[Laser] polarisation subtraction (stuart.wisher at talktalk.net)

[Dave]

On 2012-10-17 12:02 PM, laser-request at mailman.qth.net wrote:
> 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.

Mostly reasonable.

> 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.

Icelandic Spar is Calcite, which is rather strongly birefringent.  There 
are,
of course, other birefringent materials.

http://en.wikipedia.org/wiki/Birefringence
http://en.wikipedia.org/wiki/Birefringence#Examples_of_uniaxial_birefringent_materials

Plus, there's also the Brewster Angle, which could be exploited:

http://en.wikipedia.org/wiki/Brewster%27s_angle

Or, you could use a partially silvered mirror/beam splitter and two
polarizing filters.

http://en.wikipedia.org/wiki/Partially_silvered_mirror
http://en.wikipedia.org/wiki/Polarizer

> 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

Ah, yes, atmospheric absorption/scattering is a pain.  :-(

> An interesting concept though...

I have to wonder if Faraday rotation may rear its head?

http://en.wikipedia.org/wiki/Faraday_rotation

> Regards to all,
>
> Stuart, G8CYW

Dave
WA4QAL