Abstract

Single-walled carbon nanotubes (SWCNTs) are being widely investigated as saturable absorbers for initiating and maintaining mode-locking [1–3]. Among their advantages are low fabrication cost, a high damage threshold, sub-picosecond recovery times and a wide spectral range (1.0–2.5 μm). All mode-locked fibre laser systems based on SWCNT saturable absorbers, demonstrated to date, have used the first electronic transition of the nanotubes, known as E₁₁, which corresponds to the single real gap in the electron density of states (DOS) of semiconducting nanotubes [4]. It was expected that only this transition could be used for saturable absorption. However, in a couple of recent papers [5,6], the mode-locking regime has been realized in bulk lasers at 1.06 μm with arc-synthesized nanotubes, but the authors did not pay much attention to the fact that this wavelength belongs to the second absorption band, corresponding to the E₂₂ pseudo-gap [4]. This means that after excitation the electron falls to the lower energy transition, corresponding to the real gap E₁₁. In this work we clearly demonstrate saturable absorption on the E₂₂ transition by using it to mode-lock a fibre laser for the first time. This discovery should enable the use of SWCNT saturable absorbers at much shorter wavelengths, into the visible spectral region.

© 2009 IEEE