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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