Abstract
The mode-coupling processes of long-period gratings (LPGs) fabricated
in hollow-core photonic bandgap fibers are investigated. The LPGs are formed
by periodic structural deformations induced by local heating generated by
use of a pulsed CO$_{2}$ laser. Highly polarization-dependent grating resonances are observed
in the transmission spectrum of the LPGs and found to be due to coupling from
the fundamental to higher order core modes. The mode-coupling process is understood
based on coupled local-mode theory: A single deformation of the holey cladding
modifies the local-mode field profiles and induces fractional energy transfer
between the fundamental and the higher order modes. Resonant coupling between
a phase-matched fundamental/higher order mode pair is excited by having multiple
such deformations arranged periodically along the fiber. The LPG spectrums
are numerically modeled by solving the coupled local-mode equations at different
wavelengths and found in agreement with the experimentally measured results.
© 2011 IEEE
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