High-order dispersion introduced by Gires–Tournois interferometer mirrors usually causes spectral sidebands in the near-zero dispersion region of mode-locked fiber lasers. Here, we demonstrate a sideband-free Yb-doped mode-locked fiber laser with dispersion-compensating Gires–Tournois interferometer mirrors. Both the simulation and the experiment demonstrate that the wavelength and energy of the sidebands can be tuned by changing the transmission coefficient of the output mirror, the pump power, and the ratio of the net cavity dispersion to the net third-order dispersion in the cavity. By optimizing these three parameters, the laser can generate a sideband-free, Gaussian-shaped spectrum with a 13.56-nm bandwidth at -0.0232ps2 net cavity dispersion, which corresponds to a 153-fs pulse duration.

目前，飞秒激光脉冲因脉冲宽度窄和峰值功率高的特点被广泛运用在多种领域中。其中，色散管理光纤锁模激光器因其特有的腔内呼吸机制使输出的激光脉冲能量更高，光谱更宽、脉宽更窄。使用啁啾布拉格光纤光栅进行色散管理的光纤锁模激光器能够实现真正的全光纤结构，提升激光器的紧凑性和稳定性，因此基于啁啾布拉格光纤光栅进行色散管理的光纤锁模激光器具有更加实际的应用意义。采用数值模拟的方法，研究了基于啁啾布拉格光纤光栅进行色散管理的掺镱光纤锁模激光器中单模光纤在腔内的不同分布对脉冲动力学过程和输出脉冲参数的影响。系统分析了谐振腔内净色散值不同时，腔内单模光纤的分布对脉冲在腔内的动力学过程的影响。模拟结果表明，在腔内净色散值为负时，啁啾布拉格光纤光栅与增益光纤间的单模光纤越短，光纤激光器维持稳定单脉冲运行的最大泵浦强度更高且输出光谱更宽，从而能够获得脉宽更窄的去啁啾脉冲；腔内净色散值越接近零时，啁啾布拉格光纤光栅与增益光纤间的单模光纤长度对输出脉冲参数作用的影响越显著；腔内净色散值为正时，单模光纤在腔内的分布对输出脉冲影响逐渐减弱，优化单模光纤分布提升锁模激光器性能并不明显。最后，提出了一种通过改变单模光纤在腔内的分布来提高激光器输出性能的优化方法。

Ultrafast lasers with high repetition rate, high energy, and ultrashort pulse duration have enabled numerous applications in science and technology. One efficient route to generate such pulses is postcompression of high-power Yb-doped lasers. Here, we report on the generation of 24.5 fs pulses with an output energy of 1.6 µJ and a repetition rate of 500 kHz. The pulses are obtained by using a hybrid cascaded nonlinear compression of the pulses delivered by a Yb-based fiber chirped pulse amplification (CPA) system. In the first stage, the initial 390 fs laser pulses are compressed to 100.7 fs based on spectral broadening in three fused silica plates. In the second stage, the pulses have been shortened to sub-30 fs by means of nonlinear compression in a hollow-core fiber. Overall, we could achieve ∼16 times temporal shortening with the proposed approach. The results show that our system can effectively generate few-cycle pulses at a relatively high repetition rate and high energy, which can benefit future possible applications.

随着激光雷达技术的发展和测距精度需求的提高，对发射和接收光学系统提出了新的要求，需具有光束可调节、测量光斑小、回波效率高等特性。设计一种工作于1 550 nm光通信波段的收发一体光学系统，发射与接收模块共用部分光路，以减小接收视野盲区，同时有利于结构小型化。为解决不同测量距离、不同表面倾角造成的回波能量差异问题，将光学系统的扩束组件设计成放大倍率为2^×～3.5^×的连续可调结构；使用两组双胶合透镜进行色差校正，以降低光谱宽度对系统传播距离的影响。经设计优化，系统准直后的激光发散角小于0.3 mrad，出射光斑直径在6.26 mm～10.20 mm连续可调，对于50 m内的测量目标，照射光斑直径均小于20 mm，且在不同变焦位置发散角和光斑直径均满足设计要求。

We demonstrate the stabilization of an optical frequency comb (OFC) using a segment of fiber delay line as a reference. A mode-locked Er-doped fiber laser is phase locked to a kilometer-long fiber delay line using three different schemes. The short-term stability of the comb modes in the OFC stabilized by these schemes is obviously enhanced, down to the 10-12 level at millisecond average time. Among these three schemes, phase locking two bunches of comb modes in the OFC to the same fiber delay line exhibits the lowest residual phase noise. Fiber-delay-line-referenced OFCs can provide reliable laser sources in precise metrology owing to the advances of low cost, compactness, and high integration.

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.

针对精密测量的实际应用需求，开发了具有6 MHz重复频率调谐范围的掺镱锁模光纤激光器。该光纤激光器利用啁啾光纤光栅实现色散补偿，可以实现不同色散域的锁模。实验中，系统研究了不同腔内净色散对锁模激光器输出特性和稳定性的影响，发现当具有一定负腔内净色散时，在不同的重复频率下都可以输出相同的光谱，同时输出光谱具有较好的高斯型。根据以上研究，特殊设计了稳定的简化腔结构和空间延迟线，同时优化了腔参数，保证了大范围的重复频率调谐和稳定的锁模运转。当中心重复频率为26 MHz 时，调谐比率达到23%。激光器稳定输出平均功率为3.23 mW的飞秒激光脉冲，去啁啾后脉冲宽度为347 fs。基于此，将光纤锁模激光器重复频率锁定于铷原子钟，在1 s的平均时间内获得了2×10⁻¹⁰的艾伦方差。

飞秒激光器的时间抖动（或定时抖动）是指其输出脉冲的时域位置相对于理想周期信号的短期随机偏差。在毫秒量级的时间尺度上，飞秒激光器的脉冲序列具有严格的一致性，其定时抖动甚至低至阿秒量级。飞秒激光器的这种独特性质及其支持的前沿应用构成了“阿秒时间精度的超快光子学”这一全新的超快研究分支。文中回顾了近年来飞秒激光器定时抖动研究进展、高时间分辨率的定时抖动测量技术、以及不同类型的飞秒激光源能够达到的最低抖动水平。最后介绍了低抖动飞秒激光器在大科学装置同步、高速模数转换、绝对测距、相干脉冲合成等领域的应用。

Internal motions in femtosecond soliton molecules provide insight into universal collective dynamics in various nonlinear systems. Here we introduce an orbital-angular-momentum (OAM)-resolved method that maps the relative phase motion within a femtosecond soliton molecule into the rotational movement of the interferometric beam profile of two optical vortices. By this means, long-term relative phase evolutions of doublet and triplet soliton molecules generated in an all-polarization-maintaining mode-locked Er-fiber laser are revealed. This simple and practical OAM-resolved method represents a promising way to directly visualize the complex phase dynamics in a diversity of multisoliton structures.