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

[Gregory Maxwell]

On Tue, Jan 13, 2015 at 5:46 PM, 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

See Sams Laser Faq section on stabalized HeNe Lasers:
http://www.repairfaq.org/sam/laserchn.htm#chnsshnl1

This uses zeeman splitting to get two different polarization modes
lasing at slightly different wavelengths. (this is described in more
detail elsewhere in the FAQ about some commercial lasers that use this
effect.)

There are do it yourself at home grade (/ easily available surplus
parts) things you can do to get the short term linewidth down to about
4MHz or so, sadly 4MHz out of 470THz is only 1e-8 or so, so not super
competitive with some off the shelf OCXO.

You're also then stuck with an optical standard, and the down
conversion to microwave is decidedly more complex. It would probably
be a fun time-nuts project, even given the not amazing performance, if
not for the difficulty in down-converting to something that could feed
a counter.