[1] Richardson D J, Fini J M, Nelson L E. Space-division multiplexing in optical fibres[J]. Nature Photonics, 7, 354-362(2013).

[2] Essiambre R J, Kramer G, Winzer P J et al. Capacity limits of optical fiber networks[J]. Journal of Lightwave Technology, 28, 662-701(2010).

[3] Essiambre R J, Tkach R W. Capacity trends and limits of optical communication networks[J]. Proceedings of the IEEE, 100, 1035-1055(2012).

[4] Li G F, Bai N, Zhao N B et al. Space-division multiplexing: the next frontier in optical communication[J]. Advances in Optics and Photonics, 6, 413-487(2014).

[5] Zhang Z Z, Guo C, Cui L et al. 21 spatial mode erbium-doped fiber amplifier for mode division multiplexing transmission[J]. Optics Letters, 43, 1550-1553(2018).

[6] Kang Q Y, Lim E L, Jung Y et al. Accurate modal gain control in a multimode erbium doped fiber amplifier incorporating ring doping and a simple LP01 pump configuration[J]. Optics Express, 20, 20835-20843(2012).

[7] Zhao Q H, Zhang Z Z, Zhao N B et al. Optimizing the erbium doped profile to equalize the modal gain in few mode erbium doped fiber amplifier[J]. Laser & Optoelectronics Progress, 53, 030602(2016).

[8] Fang Y H, Zhang C, Huang X H et al. Simultaneous realization of high gain and low DMG of four-mode EDFA under bidirectional hybrid-mode pump[J]. Optics Express, 32, 9332-9342(2024).

[9] Xu Y, Wu B J, Jiang X R et al. Research on simulation method for FM‑EDFAs by effective erbium doping concentration[J]. Chinese Journal of Lasers, 50, 0501003(2023).

[10] Naik S K, Amin I, Qazi G. Design of parabolic refractive index 2 mode-EDFA with ultra-low deviation in gain and noise figure across 2×16 channels for MDM-WDM system[J]. Optics & Laser Technology, 174, 110693(2024).

[11] Chang Y B, Pei L, Zheng J J et al. Demonstration of an all-fiber cladding-pumped FM-EDFA with low differential modal gain[J]. Optics & Laser Technology, 155, 108446(2022).

[12] Wang B Y, Yan F P, Ren G B et al. Design of four-mode erbium-doped fiber amplifier based on annealing algorithm[J]. Acta Optica Sinica, 43, 2206007(2023).

[13] Zhang A P, Chen X W, Yan J H et al. Optimization and fabrication of stitched long-period gratings for gain flattening of ultrawide-band EDFAs[J]. IEEE Photonics Technology Letters, 17, 2559-2561(2005).

[14] Yang J R, Meng X Y, Liu C Y. Accurately control and flatten gain spectrum of L-band erbium doped fiber amplifier based on suitable gain-clamping[J]. Optics & Laser Technology, 78, 74-78(2016).

[15] Thyagarajan K, Kaur J. A novel design of an intrinsically gain flattened erbium doped fiber[J]. Optics Communications, 183, 407-413(2000).

[16] Anand J, Anand J K, Sharma E K. Study of the amplification characteristics of a coaxial EDF with varying coupling conditions[J]. Optics & Laser Technology, 44, 688-695(2012).

[17] Zhu J L, Yang Y, Zuo M Q et al. Few-mode gain-flattening filter using LPFG in weakly-coupled double-cladding FMF[J]. Journal of Lightwave Technology, 39, 4439-4446(2021).

[18] Giles C R, Desurvire E. Modeling erbium-doped fiber amplifiers[J]. Journal of Lightwave Technology, 9, 271-283(1991).

[19] Morishita K. Numerical analysis of pulse broadening in graded index optical fibers[J]. IEEE Transactions on Microwave Theory and Techniques, 29, 348-352(1981).

[20] Bai N, Ip E, Wang T et al. Multimode fiber amplifier with tunable modal gain using a reconfigurable multimode pump[J]. Optics Express, 19, 16601-16611(2011).

[21] Huang X H, Zeng Y, Fang Y H et al. Optimized few-mode EDFA of applicability for variable mode groups with an extended doping region and a PSO algorithm[J]. Journal of the Optical Society of America B, 39, 2618-2624(2022).

[22] Jiang X R, Wu B J. Experimental evaluation on OSNR and DMG performance of cascade FM-EDFA systems by equivalent input spectrum method[J]. Optics & Laser Technology, 166, 109628(2023).

[23] Gaur A, Rastogi V. Modal gain equalization of 18 modes using a single-trench ring-core EDFA[J]. Journal of the Optical Society of America B, 35, 2211-2216(2018).

[24] Ma J W, Xia F, Chen S et al. Amplification of 18 OAM modes in a ring-core erbium-doped fiber with low differential modal gain[J]. Optics Express, 27, 38087-38097(2019).

[25] Zeng Y, Zhang C, Zhang S Y et al. All-fiber spatial and wavelength gain-flattening of few-mode EDFA via mode selective coupler[J]. Optics & Laser Technology, 181, 111778(2025).

[26] Huang S L, Shen L, Qiao G et al. Low-modal-crosstalk doped-fiber amplifiers in few-mode-fiber-based systems[J]. Photonics Research, 12, 1768-1775(2024).