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论文范文
1. Introduction High-power and high-energy pulsed lasers operating at 2 μm from the 3F4→ 3H6 transition of Tm3+ ion have been extensively studied for their potential applications such as material processing, laser therapy, national defense security, free-space optical communication, and pumping for mid-infrared supercontinuum generation [1–5]. Passively Q-switched Tm-doped fiber lasers are favorable ways to generate high-energy pulses at 2 μm, where two-dimension (2D) materials such as graphene, topological insulators (TIs), and transition metal dichalcogenides (TMDCs) are usually adopted as saturable absorbers (SAs) instead of commercial semiconductor saturable absorber mirror (SESAM) due to their low cost and easy fabrication [6–13]. Black phosphorus (BP), as a newly emerged 2D material, has attracted wide attention recently for the common properties of 2D materials with wide bandwidth, ultrafast carrier dynamics, and planar characteristic [14]. More importantly, BP has a thickness-dependent direct energy bandgap from 0.3 eV (bulk) to 2.0 eV (single layer) [15], filling up the interval between the zero gap of graphene and large gap of TMDCs, which is of great importance to the optical applications. Up to now, the broadband BP saturable absorbers have been widely applied to pulsed lasers in a wide spectral range from visible to mid-infrared [16–25]. Specifically, in 2 μm Q-switched fiber lasers, Wang et al. reported a passively Q-switched Tm-doped fiber laser using a BP deposited microfiber, delivering the maximum pulse energy of 154 nJ [26]; Jiang et al. deposited the BP powders onto a side-polished fiber and achieved Q-switched operation with the maximum pulse energy of 276 nJ [27]. The output pulse energy was further improved up to 632 nJ with BP SA fabricated by optical deposition method [28]. Although BP has been confirmed to be a reliable and excellent SA for 2.0 μm Q-switched fiber lasers, the output pulse energy is limited below micro joule level due to the employment of single-mode fibers. Large-mode-area (LMA) fiber fabricated via sol-gel method combined with high temperature sintering will be a choice for generating high-energy laser pulses. The sol-gel method has a great merit of higher doping homogeneity for rare-earth ions, preventing the cluster of the rare-earth ions and the fluorescence quench effect. With the advance of LMA fiber fabrication, dual-cladding Tm-doped fiber with core diameter as large as 38 μm could be developed by sol-gel method combined with high temperature sintering [29], offering a good alternative for higher pulse energy generation at 2 μm. In this paper, we adopted the optimized LMA double-cladding Tm-doped fiber as gain medium with a core diameter as large as 30 μm. Based on a mechanically exfoliated BP SA, the high-energy Q-switching operation was demonstrated. The Q-switched fiber laser emitted pulses with the maximum average output power of 615 mW, the maximum pulse energy of 11.72 μJ, and the shortest pulse width of 660 ns at 1954 nm. Compared with the previously reported pulse energies in BP Q-switched Tm-doped fiber lasers, the pulse energy was improved by an order of magnitude, indicating that the sol-gel fabricated LMA Tm-doped fiber together with BP SA is an effective way to generate Q-switched fiber laser with high pulse energy at the wavelength of 2 μm. 
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