[time-nuts] Photodiodes for high frequency OPLL

[Wolfgang]

On Monday 01 April 2013, Bruce Griffiths wrote:
> Back to the original problem:
>
> An AOM could be used to generate a sideband 7GHz above (or below) the
> output of 1 laser which could then be mixed with the output of the other
> laser using a narrow bandwidth photodiode.
>
That's going to get tough. AOMs usually stop at some hundret MHz and 
even if you do quad-pass (2 polarizations, 2 directions), you will have 
a hard time getting to 7 GHz. So you'd have to cascade several ones. 

You could, however, use an EOM theoretically, but again, considering 
price and bandwidth constraints, you will only find suitable ones for 
telecom wavelengths. 

On Monday 01 April 2013, Attila Kinali wrote:
>> [APD...]
> 
> I think the gain modulated approach to downconvert the signal should work.
>
Anyway, each of these down-mixing approaches needs to resolve the 
mirror frequency problem. I.e. if you modulate your receiver with 
7.1 GHz (or use an EOM to do that optically) and detect at 100 MHz, 
you do not know whether the input signal has an offset of 7.2 GHz 
or 7.0 GHz. 
So you might be locking on the wrong one. 

The AOM would not have that issue because it is single sideband. 

--Wolfgang, DL1SKY