Abstract
We report numerical simulations based on normal coupled mode theory
of the fundamental-mode loss and backscattering induced in air-core fibers
by random longitudinal perturbations of the core diameter. To quantitatively
explain the measured loss of $\sim$24 dB/km at 1550 nm of air-core fiber HC-1550-02 from Crystal Fibre,
these simulations predict that the autocorrelation function of the perturbation
is close to an exponential and characterized by a ratio $D/\sigma^{2}$ of $\sim 2.36\times 10^{13}\ {\rm m}^{-1}$,
where $D$ is
the characteristic length and $\sigma$ the amplitude of the perturbation. This analysis yields a characteristic
perturbation length for this fiber in the range of $\sim$1 to $\sim$30 cm. That this is much shorter
than in a conventional fiber is consistent with the slower speeds at which
air-core fibers are pulled, which reduces the length of the fiber perturbations.
The same exponential perturbation and $D/\sigma^{2}$ ratio also predict that the backscattering coefficient for the
fundamental mode of this fiber is $1.5\times
10^{-9}\ {\rm mm}^{-1}$, which agrees well with a measured
value. When applied to a 19-cell air-core fiber from the same manufacturer
(HC19-1550-01) the same perturbation predicts a loss of 4 dB/km, which agrees
with the measured range of 1.2 to $\sim$10 dB/km. These independent agreements between modeled and measured
loss and backscattering coefficients and the reasonable predicted range of
perturbation lengths confirm that core dimension variations are the dominant
mechanism behind the loss and backscattering of current air-core fibers.
© 2009 IEEE
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