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Nonlinear Optics and Laser Physics:
Semiconductor
Lasers with Feedback
Semiconductor lasers currently play a central role in the massively
growing optoelectronic industry.
With external feedback they have been successfully used for
linewidth narrowing, suppression of secondary solitary modes and reduction
of modulation-induced frequency chirp.
However, under conditions of moderate feedback, these laser systems
are well known to exhibit rich forms of dynamical behaviour which have been
the subject of intense research activity in recent years, for both basic
science and application. This
programme researches, through experiment and theory, the nature of this
dynamical behaviour their physical origin and their application to secure
communication. Aspects of this work
are in collaboration with the group of A Gavrielides, Air Force Research
Laboratory, Kirtland, USA.
In particular this work addresses the phenomenon of low frequency
dropouts in these lasers, characterised by sudden average power dropouts,
followed by gradual build-up of the laser power. Our recent finding of locked states
(Synchronous Sisyphus effect) and that they underlie LFFs at low noise
levels provides new insight on the deterministic origin of LFF's, the route
of which may be understood through the new phenomenon of Coherence
Resonance. In theory our work has
shown LFF's to be a multi-mode phenomenon, not single-mode as commonly
assumed, energy transfer between solitary modes being found to be a common
effect when the laser undergoes LFFs or is in the locked state. This model description is currently being
used to gain understanding of the connection between LFF's and underlying
ultra-fast (psec) dynamics recently discovered in these lasers, also
manifested in the intermittent and chaotic behaviour shown to occur above
the solitary laser threshold.
Current work also addresses synchronisation of coupled chaotic
oscillators, a subject of active research for its application in secure
communications. Notable findings are
the new phenomena of phase- and lag- synchronisation we observe in couple
diode lasers with feedback both operated in the LFF regime and that the
concurrent fast underlying dynamics remain strongly uncorrelated. These are presently being actively
researched for the new insight they may provide on the basic nature of
nonlinear dynamical systems that function on more than one time scale. Phase space analysis of the local and
global temporal dynamics of the diode system reveal synchronisation in
local regions in phase space, a new manifestation of this phenomenon In
application, consequences of these findings to encoding and transport of
information in secure communications is being researched.
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