[time-nuts] Frequency over fiber (was WWV and legal issues)

[Anders Wallin]

FWIW the round-trip delay on our 900km White Rabbit link looks like so
(lower graphs. upper are a local Cs-clock vs. the fiber-time):
http://monitor.mikes.fi/mikes_kaja/
No fault-lines, earthquakes, or volcanoes in Finland I'm afraid. That's a
standard 2-fiber (separate TX and RX fibers) DWDM system with amplifiers,
multiplexers, and dispersion-compensation fibers - in addition to the
actual fiber-spans.
diurnals on the 10.5 ms RTT seem to be around 80ns this time of the year.
Larger jumps are fiber/equipment repairs.

The recent earthquake paper on the London-Paris link is this one I think:
https://arxiv.org/ftp/arxiv/papers/1801/1801.02698.pdf


Anders


On Sun, Sep 2, 2018 at 11:14 PM Peter Vince <petervince1952 at gmail.com>
wrote:

> Hi Magnus,
>
>      Could you please give us some idea of the magnitude of these effects?
> Even if just whether we are talking about nano, pico, or femtoseconds?
>
>      Thank you,
>
>           Peter
>
>
> On 2 September 2018 at 12:17, Magnus Danielson <magnus at rubidium.dyndns.org
> >
> wrote:
>
> > Hi Gerhard,
> >
> > I see that this became a separate thread.
> > ...
> > ...
> >
> > Acoustical sensitivity, low frequency changes.
> >
> > For optical clocks and frequency transfer, just the vibration from
> > traffic and other activity causes disturbances which disturbes the group
> > delay. What is done for these links is to actively compensate then using
> > a return path and closing the loop with a controller, very much like a
> > PLL. The length of the loop limits the bandwidth and hence how high up
> > the compensation can be done, so for longer stretches, this needs to be
> > repeated. They have now built links from PTB to SYRTE and NPL.
> >
> > Temperature shifts are slower, but also compensated though the active
> loop.
> >
> > Close proximity to strong power-currents have also been shown to cause
> > modulations, so separate from power-cables if you can.
> >
> > Remember that the end nodes have very stable clocks, so their effects
> > can be taken out of the equation. For other setups, such as telco
> > operation, that's a completely different ballgame.
> >
> > Cheers,
> > Magnus
> >
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