Imagine a glass micro-pipette whose inner diameter is two
microns; it is 9 cm long. At both ends it progressively
(over a distance of 3.5 cm) becomes a pipette whose inner
diameter is 50 microns. In other words a glass tube with
a very narrow central region.
An extremely short (tens of femto-seconds ?), and very
intense (3.9 nJ), pulse of laser light enters the tube as
highly monochromatic (about 840 nm). And what comes
out? Photons whose wavelengths have a continuous
distribution from about 400 nm to about 1500 nm.
In other words, the output is an extremely short pulse
of "white" light. I saw several articles describing similar
transformations taking place in other fiber-optics structures.
But I am not at all familiar with highly complex nonlinear
phenomena which are responsible for what is taking place.
But I am amazed.
For more details, and references, see "Supercontinuum
Generation in Tapered Fibers", by T.A. Birks et al. in
OPTICS LETTERS (October 1, 2000 / vol 25, # 19,
pages 1415 to 1417).
Ludwik Kowalski