All-Fiber Bidirectional Mode-Locked Ultrafast Fiber Laser at 2 μm

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.