[Laser] differential detector

[laser@mailman.qth.net]

I was browsing the web and found some articles on optical detection schemes.  
Most seemed to create a fuzzy spot above and behind my eyebrows ( OK, mostly 
above and beyond all of me. )  

I did get the idea that you can get some benefit from two detectors.  Trying 
to imagine how to build a device for experimenting, I thought of two 
photodiodes, cathode grounded.  The anode of each to the respective non-inverting 
inputs of a low noise FET input dual op amp.  The rest of the circuit is modeled 
after a high impedance differential input instrumentation amplifier, so that the 
differential output of the two op amps is a high gain of the difference 
between the two photodiodes, without loading either.

If you mount the two photodiodes in the focal plane of a telescope, they 
should get similar noise source input from the surroundings and the "ordinary" 
light coming in from the telescope.  If you then point the telescope at a signal 
source, such that its image falls on only one of the two photodiodes, then the 
differential circuit should respond to the signal, which is a differential.  
The common input should be rejected.

This scheme seems to have a significant benefit to small and point sources 
that are surrounded by noisy broad light sources.  An example would be a laser 
signal from across a brightly lit city.  Maybe it would work if a laser was 
pointed at a cloud.   I would think that it would not work well with a scattered 
signal as it would likely strike both photodiodes equally.  It does nothing 
about random noise that originates in the diodes and amplifiers.  In fact, it 
doubles the number of such noise sources.  ( I seem to remember if you add the 
noise from two random sources you increase the noise power by 1.414 not by 2.0 
because sometimes the two random sources cancel. )  Also, I think that you 
would need a least fairly sharp optics, not necessarily telescope image quality, 
but perhaps better than a plastic fresnel lens.

Anybody tried this?  Or suggestions?

James
N5GUI


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