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

"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 -- especially seen if the derivation of "my" eq. is the geometric one, as it is in the Berkeley Physics Lab. manual i.e. the manual makes no assumptions about the mechanism. (I pray I remember this correctly; it's been a coupala days.) So the hall of mirrors is the limit where group v is ~= C.

"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.

Returning to my early suggestion of using co-ax., RG/62U, if I remember correctly, has a different * construction from, say, 58 or 59. by making measurements on both perhaps one may separate. Unfortunately, I fear my thinking may be just muddled.

* the dielectric is a shell instead of solid, and the center conductor is sl. coiled.

bc

p.s. Am muddled, co-ax doesn't exhibit waveguide dispersion.

James McLean wrote:

Sorry for the long delay between postings....

John S. Denker wrote:

On 02/26/05 02:56, James McLean wrote:

How can it be that a fiber optic cable "works like a wave guide"?

It's a waveguide in the sense that it guides the wave.

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.)

The apparatus that we have has a single-fiber cable, the fiber being
maybe 0.75mm in diameter (I don't have it handy to measure exactly).

relevant dimension is the core diameter, which is a
whooole lot smaller.

Oh yeah, of course. Actually, I realized this about 48 hours after my
posting. Sadly, the equipment and manual for the apparatus provide no
information at all about these dimensions; not a very promising
reflection on the product...

I'm sure that the light source is probably not visible but it probably
is in the IR. So if the wavelength is 1 micron, we're in mode 1500 of
the waveguide? That doesn't sound very "scrunched" to me!

Assuming what you've got is communication fiber, it's
probably 1.06 micron light in a 50-micron core. OK,
that's not _very_ scrunched, but even for the 0,0 mode
there would be an easily-observable effect on the speed
of propagation (which is where this thread started).

Huh? Perhaps we have different ideas of what is "easily-observable"?

According to Bernard Cleyet's equation, which you seem to have condoned,
v_g = c*sqrt[1- (free space wavelength/2*waveguide dimension)^2],
I get v_g = 0.999944c. For the fiber, I suppose that this would have to
be divided by the core material refractive index.

I suppose this difference could be measured by some techniques, it being
only 1 order of magnitude less important that the difference between
vacuum and air. But it is a level of accuracy well beyond the apparatus
in question.
--
Dr. James McLean phone: (585) 245-5897
Dept. of Physics and Astronomy FAX: (585) 245-5288
SUNY Geneseo email: mclean@geneseo.edu
1 College Circle web: http://www.geneseo.edu/~mclean
Geneseo, NY 14454-1401

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