[time-nuts] LED instead of discharge lamp for Rb vapor cell standards

Bob kb8tq kb8tq at n1k.org
Wed Feb 21 13:34:35 EST 2018


There are a number of papers on LED / Laser excitation of Rb cells (and 
other gas cells). They go back quite a ways. The gotcha (as pointed out 
in PHK’s post) is that you need to stabilize the LED source. Doing that is 
a bit complex. Doing that so that the result beats a gas lamp is a bit more 
involved than just getting it working. Part of the problem is that there are 
a number of transitions you can hit and you only want the “right” one. The
next layer is the signal to noise once you get on the right transition 

Based on the examples I have seen on my bench, the ADEV of a typical 
laser based unit is not super duper compared to Corby’s modified units. 
In defense of the designers, they normally are targeting small size rather 
than super stability. That’s what the market wants to buy …. If you are 
spending tens (or hundreds) of millions of dollars, you focus on a product 
you can sell a lot of. 


> On Feb 21, 2018, at 12:49 PM, Attila Kinali <attila at kinali.ch> wrote:
> Moin,
> Has anyone ever tried using a LED instead of a discharge lamp for Rb standards?
> There are quite a few and very cheap 780nm IR LEDs available. They usually
> have a line width in the order of 20nm to 50nm (FWHM). This means that they
> still need the Rb85 filter, but they would not produce all those spectral lines
> that the discharge lamp has, thus one could get rid of the (not so cheap)
> optical filter that Corby uses for the super-5065. 
> The one thing I am not sure about is the filter efficiency of Rb85.
> Because now the light isn't two discrete lines from the Rb87 lamb
> of which one needs to be masked, but a continuous and wide spectrum.
> So the Rb85 "notch" filter might not get rid of all the unwanted light
> and some of this might depopulate the excited state of the Rb87
> that are in the RF cavity.
> As a side-effect, you also get easier regulation of the light intensity
> of the LED, thus potentially less instability due to light-shift variation.
> Beside the LED operation being much simpler than that of a discharge lamp
> and overall less heat dissipation.
> 			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
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