[time-nuts] a newbie question: where can I purchase 794.7 nm VCSEL for building CPT rubidium clock?

Attila Kinali attila at kinali.ch
Tue Jun 5 05:11:59 EDT 2018

On Mon, 4 Jun 2018 21:31:56 +0800
mimitech mimitech <mimitech at gmail.com> wrote:

> I'm planning to build a CPT (coherent-population-trapping) rubidium clock
> as my next hobby project. The main purpose is to learn the principles
> behind CPT rubidium clock, and hopefully got similar or better performance
> than commercial miniature rubidium clock such as FE-5680A.

Building a CPT clock is slightly more involved than you might think
at first. The laser diode is only one part of it. You will most likely
be able to improve on the short-term stability of the FE-5680 (which
is rather poor). But I doubt you will be able to improve much on
the long term stability, which is where things actually become interesting,
if you use a naive approach.

Nevertheless, I have not seen many 794/795nm diodes around. The only
one that I have the datasheet of is the one from Vixar.
You might want want to consider going for the D2 line instead of the
D1 line, as 780nm diodes are more commonly available than 795nm. You will
also need to buy several of those and select the ones that come closest
to the wavelength at the desired opearating conditions (usuall spread
is +/-1nm to +/-10nm). Do not assume you can tune more than 0.1nm with
temperature and current (rule of thumb is that you get about 10GHz
per °C and mA). If you need more tuning range, you will need to add an
external cavity (can give you up to 5nm range), which then needs to be
tuned to the 3.45GHz (ie it's length needs to be approximately 8-9cm).

Alternatively, you can get two S1-0780-XXX from Sacher Laser
(cost IIRC 2500€ each) and keep them 6.9GHz apart (using an optical PLL).
If you have enough money to spend, I'd go for two Cateye diode laser CEL's
from Moglabs (cost AFAIK 5000€ each)

No matter what you choose, you will need some wavelength stabilization
scheme. You can either do that with the vapor cell itself or use
an additional cell and do a DVALL or a saturated absorption locking.
Note that this addtional cell will need to be without buffer gas.
An external cell will offer better stability and thus lower noise,
which directly translates into higher stability.

As polarisation scheme, I suggest using σ+/σ- as it seems to be more
robust than the lin/lin schemes.

			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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