Controllable Circularly Polarized Vortex Beam in a Twisted Few-Mode Polarization-Maintaining Fiber

We fabricate a few-mode polarization-maintaining fiber (FM-PMF) and present a technique for generating circularly polarized (CP) vortex beam, carrying orbital angular momentum (OAM) and spin angular momentum (SAM), by twist method. By controlling the direction of twist, a right-handed circularly polarized (RCP) OAM+1 mode with a mode purity of ~96.2% and a polarization extinction ratio (PER) of ~0.75 dB, as well as a left-handed circularly polarized (LCP) OAM-1 mode with a mode purity of ~95.3% and a PER of ~0.82 dB can be selectively obtained. Additionally, the twisted FM-PMF is encapsulated as a CP mode converter, operational in the range of 1530-1560 nm. This mode converter is applied into an all-fiber narrow-linewidth single-frequency laser system, successfully generating the CP vortex laser beam at 1560 nm with an out power of 33 mW. The proposed CP mode converter has potential applications in optical communication, high-power lasers, optical sensing, high-resolution imaging, and beyond.