[time-nuts] Photodiodes for high frequency OPLL

[Attila Kinali]

On Fri, 29 Mar 2013 17:20:35 -0700
ed breya <eb at telight.com> wrote:

> I don't think that you can effectively directly mix two laser 
> wavelengths in a semiconductor light detector and get a useable IF - 
> it's hard enough just to get the tens of GHz modulation signals out 
> above the noise floor, let alone a tiny difference signal between 
> hundreds of THz.

Well, i'd like to stay at 7GHz. That should be doable.
At least i have read at least a dozen papers who claim to have
done that. Some even from the 80s.


> My knowledge of this stuff isn't up to date - maybe nowadays there 
> are detector devices and methods that take care of this directly, but 
> I don't think so.

According to some papers i've read, just pointing two lasers
at a photodiode will do that. Most of those around 800nm do
offsets of a couple 10MHz to a few 100MHz at most. The ones
with the larger offsets are usually in the 1-1.5um region
and talk about applications for long haul highspeed communication.
 
> Most really high speed diodes are optimized for the 1550 nm region 
> where EDFAs work, but maybe they have usable response at other 
> ranges. It depends on your particular application and wavelength. I 
> think detectors are usually specified over the entire IR region, so 
> datasheets may tell enough.

Wavelenght is 780-800nm which is outside of the detection range of
those communcation photodiodes. 

> 
> Here's link to some good info, but not current state of the art:
> 
> http://e-collection.library.ethz.ch/eserv/eth:28429/eth-28429-02.pdf

Thanks. I have read this already.
I thought about trying to contact the author, and ask him a few
questions about this project.

> There are various methods that use lower frequency modulation 
> techniques so that regular detectors can be used directly. If you 
> study up on related patents, you may find some ideas and leads to 
> appropriate actual devices.

I have not come accross any that seemed simpler.
Can you give me some pointers?

			Attila inali

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