Self-referenced f -to-2 f beat note of a period-doubling mode-locked Yb-fiber laser

We demonstrated a period-doubling mode-locked (PD-ML) state with the pulse train locked to the twice of cavity roundtrip time in the Yb-fiber laser. The overall comb tooth of such PD-ML laser is characterized through self-referenced f -to-2f beat note. In contrast to normally fundamental mode-locking or harmonic mode-locking, the carrier-envelope offset frequency ( f₀ ) was observed to shrink into half of the corresponding fundamentally mode-locked laser in the frequency domain. Meanwhile, the regenerated half harmonics of a repetition frequency exhibited an amplitude tunable property. Although the f₀ signal slightly deteriorated, a modulation rate of 4.65 MHz/mA within a linear tuning range of 100 MHz was achieved in our PD-ML laser.