Fig. 1. Reasons that make the capacity of single-mode fiber approach Shannon limit. (a) Nonlinear Shannon limit^[3]; (b) fiber fuse damage phenomenon^[4]; (c) transmission window of fused-silica fibers, where black region represents the standard optical amplification band^[5]

Fig. 2. Comparison of crosstalk between Homo-MCF and Hetero-MCF. (a) Crosstalk of Homo-MCF dependence on bending radius and change of effective index under bending condition; (b) crosstalk of Hetero-MCF dependence on bending radius and change of effective index under bending condition

Fig. 3. Classification of single-mode multi-core fiber based on the assisted structure around core^[20]. (a) Refractive index profile of trench-assisted core; (b) cross-section of trench-assisted multi-core fiber; (c) refractive index profile of air-hole-assisted core; (d) cross-section of air-hole-assisted multi-core fiber; (e) refractive index profile of photonic crystal core; (f) cross-section of photonic crystal multi-core fiber

Fig. 4. Core selection of outer core 1^[42]. (a) Cross-section of a Hetero-TA-7-core fiber; (b) n_eff and A_eff as functions of r₁ and Δ₁ at λ =1550 nm and Λ=35 μm for the first outer cores in Hetero-TA-7-core fiber

Fig. 5. Core selection of outer core 2^[42]. (a) Cross-section of a Hetero-TA-7-core fiber; (b) n_eff and A_eff as function of r₁ and Δ₁ at λ=1550 nm and Λ=35 μm for the second outer cores in Hetero-TA-7-core fiber

Fig. 6. XT between core 1 and the undetermined core as a function of n_eff,q and K^[42], where n_eff,p and n_eff,q in Eq. (5) are effective indexes of refraction of core 1 and the undetermined core, respectively

Fig. 7. n_eff and A_eff as functions of r₁ and Δ₁ at λ=1550 nm and λ=35 μm with CSR of center core 2 in Hetero-TA-7-core fiber^[42], where ${l_{\mathrm{XT}}}_{,\max }$

Fig. 8. Conversion relationship between OAM, CV and LP modes

Fig. 9. Reported inner-depressed step-index few mode fibers with the weak coupling. (a) Inner-depressed step-index 6-LP-mode fiber with the weak coupling^[49]; (b) inner-depressed step-index 7-LP-mode fiber with the weak coupling^[50]; (c) inner-depressed step-index 4-LP-mode fiber with the weak coupling^[51]; (d) inner-depressed step-index 6-LP-mode fiber wi

Fig. 10. Fiber solutions for reducing DMGD between modes. (a) Trench-assisted dual-cladding step fiber with low refractive index^[54]; (b) trench-assisted graded-index fiber with low refractive index^[55]; (c) 0DMGD compensation method to connect positive DMGD fiber (p) and negative DMGD fiber(n) with low refractive index ^[57]

Fig. 11. OAM mode groups multiplexing transmission system. (a) Low crosstalk and low attenuation ring-core fiber with 4×4 MIMO based OAM multiplexing transmission experiment involving ten wavelengths and eight OAM modes over a distance of 100 km, transmitting 16-Gbaud QPSK signals over all 80 channels^[65]; (b) graded-index multi-mode fiber without MIMO based OAM multiplexing transmission over 2.6 km transmitting 4 OAM mode groups^{[Download full size}

Fig. 12. Cross-section, refractive index distribution and dispersion curves of step-index fiber. (a) Cross-section and refractive index distribution of step-index fiber; (b) dispersion curves of weakly-guiding step-index fiber^[68]

Fig. 13. Cross-section, refractive index distribution and dispersion curves of ring-core fiber^[68]. (a) Cross-section and refractive index distribution of ring-core fiber; (b) dispersion curves of weakly-guiding ring-core fiber, when d/(2a)=0.20; (c) dispersion curves of weakly-guiding ring-core fiber, when d/(2a)=0.25; (d) dispersion curves of weakly-guiding ring-core fiber, when d/(2a)=0.30

Table 1. Core arrangement of the reported trench-assisted single-mode multi-core fibers

Table 2. Reported OAM strongly-guiding and weakly-coupling ring-core fibers

Table 3. Correction of propagation constant for each CV mode

Table 4. Comparison among three SDM fiber schemes