[time-nuts] Photodiodes for high frequency OPLL

Attila Kinali attila at kinali.ch
Sat Mar 30 14:51:51 EDT 2013

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

The people on 4chan are like brilliant psychologists
who also happen to be insane and gross.
		-- unknown

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