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

[Attila Kinali]

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

> I had a brief read. Equation 1 made me wonder what could be achieved
> with a cheap HeNe laser. It should be fairly easy to mix a couple of
> HeNe lasers on a photodiode and look at the difference frequency
> between them, so gaining insight into their stability.  A quick check
> on Wikipedia
> 
> https://en.wikipedia.org/wiki/Helium%E2%80%93neon_laser
> 
> indicates a spectral width of 0.002 nm.
> 
> The common 632.8 nm laser has a frequency of 4.7 x 10^14 Hz, or 470 THz.

Oops.. i just reread the part of the paper an realized that i answered
in a completely meaningless way. Sorry about that.

For the real answer: You can use a HeNe laser to provide with a
stable wavelength, but as with all lasers, the linewidth and stability
are determined by the cavity, not by the atomic transition. The width
of the atomic transition is much wider than the cavity free spectral range.
Usually, these lasers have a 2-10cm long cavity, which results in a free
spectral range of 60GHz to 3Ghz. The exact wavelength depends on the length
of the cavity. Which means that any temperature change will shift the
laser wavelength around. And we are not yet talking about possible mode hops.

All this together will lead to laser wavelengths of different HeNe laser
tubes that are so far apart that a simple diode detection of the beat
frequency will not work (the beat frequency could be in the several 10GHz
and/or wander around very quickly). 

When stable lasers are needed, they are locked to something that does
not change. For example a vapor cell (DAVLL) or to a high stability
fabry-perot cavity (Pound-Drever-Hall). Both, the vapor cell or the
cavity are temperature stabilized. Locked also means that the laser
is steered, ie the wavelength of the laser is controlled electronically
using the feedback of the reference element.

Long story short, a simple HeNe laser will not give you anything stable,
without some additional stabilization.

			Attila Kinali

-- 
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