[time-nuts] Photodiodes for high frequency OPLL

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Mar 30 18:43:48 EDT 2013

The detectors don't have to be fast enough to keep up with optical 
carrier frequency as long as the incident optical power has a component 
at a much lower frequency.


ed breya wrote:
> 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.
> Ed
> 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
> 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.
> >
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