Intensity discrimination with twisted birefringent optical fibers

Abstract

Intensity-dependent polarization effects in birefringent optical fibers have been used by several workers to achieve pulse shaping and intensity discrimination. In that application, the low intensity parts of a pulse transmitted through the fiber are blocked by an output polarizer while the high intensity part, which undergoes nonlinear polarization rotation, is transmitted. We recently reported a theory of this effect which shows that for fibers with well-defined principal axes, nonlinear polarization changes cannot occur if the input polarization is along a principal axis and the intensity is below the critical value for the fast axis instability. Experimentally, it has been noted that the virtually unavoidable twists present in any real fiber play an important role in determining the output polarization state. In this paper we present a theory of intensity-dependent polarization changes in twisted birefringent fibers. The theory takes into account the twist-induced circular birefringence and shows that twist can enhance the nonlinear transmission of fiber-optic devices that rely on nonlinear birefringence.