Transverse mode switchable fiber laser with a multimodal interference-based beam shaper.

We propose a Yb-doped fiber laser with an all-fiber beam shaper based on a single-mode-graded-index multimode-few-mode fiber (SMF-GIMF-FMF) structure. The excitation coefficients of the mode can be adjusted continuously by changing the GIMF length. Numerical simulations are performed to investigate the beam shaping dynamics in the fiber structure. Through adding the simple device geometry in the laser cavity, the switchable output between the fundamental transverse (LP) mode and the second-order transverse (LP) mode can be achieved. Cylindrical vector beams with high mode purity are also shown by removing the degeneracy of the LP mode.