**6. Conclusions and prospects**

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

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

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

In the primary experimental operation based on this model, the 2 μm DS mode-locked Tm-doped fiber laser with a linear cavity delivers 4.9 nJ DSs with pulse duration of 579 fs after being dechirped. Then, through increasing pump power or managing the cavity dispersion map, the pulse energy of this DS fiber is improved to ~12 nJ. We also observe that highpulse-energy harmonic mode-locked DSs from 2 μm Tm-doped fiber lasers, with single pulse energy of 6.27, 4.32, and 3.29 nJ for the second- to the fourth-order harmonics. Thereafter, DS mode-locking of 2 μm TDFL with 2D material (multilayer MoS2 ) is investigated, and through decreasing the pulsing frequency, the pulse energy is scaled to 15.5 nJ. This improves the pulse energy of 2 μm mode-locked single-mode fiber lasers to approaching the 1 and 1.5 μm counterparts. All these results show that CGFML DS can be an efficient way to produce highenergy ultrafast pulses from 2 μm TDFLs.

To further scale the pulse energy of the CGFML DS in 2 μm TDFLs, more condensed GFs (which has been available currently) should be adopted, and the total cavity dispersion map should be optimized. Therefore, with higher pump power, more condensed GFs, and further optimized parameters, ultrafast 2 μm pulses with even higher energy are readily feasible.

This CGFML model can be readily extended to beyond 2 μm, e.g., mid-infrared fiber lasers (usually with anomalously dispersive gain media) to scale DS energy and thus is an efficient pulse energy scaling route for anomalous dispersive fiber lasers.
