A stable mode-locked fiber laser (MLFL) employing multi-layer graphene as saturable absorber (SA) is presented. The multi-layer graphene were grown by chemical vapor deposition (CVD) on Ni close to A-A stacking. Linear absorbance spectrum of multi-layer graphene was observed without absorption peak from 400 to 2000 nm. Optical nonlinearities of different atomic-layers (7-, 11-, 14-, and 21- layers) graphene based SA are investigated and compared. The results found that the thicker 21-layer graphene based SA exhibited a smaller modulation depth (MD) value of 2.93% due to more available density of states in the band structure of multi-layer graphene and favored SA nonlinearity. A stable MLFL of 21-layer graphene based SA showed a pulsewidth of 432.47 fs, a bandwidth of 6.16 nm, and a time-bandwidth product (TBP) of 0.323 at fundamental soliton-like operation. This study demonstrates that the atomic-layer structure of graphene from CVD process may provide a reliable graphene based SA for stable soliton-like pulse formation of the MLFL.

Can laser light be used to trigger lightning? Previous attempts [1] have been unsuccessfull, because they created a dense plasma opaque to the beam by avalanche ionization with an intense infrared laser. The method under investigation here uses a fs UV pulse at 248 nm to create a moderate (1013/cm3) electron density in a narrow (a few cm in diameter) vertical column of air. The charges are injected in the high field region between ground and cloud (at least 100 m altitude). The motion of the charges in the “external” atmospheric field result in a field enhancement (up to tenfold) at the “tip” of the cylinder of charges. A long (up to 1 µs) visible laser pulse has to be sent simultaneously through the same path to prevent the creation of negative ions O− and O2−

To create a weakly conducting channel in air over 100 m, three-photon ionization is the most appropriate mechanism. A lower-order process requires too high energies to compensate for absorption. Theoretical modeling and experimental investigation of the generation of an ionized channel in air, using prechirped fs UV (248-nm) light pulses is presented. Theoretical estimates are made of the ionization cross-section of nitrogen and oxygen at 248 nm, and compared to experimental data. We calculate the optimum chirp that will produce sufficient pulse compression in normally dispersive air to compensate for the depletion due to the ionization. Important for the rapid formation of a streamer is an abrupt termination of the ionization process, resulting in a local field enhancement. A proper choice of the initial pulse parameters (duration, chirp, and energy) allows us to obtain a uniformly ionized channel of electron density to 1012/cm3 for the path—the thickness of a space-charge layer over the earth surface—which leads to the triggering of lightning.