The soliton-like interactions of two low divergence fields propagating in unbiased self-defocusing photorefractive media are examined. The interactions appear to be a result of factors that are not dissimilar to those between two steady state screening solitons. Analysis of the conditions for the interactions show a cutoff for the degree of beam divergence required for the soliton-like interactions to occur. The analysis also indicates that the strength of the interaction increases as the beam divergence decreases.
Optical spatial solitons are of interest at present due their possible application to integrated all optical circuitry where
light controls light. This optical circuitry utilises the various novel properties of the optical spatial soliton, such as
rewritable waveguides, and phase dependent interactions. Of all the types of optical spatial solitons, photorefractive
(PR) solitons are the subject of much research due to their ease of production and stability. They are readily produced in
either self-focusing PR media (photovoltaic solitons), or self-defocusing PR media with an applied external bias
(screening solitons). The external bias, typically an applied DC filed, is used to manipulate the self-defocusing PR
media to act like self-focusing PR media. However, solitons produced in self-focusing PR media run the risk of over
focusing causing permanent damage to the PR media, while applying an external bias to the PR media requires many
additional components, increasing the complexity of the system.
Recently, we outlined for the first time a theoretical model of soliton-like low divergence fields in unbiased self-defocusing
PR media. Numerical analysis of these soliton-like fields showed stability over distances well in excess of
both the confocal distance of the beam, and the physical size of the PR media. The present research examines the
existence of the low-divergence soliton-like fields and the fundamental nature of the interactions of these low-divergence
soliton-like fields in unbiased PR self-defocusing media. Here we show that low-divergence soliton-like fields can be
produced in unbiased self-defocusing photorefractive media, and that when the two of these soliton-like fields interact
within the PR media, they are forced away from each other.
Optical devices, where light controls light, are of interest to the computing and communications industries due to their
potential to vastly improve information capacity and processing speed. One such device is an optical logic gate, based on
the interactions of low divergence fields in photorefractive media. Presently, bright solitons in self-focusing
photorefractive media offer one attractive possibility. A wide variety of other low divergence fields have also been
outlined in recent literature, however, no theoretical model of a single bright soliton propagating in unbiased selfdefocusing
photorefractive media is currently available.
Evidence is presented of self-defocusing photorefractive media as an intensity dependent Gradient-Index (GRIN) lens
with a negative power. Using this model, we outline conditions for the change in the complex beam parameter, and
consequently the area and wavefront curvature, of the Gaussian beam to be minimised as it propagates through the selfdefocusing
media. This is the first instance where self-defocusing photorefractive media has been modelled as an
intensity dependent GRIN lens, and where a low divergence field propagating through unbiased self-defocusing media
with a constant complex Gaussian beam parameter has been described.
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