[Phys-L] Re: fiber optic cables and waveguides

["John S. Denker" <jsd@AV8N.COM>]

On 03/04/05 01:08, Bernard Cleyet wrote:
> "Well, in that sense a hall of mirrors could be called a waveguide.
> I suppose that it is etymologically true, but at some point it stops
> being a useful physical model.  (Here I am referring to the "solve
> Maxwell's Eq. with boundary conditions" model, as opposed to the
> "light ricochet" model.)"
>
> I think tho the mechanisms are different the equations are the same

Hmmmm.

To my way of looking at it, the mechanism is the same.  It's
just Maxwell equations with boundary conditions.  There are
some simplifying assumptions you can make in certain limiting
cases -- such as restricting attention to the 0,0 mode -- but
we are not required to make such assumptions.

> So the hall of mirrors is the
> limit where group v is ~= C.

Yes.

One could equally well characterize the hall of mirrors as
the case where the mode-number is huge, a long ways from
0,0 ... and derive v ~ C as a consequence.


> "Huh?  Perhaps we have different ideas of what is
> "easily-observable"?"
>
> I came to this conclusion ("back of envelope" calc.) and then
> realized JD was assuming the fiber was kilometers long (original
> expt. description). However, I am a bit lost on how to separate the
> index effect from the waveguide one.

Second issue answered first:  study the propagation speed
as a function of mode number.  A mode with one node will be
twice as scrunched as one with none.

As for the first issue:  Build an interferometer with fiber
in one arm and something else (free space, or just a different
fiber with a larger core) in the other arm.  The fringes
will shift as a function of mode number.  You will need to
use *less than one meter* of fiber because the effet is so
large compared to the sensitivity of the method.

A key part of the experiment is controlling which mode you're
injecting into.  There are standard techniques for doing this.

I know somebody who likes to pipe her lasers through a
single-mode fiber ... to achieve the best possible spatial
filtering.  This is for atom-trapping experiments;  she's
not interested in measuring the fiber, just using it as an
easy trick of the trade.

If you want to know my idea of what's easy and what's hard:
atom trapping is hard.  The single-mode fiber trick is
less than 0.01 percent of the trickiness required to make
the atom trap work.


> Returning to my early suggestion of using co-ax., RG/62U
....
>  co-ax doesn't exhibit waveguide dispersion.

Indeed!  Coax physics is different from waveguide physics.
No cutoff frequency, for starters.
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