In this paper, we demonstrated an all-fiber bidirectional ultrafast thulium-doped fiber laser at 2 μm with a single-wall carbon nanotube presented as saturable absorber. We successfully obtained bidirectional mode-locking operations with pulse repetition frequency adjustable from 35 to 122 MHz by shortening the cavity length. Meanwhile, with the reduction of the intracavity dispersion, the number of Kelly sidebands was significantly decreased and the output energy was more concentrated on the soliton pulse. Besides, by manipulating the pump power and polarization controller, the two results of bidirectional mode-locked pulses with the same repetition frequency and differential repetition frequency were observed. We found that the repetition rate difference between the clockwise and counterclockwise pulse trains was adjustable by changing the pump power or controlling the intracavity polarization. When the pulse repetition frequency was 35 MHz and 122 MHz, the adjustment range of repetition rate difference was 764–922 Hz and 540–2000 Hz, respectively. It is believed that this novel laser source can support free-running dual optical comb spectroscopy to detect important air gases like H2O or CO2 in the future.

中文翻译：

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2 μm 全光纤双向锁模超快光纤激光器

在本文中，我们展示了一种全光纤双向超快掺铥光纤激光器，其波长为 2 μm，单壁碳纳米管作为可饱和吸收体。通过缩短腔长，我们成功地获得了脉冲重复频率从 35 到 122 MHz 可调的双向锁模操作。同时，随着腔内色散的减少，凯利边带的数量显着减少，输出能量更多地集中在孤子脉冲上。此外，通过操纵泵浦功率和偏振控制器，观察到了具有相同重复频率和不同重复频率的双向锁模脉冲的两个结果。我们发现顺时针和逆时针脉冲序列之间的重复率差异可以通过改变泵浦功率或控制腔内极化来调节。当脉冲重复频率为35 MHz和122 MHz时，重复率差的调节范围分别为764-922 Hz和540-2000 Hz。相信这种新型激光源将来可以支持自激双光梳光谱检测重要的空气气体，如 H2O 或 CO2。