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11
Coherent Structures in Dissipative
Nonlinear Optical Systems
. Moloney
ABSTRACT Critical self-focusing can manifest itself in many different
ways in nonlinear optical systems. In transparent materials, a continuous
wave laser beam can collapse to a localized hot spot after propagating
some finite distance. Here the nonlinear optical interaction is local and as-
sumed to be instantaneous. In a wide aperture semiconductor laser, the
self-focusing nonlinearity is sluggish, responding to the relatively slow mo-
tion of the total carrier density within the lasing medium. In both situa-
tions, large scale coherent structures are generated. In this article, we will
review some novel manifestations of the self-focusing nonlinearity in which
dissipation plays a prominent role. Our first example will address the issue
of the propagation of an intense femtosecond-duration laser pulse which can
undergo critical collapse in a transparent bulk material. Here weak plasma
generation in the vicinity of the focal spot regularizes the collapse singu-
larity. In the second example, we will illustrate how dynamic filamentation
instabilities in wide aperture semiconductor lasers can be stabilized via an
all-optical spatial/spectral filtering delay feedback scheme.
1 Introduction
The critical self-focusing nonlinearity is well established in nonlinear op-
tics. Typically experimentalists go to great lengths to avoid this instability
as its consequences are often catastrophic. For an excellent review of the
early developments in the field, the reader should consult the articles by
Shen [21] and Marburger [16]. In a transparent bulk dielectric medium, a
continous wave (CW) laser beam or long laser pulse can undergo critical
self-focusing when its power exceeds some critical value. The high fields
generated at the focus lead to optical breakdown of the material. The op-
tical breakdown mechanism depends on t