Observation of 550 MHz passively harmonic mode-locked pulses at L-band in an Er-doped fiber laser using carbon nanotubes film

We demonstrate a passively harmonic mode-locked（PHML） fiber laser operating at the L-band using carbon nanotubes polyvinyl alcohol（CNTs-PVA） film. Under suitable pump power and an appropriate setting of the polarization controller（PC）, the 54^（th） harmonic pulses at the L-band are generated with the side mode suppression ratio（SMSR） better than 44 dB and a repetition frequency of 503.37 MHz. Further increasing the pump power leads to a higher frequency of 550 MHz with compromised stability of 38.5 dB SMSR. To the best of our knowledge, this is the first demonstration on the generation of L-band PHML pulses from an Er-doped fiber laser based on CNTs.

Higher Harmonics Induced by Waves Propagating over A Submerged Obstacle in the Presence of Uniform Current

To investigate higher harmonics induced by a submerged obstacle in the presence of uniform current, a 2D fully nonlinear numerical wave flume（NWF） is developed by use of a time-domain higher-order boundary element method（HOBEM） based on potential flow theory. A four-point method is developed to decompose higher bound and free harmonic waves propagating upstream and downstream around the obstacle. The model predictions are in good agreement with the experimental data for free harmonics induced by a submerged horizontal cylinder in the absence of currents. This serves as a benchmark to reveal the current effects on higher harmonic waves. The peak value of non-dimensional second free harmonic amplitude is shifted upstream for the opposing current relative to that for zero current with the variation of current-free incident wave amplitude, and it is vice versa for the following current. The second-order analysis shows a resonant behavior which is related to the ratio of the cylinder diameter to the second bound mode wavelength over the cylinder. The second-order resonant position slightly downshifted for the opposing current and upshifted for the following current.

Analysis of generation of cumulative second harmonics of Lamb modes in a layered planar structure

A technique for analyzing the nonlinear generation of the cumulative second har-monics of generalized Lamb modes in a layered planar structure is developed. A theoretical model for nonlinear generalized Lamb mode propagation in a layered planar structure has been established, based on a partial plane wave approach. The nonlinearity is treated as a second-order perturbation of the linear elastic response. This model reveals some interesting features of the physics of the cumulative second harmonic generation. Although Lamb mode propagation is dispersive in a layered structure, the results of this analysis show that the amplitudes of the second harmonics do accumulate with propagation distance under certain special conditions. On the basis of the boundary and initial conditions of excitation, the formal solution of the cumulative second harmonic has been derived. Using the formal solution, we have performed some numerical simulations and obtained the cumulative second harmonic field patterns, illus-trating the distortion effect along the propagation distance, as well as the dependence of the field patterns on the position of the excitation source.