Fig. 1. Cross section of an MSC. Fiber 1 is an MMF, fiber 2 is an SMF, and they have the same cladding. The distance between them is d.

Fig. 2. Effective refractive indices of the modes in a fiber. The dashed line is phase matched between different modes at neff=1.449. The wavelength, and the indices of the core and the cladding are 1.55 μm, nco=1.4546, and ncl=1.4446, respectively, and the core radius of fiber 1 and fiber 2 are 12 and 3 μm, respectively.

Fig. 3. Relative coupling coefficient between fiber 1 and fiber 2 as a function of the azimuthal angle α in an MMF.

Fig. 4. Cross section of the three-core MSC; fiber 2 and fiber 3 are SMFs, and fiber 1 is an MMF. θ is the angle between fiber 2 and fiber 3.

Fig. 5. (a) Normalized coupled power between fiber 2 (LP01 mode) and fiber 1 (LP11s mode); (b) normalized coupled power between fiber 3 (LP01 mode) and fiber 1 (LP11c mode). P2 and P3 are incident powers in fiber 2 and fiber 3, and P1 is the coupled power in fiber 1. zc=1.51  cm.

Fig. 6. (a) Normalized coupled power between fiber 2 (LP01 mode) and fiber 1 (LP31s mode); (b) normalized coupled power between fiber 3 (LP01 mode) and fiber 1 (LP31c mode); (c) the intensity generated in fiber 1 and the phase profiles of the coupled mode in fiber 1 with topological charge equal to three. P2 and P3 are incident powers in fiber 2 and fiber 3, and P1 is the coupled power in fiber 1. zc=4.92  cm.