[time-nuts] Fiber propagation delay tempco

[Dr Bruce Griffiths]

Hal in July you asked
> >/ With the fibre-based two-way time transfer. For shorter distances you
> />/ can do well on coax, but for the distance range you require you really
> />/ want to go fibre. That world is a bit different but can be made sense
> />/ off. 
> /
> How do I setup 2 clocks so they are ticking within 1 ns of eachother?
>
> If both clocks were next to eachother on a bench, I'd connect them to a scope 
> (or equivalent box) and adjust a knob until the signals lined up.  Note that 
> 1 ns is small enough so that I have to make sure the cables are the same 
> length and the amplifier delays are matched.  I can swap inputs to check that.
>
> But what if the clocks are 10 km apart and all I have is two fibers between 
> the sites?  Is there some common recipe for synchronizing this setup?
>
> Let's assume the fibers are the same length.  I'm not sure that's accurate at 
> the ns level.  You can probably swap fibers to check.
>
> One approach is to make a symmetrical setup: send your signal to the other 
> site, compare the signal from the other site with yours, adjust one knob 
> (pick one) until the offsets match.  That's ugly since you now have to 
> measure an offset rather than tune for a null.  Is there a way to avoid that?
>
>
>
> What's the thermal coefficient of delay for fibers?
>
>
Some info on the tempco of the propagation delay for fiber can be found at some of the Radio interferometer array sites where buried fiber 
is used to distribute a reference frequency to all the antennas. Some fibers have very low tempcos, much better than for coax. 
In particular teflon dielectric coax exhibits a discontinuity in the phase tempco at 15C. This stuff should be avoided like the plague, Polyethylene 
dielectric coax is far better behaved.

Diurnal fluctuations in fiber delay due to thermal expansion of the fiber is one of the largest contributors to variations in fiber delay.
Even the 5E-7 thermal expansion tempco of fused silica produces a 5mm increase in length per degree C increase in fiber temperature for a 10km fiber length.
This is equivalent to a propagation delay increase of around 25 picosec. If the fiber dopant is selected so that the tempco of its refractive index compensates 
for the fiber expansion a somewhat lower propagation delay tempco is possible. Suitable single mode fibers are available from Sumitomo. 
with a low propagation delay tempco.
Jacketed fibers may have higher tempco than loose fibers.

Alternatively one can use fiber stretchers and an optical interferometer to compensate for fiber propagation delay variations:

http://www.alma.nrao.edu/memos/html-memos/alma483/memo483.pdf#search=%22fiber%20radio%20astronomy%20frequency%20distribution%22

Some data on fiber propagation delay is given here:

http://astro.berkeley.edu/ral/ata/memos/memo55.pdf#search=%22fiber%20radio%20astronomy%20frequency%20distribution%22

Bruce