[Laser] sensor vs focal plane

[TWOSIG at aol.com]

In a previous post, Tim Toast  stated:  
 
",,,I had read somewhere that under-illuminated diodes (when the focal spot  
is smaller than the total area of the diode) would have more noise than if you 
 used the whole surface area -..."

I am wondering if anyone has more information on the issue of trying to  
illuminate a "large" area of a photo sensor.  Tim, if you can find a  reference on 
under-illumination, I would like to check into it further.
 
As I see it, if you put the photo sensor at the focal plane, the incoming  
signal forms a small dot that is limited by the apparent size of the source and  
the quality of the optics in the collector.  If you move the sensor forward  
or aft of the image plane, then the dot gets bigger.  As long as the  dot 
still falls entirely on the sensor, you still capture all of the  photons.
 
The first question I have under those conditions is: does the photo sensor  
work better, worse, or the same, when the same number of photons are clustered  
together into a small dot, or if they are spread out over most of the surface 
of  the sensor.
 
If your signal has neighboring noise sources in your field of view, there  
may be a way to use the sensor off of the focal plane to capture all of the  
signal photon, but reduce the noise photons.  Consider two types of noise  
sources, point sources and area sources.  In what follows, consider an area  source 
to be a large number of proportionately weaker point sources scattered  over 
the area.
 
First take the case of a single point source.  Removing the noise  should be 
as simple as shifting the direction of the optic path so that  the offender is 
no longer in the field of view.  Shifting the sensor  out of the focal plane 
is not needed.
 
That does not work if you have multiple point sources that come into the  
field of view as you move some of them out of the field of view.  To deal  with 
this problem, one thing that you can do is to reduce the field of view,  either 
by increasing the effective focal length of the optical path, or masking  off 
part of the sensor.  Either way the device will be harder to aim.   It may be 
that if the photo sensor is more effective at converting the photons  into 
electricity when they are bunched together, then it will be the better way  to 
attack the problem of neighboring noise sources.
 
On the other hand, if the photo sensor is more effective when the photons  
are dispersed over most of its surface, the there are two advantages placing the 
 sensor further away from the objective lens.  If the signal source is  
expanded to an area nearly equal to the sensor, then a noise source will be  
expanded to a similar size, and unless it is very close to the desired signal,  some 
of its photons will fall beyond the boundaries of the sensor.  Hence,  the 
noise power will be reduced.  The increased distance from the objective  will 
result in a proportional increase in the spacing of the signal source from  the 
noise source on the sensor area.  That in turn will increase the spill  over 
of the noise without increasing the loss of signal.  There will be a  small 
narrowing of the field of view,, but it will not be as dramatic as masking  the 
sensor or increasing the focal length.
 
There is another advantage to moving the sensor away from the focal  plane.  
The larger dot will transition across the boundary of the sensor  slower.  If 
the system has a sharp focus, the photons in the signal will be  more likely 
to be completely on or completely off the sensor.  A large dot,  is more likely 
to have some of the photons spill off the sensor, resulting in a  fade 
instead of a sharp signal loss.
 
There is still one more way to make use of a sensor that is further away  
from the objective.  That is to put an adjustable iris at the focal  plane.  To 
acquire the signal you open it up so that you have maximum field  of view.  
Once the system is locked onto the signal source, you stop down  the iris, which 
will limit the field of view.  You can then use fine  adjustments to point the 
system more accurately at the target, and perhaps stop  down the iris even 
further, to eliminate more noise.  The noise that this  system will eliminate 
probably will not cause problems finding and tracking the  signal, but would 
limit the data transfer rate.
 
In all of this discussion, I would probably prefer to use a bigger  objective 
lens (more signal received) and longer focal length (smaller  field of view, 
less noise), and more accurate pointing (needed because the field  of view is 
more limited), to improve signal to noise characteristics.   However, these 
ideas may be able to squeeze a little more performance out of a  system that 
cannot be modified in size and focal length.
 
 
James
N5GUI