Fig. 1. Characterizations of Fe3O4 nanoparticle dispersion: (a) atomic structure, (b) TEM image with a 50 nm scale, (c) HRTEM image with a 5 nm scale, and (d) X-ray diffraction pattern.

Fig. 2. Nonlinear optical absorption measurement: (a) experimental setup and (b) results at 2.87 μm.

Fig. 3. Experimental setup of the tunable passively Q-switched Dy3+-doped ZBLAN fiber laser using Fe3O4 nanoparticles as the SA. L1-L4, four lenses; DM1 and DM2, two dichroic mirrors; HT, high transmittance; HR, high reflectance.

Fig. 4. Q-switched pulse train and single pulse waveform (inset) at the launched pump powers of (a) 1.62 W and (b) 2.89 W; (c) optical and (d) RF spectra at the launched pump power of 2.89 W.

Fig. 5. (a) Pulse width and repetition rate and (b) pulse energy and output power as a function of the launched pump power.

Fig. 6. (a) Output spectrum, output power, and pulse energy, and (b) repetition rate and pulse width as a function of the wavelength at the launched pump power of 2.89 W.