Long-cavity mode-locked thulium-doped fiber laser for high pulse energy

•Two thulium-doped fiber gains are deployed separately for effective pulse amplification.•Hybrid mode-locking is attained by the coaction of a graphene saturable absorber and nonlinear polarization rotation.•Numerical simulations determine the maximally extractable pulse energy and laser stability.•A pulse energy of 54.6 nJ is reached, the highest as a seed femtosecond pulse to the best of our knowledge.•A pulse rate of 427 kHz is achieved, the lowest among mode-locked thulium-doped fiber lasers ever reported.