Mitigation of lamp oven and cavity oven temperature-induced frequency variation in rubidium atomic clock
Just published in Review of Scientific Instruments:
https://doi.org/10.1063/5.0130524
ABSTRACT
The long-term frequency stability of the rubidium atomic clock is primarily
affected by temperature variations in the lamp oven and the cavity oven,
which cause changes in light intensity, which are then converted into
frequency variations. Therefore, we propose using light intensity
variations to actively improve the cavity oven and lamp oven temperature
sensitivity of the rubidium atomic clock. This is accomplished through
research into the theory of the rubidium atomic frequency standard,
specifically the effect of light intensity, lamp oven temperature, and
cavity oven temperature on the frequency deviation. In previous work, we
discovered the relationship between the light intensity and frequency
deviation by combining this with our engineering expertise. Furthermore,
some related experiments show that the method is feasible with the lamp
oven and cavity oven temperature sensitivity of the rubidium atomic clock
greatly improved, providing an effective way to improve the rubidium atomic
clock’s long-term frequency stability.
On Wed, 1 Feb 2023 13:33:41 -0500
Mark Kahrs via time-nuts time-nuts@lists.febo.com wrote:
Just published in Review of Scientific Instruments:
https://doi.org/10.1063/5.0130524
Some small comments on this paper:
One important topic they do not describe is why the light
intensity shifts when the temperature of the lamp or the
cavity changes. This is due to the change in vapor pressure,
ie the number of atoms that evaporate and stay in the gaseous
phase increases with temperature [1].
In the lamp this has obviously the effect of having more atoms
that can emit light. Thus higher temperature means more light.
What they do not mention is that this also comes with a shift
in the spectrum which also has an effect on the gas cell frequency
as wel. The effect of this shift would need to be measured and
quantified as well. See [2] for an example that did this work,
though UniNE, like all research groups that work on state of
the art vapor cell Rb standards use lasers, which make
analysing frequency shifts due to light intensity and light
frequency a lot easier as they can be independently tuned.
Similarly, higher temperatures in the cavity mean more atoms
in the vapor cell, means more absorption, thus less light passing
through. In a lot of papers you will read a phrase like "optically
dense" or similar. Which is saying that the amount of rubidium
vapor is so high and the path through the cell so long, that
the amount of light passing through is limited by the absorption
of the rubidium atoms. Any increase in length does not increase
the SNR of the signal any further because of the increased absorption.
The frequency shift in the vapor cell is mostly due to buffer gas
colision. This shift, depending on the gas mixture, has usually
a parabolic shape. I.e. it is possible to choose the mixture such
that the temperature dependend frequency shift is approximately
zero at the desired temperature. A technique that has been known
for at least 30 years (c.f. [3]), see [4] for a more modern
publication on that topic. I do not know why the authors of this
paper chose not to adjust the gas mixture such that they would
get a df/dT null for the vapor cell at their operating temperature,
respectivelly adjusting the operating temperature that such that
they are at the df/dT null. The authors do not discuss why they
do not implement this standard procedure.
It is not clear either why they chose to implement a C-field
adjustment using analog means (which are hard to adjust and
require manual testing and adjustment for each device) instead
of a digital frequency adjustment in the synthesis chain (see [5]
for a contemporary example of a synthesis chain that is widely
used by many research groups) that can be calibrated in an automated
fashion and can even have non-linear adjustments using a lookup
table. Also using light intensity as detection mechanism is iffy
at best. Not only is light intensity one of the factors that shift
frequency, the intensity that hits the sensor is modulated by two
independent processes with different causes that have different
frequency shift characteristics. I.e. the compensation is limited
by the correlation of the light intensity vs frequency shift
characteristic at calibration time vs during use in the field.
It would have been much better to use independent temperature
sensors at the lamp and vapor cell oven, then do a principle
component analysis of the shifts due to temperature change
of the two ovens and the light intensity. This way all three
factors could have been independently compensated.
Attila Kinali
[1] "Rubidium 87 D Line Data" by Daniel Adam Steck, 2021,
Figure 1 on page 25
https://steck.us/alkalidata/rubidium87numbers.pdf
[2] "Long-Term Stability Analysis Toward <1e−14 Level for a
Highly Compact POP Rb Cell Atomic Clock", by Almat, Gharavipour,
Moreno, Gruet, Affolderbach and Mileti, 2020
https://doi.org/10.1109/TUFFC.2019.2940903
https://www.unine.ch/files/live/sites/ltf/files/shared/Publications/2020/2020_Almat_Long-term_stability_TUFFC_67_207.pdf
[3] "On the light shift and buffer gas shift in passive rubidium
frequency standard", by Vanier, Kunski, Aulin, Savard, Tetu and Cyr, 1982
https://doi.org/10.1109/FREQ.1982.200593
[4] "Short and medium term frequency stability of a laser pumped
"rubidium gas-cell frequency standard for satellite navigation",
by Besedina, Berezovskaya, Gevorkyan, Mileti, and Zholnerov, 2006
https://ieeexplore.ieee.org/document/6230986
Also somewhere in https://www.eftf.org/fileadmin/documents/eftf/documents/Proceedings/proceedingsEFTF2006.pdf
[5] "Simple-design ultra-low phase noise microwave frequency
synthesizers for high-performing Cs and Rb vapor-cell atomic clocks",
by François, Calosso, Abdel Hafizz, and Boudot, 2015
http://dx.doi.org/10.1063/1.4929384
--
In science if you know what you are doing you should not be doing it.
In engineering if you do not know what you are doing you should not be doing it.
-- Richard W. Hamming, The Art of Doing Science and Engineering
On Tue, 21 Feb 2023 21:21:25 +0100
Attila Kinali attila@kinali.ch wrote:
Not only is light intensity one of the factors that shift
frequency, the intensity that hits the sensor is modulated by two
independent processes with different causes that have different
frequency shift characteristics.
Sorry, small correction here, as this is not quite right.
There are quite a few more processes that change the amount
of light hitting the sensor:
- RF power of the lamp
- RF frequency of the lamp
- Lamp temperature
- Lamp bulp absorption of rubidium into the glass
- Vapor cell temperature (i.e. vapor pressure)
- Vapor cell absorption of rubidium into the glass
- RF power of the vapor cell
- RF frequency of the vapor cell
to name a few....
Some of these are usually adjusted/kept constant in modern
devices. Others are not or cannot be measured/compensated.
To make a Rb vapor cell standard stable, these influences have
to be accounted for independently as they have different
effect magnitudes on the resulting frequency shift. Mushing
them all into a single measurement values forgoes a lot of
potential for additional stability at best. At worst it makes
the system less stable.
Attila Kinali
--
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always
wonder about. And that's why we do research.
-- Kobayashi Makoto
Attila Kinali via time-nuts writes:
like all research groups that work on state of
the art vapor cell Rb standards use lasers, which make
analysing frequency shifts due to light intensity and light
frequency a lot easier as they can be independently tuned.
Is this lasers stabilized and tuned by temperature, or are we into optical comb territory here ?
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
On Wed, 22 Feb 2023 07:12:40 +0000
"Poul-Henning Kamp" phk@phk.freebsd.dk wrote:
Is this lasers stabilized and tuned by temperature, or are we into optical comb territory here ?
Currently it's just tunable lasers. I think the most common ones are DFB and DBR lasers
that are tuned using their lasing current and temperature. But there are also quite
a few ECDL with either gratings or cats-eye mirrors going around.
I don't think that optical combs will enter the Rb vapor cell market any soon.
These devices still cost more than what a complete Rb standard costs today.
And almost all of them are quite finiky devices that need constant pampering
(I know of only one manufacturer who makes them sturdy enough that they can run
long term without problems and are even immune to being hit by a hammer)
Attila Kinali
--
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always
wonder about. And that's why we do research.
-- Kobayashi Makoto
I don't think that optical combs will enter the Rb vapor cell market any soon.
Just to add on top that efforts are being made in this direction, see for example this recent project:
https://ec.europa.eu/info/funding-tenders/opportunities/portal/screen/how-to-participate/org-details/999999999/project/101047289/program/43108390/details
The idea is to integrate directly the Rb cell in the comb, being self-referenced, to produce a clock.
This will become interesting if the device can output the clock signal as well as some comb light!
Best regards,
Edgar
Edgar Fernandes
Menhir Photonics AG
-----Original Message-----
From: Attila Kinali via time-nuts time-nuts@lists.febo.com
Sent: Wednesday, 22 February 2023 08:17
To: Poul-Henning Kamp phk@phk.freebsd.dk
Cc: Discussion of precise time and frequency measurement time-nuts@lists.febo.com; Attila Kinali attila@kinali.ch
Subject: [time-nuts] Re: Mitigation of lamp oven and cavity oven temperature-induced frequency variation in rubidium atomic clock
On Wed, 22 Feb 2023 07:12:40 +0000
"Poul-Henning Kamp" phk@phk.freebsd.dk wrote:
Is this lasers stabilized and tuned by temperature, or are we into optical comb territory here ?
Currently it's just tunable lasers. I think the most common ones are DFB and DBR lasers that are tuned using their lasing current and temperature. But there are also quite a few ECDL with either gratings or cats-eye mirrors going around.
I don't think that optical combs will enter the Rb vapor cell market any soon.
These devices still cost more than what a complete Rb standard costs today.
And almost all of them are quite finiky devices that need constant pampering (I know of only one manufacturer who makes them sturdy enough that they can run long term without problems and are even immune to being hit by a hammer)
Attila Kinali
--
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always wonder about. And that's why we do research.
-- Kobayashi Makoto
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