Fig. 1. Structure of elliptical-core fiber

Fig. 2. Normalized electric field distributions and polarization states of six vector modes varying with elliptical parameters. (a1)-(a6) η=1.000; (b1)-(b6) η=1.001; (c1)-(c6) η=1.050

Fig. 3. am and am-1 varying with elliptical parameter. (a) am of TE₀₁ and TM₀₁; (b) am-1 of HE_21o and HE_21e

Fig. 4. Effective refractive index and effective refractive index difference between six modes varying with elliptical parameter.(a) Effective refractive index; (b) effective refractive index difference

Fig. 5. Structural diagram of FM-EDFA

Fig. 6. Emission and absorption spectra of erbium-doped fiber

Fig. 7. Schematic diagram of Erbium ion distribution at energy levels in few-mode fiber

Fig. 8. Curves of mode gain and gain error calculated by LP mode approximation and accurate mode field. (a) Mode gain; (b) gain error

Fig. 9. Influence of am of different modes on elliptical parameter and refractive index difference. (a) TM₀₁; (b) HE_21e; (c) TE₀₁; (d) HE_21o

Fig. 10. Mode gain error caused by LP-mode approximation varying with elliptical parameter