[time-nuts] Photodiodes for high frequency OPLL

[Bruce Griffiths]

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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