[time-nuts] Photodiodes for high frequency OPLL

[ed breya]

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. You need an optical interference or nonlinear device 
up front to do the "mixing" and get the wavelength discrimination, 
while the optical detector(s) serve as the first IF O-E transducer.

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.

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.

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

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.

Ed