We report the first mode-locked fiber laser to operate in the femtosecond regime well beyond 3 {mu}m. The laser uses dual-wavelength pumping and non-linear polarisation rotation to produce 3.5 {mu}m wavelength pulses with minimum duration of 580 fs at a repetition rate of 68 MHz. The pulse energy is 3.2 nJ, corresponding to a peak power of 5.5 kW.
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