Wideband-tuneable, nanotube mode-locked, fibre laser

Ultrashort-pulse lasers with spectral tuning capability have widespread applications in fields such as spectroscopy, biomedical research and telecommunications(1-3). Mode-locked fibre lasers are convenient and powerful sources of ultrashort pulses(4), and the inclusion of a broadband saturable absorber as a passive optical switch inside the laser cavity may offer tuneability over a range of wavelengths(5). Semiconductor saturable absorber mirrors are widely used in fibre lasers(4-6), but their operating range is typically limited to a few tens of nanometres(7,8), and their fabrication can be challenging in the 1.3-1.5 mu m wavelength region used for optical communications(9,10). Single-walled carbon nanotubes are excellent saturable absorbers because of their subpicosecond recovery time, low saturation intensity, polarization insensitivity, and mechanical and environmental robustness(11-16). Here, we engineer a nanotube-polycarbonate film with a wide bandwidth (> 300 nm) around 1.55 mu m, and then use it to demonstrate a 2.4 ps Er3+-doped fibre laser that is tuneable from 1,518 to 1,558 nm. In principle, different diameters and chiralities of nanotubes could be combined to enable compact, mode-locked fibre lasers that are tuneable over a much broader range of wavelengths than other systems.