Twisted single-mode fibers show a high degree of polarization for any incident polarization state, when phase compensation is performed at the fiber output. The performance is demonstrated in a 1.14 km long fiber with a twist rate of 2 turns/m. Polarization degree, output polarization ellipticity, and output polarization major axis angle were studied experimentally and theoretically in terms of the fiber twist rate. Preservation of circular polarization without phase compensation, in particular, is also verified in the twisted fiber. Polarization characteristics are stable against external perturbations. It was also confirmed that proper orthogonal elliptical polarization states exist, in genaral, which are maintained over a 5 km long fiber.

The phenomena of the wave propagation in a nonlinear transmission line are analyzed theoretically by the derivative expansion method. Each section of this line is constructed with a series inductor L1 and the shunt circuit consisting of a seriesed L2 - C2 element and a nonlinear capacitor C (V) in which V is a line voltage. There exist two modes; L.F. mode propagating in the frequency range 0ωωt and H.F. mode in the range ωpω, where (L2 C2)1 and (1C2/C(0)). In the strongly dispersive region the asymptotic behavior of the nonlinear waves is described by the nonlinear Schrödinger equation and by the Korteweg-de Vries equation in the weakly dispersive region. Critical frequencies ω10 and ω20 (ωpω20ω10) decompose the H.F. mode into three regions (1) ωpωω20, (2) ω20ωω10 and (3) ω10ω. The resonant interaction between the L.F. mode and the H.F. mode occurs in the frequency ω10, while the lowest order nonlinear interaction disappears in the frequency ω20. In the regions (1), (3) and the L.F. mode the plane wave is stable under the wave modulation, while the plane wave is unstable in the region (2). We derive the basic equations describing the three wave interactions and find that the plane wave with a large amplitude becomes unstable through the parametric decay instability. The wave number and maximum growth rate of the excited waves are determined.

Polarization evolution and the degree of polarization in twisted optical fiber are measured using an identical fiber sample. Measured polarization degree is consistently explained using independently measured linear birefringence, polarization dispersion and light source spectral data. As the result, polarization characteristics in twisted optical fibers could be comprehensively understood by revealing the mutual relation between several characteristics. Twisted optical fiber is designed based on equations supported by experiment. To get good polarization preserving fiber with a twist rate as low as several turns/m, core ellipticity of starting fiber should be less than about 1%. Linear birefringence and polarization dispersion data are compared with theoretical values derived from fiber parameters and the sources of discrepancies are discussed.