[time-nuts] laser locking
Paul Boven
p.boven at xs4all.nl
Mon Apr 29 19:20:43 UTC 2013
Hi John,
On 04/29/2013 08:16 AM, John Pease wrote:
> I'm asking mostly out of interest, because i have no clue how this
> is done. Although i've read many papers on laser spectroscopy and
> how to acheive even better atomic clocks using lasers, none of those
> papers mentiones how to get the laser there where it should be and
> how to keep it there.
There are three parts to a modern optical atomic clock:
1.) The laser itself, which generally has a linewidth of only a few Hz
to begin with. It is stabilized by using a cavity of extremely stable
glass between the mirrors. The glass is 'ULE', ultra low expansion,
which results in linewidths of the order of a few Hz. This gives great
stability on timescales up to a few seconds.
2.) The laser light is then modulated to form femtosecond pulses. This
generates a whole comb of lines in the spectrum, and through special
locking techniques, these lines can be spaced e.g. 1GHz apart. This can
also tie the optical frequency to a microwave atomic clock.
3.) By selecting the right lines from the comb spectrum, the light from
the laser can be used to interrogate an atomic specimen in a trap. This
can be either a single atom or a few of them, neutral or ionized. By
looking at the fluorescence, the laser itself can be locked to a very
narrow atomic transition, which gives the clock its long-term frequency
stability. This measurement in itself is often a 'batch' process, but
the laser + cavity function as a flywheel in between measurements.
Combined, this can lead to Allan deviations down to the 10E-18. This is
such a mindbogglingly high accuracy that some people call these devices
'Einstein clocks': a change in height of just a few cm causes enough of
a change in gravitation that the clock measurably changes speed.
I hope this gives you enough keywords to do a bit of Googling :-)
Regards, Paul Boven.
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