[Laser] sensor vs focal plane

[TWOSIG at aol.com]

Good point about fresnel lenses.  They may well have a focal plane dot  that 
is bigger than the photosensor.  All optical systems have some  limitations to 
the dot size.  For optical communications, the acceptable  errors are much 
bigger than for cameras.  So if this technique has any  value, the sensor size 
would need to be bigger than the minimum dot size.
 
Why this discussion has me thinking about reflector optical systems I don't  
know, but I have two comments:
 
In amateur telescope making, it is well known that it is much easier to  make 
a spherical mirror than one that has better optical qualities.  What  would 
be a terrible scope for viewing would be many times the precision needed  for 
collecting photons on an optical communications sensor.  The mirror  shape is 
not as critical, and distortions caused by mechanical and thermal  stresses in 
the material of the mirror probably would not be significant.
 
 
In the history of reflector telescopes, one of the early materials used to  
make telescopes was a copper alloy.  It could be cast, ground to spherical  
shape, and polished before the techniques were developed to silver and now  
aluminize glass mirrors.  However, I found out that the reflective  characteristics 
of copper are almost as good as aluminum, if you limit the light  frequency 
to the red and infra-red spectrum.  Maybe copper or brass would  be an 
acceptable material for a reflector optical system for  communications.
 
James
N5GUI
 
 
 
 
 
 
In a message dated 3/26/2005 12:30:22 A.M. Central Standard Time,  
n9jim-6 at pacbell.net writes:
Note that the fresnel lenses usually do not focus  quite as well as some of 
the
full size lenses. The result is a smearing of  the focus, resulting in 
slightly
larger area.

Another attribute to the  fresnel, is that they have a fair amount of 
chromatic
distortion. (focal  point varies with light color or frequency) Be carefull to
focus the correct  color to the sensor.

Jim
N9JIM/6

--- TWOSIG at ol.com  wrote:

> 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