基于差频产生的中红外飞秒光源具有波长调谐范围宽（6~20 μm）、覆盖范围广（整个“指纹区”）和系统复杂程度低等优势，超快光纤激光器驱动的中红外飞秒光源只有差频部分采用了空间光路，进一步提高了系统的稳定性。文中介绍基于超快光纤激光器驱动的光学差频产生长波中红外飞秒脉冲的技术路线，阐述在差频过程中如何通过非线性光纤光学技术（包括超连续谱产生、孤子自频移和光谱滤波技术）产生合适的信号脉冲，并从理论上详细介绍差频过程中提高中红外脉冲功率的方法。

We numerically and experimentally investigate the multi-pulsing mechanism in a dispersion-managed mode-locked Yb-doped fiber laser. Multi-pulsing occurs primarily owing to the inherent filtering effect of the chirped fiber Bragg grating. The spectral filtering effect restricts the spectral broadening induced by self-phase modulation and causes extra loss, leading to a decreased pump power threshold for the multi-pulsing state. Numerical simulations show that multi-pulsing emerges at a lower pump power when the spectral filter bandwidth becomes narrower. In the experiment, the spectral width increases as the net cavity dispersion approaches zero. Pulses with wider spectral widths experience more loss from the spectral filtering effect, leading to a decreased pump power threshold for multi-pulsing. Therefore, the net cavity dispersion also has an impact on the multi-pulsing threshold. Based on this conclusion, we devise a strategy to obtain single-pulsing operation with the shortest pulse width and the highest pulse energy.

The performance of fiber mode-locked lasers is limited due to the high nonlinearity induced by the spatial confinement of the single-mode fiber core. To massively increase the pulse energy of the femtosecond pulses, amplification is performed outside the oscillator. Recently, spatiotemporal mode-locking has been proposed as a new path to fiber lasers. However, the beam quality was highly multimode, and the calculated threshold pulse energy (>100 nJ) for nonlinear beam self-cleaning was challenging to realize. We present an approach to reach high energy per pulse directly in the mode-locked multimode fiber oscillator with a near single-mode output beam. Our approach relies on spatial beam self-cleaning via the nonlinear Kerr effect, and we demonstrate a multimode fiber oscillator with M² < 1.13 beam profile, up to 24 nJ energy, and sub-100 fs compressed duration. Nonlinear beam self-cleaning is verified both numerically and experimentally for the first time in a mode-locked multimode laser cavity. The reported approach is further power scalable with larger core sized fibers up to a certain level of modal dispersion and could benefit applications that require high-power ultrashort lasers with commercially available optical fibers.

Rogue waves (RWs) are rare, extreme amplitude, localized wave packets, which have received much interest recently in different areas of physics. Fiber lasers with their abundant nonlinear dynamics provide an ideal platform to observe optical RW formation. We review recent research progress on rogue waves in fiber lasers. Basic concepts of RWs and the mechanisms of RW generation in fiber lasers are discussed, along with representative experimental and theoretical results. The measurement methods for RW identification in fiber lasers are presented and analyzed. Finally, prospects for future RW research in fiber lasers are summarized.