**1. Introduction**

In recent years, ultrafast 2 μm fiber lasers have attracted considerable attention around the world and have found extensive application in areas like LIDAR, surgical operation, molecule

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

spectroscopy, optical sensing, medical treatment, material processing, and nonlinear microscopy [1–8]. In application, ultrashort pulses are usually required to have high pulse energy, which is very important for both scientific and industrial aims. In addition, achieving high energy short pulses at various wavelengths is the persistent pursuit of laser scientists.

energy of such mode-locked fiber laser can be improved to over 12 nJ, which is comparable to that of conventional solid-state mode-locked lasers. Under high pumping levels, pulsing dynamics of the CGFML fiber laser are studied, and high-energy harmonic mode-locking is realized. Finally, we present high-energy mode-locking of 2 μm fiber laser with new developed 2D materials (MoS2) and also achieve over 15 nJ pulse energy. These research results indicate that the CGFML is a new route to develop high-energy fiber lasers with short pulse

Developing High-Energy Dissipative Soliton 2 μm Tm3+-Doped Fiber Lasers

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Based on detailed simulation of the dynamics of short pulse propagating in various fiber circumstances, we found that the main factor that limited the pulse energy in 2 μm DS fiber lasers was related with nonlinear phase shift, which was primarily accumulated in the gain fiber. If we can efficiently control the nonlinear phase shift generated in the gain fiber, then the pulse energy of 2 μm DS fiber lasers probably can be significantly scaled. Therefore, we propose a condensed-gain fiber model where the gain fiber should be shortened as much as possible, and in the following, we give a detailed description about the model and carry out

A simple schematic diagram for the CGFML model is shown in **Figure 1(a)** [53]. The fiber laser cavity mainly includes five elements: output coupler (OC), single-mode fiber (SMF), gain fiber (GF), dispersion-compensating fiber (DCF), and saturable absorber (SA). Here, we use a single-mode highly doped 2 μm thulium fiber as the GF. Light evolution (pulse shape, pulse intensity, and spectrum) in the laser cavity is traced through solving the well-known

**Figure 1.** (a) Schematic diagram of the condensed-gain fiber laser model shows the light flow in the cavity. (b) Experimental setup of the SESAM mode-locked fiber laser system with a linear cavity. OC, output coupler; SMF, single-mode fiber; GF,

gain fiber; DCF, dispersion-compensating fiber; SA, saturable absorber [53].

**2. Condensed-gain dissipative soliton model and simulation for** 

simulation about the pulse dynamics happened in a 2 μm DS fiber laser.

nonlinear Schrodinger equation (NLSE) [27], which needs the original equation:

duration in the 2 μm wavelength region.

**2 μm fiber lasers**

Compared to traditional solid-state lasers (SSLs), fiber lasers (FLs) are better candidates for generation of ultrashort laser pulses due to their advantages of compactness, robustness, and good laser beam quality. Conventionally, generation of short pulses from fiber systems is achieved by the soliton mode-locking mechanism. Various passive mode-locking techniques can be employed, including the nonlinear polarization rotation (NPR) [9, 10], the nonlinear loop mirror [11, 12], and the saturable absorber method [13, 14]. However, pulse energy of traditional solitons (with anomalous net cavity dispersion), which is based on the balance of dispersion and nonlinearity, is usually limited by the soliton area theorem [15, 16] or the pulse peak power clamping effect [17, 18] to less than 1 nJ. Therefore, fiber lasers still produce much lower pulse energy than their solid-state counterparts [19].

To improve the pulse energy of fiber lasers, many techniques have been proposed and explored [20–36], among which four kinds of mechanisms have played important roles: dispersionmanaged soliton [20–22, 37, 38], all normal dispersion mode-locking [39], self-similar soliton [27–30], and dissipative soliton (DS) [31–36]. By taking advantage of the balance between not only nonlinearity and dispersion but also gain and loss, DS mode-locked fiber lasers have realized pulse energy 1–2 orders of magnitude larger than those from conventional soliton mode-locking [31, 32]. However, although the DS pulse energy from 1 to 1.5 μm fiber lasers has exceeded 10 nJ [40–42] and even over 20 nJ [33–35], pulse energy of 2 μm DS fiber lasers still remains at a low level. This is because the currently available gain fibers (GFs) in the 2 μm region show relatively large anomalous dispersion, resulting in conventional soliton modelocking operation of 2 μm fiber lasers [43–48]. Therefore, the pulse energy is still governed by the soliton area theorem and clamped by peak power [15, 17].

DS mode-locking has been widely adopted as an efficient method to improve the pulse energy of 2 μm fiber lasers. To implement DS mode-locking, the whole cavity's dispersion has to be pushed into the normal dispersion region. To that end, various methods have been proposed, e.g., inserting a chirped fiber Bragg grating into the cavity to provide normal dispersion [49] or incorporating specially designed dispersion-compensating fibers (DCFs) into the cavity [50–52]. However, these methods only improve pulse energy to around 1 nJ, and the great potential of DS mode-locking mechanism has not been fully explored.

Here, we will first present a new model to investigate the intracavity pulsing dynamics of a 2 μm DS mode-locked fiber laser and show that (different from the 1 to 1.5 μm counterparts) the pulse energy of 2 μm DS fiber lasers is mainly limited by the nonlinear phase shift caused by the gain fiber, and thereafter we propose that the anomalous dispersive GF should be condensed as short as possible to efficiently decouple gain from dispersion and nonlinearity. We name it the condensed-gain fiber mode-locking (CGFML). By avoiding too much phase accumulation, numerical simulations show that over 10 nJ DSs at 2 μm are readily feasible. After that, we carry out experimental operation of such CGFML of a 2 μm fiber laser, and a 4.9 nJ DS with 579 fs dechirped pulse duration is achieved. By further optimizing the cavity, the pulse energy of such mode-locked fiber laser can be improved to over 12 nJ, which is comparable to that of conventional solid-state mode-locked lasers. Under high pumping levels, pulsing dynamics of the CGFML fiber laser are studied, and high-energy harmonic mode-locking is realized. Finally, we present high-energy mode-locking of 2 μm fiber laser with new developed 2D materials (MoS2) and also achieve over 15 nJ pulse energy. These research results indicate that the CGFML is a new route to develop high-energy fiber lasers with short pulse duration in the 2 μm wavelength region.
