Geometry assessment of ultra-short pulsed laser drilled micro-holes

Ultra-short pulsed laser ablation enables a defined generation of micro-holes. A parameter study on the ablation characteristics of copper clearly reveals a benefit for green wavelength with lower threshold fluence, simultaneously increasing the Rayleigh length. The use of a circular drilling method allows a defined manufacturing of micro boreholes and micro through-holes with 35 μm diameter of up to 165 μm and 300 μm length. Introducing high-resolution micro-computed X-ray tomography studying the micro-hole evolution and adjacent geometrical transformations reveals micrometer resolution and high usability. The conical geometry evolving up to an aspect ratio of 5:1 fits well to established models known for percussion drilling. However, increasing the number of pulses leads to non-conical geometry evolution, and this resulting geometry is studied for the first time. Henceforth, the exact geometrical evolution from conical to cylindrical shape upon laser drilling can be resolved revealing the impact of multiple reflections at the generated steep flanks.

Diffraction-ray optics of femtosecond laser pulses under normal dispersion conditions in air

The results of a theoretical study of the propagation of femtosecond pulses of a Ti:Sa laser under normal dispersion conditions in air are presented. The use of the diffraction-beam tube method for the analysis of numerical solutions of the nonlinear Schrödinger equation in a dispersion medium with Kerr-plasma nonlinearity made it possible to determine the basic regularities of femtosecond laser pulses self-focusing and filamentation in air at various pulse lengths, initial beam radius, and peak powers. It is shown that in the case of the group velocity dispersion influence with an increase in the initial laser beam radius, the filamentation breaks down even at large supercritical powers. It is shown that with an increase in the dispersion distortions of the pulse the radius of the energetically replenishing diffraction beam tube, the angular divergence of the post-filamentation light channel, and the nonlinear focus coordinate normalized to the Rayleigh length for the central time layers of the laser pulse and the integral picture are increasing.

Role of the Features of Focused Laser Beam at Pulsed Laser Cutting

In this article investigation of the roles of two important factors of focused laser beam, the focal spot diameter and the Rayleigh length as determining variables of the beam quality were made. The equations of these two factors are based on those most commonly used in the literature. The exchange between three different beam quality numbers were shown. It is proven on the basis of the scientific literature, that the beam quality degrades compared to the original data given by the factory of laser. The causes of the beam quality degradation are lens aberrations in the optical path of the given laser, and the shifting of the beam propagation ratio (M2) to higher values. A new equation for estimation of the new, lowest value for M2 factor is presented, based on the comparison of the laser cut material thickness to the depth of focus, which is two times the Rayleigh length.

Green Generation by Single-Pass Frequency-Doubling in a Periodically Poled MgO:LiNbO3 at Room Temperature

122mW green emission at 532.3nm with a conversion efficiency of 1.2% was measured by single-pass second-harmonic generation in a 10mm long periodically poled MgO:LiNbO3 (MgO:PPLN) crystal is reported. A continuous-wave Yb-doped fiber laser operating at 1064.6nm with narrow bandwidth of 0.1nm is used as pumping source. The experimental temperature acceptance bandwidth ΔT=4.6°C is a little higher than the simulation data of 4°C. Output power instabilities or variations of the green beam pattern were not observed during experiments. In this work, the optimized efficiency was achieved when the waist located at the center of MgO:PPLN and Rayleigh length equal to its length, and the lens’ location in the system was calculated.