[time-nuts] optically pumped frequency standards

[Bruce Griffiths]

Magnus Danielson wrote:
> On 01/14/2011 07:24 PM, Bruce Griffiths wrote:
>> ehydra wrote:
>>> Hi!
>>>
>>> By modulating the laser diode current you can modulate the diode
>>> temperature. And that changes the wavelength.
>>>
>>> - Henry
>>>
>>>
>> At 3.4GHz??
>> Modulation at 3.4 GHz required by an Rb standard so that the frequency
>> difference between the 2 first order sidebands is 6.8GHz.
>>
>> Actually all you need to do is phase modulate the output.
>
> What they actually do is set the DC current for wavelength and AC 
> modulate at 3.4 GHz for locking frequency. It's obvious if just 
> looking into one of the papers I linked earlier.
>
> If you look at the Symmetricom paper for the SA.3X they also describe 
> the overall algorithms of locking in the wavelength etc.
>
> Will have to read up on it.
>
> Cheers,
> Magnus
>
The modulation mechanism has to be other than thermal expansion to 
achieve the required bandwidth.
Modulating the current density will also modulate the refractive index 
of the laser crystal.
To achieve the required modulation bandwidth one has to use something 
like a VCSEL.
With other lasers such as an ECDL which dont have a sufficiently large 
modulation bandwidth an external electrooptic phase modulator is required.

Bruce