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

[sampere <sampere@PHYSICS.SYR.EDU>]

Another excellent question for tap-l! Cliff Bettis from the University
of Nebraska has been doing a great speed of light experiment for years.
He pulses a diode and triggers a scope with the pulse.  The pulse
reflects off an elaborate reflector - one purchased from an auto store
and returns to a photodiode. The time between pulses is easily measured.
You don't need fiber, nor waveguides, nor ... Just a pulse generator
(already have that), a fast scope such as a tek TDS series, a fast diode
and photodetector.

Sam

Bernard Cleyet wrote:

> Very quick answer:  (others will probably criticize and amplify.)
>
> My understanding is that th fiber does work like a wave guide.  I
> suspect this is true if the relative dimensions are the same.  A quick
> search of the webb will reveal the truth or falsity.  [aside, I think
> the graded N is so total internal reflection will occur at shallower
> angles than the Brewster's (right word?) determined for 1.5]
>
> The chase:  the formula for the group speed down the guide  is v sub g =l.
> guide dimension is for a rectangular guide with E perpendicular (lowest
> mode).
>
> Berkeley Phys. Lab. B 10 explores this including deriving the group and
> phase speeds.
>
> bc who suspects the experiment is a fraud.   A less fraudulent
> experiment is to use coaxial cable driven by the pulse generator.
> Compare the predicted from the cable's Z w/ measured.   A very common
> xpt. uses a fast pulser, corner reflector (plastic bicycle or shop
> entrance ones will do) and diode and detector.  A little of the outgoing
> light is fed (beam splitter) immediately into the detector to start the
> counter or sweep and the return stops the counter or is seen on the scope.
>
> p.s. better use for the apparatus is to determine N.  In the case of a
> commercial communication fiber it'll be an equivalent N.  there  are
> many books on fiber optics both very technical and for the amateur
> experimenter.  Sonoma state (Cal. State U @) has a course just on fiber
> optics.
>
> Joseph Bellina wrote:
>
>
>
> > I can't answer you questions, but I will tell you I've done a similar
> > experiment in air...I used a modulated laser.  Perhaps the intensity is
> > sufficient to do the same.
> >
> > cheers,
> >
> > joe
> >
> >
> >
> > Michael Edmiston wrote:
> >
> >
> >
> >
> >
> > > We have an apparatus for measuring the speed of light in which a red LED
> > > is pulsed at about 800 kHz; the pulses travel through a 20-meter fiber
> > > optic cable; an oscilloscope is used to measure the time the light
> > > pulses take to go through the cable.
> > >
> > > The company advertizes the refractive index for the fiber is 1.50 for
> > > the LED being used.  The directions for the students say to calculate
> > > the observed velocity as delta-x over delta-t using the length of the
> > > cable for delta-x.  Then find c = nv using 1.50 for n.
> > >
> > > The students originally think a fiber optic cable works by multiple
> > > reflections, and therefore suspect the actual pathlength of the light is
> > > much greater than 20 meters.  I have told them that fiber optic cables
> > > work more like a wave guide; however, I realized I don't know exactly
> > > what that means in terms of path length because I have no experience
> > > working with wave guides.
> > >
> > > My first question for this forum is... when a wave goes through a
> > > waveguide, what is the effective length of the guide.  Is it just the
> > > physical length?
> > >
> > > My second question is... it is my understanding that an optical fiber is
> > > constructed so the refractive index is continuously graded from a higher
> > > refractive index at the center to a lower refractive index near the
> > > outside.  If this is true, what does it mean to refer to a cable as
> > > having a particular refractive index?
> > >
> > > 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?
> > >
> > >
> > > Michael D. Edmiston, Ph.D.
> > > Professor of Physics and Chemistry
> > > Bluffton University
> > > Bluffton, OH 45817
> > > (419)-358-3270
> > > edmiston@bluffton.edu
> > --
> > Joseph J. Bellina, Jr. Ph.D.
> > 574-284-4662, 4968
> > Saint Mary's College
> > Dept. of Chemistry and Physics
> > Notre Dame, IN, 46556