The Evolution of Polarization State in Two-mode Optical Fibers

摘要

Two-mode optical fibers have been studied extensively for telecommunication device and sensor application. This paper investigates the evolution of polarization state in two-mode optical fibers. Two-mode Fibers usually support four modes, i.e. LP_(01)~x, LP_(01)~y, LP_(11)~x (even), LP_(11)~y (even). The propagation constants of the four modes are different so that the polarization state of an incident light will not maintain when the light propagating along the fiber. The evolution of the polarization state in a two-mode optical fiber is analyzed in theory, the result shows that unlike a conventional highly birefringent optical fiber, the polarization state of the incident light is not recurred at any length of the fiber; i.e. strictly speaking, the beat length is not existed in a two-mode optical fiber. However, the every modal birefringence index of two modes can be found by a wavelength scanning method. The power of the output light in a special polarization direction is detected; it is a multi-cycle function, which depends on the wavelength of the incident light. Six periods can be given by using the Fourier transform spectrum of the output light; they are in direct proportion to modal birefringent indices and the fiber length, so three modal birefringence indices can be got. It is a most important property of the two-mode optical fibers for device and sensor application.