Abstract

A >600 MHz harmonic mode-locked femtosecond holmium-doped (Ho-doped) fiber laser is presented by using the nonlinear polarization rotation (NPR) method. When the fiber laser works in fundamental mode-locking (FML) regime at 2063.1 nm, a pulse repetition rate of 20.8 MHz is obtained with a corresponding pulse duration of 983 fs. By increasing the pump power, 29th-order harmonic mode-locking (HML) operation of the Ho-doped fiber laser is realized for the first time, to the best of our knowledge. The maximum pulse repetition rate of 603 MHz at 2063.7 nm is achieved with a harmonic pulse duration of 996 fs, corresponding to a super-mode suppression radio (SMSR) of as high as 33 dB.