Abstract: A mid-infrared chalcogenide few-mode fiber with cladding made by LiNbO₃ crystals in which the optical axis is running along the axis of optical fiber is proposed. Besides, the effect of the adding electric field on the rate of the extraordinary to the ordinary ray refractive index of LiNbO₃ crystals and the transmission characteristics of optical modes are studied by using the full-vector finite element method. The results show that the effective refractive index and power confinement factor both decrease with increasing wavelength, but the differential mode delay shows an increasing trend at the wavelength of 2.25 ~ 3.85 μm. Besides, the external axial electric field not only reduces the effective refractive index and differential mode delay of each optical mode, but also increases energy confinement for each mode. And the higher the mode orders is, the stronger energy confinement becomes. The dispersion of the HE₁₁ mode rises with wavelength drop without an external electric field, whilst the HE₂₁, TE₀₁, and TM₀₁ modes exhibit parabolic distribution with the wavelength increasing, and there exist two zero dispersion wavelengths. The dispersions of optical modes increase as the electrical field increases, which contributes to a blue shift of the dispersion zero point. In comparison with and without an electric field of 4 ×10⁹ V/ m, the zero dispersion wavelengths at the short wavelengths of HE₁₁, HE₂₁, TE₀₁ and TM₀₁ modes are blue-shifted by 0.569 5 μm, 0.391 5 μm, 0.386 2 μm and 0.559 4 μm, respectively.

Key words: few-mode fiber, LiNbO₃, As₂S₃, electro-optic effect, dispersion