We propose and demonstrate experimentally multimode Q-switched mode-locking (QML) fiber laser signal generation based on a long few-mode Er-doped fiber (FM-EDF). The erbium-doped profile of the fiber has been optimized to equalize the modal gain. The fiber supporting six modes has a total length of 5.5 m, a part of the fiber is used as a gain medium, and the unpumped part of the fiber is used as a saturable absorber. Multimode oscillation lasers in the state of the continuous wave laser operation, Q-switching, and QML state pulse laser operation were observed and measured at different pump powers by adjusting the polarization controller. A 1570-nm multimode QML pulse train was obtained, when the pump power was about 2.4 W. The width of the Q-switched envelope was 19 μs, and the repetition rate was 17.4 kHz. The stable mode-locked pulses repetition rate in Q-switched envelopes was 13.18 MHz. These temporal pulses can also be observed under the case of partly blocking the beam profile, and the part of the spectral signal gradually disappeared. The mode number in the laser was checked by a homemade mode selection photonic lantern. The results showed that the fiber laser had multimode oscillation, and the modes in generated signal were ≥5. The multiple transverse modes of the generated lasers are all in QML state at the same time. To the best of our knowledge, this is the first experimental demonstration of a multimode QML fiber laser using a long FM-EDF.

中文翻译：

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基于增益均衡少模掺Er光纤的多模振荡调Q锁模光纤激光器

我们提出并演示了基于长的少数模掺Er光纤（FM-EDF）的实验性多模Q开关锁模（QML）光纤激光信号生成。光纤的掺-轮廓已得到优化，以补偿模态增益。支持六种模式的光纤总长度为5.5 m，一部分光纤用作增益介质，另一端未抽出的光纤用作可饱和吸收器。通过调节偏振控制器，可以观察并测量处于连续波激光操作，Q开关和QML状态脉冲激光操作状态的多模振荡激光器，并在不同的泵浦功率下进行测量。当泵浦功率约为2.4 W时，获得了1570 nm多模QML脉冲序列。调Q包络的宽度为19μs，重复频率为17.4 kHz。调Q包络中的稳定锁模脉冲重复率为13.18 MHz。在部分阻挡光束轮廓的情况下，也可以观察到这些时间脉冲，并且光谱信号的一部分逐渐消失。激光中的模式编号由自制的模式选择光子灯笼检查。结果表明，光纤激光器具有多模振荡，所产生信号的模≥5。所产生的激光器的多个横向模式都同时处于QML状态。据我们所知，这是使用长FM-EDF的多模QML光纤激光器的首次实验演示。激光中的模式编号由自制的模式选择光子灯笼检查。结果表明，光纤激光器具有多模振荡，所产生信号的模≥5。所产生的激光器的多个横向模式都同时处于QML状态。据我们所知，这是使用长FM-EDF的多模QML光纤激光器的首次实验演示。激光中的模式编号由自制的模式选择光子灯笼检查。结果表明，光纤激光器具有多模振荡，所产生信号的模≥5。所产生的激光器的多个横向模式都同时处于QML状态。据我们所知，这是使用长FM-EDF的多模QML光纤激光器的首次实验演示。