[Laser] differential detector

Wed, 21 Apr 2004 09:27:49 -0400

Yes. I did some experimenting with this idea. I used two of the OPT101
detectors feeding a LT1920 (620 type instrument amp). Inside without optics,
the noise from the florescent light was canceled. The OP101s and the LT1920
was run with gain. I didn't put the detectors behind optics yet.
	The idea is that the "noise" that both detectors see is COMMON MODE.
Instrument amps are used when you need a lot of common mode rejection. It is
very important to have the two input circuits identical (two matched PGP
front ends may work well). If you are operating where there is a lot of
"noise" from city lights etc. There may be a large benefit from the common
mode rejection at the sacrifice of the lowest noise figure. I was thinking
of placing one detector at the center of the telescopes optical axis and the
second slightly off center with a short baffle between them (to block light
scattered off the detectors).
	Optics better than a fresnel lens would be needed. The short focal length
fresnel lenes have a huge amount of spherical aberration. This is easy to
see by making a mask with a series of holes, 1/4 in, across the face. Then
point at the sun and focus on a white surface. The focal point of each pair
of holes falls at a different distance from the lens. At a particular
distance the light from all the holes is spread out. It can be so wide that
most of the aperture is wasted on most sensors.
	I think its a great idea and would like to hear how it works out.

walt
-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of [email protected]
Sent: Tuesday, April 20, 2004 10:22 PM
To: [email protected]
Subject: [Laser] differential detector

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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