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A simple but novel concept of switchable and tunable dual-line erbium-doped fiber ring laser in the C - plus L -band is proposed and demonstrated. It is based on concurrence of the Sagnac and modal interferences in a fiber loop mirror including cascaded high birefringence few-mode fiber (HB-FMF) and polarization controller (PC). Random combination of the two lasing lines, one in the C-band and the other in the L-band, can be realized by adjusting only the PC next to the HB-FMF in the loop arm. In addition, they can be simultaneously tuned at a discrete step equal to the channel spacing of the modal interference. The output power flatness is better than ±0.5 dB for both the single- and dual-line emissions by carefully choosing the HB-FMF length with the 3-dB spectral linewidth narrower than 0.03 nm and signal-to-noise extinction ratio higher than 40 dB during the whole switching and tuning processes.