
1400 km of optical fiber connect optical clocks in France and Germany - upen
http://sciencebulletin.org/archives/4098.html
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Faaak
Could they, connecting another clock, act as a "large-scale" LIGO ? Or is the
precision not enough ?

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analog31
I suppose they could connect the two clocks via fibers following different
paths, to create a large interferometer. For that matter, it might be possible
to cover a very large area with a bunch of clocks interconnected by a "web" of
fibers.

But fibers do have problems. Their optical path length depends on temperature,
and on how much the fiber is stretched. Understanding those issues is probably
good for a dissertation or two. ;-)

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UweSchmidt
Just a wild guess - the path length due to temperature and wear should grow
and shrink at a slow rate, no? The gravitational wave however should be a
spike, affecting the lag that could be potentially be measured.

The LIGO-lasers are probably much more precise, but we have way more fiber and
clocks than LIGO observatories. Thoughts?

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analog31
Thinking about it more, it was mentioned that the laser clocks might have a
fair amount of phase noise, and might not be good for picking up fast time lag
signals. On the other hand, you can feed multiple lasers down the same fiber,
so a better experiment might be to wait for the fiber network to be built, and
use a separate laser color from a specialized laser with excellent short term
stability.

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amelius
> frequency fluctuations added during the propagation along the fibre are
> actively suppressed by up to 6 orders of magnitude

Where could these fluctuations originate from?

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walrus01
I am pretty sure that this is an attempt by somebody who doesn't work in
DWDM/optical transport systems to describe industry standard dispersion
compensation modules (DCM). Such as you would install for a 40 x 10 Gbps
wavelength DWDM link on two strands between two sets of chassis and linecards.

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smp1234
This is not dispersion. The light sent in fiber is essentially
monochromatic,hence there is no need to cancel dispersion.

A whole bunch of processes can add phase noise to the light as it propagates
in fiber. Refractive index changes, temperature fluctuations, stresses in
fiber due to bending, etc. This is suppressed by sending the light back
through the same fiber and actively locking the phase of the light that had
travelled through the fiber wrt the incident light.

[https://jila.colorado.edu/yelabs/sites/default/files/uploads...](https://jila.colorado.edu/yelabs/sites/default/files/uploads/sArticle_1994_11_FiberNoiseCancel.pdf)

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madengr
That interesting because in most metrology clock applications, phase noise is
not a concern, rather long term drift. Here they are going to the difficulty
to maintain low phase noise (jitter).

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acqq
I compare it with what I played with personally: NTP. The drift is covered
here by the very clock technology, the jitter of the line, just like in NTP,
is why the cables are specially tuned. Both are needed.

This is just my guess, however, and if somebody knows more, please write!

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MasterScrat
> This will benefit various research areas, with applications in fundamental
> physics, astrophysics and geoscience.

Does anyone know of some concrete application examples?

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pm90
I'm just guessing here but I think an accurate log of astronomical events
might be something for which this would be useful. If you have observatories
distributed throughout the world, then an accurate time log would perhaps be
invaluable in making very accurate measurements.

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madengr
Probably also useful for frequency synchronization for large baseline radio
telescopes. I believe now they use hydrogen masers. One needs to maintain
coherent receivers to several GHz over thousands of km.

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madengr
Is time dilation measurable between the two (due to gravity differences
between locations)?

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eeZi
Yes it's in the article. Pretty exciting.

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madengr
Thanks; I glossed right over it.

"Also, the apparent rate of a clock depends on the local gravitational
potential: comparing two clocks measures the gravitational redshift between
them, and thus yields their height difference. Such measurements provide data
points for the geodetic reference surface, the so-called “geoid”. This
research approach is pursued jointly by physicists and geodesists in the
Collaborative Research Centre 1128 (“geo-Q”) of the German Science Foundation
(DFG)."

