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

We characterize the saturable absorption of the second (E 22 ) 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 E 11 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 E 11 transition, of the same nanotubes, with an erbium fiber ring laser, producing 1.1 ps pulses. Using the E 22 transition should enable the use of carbon nanotube saturable absorbers at shorter wavelengths than currently possible with the E 11 transition, which are limited by the smallest achievable nanotube dimensions.