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. Mode-locking was achieved using a single wall carbon nanotube saturable absorber. 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. The considerable variation of pulse duration for each cavity length is achieved by adjusting the polarization controller and the pump power. These results demonstrate a new regime of mode-locked fibre laser.