[1] [1] Pal A, Sen R J, Bremer K, et al. “All-fiber” tunable laser in the 2 μm region, designed for CO2 detection[J]. Appl Opt, 2012, 51(29): 7011–7015.

[2] [2] Hudson D D. Invited paper: short pulse generation in mid-IR fiber lasers[J]. Opt Fiber Technol, 2014, 20(6): 631–641.

[3] [3] Rudy C W, Digonnet M J F, Byer R L. Advances in 2-μm Tm-doped mode-locked fiber lasers[J]. Opt Fiber Technol, 2014, 20(6): 642–649.

[4] [4] Mingareev I, Weirauch F, Olowinsky A, et al. Welding of polymers using a 2 μm thulium fiber laser[J]. Opt Laser Technol, 2012, 44(7): 2095–2099.

[5] [5] Nishizawa N, Nozaki Y, Itoga E, et al. Dispersion-managed, high-power, Er-doped ultrashort-pulse fiber laser using carbon-nanotube polyimide film[J]. Opt Express, 2011, 19(22): 21874–21879.

[6] [6] Lan Y, Song Y J, Hu M L, et al. Enhanced spectral breathing for sub-25 fs pulse generation in a Yb-fiber laser[J]. Opt Lett, 2013, 38(8): 1292–1294.

[7] [7] Wang Q, Geng J, Luo T, et al. Mode-locked 2 μm laser with highly thulium-doped silicate fiber[J]. Opt Lett, 2009, 34(23): 3616–3618.

[8] [8] Wang Q, Geng J H, Jiang Z, et al. Mode-locked tm–ho-codoped fiber laser at 2.06μm[J]. IEEE Photonics Technol Lett, 2011, 23(11): 682–684.

[9] [9] Ma W Z, Zhao D S, Liu R M, et al. Observation and optimization of 2 μm mode-locked pulses in all-fiber net anomalous dispersion laser cavity[J]. Opto-Electron Adv, 2020, 3(11): 200001.

[10] [10] Kong L C, Xie G Q, Yuan P, et al. Passive Q-switching and Q-switched mode-locking operations of 2 μm Tm:CLNGG laser with MoS2 saturable absorber mirror[J]. Photonics Res, 2015, 3(2): A47–A50.

[11] [11] Zhang X J, Li W W, Li J, et al. Mid-infrared all-fiber gain-switched pulsed laser at 3 μm[J]. Opto-Electron Adv, 2020, 3(5): 190032.

[12] [12] Yan Z Y, Li X H, Tang Y L, et al. Tunable and switchable dual-wavelength Tm-doped mode-locked fiber laser by nonlinear polarization evolution[J]. Opt Express, 2015, 23(4): 4369–4376.

[13] [13] Wang Q, Chen T, Zhang B, et al. All-fiber passively mode-locked thulium-doped fiber ring oscillator operated at solitary and noiselike modes[J]. Opt Lett, 2011, 36(19): 3750–3752.

[14] [14] Jin X X, Wang X, Wang X L, et al. Tunable multiwavelength mode-locked Tm/Ho-doped fiber laser based on a nonlinear amplified loop mirror[J]. Appl Opt, 2015, 54(28): 8260–8264.

[15] [15] Li J F, Zhang Z X, Sun Z Y, et al. All-fiber passively mode-locked Tm-doped NOLM-based oscillator operating at 2-μm in both soliton and noisy-pulse regimes[J]. Opt Express, 2014, 22(7): 7875–7882.

[16] [16] Wang Z K, Wang D N, Yang F, et al. Stretched graded-index multimode optical fiber as a saturable absorber for erbium-doped fiber laser mode locking[J]. Opt Lett, 2018, 43(9): 2078–2081.

[17] [17] Zhu X, Schülzgen A, Li H, et al. Single-transverse-mode output from a fiber laser based on multimode interference[J]. Opt Lett, 2008, 33(9): 908–910.

[18] [18] Li H H, Wang Z K, Li C, et al. Mode-locked Tm fiber laser using SMF-SIMF-GIMF-SMF fiber structure as a saturable absorber[J]. Opt Express, 2017, 25(22): 26546–26553.

[19] [19] Nazemosadat E, Mafi A. Nonlinear multimodal interference and saturable absorption using a short graded-index multimode optical fiber[J]. J Opt Soc Amer B, 2013, 30(5): 1357–1367.

[20] [20] Fu S J, Sheng Q, Zhu X S, et al. Passive Q-switching of an all-fiber laser induced by the Kerr effect of multimode interference[J]. Opt Express, 2015, 23(13): 17255–17262.

[21] [21] Wang Z K, Wang D N, Yang F, et al. Er-doped mode-locked fiber laser with a hybrid structure of a step-index–graded-index multimode fiber as the saturable absorber[J]. J Lightwave Technol, 2017, 35(24): 5280–5285.

[22] [22] Yang F, Wang D N, Wang Z K, et al. Saturable absorber based on a single mode fiber–graded index fiber–single mode fiber structure with inner micro-cavity[J]. Opt Express, 2018, 26(2): 927–934.

[23] [23] Zhang H W, Jin L, Zhang H, et al. All-fiber nonlinear optical switch based on polarization controller coiled SMF–GIMF–SMF for ultrashort pulse generation[J]. Opt Commun, 2019, 452: 7–11.

[24] [24] Uzma M, Zaheer M, Asif A, et al. Effect of phase shifting on self-imaging in optical multi-mode interference devices[C]//Proceedings of 2016 19th International Multi-Topic Conference, Islamabad, Pakistan, 2016: 1–5.

[25] [25] Mehta A, Mohammed W, Johnson E G. Multimode interference-based fiber-optic displacement sensor[J]. IEEE Photonics Technol Lett, 2003, 15(8): 1129–1131.

[26] [26] Wei L, Khattak A, Martz C, et al. Tunable multimode fiber based filter and its application in cost-effective interrogation of fiber-optic temperature sensors[J]. IEEE Photonics J, 2017, 9(2): 7101808.

[27] [27] Khattak A, Tatel G, Wei L. Tunable and switchable erbium-doped fiber laser using a multimode-fiber based filter[J]. Appl Sci, 2018, 8(7): 1135.

[28] [28] Zhu X, Schülzgen A, Li H, et al. Detailed investigation of self-imaging in largecore multimode optical fibers for application in fiber lasers and amplifiers[J]. Opt Express, 2008, 16(21): 16632–16645.

[29] [29] Daniault L, Hanna M, Lombard L, et al. Coherent beam combining of two femtosecond fiber chirped-pulse amplifiers[J]. Opt Lett, 2011, 36(5): 621–623.