[time-nuts] Improving the stability of crystal oscillators

[Magnus Danielson]

From: "John Miles" <jmiles at pop.net>
Subject: Re: [time-nuts] Improving the stability of crystal oscillators
Date: Sun, 14 Oct 2007 14:15:10 -0700
Message-ID: <PKEGJHPHLLBACEOICCBJOECGBPAB.jmiles at pop.net>

> ); SAEximRunCond expanded to false
> Errors-To: time-nuts-bounces+magnus=rubidium.dyndns.org at febo.com RETRY
> 
> 
> > The required depth depends on the soil diffusivity and the temperature
> > stability required.
> > It is instructive to install thermometers at depth intervals of a foot
> > or so and record the temperature fluctuations experienced by each
> > thermometer.
> > This was first done around 1860 by Forbes.
> > I repeated the experiment in 1966.
> 
> There was an interesting bit in the last Agilent Measurement Journal about a
> product that uses an ordinary communications-grade fiber as a thermometer.
> >From what I remember, they send a laser pulse down the fiber, then look at
> the backscatter, correlating time-of-flight with the Raman-scattering lines
> (Stokes and anti-Stokes).  One of those spectral lines is
> temperature-dependent while the other isn't, so by recording the separation,
> they end up with is a graph of temperature versus distance along the fiber,
> gathering up to a few kilometers' worth of data with what looked like
> sub-meter resolution.
> 
> No doubt this effect is old hat to physicists on the list, but I'd never
> heard how it worked before.  So if you buried a fiber like this, you'd
> presumably get a great picture of what happens with temperature at various
> depths.  Plotting the temperature-versus-distance on a waterfall display
> gives a nice diurnal picture.  The article used it to study water
> temperature along the course of a stream, but you could think of plenty of
> other uses for 2D remote temperature sensing.

Bell Labs have made measures on soil temperature at different deps and also
fiber-delay as it varies with temperature. As it happends, the delay is part
due to fiber length change due to temperature and most part due to change of
delay due to change in dielectrics which converts into changes in the wave-
equation.

It naturally depends on temperature, laser wavelength and accumulates over
distance. I haven't seen any paper on non-chromatic delay shifts, but I haven't
looked too closely.

Cheers,
Magnus