[time-nuts] Current state of optical clocks and the definition of the second

Attila Kinali attila at kinali.ch
Tue Jan 13 13:37:05 EST 2015

On Tue, 13 Jan 2015 17:46:01 +0000
"Dr. David Kirkby (Kirkby Microwave Ltd)" <drkirkby at kirkbymicrowave.co.uk> wrote:

> Of course I'm not suggesting a HeNe would provide the stability of
> cutting edge research laser optical clock, but they are easily within
> the budget of a hobbyist and could be a bit of fun to try to measure
> their stability/phase noise. The tricky bit would be getting 470 THz
> down to 10 MHz, but a cheap and quick experiment would prove whether
> it is a total waste of time or not.

Actually, you can get better than that, and cheaper.

If you use a standard laser diode, these have a linewidth of
around 20-100MHz. If you provide them the slightest feedback,
they go down to 1MHz easily (ie just by adding some window glass
infront of the laser, that reflects a tiny bit back).
Using a more sophisticated scheme with a grating and you get into
the range of 1-100kHz, which is pretty darn good, and enough for
vapor cells with their broad lines. Wieman wrote a couple of papers
on how to build such laser system [1-3]. Also worth a look are the two
papers by Libbrecht [4,5]. For those who need some theory for
calculation or as background [6] will be a good start. It also
contains a lot of usefull references.

That said. For just doing something like CPT, you don't need
to narrow the linewidth of a standard laser diode. Eg the CSAC
uses just a standard VCSEL without any additional optics.
The disadvantage is, that your only means to control the laser
wavelength is temperature and current. Also, you will see mode
jumps within the range you are interested in.

			Attila Kinali

[1] "Using diode lasers for atomic physics", by Wieman, Holberg, 1990

[2] "A narrow-band tunable diode laser system with grating feedback,
and a saturated absorption spectrometer for Cs and Rb", 
by MacAdam, Steinbach, Wieman, 1992

[3] "Inexpensive laser cooling and trapping experiment for
undergraduate laboratories", by Wieman, Flowers, Gilbert, 1994

[4] "Teaching physics with 670nm diode lasers - construction of
stabilized lasers and lithium cells", by Libbrecht, et al, 1995

[5] "Teaching physics with 670nm diode lasers - experiments
with fabry-perot cavities", by Boyd, Bliss, Libbrecht, 1996

[6] "Mode stability of external cavity diode lasers", 
by Saliba, Junker, Turner, Scholten, 2009

It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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