[time-nuts] laser locking

[Paul Boven]

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.