All-fiber switchable orbital angular momentum mode-locked laser based on TM-FBG

In this paper, a simple all-fiber switchable orbital angular momentum (OAM) mode-locked laser is demonstrated. The laser is mainly composed of a single-mode fiber Bragg grating (FBG), a two-mode fiber Bragg grating (TM-FBG), a two-mode circulator, and a nonlinear polarization rotation system. The coupling properties of the TM-FBG are verified, and an OAM mode-locked laser with switchable topological charges of −1, 0, and 1 is realized. When the pump power is 462 mW, the output powers of the fundamental mode and OAM±1 mode-locked lasers are 9.750 and 2.707 mW, respectively. Their repetition rates are both 10.16 MHz, and the signal-to-noise ratios are 60 and 59 dB. When the pump power is increased to 774 mW, the mode-locked laser can operate in the single-pulse, double-pulse, and triple-pulse states. Their output powers are 5.1, 7.4, and 10.1 mW, respectively. The OAM mode purity higher than 98.9% is experimentally realized.