Abstract
A major approach investigated recently for stimulated Brillouin scattering
(SBS) suppression in fiber amplifiers for high power single frequency fiber
lasers is to explore designs of acoustic waveguide in optical fibers. This
acoustic waveguide can be implemented to some extent independent of the
optical waveguide by using a combination of dopants which modify the host
glass by varying levels in acoustic and optical properties. Although this
approach provides some SBS suppressions, the new analysis described in this
work, considering the often omitted leaky acoustic modes, demonstrates its
limit. A complex acoustic mode solver, reported in details elsewhere, was
recently developed to find solutions for simultaneous longitudinal and shear
acoustic wave equations which satisfy rigorous boundary conditions in an
arbitrary circular acoustic waveguide. By taking advantage of this new tool,
it is possible to find the leaky acoustic modes for acoustic waveguides
designed for SBS suppressions. In these acoustic waveguides, the guided
acoustic modes often have a reduced overlap with the optical modes by design
and, sometimes, do not exist at all, while the leaky acoustic mode can have
a significant stronger overlap with the optical modes and, consequently, can
play a dominant role in the SBS gain spectrum. It is, therefore, critical to
consider the leaky acoustic modes in these cases. Two prominent transverse
acoustic waveguide designs proposed recently for SBS suppression in optical
fibers are studied here. This work reports, for the first time, the limits
of these designs for SBS suppression. With consideration of the leakage
acoustic modes, some new insights are provided on the underlying physics as
well as on how to optimize these designs for optimal SBS suppression.
© 2010 IEEE
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