Intracavity Ultrafast Fiber Lasers With On‐Demand and Reconfigurable Structured Modes

Structured light enables tailored optical fields through precise amplitude, phase, and polarization modulations, giving rise to beams with unique spatial characteristics, such as Laguerre–Gaussian (L–G) modes carrying orbital angular momentum (OAM), petal‐like LG superpositions, and Hermite–Gaussian (H–G) beams with desired symmetries. Spatial light modulators (SLM) are commonly employed to generate structured light on demand within digital laser cavity, but their role has been limited to passive linear modulation in continuous‐wave (CW) lasers. Here, we experimentally demonstrate active spatiotemporal modes control of an ultrafast fiber laser through the synergistic integration of an SLM and a polarization beam splitter (PBS) inside the laser cavity. The laser delivers femtosecond pulses (∼800 fs) with on‐demand switching among a wide range of LG and HG modes. A polarization‐dependent mode‐locking mechanism was revealed both experimentally and numerically, producing a total of 84 structured modes, which is an order of magnitude higher than the conventional spatiotemporal lasers. This method is capable of generating arbitrary higher‐mode light limited only by the available resolution of the SLM. This work establishes a versatile platform for intracavity ultrafast structured light generation, opening new avenues for applications in high‐resolution imaging, optical manipulation, laser micro/nanofabrication and optical communications.