A photodiode is in fact a nonlinear device for optical fields as it is
essentially a linear optical power detector.
The output is proportional to the incident optical power not the field
amplitude.
Photomixers are routinely used in wide range of diverse application such
as translating the frequency fluctuations of the (Mie) scattered light
due to Brownian motion of the colloidal particle sizes to baseband. The
size of the scattering particles can be inferred from the shape of the
resultant frequency spectrum.
An interferometer of itself (without a detector) is a linear device that
merely superimposes optical fields and will of itself produce no
difference frequency output.
Bruce
ed breya 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. 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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