Polarization Mode Coupling and Spatial Power Spectrum of Fluctuations Along a Highly Birefringent Holey Fiber

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.