High-order mode erbium-doped fiber laser based on cavity LPFG converters

We experimentally investigate high-order mode (HOM) generation at a wavelength of 1.5 µm in an all-fiber erbium-doped laser based on a long-period fiber grating (LPFG). The CW laser emission is achieved when the pump power is above the threshold of 10 mW. An LPFG with a 15 dB bandwidth of 147.76 nm from 1502.76 nm to 1650.52 nm is used as a mode converter inside the cavity. The generation of the broadband L P ₁₁ mode is ultimately obtained. By using a few-mode output coupler, we can obtain the intracavity conversion of the linear polarization mode. Single-, dual-, triple-, and quadruple-wavelength operations can be achieved by changing the polarization state of the polarization controllers in the cavity. The tunable range of the output wavelengths is up to ∼23.20n m. The output power and slope efficiency of the HOMs are presented and discussed. We believe our work might benefit the investigation of HOM fiber lasers, and might be further applied to the intracavity conversion of the linear polarization mode or orbital angular momentum beams.