All-fiber laser with flattop beam output using a few-mode fiber Bragg grating

We report a novel method of beam shaping in an all-fiber laser. Flattop beams are first generated from the all-fiber laser using a few-mode fiber Bragg grating by superposing the fundamental mode (LP01) with a solid central beam distribution, and the second-order mode (LP11) gives rise to a donut-shape beam distribution. Two modes are obtained in the fiber laser simultaneously, and the wavelengths of the two modes are 1053.7 nm and 1055.4 nm, respectively. An experiment has been carried out, and a high-quality flattop profile is obtained for the normalized root-mean-square of the light intensity less than 5% under no numerical aperture illumination, consistent with the theoretical prediction.