[2] [2] RADEMACHER G, LUIS R S, PUTTNAM B J, et al. High capacity transmission with few-mode fibers[J]. Journal of Lightwave Technology, 2019, 37(2): 425-432.

[3] [3] HU T, LI J, GE D, et al. Weakly-coupled 4-mode step-index FMF and demonstration of IM/DD MDM transmission[J]. Optics Express, 2018:8356-8363.

[4] [4] WAKAYAMA Y, SOMA D, BEPPU S, et al. 266.1-Tbit/s transmission over 90.4-km 6-mode fiber with inline dual C+L-band 6-mode EDFA[J]. Journal of Lightwave Technology, 2019, 37(2): 404-410.

[5] [5] GE D W, GAO Y Y, YU Y, et al. A 6-LP-mode ultralow-modal-crosstalk double-ring-core FMF for weakly-coupled MDM transmission[J]. Optics Communications, 2019, 45: 97-103.

[6] [6] SHIBAHARA K, MIZUNO T, KAWAKAMI H, et al. Full C-band 3060-km DMD-unmanaged 3-mode transmission with 40.2-Tb/s capacity using cyclic mode permutation[J]. Journal of Lightwave Technology, 2020, 38(2): 514-521.

[8] [8] FATADIN I, SAVORY S J, IVES D. Compensation of quadrature imbalance in an optical QPSK coherent receiver[J]. IEEE Photonics Technology Letters, 2008, 20: 1733-1735.

[9] [9] GARDNER F M. A BPSK/QPSK timing-error detector for sampled receivers[J]. IEEE Transactions on Communications, 1986, 34(5): 423-429.

[10] [10] XU K, CHEN M H, JIA Z S, et al. Measurement methods and statistics of polarization mode dispersion in optical transmission systems with polarization dependent loss[C]//SPIE. Proceedings of SPIE. Beijing: SPIE,2001: 202-207.

[11] [11] XIE X, YAMAN F, ZHOU X, et al. Polarization demultiplexing by independent component analysis[J]. IEEE Photonics Technology Letters, 2010, 22(11): 805-807.

[12] [12] CARTWRIGHT K V, KAMINSKY E J. A simple improvement to the viterbi and viterbi monomial-based phase estimators[C]//IEEE. Proceedings of globecom 2006. San Francisco: IEEE, 2006:1-6.

[13] [13] VITERBI A J, VITERBI A M. Nonlinear estimation of PSK modulated carrier phase with application to burst digital transmission[J]. IEEE Trans. Inform. Theory, 1983, IT-29: 543-551.