Soliton Mode-Locked Fluoride Fiber Laser at ∼3.5 μm Using an InAs/GaSb Superlattice SESAM

In this work, we report a soliton mode-locked fluoride fiber laser operating around 3.5 μm, based on the broadband laser platform (i.e., red-diode-pumped Er3+/Dy3+ codoped fluoride fiber) and an advacned optical switch (i.e., InAs/GaSb superlattice semiconductor saturable absorber mirror (SESAM)). As far as we know, this is the first experimental report on soliton generation from a saturable absorber (SA) mode-locked fluoride fiber laser in the >2.5 μm mid-infrared (MIR) spectral region. Operating at the fundamental frequency of 17.2 MHz, stable mode-locking with a pulse width of 1.52 ps has been achieved, yielding a pulse energy of 2.9 nJ and a peak power of 1.68 kW. By controlling the pump power and position of the SESAM, harmonic mode-locking with a continously variable order from 2nd to 13th has been also acquired. At the highest repetition rate of 223.8 MHz, it exhibits a side mode suppression ratio of 41.5 dB and a slightly larger pulse width of 2.27 ps. To our knowledge, the pulse width and repetition rate are both the current records of SA mode-locked MIR fluoride fiber lasers. This work not only narrows the performance gap between the SA and state-of-the-art nonlinear polarization rotation modelocked fluoride fiber lasers in the MIR, but also offers an optional platform for MIR soliton dynamics research.