Generating Femtosecond Visible Doughnut Beams Based on Transverse-Mode Modulation

Ultrafast visible doughnut beams carrying cylindrically symmetric states of polarization or orbital angular momentum provide new capabilities for super-resolution microscopy, visible light communications, and material processing. Visible laser pulses typically originate from solid-state lasers, and the generation of ultrashort visible light beams using an all-fiber layout remains challenging. In addition, the generation of doughnut beams usually relies on free-space optics, which are costly and require precise alignment. Here, an integrated fiber method for producing a femtosecond visible doughnut beam is proposed by integrating nonlinear frequency conversion and in-fiber mode conversion techniques. This approach takes advantage of nonlinear frequency conversion and in-fiber transverse-mode modulation, which creates a new manipulation technique of visible doughnut beams of ultrashort pulses. The dynamic spatial manipulation of vector visible beams enabled by polarization control in a polarization-maintaining fiber (PMF) or by mode conversion through a quarter-wave plate and a polarizer experimentally proves the mode-controllable visible doughnut light generation.