  Black phosphorus, a newly emerged two-dimensional material, has attracted
wide attention as novel photonic material. Here, multi-layer black phosphorus
is successfully fabricated by liquid phase exfoliation method. By employing
black phosphorus as saturable absorber, we demonstrate a passively Q-switched
Er-doped ZBLAN fiber laser at the wavelength of 2.8 {\mu}m. The modulation
depth and saturation fluence of the black phosphorus saturable absorber are
measured to be 15% and 9 {\mu}J/cm2, respectively. The Q-switched fiber laser
delivers a maximum average power of 485 mW with corresponding pulse energy of
7.7 {\mu}J and pulse width of 1.18 {\mu}s at repetition rate of 63 kHz. To the
best of our knowledge, this is the first time to demonstrate that black
phosphorus can realize Q-switching of 2.8-{\mu}m fiber laser. Our research
results show that black phosphorus is a promising saturable absorber for
mid-infrared pulsed lasers.
