Ultrafast visible fiber laser mode-locked by frequency-shifted feedback of an α-BaTeMo2O9 crystal acousto-optic modulator.

We report the first demonstration, to the best of our knowledge, of visible mode-locked fiber laser using frequency-shifted feedback (FSF) with a visible α-BaTeMo₂O₉ (α-BTM) crystal acousto-optic modulator (AOM). First, an α-BTM crystal is used as the visible high-quality AOM with a high diffraction efficiency of 85%, a fast rise/fall time of 79/98 ns, and a low insertion loss of 0.2 dB at 635 nm. Then, the 635 nm FSF mode-locked fiber laser is achieved using a Pr³⁺:doped ZBLAN double-clad fiber as a visible gain medium and the α-BTM AOM as a frequency-shifting element. Self-starting mode-locking at 635 nm directly generates red laser pulses with a pulse duration of 45 ps and a repetition frequency of 31.8 MHz. Furthermore, we investigate the evolution of mode-locking dynamics as the α-BTW-AOM feedback-frequency, which helps further understand the FSF dynamics by directly generating visible ultrashort pulses.