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
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