Re: Optical dispersion in fibers

[John Denker <jsd@MONMOUTH.COM>]

At 07:53 PM 1/28/00 -0600, brian whatcott wrote:

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

The peripheral rays issue has to do with the spatial degrees of
freedom.  That's a problem, even in the absence of dispersion.

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

--------

At 06:53 PM 1/28/00 -0500, Ludwik Kowalski wrote:

> Dispersion does prevent long range communication. Right?

It doesn't prevent it, but for long fibers (e.g. transoceanic) it is a
major issue that must be dealt with.

> In fact, optical fibers would not work for long distances if
> the modulated light were not produced by a laser, or another
> highly monochromatic light sources.

The whole point is that you want to modulate the laser at GHz rates or
more.  Therefore
  a) The laser had better have a natural (unmodulated) linewidth small
compared to the modulation rate, and
  b) During use, the modulation guarantees that the light you care about
isn't monochromatic.  Dispersion will occur.

At 10:55 PM 1/28/00 -0500, Ludwik Kowalski wrote:
>
> It also evokes achromatic lenses in which inventivness was
> used to overcome limitations imposed by optical dispersion.
> Two dispersions nearly cancelling each other.

That's a much better way to look at it.  At the transmitter and the
receiver, all sorts of trickery is used to undo the effects of dispersion.