Enhancing pulse energy in a mode-locked thulium-doped figure 9 fiber laser by incorporating a few-mode fiber saturable absorber

Abstract The figure 9 fiber laser offers robust mode-locking performance, however, its pulse energy is often limited by pulse splitting at high pump power. To address this limitation, we propose and experimentally demonstrate an innovative figure 9 laser design that incorporate a few-mode fiber (FMF) saturable absorber (SA) within the linear arm to enhance pulse energy. Transmittance curve of the modified figure 9 fiber laser reveal a marked improvement in nonlinear tolerance, effectively suppressing pulse splitting. Experimental results further validate the efficacy of this proposed design. Under noise-like operation regime, the laser maintains stable single-pulse operation even at high pump levels, achieving a maximum output power of 823 mW and a corresponding pulse energy of 54 nJ. This represents a significant enhancement compared to the 34 nJ obtained without the FMF SA. Overall, our work provides a promising approach for scaling pulse energy in figure 9 fiber lasers while preserving single-pulse mode-locking stability.