[time-nuts] Pressure sensitivity of Rb vapor cell standards

[Attila Kinali]

On Tue, 18 Feb 2020 11:56:48 -0600
Dana Whitlow <k8yumdoober at gmail.com> wrote:

> What's the optimum wavelength for the Rb light source?

The one that pumps the electrons of one of the hyperfine ground
states of the 5S orbital into the 5P orbital. Which is 780.241xxx nm
(see [1])

>  A laser would certainly have a much narrower linewidth than an LED,
> provided one can find a laser with the correct wavelength.

You will not be able to find one with the correct wavelength.
An unstabilized laser diode has an output bandwidth in the 50-300MHz
range. Which means you need to hit the right wavelength with the
accuracy of the bandwidth of the laser. For a bandwidth of 100MHz
at 780nm that means you have to hit the right wavelength with
an accuracy of 0.2pm. The slightes vibration, the slightest change
in temperature, the slighest change of current through the laser
diode or a fly coughing and you are off. That's why you have to
actively steer the laser diode such that its output wavelength stays
where you want it to be. If you are using a laser, there is no way
around it.

That's why I suggested using a LED instead of a laser.
It's broad enough that you don't have to worry about hitting
the right wavelength. You only have to worry about not hitting
the other wavelength that depopulates the state you want to have,
which is 6.9GHz away. But the 85Rb filter does a good job at that.


>  Many diode lasers have a detector integrated into the same
> package, making tightly-stabilized output power a snap to achieve.

Yes, it makes it easy to keep the intensity stable, if you don't
care about the wavelength. The laser current is one of the parameters
that change the laser wavelength and the fastest of all the ways
you can work on the laser. I.e. you have to change it to keep the
laser on the right wavelength as changing the temperature has a too
long time constant to lead to a stable control loop. The only
alternative is to use an extended cavity with an piezo actuator,
but even there you want to have it as the piezo is slow (reacts in
milli seconds) as it has to move a mirror. In general most extended
cavity laser diode systems use the current as the fast loop and the piezo
as the slow loop. The temperature is usually set such, that you get
mode hop free operation in the range you are working in.

Yes, doing the laser thing is not easy.


> Also: at 60C is the vapor pressure of Rb high enough to work with vis-a-vis
> achieving a quality lock to the atomic transition?

You don't need a high vapor pressure. You need a sufficient amount
of atoms in the light path. But yes, 60°C is enough. Most commercial
rubidiums keep the cell around 60-80°C, mostly depending on how close
the Rb lamp has to be, which is around 120-200°C. Eg the PRS10 has a
1-1.5cm gap between the lamp and the cell, in order to keep them thermaly
isolated. As you are doing differential detection, you don't need that
high of a signal. Most reported constrast values are between 5% and 30%.
You want to have an as high value as possible, though, as the SNR
goes directly into the achievable short term stability.

BTW: for a classical Rb vapor cell standard, the PRS10 is by far my
favorite design. The SRS people really thought hard how to get the
best performance out of this small package, while still
being reasonably cheap to manufacture.


			Attila Kinali

[1] "Rubidium 87 D Line Data" by D.A. Steck
https://steck.us/alkalidata/ 
https://steck.us/alkalidata/rubidium87numbers.pdf
-- 
<JaberWorky>	The bad part of Zurich is where the degenerates
                throw DARK chocolate at you.