  We demonstrate ultrashort pulse generation from a fluoride fiber laser
co-doped with holmium and praseodymium. To date the majority of work focused on
short pulse generation from this class of fiber laser has employed loss
modulators in the cavity, both real and artificial. In this work we
alternatively employ a frequency shifting element: an acousto-optic modulator
(AOM) in the cavity. This results in mode-locked output of sub-5 ps pulses with
10 nJ of energy at a center wavelength of 2.86 {\mu}m, and a pulse repetition
frequency of 30.1 MHz, equating to a peak power of 1.9 kW. Additional
experimental investigation of the relationship between frequency shift and
cavity round trip offer insight into the complex underlying dynamics. As a
complementary mode-locking technique to conventional loss modulation, this
method of pulse formation may greatly expand the design flexibility of pulsed
mid-infrared fiber lasers.
