Cylindrical vector beam generation from a hybrid multimode Q-switched mode-locked fiber laser

We demonstrate a hybrid multimode Q-switched mode-locked fiber laser capable of delivering cylindrical vector beams (CVBs). The wavelength-tunable Q-switching, conventional mode-locking, Q-switched mode-locking (QML), and CVB generation under QML operation are respectively achieved in this hybrid multimode cavity. The mode-locking mechanism is based on carbon nanotubes as saturable absorber and two interferometric filtering structures, i.e., multimode fiber-single-mode fiber-multimode fiber and multimode fiber-few-mode fibermultimode fiber. The mode-selective coupler is used as a mode selector converting the fundamental mode to higher-order mode and as an output coupler to obtain CVB. The Q-switched pulses allow for tunable wavelengths in the range of 1028.2 nm to 1038.2 nm, with a frequency of about 50 KHz. The conventional mode-locking is achieved with a repetition frequency of 15.6 MHz and a pulse interval of 66.7 ns. Specifically, the introduction of dual interferometric filtering structures in the cavity results in noteworthy nonlinear effects, facilitating the generation of third-harmonic mode-locking under the single-wavelength QML operation. These experimental findings highlight the promising applications such as mode-division multiplexed optical communication and laser processing.