Tunable mode-locked cylindrical vector beam fiber laser based on nonlinear multimode interference in a linear cavity

A tunable passively mode-locked fiber laser with cylindrical vector beams (CVB) generation in a linear cavity was presented. The mode-locking mechanism is based on the nonlinear multimode interference effect of the graded-index multimode fiber (GIMF). The Sagnac loop mirror works as a filter. An ultra-broadband long-period fiber grating (UBLPFG) with a high conversion efficiency < 90% from 1004 nm to 1068 nm is used to covert fundamental mode to higher-order modes. The laser can achieve a continuously tunable wavelength range from 1032.344 nm to 1042.556 nm by changing the reflection spectrum of the Sagnac loop mirror thermally. When the laser operates at the central wavelength of 1042.556 nm, the output pulse trains have a period of 92.08 ns with pulse duration of 1.15 ns. The slope efficiency of the laser is about 4.73% with the mode-locking threshold of 167.7 mW. The purity of the radially and azimuthally polarized beams is measured to be higher than 93.8%.