Using the E₂₂ transition of carbon nanotubes for fiber laser mode-locking

We characterize the saturable absorption of the second (E₂₂) electronic transition of a sample of single-walled carbon nanotubes and use it to mode-lock an ytterbium fiber ring laser. The modulation depth of ∼ 15% was found to be similar to the corresponding E₁₁ transition (∼ 13%), but the saturation intensity (∼ 220 MW cm^-2) about an order of magnitude larger (∼ 10 MW cm^-2). We achieved a 15 MHz mode-locked pulse train with an output pulse duration of 6.5 ps. For comparison we also demonstrate stable mode-locking on the E₁₁ transition, of the same nanotubes, with an erbium fiber ring laser, producing 1.1 ps pulses. Using the E₂₂ transition should enable the use of carbon nanotube saturable absorbers at shorter wavelengths than currently possible with the E₁₁ transition, which are limited by the smallest achievable nanotube dimensions.