Re: Optical dispersion in fibers

[Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>]

brian whatcott wrote:

> One way has been the graded fiber which refracts
>  peripheral rays towards the axis in a time locked
> fashion.

Is it not true that the geometrical idea of "axial rays
arriving there before peripheral rays" does not apply
to monomode fibers? If true then the graded fiber
approach works only for older multimode fibers.
The geometrical path limitation has nothing to do
with dispersion, even ideally monochromatic light
is subjected to that limitation.

And, as John wrote,
> Solving the spatial problem doesn't get rid of dispersion
> (which occurs in frequency-space, even in a fiber with
> a single spatial mode).

The optical amplification, however, should be useful in
overcoming limits imposed by dispersion.

> > But I find the distributed optical amplifiers exciting,
> > in that the need for extraordinary measures to reduce
> > dispersion can be avoided by regenerating the pulse
> > train.

Brian is not referring to amplifiers which need electrical
power supplies, they are to be powered by light (whose
wavelength is different from that of the information carrier)
Optical amplifiers are inserted pieces of fiber which operate
like lasers; they receive weak signals at the input and produce
strong signal at the output.

Can somebody describe these optical amplifiers (doped
fibers) for us at the level appropriate for an introductory
physics course? I remember reading about them about two
years ago.  Are they already in use?
Ludwik Kowalski