  We experimentally demonstrate a passively Q-switched erbium-doped fiber laser
(EDFL) operation by using a saturable absorber based on Fe3O4 nanoparticles
(FONP) in magnetic fluid (MF). As a kind of transition metal oxide, the FONP
has a large nonlinear optical response with a fast response time for saturable
absorber. By depositing MF at the end of optical fiber ferrule, we fabricated a
FONP-based saturable absorber, which enables a strong light-matter interaction
owing to the confined transmitted optical field within the single mode fiber.
Because of large third-order optical nonlinearities of FONP-based saturable
absorber, large modulation depth of 8.2% and non saturable absorption of 56.6%
are demonstrated. As a result, stable passively Q-switched EDFL pulses with
maximum output pulse energy of 23.76 nJ, repetition rate of 33.3 kHz, and pulse
width of 3.2 {\mu}s are achieved when the input pump power is 110 mW at the
wavelength of 980 nm. The laser features a low threshold pump power of ~15 mW.

Optics