Conditions for invariant polarization in a single‐mode optical fiber in pulse wave transmission

Abstract

AbstractConditions are determined for maintaining polarization when pulse waves propagate in a single‐mode fiber in which both non‐uniformity in distribution of the core dielectric constant and cross sectional deformation exist. The coupling coefficient between the waves with left‐ and right‐handed circular polarizations is denoted by β( = β1 − β2). Deformation of the core cross section from a perfect circle and deviation from uniform dielectric distribution can always be expanded into circumferential Fourier series. The values of β1 and β2 are determined by the second‐order Fourier coefficients. The condition of invariant polarization in the fiber is β1 = β2 = 0. The condition for time‐invariance of the plane of polarization is frequency independency of β1 and β2. We also study the effect of twisting of fibers satisfying these condtions. Denoting the rigidity by G, the optoacoustic constant by C and the index of refraction by N, we show that if G·C/N is frequency‐independent, the degree of polarization is maintained in the fiber with invariant polarization although the plane of polarization rotates. When a fiber time‐invariant polarization plane time is twisted, the characteristics remain unchanged. We show several examples of core cross‐sectional shapes and dielectric distributions needed for maintaining the polarization.