Vortex generation from a fiber laser by a polarization rotation technique

Vortex beams have wide applications in optical tweezers, optical communications and so on. Long period fiber gratings, mode selective couplers, few mode fiber Bragg gratings or specially designed fibers are often used to generate vortex beams from fiber lasers. New methods of mode selection in fiber lasers are always attractive. When transverse modes propagate along a multimode fiber, the polarization state of each mode rotates differently due to birefringence. In this paper, a linear cavity fiber laser and a ring cavity fiber laser using a polarization rotation technique are proposed to select the output laser mode. Three waveplates and a polarizing beam splitter (PBS) are used as a mode loss controller inside the laser cavity. By rotating the waveplates inside the cavity, different laser modes can be selected and achieved from the laser cavity by the PBS. Experimentally, the fundamental LP₀₁ mode, the LP₁₁ mode, the LP₂₁ mode and vortex beams with ±1 topological charges were achieved. (letter)