In the last few decades, only temporal modes have been considered for mode-locked fiber lasers using single-mode fibers. Mode-locked single-mode fiber lasers offer advantages due to their high-gain doping, intrinsically single-spatial mode, and compact setups. However, in terms of power levels, mode-locked fiber lasers suffer from high nonlinearity, which is introduced by the small core size of the single-mode fibers. Researchers from École Polytechnique Fédérale de Lausanne, Switzerland (EPFL) recently developed a new approach for generating high-energy, ultrashort pulses with single-mode beam quality: nonlinear beam cleaning in a multimode laser cavity.
source https://phys.org/news/2020-12-nonlinear-spatiotemporally-mode-locked-lasers.html
