[Laser] Polarization Subtraction

[Tim Toast]

More about the camera and adapting the concept to optical comms.

At first i thought of the rotating or electronically switched 
(LCD)filters too but i guess it's not that complicated really. 
At least in the case of this special camera. Just a lens + plane 
beam splitter with two fixed polarization filters at right angles 
and two CCD's in their camera. 

The scheme appears to work over whatever the exposure time they 
used to take the photo's, 1/1000th to 1/10th a second or so. I 
suspect in the case of our optical comms, with photodiodes instead 
of CCD's, it would work continuously, with the two polarized 
signals being continuously subtracted in real time. And with 
enough signal to make the loss in light level tolerable due to 
the splitter (-3 dB?) and the filters (-4 or -5dB ?) I think some 
only let 30 percent of the light through. It seems that this could 
not work with very weak signals. It could be that this thing is 
only usefull in fog with moderate optical signals - wouldn't that 
be strange?!

It may be that the path length from the single lens optic to the 
focal points of both beams are important. With a plane ordinary 
beam splitter, the beams are going to be exactly the same 
length assuming the photodiodes are positioned well enough 
- normally within a millimeter or so either side of the focal 
point. I think light travels about a foot per nanosecond. So the 
two beams would be well within that distance. There would need to 
be a whole foot difference in the path length to have a one 
nanosecond difference in arival times for the photons. This is 
very - very simultaneous! A few millimeters of light travel time 
- juat a few picoseconds.  
Not that the polarization in the scene is changing at ultra fast 
rates but if it were, this capture device should still get a 
"temporal frame of any length" nearly simultaneously - within 
a few picoseconds. I think most of the polarization info is 
almost static anyway and not changing very fast other than 
being jumbled around a bit by the air pockets or cells that 
cause scintillation sometimes. The fog or haze is scattering the 
light photons randomly but is it scattering the polarization 
angles too? Or is it affecting the scene polarizations in other 
ways? IDK. Maybe the polarization info in the image or scene is 
not affected by scattering fog at all. Meanwhile, the degree 
of simultaneousness in the camera exceeds all that by an 
enormous amount.


-toast