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A reconfigurable multiwavelength erbium-doped fiber laser based on an all-fiber multimode interferometer (MMI) is proposed and experimentally demonstrated. The interferometer is constructed by sandwiching a section of highly germanium-doped fiber (HGDF) between two sections of single-mode fiber. The insertion loss of the interferometer is as low as 2 dB. Due to the polarization-dependent spectral filtering effect formed by the MMI, by rotating the intracavity polarization controller, the laser output can be switched among single-, dual-, and triple-wavelength lasing states with optical signal-to-noise ratio up to 50 dB. In particular, the obtained dual-wavelength state shows high stability with wavelength shift within $ \pm {0.04}\;{\rm nm}$±0.04nm, wavelength spacing variation within $ \pm {0.03}\;{\rm nm}$±0.03nm, and power fluctuation within $ \pm {0.04}\;{\rm dB}$±0.04dB by monitoring the output spectra over 8 h at room temperature. By changing the length of the HGDF, the wavelength spacing can also be flexibly manipulated. Taking the advantages of reconfiguration, low cost, and easy fabrication, this fiber laser may have great potential in various optical applications.

This publisher's note amends the spelling of the second author's name in Appl. Opt.57, 6388 (2018)APOPAI0003-693510.1364/AO.57.006388.

We investigate mode-area-scaling and bending performances of a Yb-doped large-mode-area fiber with an elongated non-circular core. Such fiber can be bent in the plane of its short axis to suppress bending effects, such as mode area reduction and mode profile distortion. Meanwhile, the other orthogonal axis can be stretched for mode area scaling. Calculations show that for fibers with the same mode area, the higher the aspect ratio between the long axis and short axis, the less sensitive the fiber will be to bending effects. However, mode area scaling is limited by the increased beat length (BL) between the fundamental mode (FM) and the second-order mode, leading to mode degeneracy at higher aspect ratios. Within the 100 mm BL, the FM area is scalable to 3000  μm 2 in a bent fiber. To facilitate FM operation, we study mode-selective gain through confined doping. Thanks to the small bending distortions, the confined-doping approach works well in the bent large-mode-area fiber. In addition, the advantage of tandem pumping is also discussed in terms of preferential modal gain. A non-circular core fiber with a 41 μm short axis and 120 μm long axis was fabricated in-house. We evaluated the fiber in a linear laser cavity pumped by a 975 nm laser diode. The maximum output power obtained was 191 W, with slope efficiency of approximately 67% with respect to launched pump power. The output signal has good beam qualities with M 2 of ∼1.5 and ∼3.1, respectively, along the short and long axis.