[time-nuts] Photodiodes for high frequency OPLL

jmfranke jmfranke at cox.net
Sat Mar 30 13:46:51 EDT 2013

I used UDT PIN10 photodiodes to observe the mode spacings in HeNe lasers. 
The typical mode spacings were around 600 MHz.

John  WA4WDL

From: "ed breya" <eb at telight.com>
Sent: Friday, March 29, 2013 8:20 PM
To: <time-nuts at febo.com>
Subject: Re: [time-nuts] Photodiodes for high frequency OPLL

> 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
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