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Using amplitude modulation in a semiconductor saturable absorber mirror induced by optical clock signal, we have synchronized 600-fs pulse train from a mode-locked thulium-holmium fiber laser operating at 1.99 μm to an external reference source. The optical source based on 1.56- μm distributed-feedback diode laser driven by the clock signal was used as a seed source. The amplified seed pulses launched into the fiber cavity synchronized the mode-locked pulses through amplitude modulation in the absorber. The synchronization method allows us to operate the mode-locked laser at multiples of cavity harmonics and to increase the repetition rate.