[time-nuts] Photodiodes for high frequency OPLL

ed breya eb at telight.com
Sat Mar 30 18:29:11 EDT 2013

Ooops - never mind. I wrote before my memory was updated. My 
experience in E-O stuff was years ago using AM at relatively low 
frequency, and nowhere near the lasers and microwave/gigabit/sec 
stuff - I didn't think the detectors were fast enough to actually 
keep up with the optical carrier frequency. I was also picturing 
wavelength/spatial separation with interference in order to allow 
relatively slow detectors to see it, or mixing in nonlinear optical materials.


Bruce Griffiths wrote:

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
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.


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.

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