Fig. 1. Cross section of the proposed elliptical-ring core fiber with a central circular air hole. Key waveguide parameters are the cladding diameter dcladding, the air hole radius r, the major axes of the ring core, bx and by, the minor axes, ax and ay, the cladding’s and core’s materials, SiO2 and GeO2-doped SiO2.

Fig. 2. In the situation that nring=1.478, ncladding=1.444, dcladding=125  μm, η=1.4, ρ=0.67, bx=5.06  μm, and λ=1.55  μm: (a) the Δneff between the 10 adjacent modes as a function of the air hole radius r; (b) magnified image of the light orange rectangle in (a); (c) the Δneff between the first seven adjacent modes with increasing of r; (d) the Δneff between the last four adjacent modes with increasing of r. Note that the lines with symbols are the Δneff between orthogonal polarization of the same mode (modal birefringence), while the lines without symbols are the mode group separation.

Fig. 3. Transverse electrical fields, amplitudes, and directions of the vector modes at 1.55 μm for different sizes of air hole: (a) r=0  μm, (b) r=0.7  μm, (c) r=0.9  μm, (d) r=1.2  μm, and (e) r=1.7  μm.

Fig. 4. (a) Effective refractive indices (neff) of all guided modes. (b) Chromatic dispersions of all guided modes. (c) Δneff between all adjacent guided modes, as a function of the wavelength of λ, for the 10-mode PMF with r=1.7  μm, nring=1.478, ncladding=1.444, dcladding=125  μm, η=1.4, ρ=0.67, and bx=5.06  μm.

Fig. 5. Transverse electrical fields, amplitudes and directions of the vector modes at 1.55 μm for the optimum air hole radius r=1.8  μm.

Fig. 6. (a) Effective refractive indices (neff) of all guided modes. (b) Chromatic dispersions of all guided modes. (c) Δneff between all adjacent guided modes, as a function of the wavelength of λ, for the 14-mode PMF.