[1] R.-J. Essiambre, A. Mecozzi. Capacity limits in single-mode fiber and scaling for spatial multiplexing. Optical Fiber Communication Conference and Exposition/National Fiber Optics Engineers Conference (OFC/NFOEC), 1-3(2012).

[2] B. Mukherjee. WDM optical communication networks: progress and challenges. IEEE J. Sel. Areas Commun., 18, 1810-1824(2000).

[3] D. M. Spirit, A. D. Ellis, P. E. Barnsley. Optical time division multiplexing: systems and networks. IEEE Commun. Mag., 32, 56-62(1994).

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

[5] B. Franz, H. Bülow. Mode group division multiplexing in graded-index multimode fibers. Bell Labs Tech. J., 18, 153-172(2013).

[6] Y. Fazea, V. Mezhuyev. Selective mode excitation techniques for mode-division multiplexing: a critical review. Opt. Fiber Technol., 45, 280-288(2018).

[7] C. Li, D. Liu, D. Dai. Multimode silicon photonics. Nanophotonics, 8, 227-247(2018).

[8] C. Sun, W. Wu, Y. Yu. De-multiplexing free on-chip low-loss multimode switch enabling reconfigurable inter-mode and inter-path routing. Nanophotonics, 7, 1571-1580(2018).

[9] L.-W. Luo, N. Ophir, C. P. Chen. WDM-compatible mode-division multiplexing on a silicon chip. Nat. Commun., 5, 3069(2014).

[10] G.-H. Chen, C.-W. Chow, C.-H. Yeh. Mode-division-multiplexing (MDM) of 9.4-Tbit/s OFDM signals on silicon-on-insulator (SOI) platform. IEEE Access, 7, 129104-129111(2019).

[11] D. Dai, S. Wang. Asymmetric directional couplers based on silicon nanophotonic waveguides and applications. Front. Optoelectron., 9, 450-465(2016).

[12] W. Zhao, Y. Peng, X. Cao. 96-channel on-chip reconfigurable optical add-drop multiplexer for multidimensional multiplexing systems. Nanophotonics, 11, 4299-4313(2022).

[13] H. Shu, B. Shen, Q. Deng. A design guideline for mode (de)-multiplexer based on integrated tapered asymmetric directional coupler. IEEE Photon. J., 11, 6603112(2019).

[14] Z. Tao, B. Wang, B. Bai. An ultra-compact polarization-insensitive slot-strip mode converter. Front. Optoelectron., 15, 5(2022).

[15] Q. Deng, Q. Yan, L. Liu. Highly compact polarization insensitive strip-slot waveguide mode converter. CLEO: Applications and Technology, JTu5A-53(2015).

[16] H. Xu, Y. Shi. Subwavelength-grating-assisted silicon polarization rotator covering all optical communication bands. Opt. Express, 27, 5588-5597(2019).

[17] Q. Huang, K. S. Chiang. Polarization-insensitive ultra-broadband mode filter based on a 3D graphene structure buried in an optical waveguide. Optica, 7, 744-745(2020).

[18] I. Zegaar, A. Hocini, A. Harhouz. Design of a double-mode plasmonic wavelength filter using a defective circular nano-disk resonator coupled to two MIM waveguides. Prog. Electromagn. Res. Lett., 104, 67-75(2022).

[19] M. Salsi. Challenges of few mode amplifiers. Optical Fiber Communication Conference (OFC), 1-3(2014).

[20] T. Xu, T. Gao, Y. Wang. High-gain integrated in-line few-mode amplifier enabling 3840-km long-haul transmission. Photon. Res., 10, 2794-2801(2022).

[21] Y. Liu, Z. Qiu, X. Ji. A photonic integrated circuit–based erbium-doped amplifier. Science, 376, 1309-1313(2022).

[22] J. Mu, M. Dijkstra, J. Korterik. High-gain waveguide amplifiers in Si₃N₄ technology via double-layer monolithic integration. Photon. Res., 8, 1634-1641(2020).

[23] J. Rönn, W. Zhang, A. Autere. Ultra-high on-chip optical gain in erbium-based hybrid slot waveguides. Nat. Commun., 10, 432(2019).

[24] B. Wang, P. Zhou, X. Wang. A low-fabrication-temperature, high-gain chip-scale waveguide amplifier. Sci. China Inf. Sci., 65, 162405(2022).

[25] W. A. Hendriks, L. Chang, C. I. Van Emmerik. Rare-earth ion doped Al₂O₃ for active integrated photonics. Adv. Phys. X, 6, 1833753(2021).

[26] Y. Zhou, Y. Zhu, Z. Fang. Monolithically integrated active passive waveguide array fabricated on thin film lithium niobate using a single continuous photolithography process. Laser Photon. Rev., 17, 2200686(2023).

[27] P. Zhou, B. Wang, X. Wang. Design of an on-chip electrically driven, position-adapted, fully integrated erbium-based waveguide amplifier for silicon photonics. OSA Contin., 4, 790-814(2021).

[28] X. Zhang, J. Yan, C. Wang. Design of a few-mode erbium-ytterbium co-doped polymer optical waveguide amplifier with low differential modal gain. J. Lightwave Technol., 39, 3201-3216(2021).

[29] C. Yu, G. Hu, F. Wang. Few-mode polymer optical waveguide amplifier for mode-division multiplexed transmission. Opt. Lett., 46, 5509-5512(2021).

[30] X. Sun, Y. Wang, L. Zhong. High-order mode waveguide amplifier with high mode extinction ratio written in an Er³⁺-doped phosphate glass. Opt. Express, 31, 5812-5819(2023).

[31] Q. Wilmart, S. Guerber, J. Faugier-Tovar. A device library for the ultra-low loss Si₃N₄ platform. Proc. SPIE, 12006, 120060D(2022).

[32] M. A. Gaafar, K. Wang, M. Ludwig. Photonic-chip integrated large-mode-area high-power CW optical amplifier. EPJ Web Conf., 287, 01009(2023).

[33] D. B. Bonneville, H. C. Frankis, R. Wang. Erbium-ytterbium co-doped aluminium oxide waveguide amplifiers fabricated by reactive co-sputtering and wet chemical etching. Opt. Express, 28, 30130-30140(2020).

[34] R. Wang, H. C. Frankis, H. M. Mbonde. Erbium-ytterbium co-doped aluminum oxide thin films: co-sputtering deposition, photoluminescence, luminescent lifetime, energy transfer and quenching fraction. Opt. Mater., 111, 110692(2021).

[35] M. Zhang, J. Lu, Y. Chen. Study on Er³⁺-Yb³⁺ co-doped La₂O₃–Al₂O₃ glasses for C-band optical waveguide amplifier with high luminous efficiency and low pump threshold. Ceram. Int., 48, 32236-32240(2022).

[36] Z. Zhang, S. Li, R. Gao. Erbium-ytterbium codoped thin-film lithium niobate integrated waveguide amplifier with a 27  dB internal net gain. Opt. Lett., 48, 4344-4347(2023).

[37] A. C. Liapis, V. Pelgrin, P. Liu. Wafer-scale atomic-layer-deposition of Er³⁺-and Yb³⁺-doped gain materials for integrated photonics. Conference on Lasers and Electro-Optics (CLEO), 1-2(2023).

[38] J. Wang, S. He, D. Dai. On-chip silicon 8-channel hybrid (de)multiplexer enabling simultaneous mode-and polarization-division-multiplexing. Laser Photon. Rev., 8, L18-L22(2014).

[39] Y. He, Y. Zhang, X. Jiang. On-chip silicon three-mode (de)multiplexer employing subwavelength grating structure. European Conference on Optical Communication (ECOC), 1-3(2017).

[40] J. Chen, J. Xiao. Ultracompact silicon-based polarization splitter and rotator based on asymmetric directional couplers with subwavelength gratings. J. Opt. Soc. Am. B, 39, 345-354(2022).

[41] Y.-D. Yang, Y. Li, Y.-Z. Huang. Silicon nitride three-mode division multiplexing and wavelength-division multiplexing using asymmetrical directional couplers and microring resonators. Opt. Express, 22, 22172-22183(2014).

[42] X. Han, Y. Jiang, A. Frigg. Mode and polarization-division multiplexing based on silicon nitride loaded lithium niobate on insulator platform. Laser Photon. Rev., 16, 2100529(2022).

[43] Z. Dong, Y. Zhao, Y. Wang. Gain optimization of an erbium-ytterbium co-doped amplifier via a Si₃N₄ photonic platform. Opt. Express, 31, 35419-35430(2023).