Bawden, N.Henderson-Sapir, O.Jackson, S.D.Ottaway, D.J.2021-06-012021-06-012021Optics Letters, 2021; 46(7):1636-16390146-95921539-4794http://hdl.handle.net/2440/130491Published 23 March 2021We report, to the best of our knowledge, the first mode-locked fiber laser to operate in the femtosecond regime well beyond 3 µm. The laser uses dual-wavelength pumping and nonlinear polarization rotation to produce 3.5 µm 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.enJournal © 2021 Optical Society of AmericaJournal article100003844310.1364/ol.4181620006366127000302-s2.0-85102983006561613Henderson-Sapir, O. [0000-0002-1613-9985]Ottaway, D.J. [0000-0001-6794-1591]