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" <e...@telight.com>
Sent: Friday, March 29, 2013 8:20 PM
To: <time-nuts@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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