Re: Resolution Beyond the Classical Limit

"Skywise" <into@xxxxxxxxxxxxxxxxxxxx> wrote in message
news:13tel70p8qsuc14@xxxxxxxxxxxxxxxxxxxxx
"Marc Reinig" <Marco@xxxxxxxxxxxxxxxxx> wrote in 47d6d523$1@darkstar:">news:47d6d523$1@darkstar:

However, you can also use a prism and shine a beam into it. Under total
reflection, there is an evanescent wave at the air-glass reflecting
interface. If this interface is placed near the core of a fiber (within
a
wavelength but not touching and assuming the fiber can support a mode for
the light) the wave will coupled into the fiber and propagate.

So, does all the light couple? If so, I presume the total reflection
disappears.

The light couples and the amount of coupling and the corresponding drop in
reflection depends on the distance of the two surfaces.

Also, is this more advantageous than direct coupling with a lens?
I presume it must be easier to align or something?

Definitely much easier considering a single mode fiber might have a core
diameter of 6 um and you need to focus your input beam on that. If you are
using high power lasers and all the energy is focused on a 6 um spot you
have a very high energy density. If you get misaligned and wander off the
end a little you could have a very brief, spectacular end to the end of your
fiber. ;=)

In evanescent coupling, if you misalign, less than optimal coupling is
achieved and the energy not coupled is just reflected (where presumably you
have an absorber any way) and no energy is coupled to places you don't want
it coupled.

Is there a name for this technique so I can go Google it?

I think it is just called evanescent coupling. On the other side it is
called frustrated total internal reflection (FTIR). The effect is used a
lot in fiber to fiber coupling.

Here are a few references. I think it is also used in some beam splitters,
where two prisms are separated by a sub wave length space, allowing the
relative intensities of the splitter to be modulated by changing the
distance (perhaps only in manufacture).

http://cirl.com/evanescent.php
http://www.photonics.com/content/spectra/2006/November/research/84974.aspx
http://fiber.kaist.ac.kr/exp/prism.pdf
http://scienceworld.wolfram.com/physics/FrustratedTotalInternalReflection.html

Marco
________________________
Marc Reinig
UCO/Lick Observatory
Laboratory for Adaptive Optics


Brian
--
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