A switchable multi-wavelength thulium-doped fiber laser (TDFL) using a four-mode fiber based Sagnac loop filter was proposed and demonstrated. The Sagnac loop, incorporating a 4.8 m long four-mode step-index fiber, acting as a comb filter was analyzed theoretically and experimentally. By adjusting two polarization controllers, eight stable single-wavelength operations and five stable dual-wavelength operations were obtained with easy switching between the different operation modes. The wavelength range covered during the single-wavelength switchable operation with a signal-to-noise ratio of up to 55 dB was ∼28.51 nm. In dual-wavelength switchable operation, the maximum and minimum wavelength spacings were ∼21.78 nm and ∼1.50 nm, respectively. Due to the abundant wavelength resources, the proposed TDFL has great potential in wavelength division multiplexing free-space optical communications, optical fiber sensing, and photonic generation of microwave signals.

中文翻译：

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使用基于四模光纤的 Sagnac 环路滤波器的可切换多波长掺铥光纤激光器

提出并演示了使用基于四模光纤的 Sagnac 环路滤波器的可切换多波长掺铥光纤激光器 (TDFL)。萨格纳克环路包含一根 4.8 m 长的四模阶跃折射率光纤，用作梳状滤波器，从理论上和实验上进行了分析。通过调整两个偏振控制器，获得了8个稳定的单波长操作和5个稳定的双波长操作，不同操作模式之间可以轻松切换。在信噪比高达 55 dB 的单波长可切换操作期间覆盖的波长范围为~28.51 nm。在双波长可切换操作中，最大和最小波长间距分别为~21.78 nm 和~1.50 nm。由于波长资源丰富，