[Laser] polarisation subtraction

[stuart.wisher at talktalk.net]

Very interesting Tim,

Having read the replies to this thread so far it seems to be getting a 
little off the original track with references to sequential imaging 
rather than parallel imaging. I have a double Fresnel lens box which 
could house two identical receivers with polarising filters at 90 
degrees to one another. I am interested in sub-carrier optical 
communications around 20 kHz and I do not think it would be a difficult 
task to  combine the signals in a differential amplifier at this 
frequency. Come to think of it, I could include a summing amplifier 
also just to see (hear?) the resulting signals.

Being cautious about this, there are some problems even with two lenses 
side by side as the scintillation would result in quite different 
signal characteristics. I have used Schlieren photography to image this 
effect and the cells are of the order of 50mm in size at the time I 
made the observation. This would result in differnt fade 
characteristics. Without the polarising filters and using a summing 
amplifier I could combine the apertures and reduce noise a little, 
effectively spatial diversity reception and not what is intended here. 
Maybe the cure to the first problem is to use the same aperture for 
both and use a beam splitter that would also act as a polarising 
filter, I am a little hazy here but I seem to remember Icelandic Spar 
has birefringent properties, I must go and remind myself on this topic.

Another problem is the lack of signal at distance here in the misty 
atmosphere in the UK, although I was impressed by the background 
appearing out of nothing on one of the photos in the article

An interesting concept though...


Regards to all,

Stuart, G8CYW