Passively Q-switched and mode-locked 1.9 m Tm-doped fiber laser based on pure water as saturable absorber

All-fiber laser with short pulses possesses many advantages, such as superior stability, compact structure, and high single pulse energy. Recently, short pulse laser working in a 1.8-2.0 mu m spectral region has received considerable attention due to its wide applications in laser spectroscopy, biomedicine, optical communications and sensing. The passive Q-switched and mode-locked operations by saturable absorber (SA) have been considered to be convenient and low-cost ways to achieve short pulses. Recently, pure water has been reported as the ideal SA because of its advantages of high damage threshold, low prices, good thermal diffusivity and stability. In this work, Tm-doped all-fiber pulse laser based on pure water as the SA is demonstrated. The pure water is fixed between two FC/PC fiber patchcord by the ceramic cannula, so we can change the loss of SA easily. The Q-switched and mode-locked operations can be obtained by adjusting the water layer thickness. The maximum output power at Q-switched state is 0.531 mW, the repetition frequency is 53.45 kHz, and the pulse width is 3.01 mu s. The maximum output power at mode-locked state is 2.28 mW, the repetition rate is 17.69 MHz, and the pulse width is 1.42 ps. To our knowledge, this is the first passive mode-locked fiber laser using pure water as a saturable absorber, and provides a new scheme for thulium-doped all-fiber pulse lasers.