Dark solitons in optical communication systems

Abstract

This thesis presents experimental and theoretical work on the use of dark optical
solitons as data carriers in communications systems. The background chapters
provide an introduction to nonlinear optics, and to dark solitons, described as
intensity dips in a bright background, with an asymmetrical phase profile. The
motivation for the work is explained, considering both the superior stability of
dark solitons and the need for a soliton solution suitable for the normal, rather
than the anomalous (bright soliton) dispersion regime.
The first chapters present two generation techniques, producing packets of dark
solitons via bright pulse interaction, and generating continuous trains of dark
pulses using a fibre laser. The latter were not dark solitons, but were suitable
for imposition of the required phase shift by virtue of their extreme stability.
The later chapters focus on the propagation and control of dark solitons. Their
response to periodic loss and gain is shown to result in the exponential growth
of spectral sidebands. This may be suppressed by reducing the periodicity of
the loss/gain cycle or using periodic filtering. A general study of the response
of dark solitons to spectral filtering is undertaken, showing dramatic differences
in the behaviour of black and 99.9% grey solitons. The importance of this result
is highlighted by simulations of propagation in noisy systems, where the timing
jitter resulting from random noise is actually enhanced by filtering. The results
of using sinusoidal phase modulation to control pulse position are presented,
showing that the control is at the expense of serious modulation of the bright
background.
It is concluded that in almost every case, dark and bright solitons have very
different properties, and to continue to make comparisons would not be so
productive as to develop a deeper understanding of the interactions between
the dark soliton and its bright background.

Date of Award	Oct 1995
Original language	English
Supervisor	Nick Doran (Supervisor)