Experimental investigation of Q-switched mode-locking pulse emission based on in-fiber acousto-optic modulator

Simultaneous Q-switching and mode-locking (QML) pulse operation is demonstrated by using an in-fiber acousto-optic bandpass modulator (AOBM). The AOBM exhibits a high rejection efficiency of 28 dB for the non-resonant wavelengths, a broad modulation bandwidth of 0.9 nm, a modulation depth of 0.74 and easy implementation in an allfiber device. The effectiveness of the modulator is demonstrated in a rig cavity, mode locking is achieved by amplitude modulation of the transmitted light and matching the modulation period with the cavity round-trip time, simultaneously, the Q factor of the cavity is controlled by superimposing a rectangular wave modulation on the acoustic signal, with variable frequency and duty cycle, allowing a QML pulse operation. Experimental results demonstrate stable QML operation at 0.5 kHz repetition rate. A maximum energy of 1.4 μJ was achieved within a burst of 110 ps mode-locked sub-pulses contained in a 6.4 μs Q-switched envelope. These results demonstrate the potential of the AOBM to perform the double function of active mode locker and Q-switching device in an all-fiber laser.