[Phys-L] Re: Travel distance in a waveguide.

[Bob Sciamanda <trebor@WINBEAM.COM>]

1) This summary of the TE01 mode of a rectangular guide will be helpful:
http://www.microwaves101.com/encyclopedia/waveguidemath.cfm#velocity
Most apropos' is the Group Delay equation, at the very end.

2) Yes, n is an "effective"  (group) index of refraction.
> From http://www.fiber-optics.info/glossary-g.htm :
"Group Index: Also called group refractive index. In fiber optics, for a
given mode propagating in a medium of refractive index (n), the group index
(N), is the velocity of light in a vacuum (c), divided by the group velocity
of the mode."

3)  Michael E.  asked:
|Putting my first two questions together, my third question becomes... is
|this apparatus a fraud?  Has the company simply said the refractive
|index of the fiber is 1.50 so the calculation for c comes out correct?
|That is, might we say n = 1.50 is an effective refractive index that we
|determine from knowing c, rather than vice-versa?

I wouldn't call this a fraud. It is easy (at least in the TE01 mode) to
directly show that the superimposed zigzagging waves add up to a wave
pattern which  is a standing wave in the transverse directions and a
traveling wave ( with the "guide" velocity) in the longitudinal direction.
I don't think assigning an "n" to this composite wave is any different from
the assignment of an "n" to the wave in a block of glass.  The velocity of
propagation in the glass block is also the the velocity of a superposition
(resultant) wave.

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
http://www.winbeam.com/~trebor/
trebor@winbeam.com