Abstract
The spatial power spectrum of fluctuations along a highly birefringent
holey fiber is evaluated using a quasi-heterodyne interferometer. Statistical
data on the nature of the power spectrum for polarization-mode coupling is
presented. We studied the dramatic change of modal birefringence with wavelength
seen in holey fibers with enlarged and normal-sized air-holes. This behavior
is very useful when investigating the mode coupling effect, since the coupled-power
distribution measured at various wavelengths can be obtained for an identical
waveguide structure. The power spectrum is the Fourier transform of the autocorrelation
function which is related to the intensity distribution yielded by the interference
between excited-mode wave launched into the fiber and the coupled-mode wave
originated along the fiber. Measurements are carried out for excited-modes
linearly polarized along the slow and fast axes of the holey fiber at wavelengths
of 972, 1312, and 1547 nm. Extinction ratio measurements are simultaneously
made at the wavelengths in order to compare the mode coupling coefficients
predicted from the power spectrum. As a result, the power spectra well explain
the data obtained from the extinction ratio measurements. The experimental
results also reveal that the power spectrum well matches a Lorenzian function
given as the polarization-mode coupling coefficient $h$ as a function of propagation constant
difference $\Delta \beta
$ between the two orthogonally polarized modes. It is clearly
seen that $h$ is
proportional to $( \Delta
\beta )^{-2}$ for $\Delta
\beta $ greater than about 50 rad/m. Furthermore, the spatial
power spectrum obtained here is similar to that of the fluctuations produced
in the fiber drawing process.
© 2009 IEEE
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