Narrow linewidth single-longitudinal-mode vortex fiber laser with switchable orbital angular momentum output.

Narrow linewidth single-longitudinal-mode (SLM) vortex fiber lasers demonstrate dual-dimensional control over both longitudinal and transverse mode properties, with this unique spectral-spatial synergy significantly enhancing their application potential in advanced photonic systems. Here, we report a simple method to generate switchable orbital angular momentum (OAM) beams in a narrow linewidth SLM vortex fiber laser. By introducing a tunable bandpass filter, a subring, and a segment of unpumped erbium-doped fiber in the cavity, stable SLM lasing with tunable wavelengths and Lorentz linewidths below 1.4 kHz is obtained. The OAM beams are output through a few-mode fiber Bragg grating (FBG) used to selectively reflect the specific transverse mode at the corresponding self-coupling Bragg wavelength from the hybrid modes excited by the asymmetric coupling structure. Vortex modes switching with OAM of 0, ± 1, ± 2 per photon are obtained by tuning the operating wavelength and regulating the polarization controllers in the cavity. Due to the self-coupling regime in the few-mode FBG adopted in our design and high wavelength stability as well as narrow linewidth property offered by the SLM lasing, the mode reflecting process between two identical transverse modes can stably occur within the few-mode FBG, resulting in the high purity of output OAM modes.