2 ns pulses from a fibre laser mode-locked by carbon nanotubes

We demonstrate strongly chirped pulses from mode-locked Yb fibre ring lasers with cavity lengths exceeding 1 km, in the presence of very high net normal dispersion, up to −36 ps nm-1. Mode-locking was achieved using a single wall carbon nanotube saturable absorber [1–3]. The output pulse durations exceed 2 ns, which is, to the best of our knowledge, the longest pulse duration produced from a mode-locked fibre laser. The long cavities allowed us to achieve sub 200 kHz repetition rates, about an order of magnitude lower than has been reported to date [4]. In the experiments reported here, no intentional spectral filtering was employed, despite the all-normally dispersive cavity. The lasing wavelength selection is therefore a result of overlap of the gain bandwidth and spectral loss profiles of the intracavity optical components, but may also be affected by a dynamic filtering effect of the saturable absorber.