Rigorous analysis on a U-shaped index fiber for generating cylindrical vector beams in an all-fiber format

We propose a novel method to generate cylindrical vector beams (CVBs) which can be applied to conventional fiber-laser system configuration. We focus on the relationship between the laser pump power threshold and the power confinement in the high index region of a fiber, and suggest a novel type of index distribution for the desirable operation. We verify our idea by computer simulations and show that the laser threshold of the CV mode can be reduced down below that of the fundamental mode by 3%. The suggested method will be useful for generating cylindrical vector beams in an all-fiber format.