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Multimode fibers (MMFs) have attracted interest because of their larger mode area and additional spatial degrees of freedom compared with single-mode fibers. Recently, MMFs have been introduced into mode-locked lasers for new cavity structures with better laser performance. Among these lasers, there are two remarkable cases in which MMFs are the key components for mode-locking: One is using a short segment of the MMF as a saturable absorber (SA), which has the advantages of high damage threshold, wide operating bandwidth, and low cost. The other is the construction of multimode cavities with MMFs. Spatiotemporal mode-locking (STML), a general form of mode-locking, can be achieved in such cavities. Herein, we review these two types of mode-locked lasers with MMFs. The principles of MMF-based SA and STML are described based on the common theory of multimode nonlinear fiber optics. Subsequently, we present a detailed overview of the recent progress regarding these lasers, including lasers mode-locked by MMF-based SAs at the 1-, 1.5-, and 2-μm bands, as well as spatiotemporal mode-locked MMF lasers. A perspective for the development of mode-locked lasers with MMFs is also provided. [ABSTRACT FROM AUTHOR]

• Both the fundamental frequency operation, 3rd harmonic, and 81st harmonic are observed. • The self-mode-locking operation has excellent stability, since the SNR of the 81st harmonic is as high as 80 dB. • The 81st harmonic has a narrow 3-dB optical spectrum bandwidth of 0.29 nm. • The maximum output power in our work is 27.63 mW. • The self-focusing caused by the Kerr-lens effect of SMF is the major mechanism of self-mode-locking. Passive ultrafast photoelectric devices based on the self-focusing effect have been deeply studied in ultrashort pulse formation, and great progress has been made in solid-state oscillators known as Kerr-lens mode-locking (KLM). The intensity-dependent change of refractive index and the 1–2 fs electronic response time of materials are the basics of KLM. Recently, the advantages of fiber lasers such as compact structure, outstanding environmental stability, excellent heat dissipation potential, and low cost have stimulated great interest among researchers to explore stable and reliable fiber lasers. Based on the feasibility of KML in fiber lasers that have been reported, we demonstrate a high-stable self-mode-locking Er-doped fiber laser in this work. Diverse stable self-mode-locking pulses phenomena can be obtained through adjusting the polarization state of the cavity, including the output pulses with a fundamental frequency of 3.93 MHz, dark-bright soliton pulses, the 3rd harmonic operation, the 81st harmonic operation with the repetition rate of 318.4 MHz and the signal-to-noise ratio of 80 dB, and dual-wavelength output. [ABSTRACT FROM AUTHOR]