Temporal and spatial resonant modes are always possessed in physical systems with energy oscillation. In ultrafast fiber lasers, enormous progress has been made toward controlling the interactions of many longitudinal modes, which results in temporally mode-locked pulses. Recently, optical vortex beams have been extensively investigated due to their quantized orbital angular momentum, spatially donut-like intensity, and spiral phase front. In this paper, we have demonstrated the first to our knowledge observation of optical vortex mode switching and their corresponding pulse evolution dynamics in a narrow-linewidth mode-locked fiber laser. The spatial mode switching is achieved by incorporating a dual-resonant acousto-optic mode converter in the vortex mode-locked fiber laser. The vortex mode-switching dynamics have four stages, including quiet-down, relaxation oscillation, quasi mode-locking, and energy recovery prior to the stable mode-locking of another vortex mode. The evolution dynamics of the wavelength shifting during the switching process are observed via the time-stretch dispersion Fourier transform method. The spatial mode competition through optical nonlinearity induces energy fluctuation on the time scale of ultrashort pulses, which plays an essential role in the mode-switching dynamic process. The results have great implications in the study of spatial mode-locking mechanisms and ultrashort laser applications.

中文翻译：

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窄线宽锁模光纤激光器涡流模式切换的实时观察

具有能量振荡的物理系统中总是具有时间和空间共振模式。在超快光纤激光器中，在控制许多纵向模式的相互作用方面取得了巨大的进步，这导致了时间锁模脉冲。最近，由于其量子化轨道角动量、空间甜甜圈状强度和螺旋相位前沿，光学涡旋光束已被广泛研究。在本文中，我们首次展示了对窄线宽锁模光纤激光器中光学涡旋模式切换及其相应脉冲演化动力学的观察。空间模式切换是通过在涡旋锁模光纤激光器中加入双谐振声光模式转换器来实现的。涡流模式切换动力学有四个阶段，包括安静、弛豫振荡、准锁模和在另一种涡旋模式稳定锁模之前的能量恢复。通过时间拉伸色散傅立叶变换方法观察切换过程中波长偏移的演化动态。通过光学非线性的空间模式竞争引起超短脉冲时间尺度上的能量波动，这在模式切换动态过程中起着至关重要的作用。该结果对空间锁模机制和超短激光应用的研究具有重要意义。通过时间拉伸色散傅立叶变换方法观察切换过程中波长偏移的演化动态。通过光学非线性的空间模式竞争引起超短脉冲时间尺度上的能量波动，这在模式切换动态过程中起着至关重要的作用。该结果对空间锁模机制和超短激光应用的研究具有重要意义。通过时间拉伸色散傅立叶变换方法观察切换过程中波长偏移的演化动态。通过光学非线性的空间模式竞争引起超短脉冲时间尺度上的能量波动，这在模式切换动态过程中起着至关重要的作用。该结果对空间锁模机制和超短激光应用的研究具有重要意义。