[Laser] Polarization Subtraction

[Tim Toast]

Thanks for all the responce to this,

The two lens idea was mine and i tossed it in there with all 
that. The article though mentions a single lens with a special 
beam splitter. Yes, i think now the slight difference in angle 
of a two lens capture would lead to a sort of non-simultaniousness
that is not intended by the article. Not to mention a totally different, although similar, scene instead of "the same scene". 
It sounds to me that they are using a single lens with a beam 
splitter to get the two polarised images. Either a polarizing 
prism type splitter that provides (I think) one polarized image 
and one unpolarized one which would need a separate filter of its 
own to provide the two orthogonal images -or- a plane beam 
splitter that each image would need a polarizing filter. 

The key element is "at the same time" and not sequential though -
which negates my thought that two lenses might work or some sort 
of alternate frame capture for a ccd. You would think though that 
any polarization changes going on would be slow ones at rates less 
than say, 100 Hz but i don't know for sure things aren't 
happening much quicker than that. Things aren't always as they 
seem of course! We've learned that over and over :)

-toast

> 
> Today's Topics:
> 
>    1. polarisation subtraction (stuart.wisher at talktalk.net)
> 
> 
> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Wed, 17 Oct 2012 06:10:29 -0400
> From: stuart.wisher at talktalk.net
> Subject: [Laser] polarisation subtraction
> To: laser at mailman.qth.net
> Message-ID: <8CF7A66C49337D1-FAC-75AE at webmail-vfrr16.sis.aol.com>
> Content-Type: text/plain; charset="us-ascii"; format=flowed
> 
> 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
>