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

Attila Kinali attila at kinali.ch
Mon Jun 11 03:52:02 EDT 2018

On Sat, 9 Jun 2018 23:28:29 +0800
mimitech mimitech <mimitech at gmail.com> wrote:

> Just do a little bit research on 1560nm telecom laser transceiver. The
> closest wavelength is 1560.61nm (DWDM channel 21) as defined by ITU.
> Usually telecom laser transceiver module uses DFB (distributed feedback)
> type laser diode instead of VCSEL laser. However, the DFB LD should be
> capable of direct modulation, so there maybe not much difference with VCSEL
> in terms of usage. For the modulation bandwidth, I guess a 2.5Gbps module
> should be OK for 1.71GHz analog modulation.

A 2.5Gbps module should be good even for 3.4GHz modulation. The 3dB frequency
of should be higher than the baud rate used. Besides, even if the 3dB frequency
is below 3.4GHz, that does not matter, it just means that your sidebands will
be damped (second order low pass, IIRC), but you don't need that much power

> So, imagine I modulated 1.71GHz microwave signal onto 1560nm laser wave,
> then used a KPT non-linear optical crystal to double the frequency,
> hopefully I could get 780nm laser with +/-3.42GHz sideband spectrum, that
> should be suitable to shine on a Rubidium vapor cell and trigger CPT
> effect. I don't know if I understand this process correctly, many details
> must be missed.

Using a non-linear element will give you lots of intermodulation products.
Ie, your sidebands will be +/-1.71GHz, +/-3.42GHz, +/-6.84GHz,...
So you will have to be a bit carefull with the laser tuning in order to
get the right harmonics. 

An alternative approach would be to use an EOM after the doubler
to get the sidebands. These have bandwidths in the order of 10-20GHz,
so applying 3.42GHz shouldn't be a problem.

BTW: a lot of the frequency doubler architectures for high stability
lasers use resonant cavities, to increase efficiency and decrease noise.
You will either have to tune the free spectral range of the cavity
to a divisor of 6.84GHz or use a non-resonant one.

> Another question is, after passing through the KPT doubler crystal, whether
> the light remains single mode and linear polarization mode or not? If
> someone could provide any relevant information and suggestion, thanks very
> much.

If I understood the optical process correctly (disclaimer: I'm not
a physicst and have never worked with optical systems), then the multiple
modes are a problem of the laser source, not of the doubler. As multi-modes
are a problem for high speed communication, I expect telecom lasers to
be quite clean. You will have to ensure that you are not operating it
close to a mode jump, though.

A side note on the doubler approach: The group at UniNE around Gaetano
Mileti and Christoph Affolderbach focuses only on double-resonant
Rb standards. That means they do not modulate the laser. As they
have gotten the whole standard to the SNR limit, they are now focusing
on getting the shifts due to laser (frequency and intensity shift)
and cavity (low Q cavity to avoid pulling) down. The current state
of affairs is quite nicely documented in their two papers at 8FSM[1,2,3].

			Attila Kinali

[1] Proceedings of the 8th Symposium on Frequency Standards and Metrology

[2] "High performance vapour-cell frequency standards", 
Gharavipour, Affolderbach, Kang, Bandi, Buret, Pellaton, Mileti, 2015

[3] "Pulsed Optically Pumped Rb clock", 
Micalizio, Levi, Godone, Calosso, François, Boudot, Affolderbach, Kang,
Gharavipour, Gruet and Mileti, 2015

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