Polarization properties of few-mode glass fiber waveguides with noncircuiar cores

An investigation was made of the dependences of the degree of polarization P of the output radiation on the wavelength of directional radiation and the length of a fiber waveguide with an oval core and a birefringence of δn = 2·10^–5. In a few-mode (V ≈ 6) regime such fibers ensure the high value of P over distances amounting to few tens of meters. An increase in the number of modes and in the length of a fiber reduced P; the first two cutoffs were accompanied by a step-like change in P. The polarization properties of birefringent fiber waveguides were explained by a difference between the polarization characteristics of the higher modes and by intermode conversion. A new polarization method was proposed for determining the cutoff wavelengths of the higher modes. The method is suitable for any type of fiber waveguide, but it is particularly convenient in the case of the waveguides with a complex refractive index profile.