Simultaneous switching of transverse mode and longitudinal mode in an all few-mode fiber laser based on polarization manipulation

We experimentally demonstrate an all-few-mode fiber laser (FMFL) capable of simultaneous transverse and longitudinal mode switching, with high mode purity and wavelength stability (wavelength drift <0.1 nm). The FMFL is enabled by the integration of a few-mode fiber Bragg grating (FM-FBG) and polarization manipulation. The conversion of the pump light from the LP 01 mode to the LP 11 mode is achieved via a mode-selective coupler. By tuning the polarization controllers, the birefringence-induced spectral shift of the FM-FBG selects different operating wavelengths, each coupled to distinct transverse modes: (i) LP 01 –LP 01 mode reflections at 1534.29 nm, (ii) LP 11 –LP 11 mode reflections at 1533.68 nm. This design offers a compact and flexible solution suitable for mode-division multiplexing and wavelength-division multiplexing systems.