Direct Generation of Tunable Femtosecond Vortex Arrays in a Yb:CALGO Laser

ABSTRACT

A novel method for directly generating ultrafast vortex arrays within the cavity of a femtosecond Yb:CALGO laser oscillator, based on transverse mode locking, is proposed and experimentally demonstrated. This approach achieves dual‐degree‐of‐freedom active control over the topological structure of vortex arrays within a single laser cavity, including the continuous tuning of the number of phase singularities and the dynamic switching of spatial topological configurations under a fixed number of singularities. To this end, the spatial gain distribution introduced by off‐axis pumping is exploited to selectively enhance the oscillation gain of higher‐order transverse modes. Meanwhile, the spatially selective loss introduced by the array‐type defect mirror is utilized to achieve precise mode selection and balancing. Through this gain–loss synergistic mechanism, the transverse mode composition involved in mode locking can be selectively controlled, thereby enabling dual active reconstruction of the ultrafast vortex array. Experimental results demonstrate that, as the pump power increases from 32 to 48 W, the number of phase singularities in the ultrafast vortex array can be continuously tuned from 1 to 3 by adjusting the position and tilt angle of the array‐type defect mirror, without disrupting the linear arrangement. Furthermore, under a fixed number of phase singularities—whether 2 or 3—different spatial topological configurations can be dynamically switched simply by fine‐tuning the defect‐mirror position.