[Laser] variable field of view for noise elimination

[TWOSIG at aol.com]

I put some ideas together for improving a light communication system.   If 
this is useful, please use it.  If I have made any funamental errors,  please 
forgive me and let corrections be known.
 
I have assumed that the signal and any external noise sources can be  treated 
as point sources at infinite distance and are not degraded by the  relative 
large area of the suggested sensor (a "large" real sensor should  respond less 
well to higher frequencies than a " small" one).   Also the sensor performs 
the same with the same number of photons striking  the sensor whether in a small 
spot or larger area.  This is geometry, not  solid state physics.
 
 
 
Suppose you are building a light communication receiver with a 100  mm
F/2 lens and a 10mm square sensor.  Assuming a transmission of 90%,  that
gives an effective aperture of 7000 sq mm.  ( Or said another way,  a
gain of 70 compared to the sensor alone. ) If the sensor is set at  the
focal distance, the field of view of the system will be 50 mR X 50 mR.  (
A "square" about 2.86 degrees on a side.  Compare that to the full  Moon
which is about a half of a degree in diameter. ) 

With the sensor  set at the focal distance, the signal photons fall on a
small area of the  sensor.  Theoretically, on a single point.  If the
signal source is  off the optical axis of the system, the response will
not change up to about  49.999 mR.  The signal will be completely gone at
50.001 mR off axis, so  the signal will degrade very sharply.  This
characteristic can used to  eliminate a strong noise source that is close
to the signal, but obviously  cannot be used to eliminate multiple noise
sources that are spaced arround  the signal.

Consider what happens if you move the sensor further away  from the lens,
that is aft of the focal distance.  The signal photons  strike the sensor
over a larger, though still well defined area, or  spot.  In this
example, the spot size will be 5 mm in diameter at a  distance of 210 mm,
or 10 mm aft of the focal distance.  The field of  view will no longer be
sharp edged.  All of the signal photons will  still fall on the sensor in
a field of view that is 23.81 mR X 23.81 mR, but  beyond that, some
photons will spill over the edge of the sensor.  All  of the photons will
miss the sensor outside a field of view of 71.43 mR  square, but the
signal will probably be well below the system noise level  much beyond 65
mR.  I find that it is interesting to note that the 50%  signal level, or
3 dB down from the peak, is a field of view that is 47.62 mR  X 47.62 mR
with rounded corners.

If you move the sensor further, to 20  mm aft of the focal distance, the
100% field of view is zero.  Only a  signal on the optical axis will
receive full gain.  The 50% FOV drops to  45.45 mR X 45.45 mR, again with
rounded corners.  The 0% FOV expands to  136.36 mR square.  

Consider one additional possibility.   Suppose that you have an
adjustable field stop at the focal distance and the  sensor is aft.  The
field stop has the same sharp edges that the sensor  exhibited when it
was at that distance.  With the stop wide open, the  sensor has wider
field of view at reduced signal strength.  These soft  edges that can be
used to peak the signal.  Then any noise sources, even  ones close to the
signal source, can be eliminated by closing down the field  stop.
 
 
By controlling where you place the sensor relative to the focal distance,  
you can control the field of view of the system, the sharpness of the edges of  
the field of view, and with a device similar to a camera iris, you can control 
 the received field of view.

James
N5gui