High-Order Mode Laser Based on a Low-Order Mode Filter in Few-Mode Fiber Ring Resonator

We proposed and demonstrated a few-mode fiber (FMF) ring laser with controllable second-order transverse mode output by constructed a low-order mode filter in a few-mode fiber ring resonator. The mode filter consists of two cascaded long-period fiber gratings, which can couple the light in the fundamental mode and LP<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1,1</inf> mode into the cladding mode, and then dissipate it. The filtering efficiency of the fundamental mode is ~90%. With a homemade symmetric mode coupler, the second-order mode light is successfully coupled out from the ring resonator. Furthermore, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{LP}_{\mathbf{2},\mathbf{1}}$</tex> mode have been obtained at 1553.5nm by adjusting the polarization controllers. And the 3dB bandwidth is 0.4nm.