Switchable multi-wavelength linearly-polarized lasing oscillations in a figure eight EDF laser based on spatial-mode beating by means of weakly-coupled FMF

A switchable multi-wavelength figure eight Erbium-doped fiber (EDF) laser with linearly-polarized (LP) beam generation in the C-band region is proposed and experimentally demonstrated. Spatial-mode beating occurs owing to effect of multi-mode interference in the weakly-coupled few-mode fiber (FMF). A bandpass filter of multiple specific wavelengths is created by incorporating a weakly-coupled FMF with a Sagnac fiber loop. The LP modes are generated by cascaded mode-selective couplers (MSCs), which are designed and fabricated by tapering the proposed FMF with single-mode fiber (SMF). Combination of the cascaded MSCs and a simple N x 1 optical switch is employed to switch the LP modes. In experiments, the proposed fiber laser delivers versatile lasing outputs in states of single-, dual-, triple-wavelength oscillations for LP01, LP11 and LP21 modes. We investigate the stabilities for LP01, LP11, LP21 modes in 24-min. Moreover, the mode intensity profiles, optical signal-noise ratios (OSNRs) of lasing output and slope efficiencies are also measured. The proposed fiber laser has promising applications in WDM-MDM optical fiber communication systems, optical fiber sensor systems, optical instruments and spectroscopy.