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

[Magnus Danielson]

Hi,

On 2020-02-18 22:10, Attila Kinali wrote:
> 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.
>
> 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.

Well, indirectly yes. The 85Rb filter works in indirect ways that get
that property, yes.

The D2 line of 87Rb and 85Rb is very close, a factor 10 closer than the
D1 lines, so 85Rb can consume energy out of D2 from 87Rb, such that
pumping makes the right conditions for state-transition to be observed
with 6,8 GHz, when engaging in the hyper-fine state.

>
>
>>  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.

For laser-cooling, one servo the laser against a separate reference
cell, which helps a lot in finding the right frequency.

The one thing that one now suffers is that the intensity changes as the
servo pulls it around, I've not seen an explanation of how that is
handled for CW operation. For side-band operation one can use the
AC-modulation amplitude, as one disperse energy from the central carrier.

I just did not have time to dig through the literature on
laser-stabilization as you have done.

Cheers,
Magnus