Near-diffraction-limited operation of step-index large-mode-area fiber lasers via gain filtering

We present, for the first time to our knowledge, an explicit experimental comparison of beam quality in conventional and confined-gain multimode fiber lasers. In the conventional fiber laser, beam quality degrades with increasing output power. In the confined-gain fiber laser, the beam quality is good and does not degrade with output power. Gain filtering of higher-order modes in 28 mu m diameter core fiber lasers is demonstrated with a beam quality of M(2) = 1.3 at all pumping levels. Theoretical modeling is shown to agree well with experimentally observed trends. (C) 2010 Optical Society of America