Mode-locked Yb-doped fiber laser emitting broadband pulses at ultra-low   repetition rates
  We report on an environmentally stable, Yb-doped, all-normal dispersion, mode-locked fibre laser that is capable of creating broadband pulses with ultra-low repetition rates. Specifically, through careful positioning of fibre sections in an all-PM-fibre cavity mode-locked with a nonlinear amplifying loop mirror, we achieve stable pulse trains with repetition rates as low as 506 kHz. The pulses have several nanojules of energy and are compressible down to ultrashort (< 500 fs) durations. 
Patrick Bowen, Miro Erkintalo, Richard Provo, John D. Harvey, and Neil   G. R. Broderick
physics.optics