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

Magnus Danielson magnus at rubidium.dyndns.org
Mon Jun 11 18:52:44 EDT 2018



On 06/11/2018 09:52 AM, Attila Kinali wrote:
> 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
> anyways.

Considering that the bandwidth of the reference receiver for SDH/SONET
is 3/4 of the baudrate, and then using a 4-pole Bessel-Thompson filter,
I beg to differ regarding the expected bandwidth. The majority of the
modulated energy will be inside those 3/4 and you don't want much more
as it causes unnecessary problems.

But sure, sidebands will be damped and you can expect 6 dB slope there
from the bare diode.

This is stuff I can actually measure at home as I come to think of it.

>> 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. 

Consider the roll-off of the diode/modulator.

Also, expect to servo the amplitude of the modulation for stabilization.
This is separate from the servo of the center of wavelength as well as
the width of the modulation. CPT has more of these loops than a
traditional rubidium.

>> 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.

The DWDM lasers is quite clean and their temperature-stabilization is
what you want to have to start with. However, I don't think any of the
ones I have will match the frequency needed, but I may be lucky.

Cheers,
Magnus



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