On Tue, 13 Jan 2015 17:46:01 +0000 "Dr. David Kirkby (Kirkby Microwave Ltd)" <drkir...@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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
