**Author details**

**6. Conclusions and prospects**

130 High Power Laser Systems

In recent years, 2 μm fiber lasers with short pulse duration have received great research interests due to great application potentials of 2 μm light sources in areas such as LIDAR, surgical operation, molecule spectroscopy, remote sensing, etc. However, applications usually require high pulse energy, which is hard to achieve with traditional soliton mode-locked fiber lasers. Although various mode-locking mechanisms have been proposed to improve the pulse energy of ultrafast fiber lasers, e.g., dispersion-managed soliton, all normal dispersion mode-locking, self-similar soliton, and dissipative soliton (DS), the pulse energy achieved with 2 μm mode-locked fiber lasers is still much lower than their 1 and 1.5 μm counterparts. This is because that currently available gain fibers and passive fibers are generally anomalous dispersive at 2 μm, which makes mode-locking lie in the traditional soliton regime, and the

DS, based on the balance of both dispersion and nonlinearity and gain and loss, provides a new route to improve the pulse energy of ultrafast fiber lasers. Up to now, pulse energy in 1 and 1.5 μm regions based on DS mode-locking mechanism has been over 20 nJ, giving pulse energies 1~2 orders of magnitude larger than that from conventional soliton mode-locking. However, it is still difficult to generate comparable high energy pulses in 2 μm DS fiber lasers,

In order to make advantage of DS mode-locking and improve the pulse energy of 2 μm modelocked fiber laser, we propose a condensed-gain fiber mode-locking (CGFML), in which the gain fiber should be as short as possible to minimize the nonlinear phase shift caused by the gain fiber. Based on this model, we give detailed exploration of the pulsing dynamics and pulse energy scaling potential of 2 μm thulium-doped mode-locked fiber lasers in several regimes and confirm that this kind of DS mode-locked fiber laser can generate pulse energy

pulse energy is thus limited by the soliton area theorem clamped by peak power.

because of large anomalous dispersion occurred in 2 μm gain fibers.

**Figure 21.** Radiofrequency spectral profile of the mode-locked Tm3+ fiber laser [6].

over 10 nJ, improving the pulse energy by 1 to 2 orders of magnitude.

Yulong Tang\*, Chongyuan Huang and Jianqiu Xu

\*Address all correspondence to: yulong@sjtu.edu.cn

Key Laboratory for Laser Plasmas (Ministry of Education), Department of Physics and Astronomy, Collaborative Innovation Center of IFSA, Shanghai Jiao Tong University, Shanghai, China
