Passively Q-switched mode-locked ytterbium-doped fiber laser based on an Fe3O4-nanoparticle saturable absorber

Abstract

A passively Q-switched mode-locked ytterbium-doped fiber laser (QMYDFL) at 1084.4 nm using ferroferric-oxide (Fe3O4) nanoparticles (FONPs) as the saturable absorber (SA) is reported. The FONPs SA exhibits a large nonlinear saturable absorption property with the modulation depth of 6.6% at the laser wavelength of 1µm band. Stable passively Q-switched pulse train with the repetition rate increasing from 5.43 kHz to 59 kHz, the corresponding pulse duration reducing from 18 µs to 2.68 µs are achieved when the input pump power increases from 100 mW to 330 mW. The maximum single pulse energy can reach 181.6 nJ. Q-switched mode-locked (QM) phenomenon is also observed by adjusting the state of the polarization controller (PC). When the input pump power is 150 mW, stable QM pulse train occurs. By continuous increasing the pump power to 320 mW, the QM pulse trains with the repetition rate increasing from 17.1 kHz to 34.3 kHz, mode-locked pulse repetition rate of 11.1 MHz and pulse duration of 880 ps are achieved. The observed phenomenon is interpreted as a mutual interaction of dispersion, non-linear effect and insertion loss. This work provides a new mechanism for fabricating cheap QMYDFL with FONPs SA.