement but it's not a very practical
modulation format. Any nonlinearity and 64-QAM starts cycle slipping like
crazy.

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aortega
At long distance, inter-symbol interference caused by dispersion are the main
causes of bit-errors. I saw a Clariphy whitepaper about a very good digital
dispersion compensation engine, based on a custom implementation of the
Viterbi algorithm.

No a lot of experience in the subject but I believe that, unlike RF, if you
need more OSNR you can always inject a lot of power in the fiber (I've heard
about 5-10 watts lasers for long-haul data links).

Also, if you need to go farther away, you can always increase the FEC
overhead.

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mmmBacon
Unfortunately you can't just increase the power into the fiber. It's
complicated but I'll try to clarify (pun not intended) why you are OSNR
challenged here.

First, fiber and just about any optical material have a nonlinear index. So as
you increase the power into the fiber, the light modifies the local index.
This creates a kind of noise. A wavelength channel can do this to itself even.
When you have lots of channels on the fiber, there is a lot of modulation of
the refractive index by all those channels. This is call cross-phase
modulation (XPM). You can think of XPM as a phase noise. As you increase the
power in the fiber, there's a point where the XPM "noise" increases faster
than the improvement in OSNR. So you have to keep the launch power low to keep
the nonlinear penalty low. This limits our launch power and ultimately the
OSNR we can achieve. Generally we try to operate near the peak of this curve.

