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The evolution of the depth of holes drilled with a single ms laser pulse combined with a series of ns laser pulses for different processing parameters was studied numerically. The results show that the achievable hole depth can be increased by increasing the intensity of the ms pulses and by saving the absorbed energy that would be lost in overheating of the melt. The melt includes the one generated between subsequent ns pulses and the un-expelled melt after the effect of shock waves induced by the previous ns pulse. The overheating of the un-expelled melt is avoided by increasing the intensity of the ns pulses. Additionally, increasing the repetition rate of the ns pulses during the processing with a single ms pulse reduces the energy wasted for overheating the generated melt and the energy transferred radially to the rim of the hole. On the basis of the numerically obtained relations between the intensity of the ns pulse and the changed depth of the hole per pulse, a semi-analytical method was developed to predict the depth of the hole drilled with combined laser pulses. The applicability of the method is discussed and verified by comparing the semi-analytical results with the numerical and experimental results. [ABSTRACT FROM AUTHOR]

The Pound–Drever–Hall (PDH) technique is a popular method for stabilizing the frequency of a laser to a stable optical resonator or, vice versa, the length of a resonator to the frequency of a stable laser. We propose a refinement of the technique yielding an "infinite" dynamic (capture) range so that a resonator is correctly locked to the seed frequency, even after large perturbations. The stable but off-resonant lock points (also called Trojan operating points), present in conventional PDH error signals, are removed by phase modulating the seed laser at a frequency corresponding to half the free spectral range of the resonator. We verify the robustness of our scheme experimentally by realizing an injection-seeded Yb:YAG thin-disk laser. We also give an analytical formulation of the PDH error signal for arbitrary modulation frequencies and discuss the parameter range for which our PDH locking scheme guarantees correct locking. Our scheme is simple as it does not require additional electronics apart from the standard PDH setup and is particularly suited to realize injection-seeded lasers and injection-seeded optical parametric oscillators. [ABSTRACT FROM AUTHOR]

High-speed laser welding is a very efficient process that results in weld seams of high quality. The process is limited by the occurrence of weld defects, such as humping and undercuts. It was found that a transition from a weld without defects to a weld with undercuts occurs when a speed limit is exceeded. It was also found that undercuts occur at slightly lower feed rates than humping. Thus, it was concluded that undercuts represent the upper process limit of high-speed laser welding and may be seen as the precursor of the humping regime. [ABSTRACT FROM AUTHOR]

The geometry of the cutting front and the cutting kerf was measured with an online high-speed x-ray diagnostic system. X-ray videos from fusion cutting of 10 mm thick stainless steel samples were recorded with a frame rate of 1000 Hz. A three-dimensional reconstruction of the time-averaged geometry of the cutting front and cutting kerf out of these images made it possible to apply ray-tracing for calculating the overall absorptance and the distribution of the absorbed irradiance at the cutting front. When increasing the feed rate, it was observed that the local inclination of the lower part of the cutting front with respect to the laser beam increased as well as the locally absorbed irradiance on the cutting front. This also leads to an increase in the overall absorptance with increasing feed rates. [ABSTRACT FROM AUTHOR]

Aging, growth of whisker, and thermal damage are the major issues when soldering electronic components. The authors present investigations to determine the influence of process times on the threshold temperature leading to thermal damage in epoxy-based printed circuit boards (PCBs). Experimentally verified numerical modeling was used to determine the temperature distribution at the interface of copper and the epoxy resin. The results show that the threshold temperature significantly increases with decreasing process times. With interaction times shorter than 5 ms, the threshold temperatures are found to be 900 K higher than the ones specified for longer heating times according to the data sheet for PCBs with FR4 epoxy resin. [ABSTRACT FROM AUTHOR]

Today's methods for the inspection of the position of weld seams in glass, which has a decisive effect on the strength and thus on the quality of the permanent bond, are costly and often carried out manually in a destructive manner. The present paper proposes the application of optical coherence tomography to determine the position of the weld seam by evaluation of the signals caused by the voids within weld seams. Blind welds of fused silica and alkali-aluminosilicate glass were investigated, and a method for data evaluation is presented. The method is predominantly suitable for glass that exhibits the formation of distinct voids during welding and provides an approach for online monitoring and control of glass welding processes. [ABSTRACT FROM AUTHOR]

A high-speed x-ray imaging system was built to investigate the fundamentals of laser material processing, such as laser welding. The system allows visualizing and analyzing the laser-induced process dynamics occurring also inside optically nontransparent workpieces. The details of the setup are presented together with the resulting performance with regard to edge sharpness, contrast, contrast transfer, and spatial as well as temporal noise. For voids with a depth (in respect to the x-ray radiation) of 100–500 μm, which is a typical range for focal diameters used in laser beam welding, a maximum contrast of 3%–19%, respectively, can be achieved. Due to the edge blurring, caused by the imaging system, the contrast is further decreased by 20% for voids with a lateral size of 100 μm and less than 10% for voids with a lateral size of 500 μm, when a magnification of 5.4 is used. The image intensifier, needed to increase the number of photons to saturate the camera, induces a noise floor of ± 15 % in respect to the mean gray value and results in a maximum achievable signal to noise ratio for a 100 μm step of 1.2. [ABSTRACT FROM AUTHOR]

Avalanche photodiodes are commonly used as detectors for low energy x-rays. In this work, we report on a fitting technique used to account for different detector responses resulting from photoabsorption in the various avalanche photodiode layers. The use of this technique results in an improvement of the energy resolution at 8.2 keV by up to a factor of 2 and corrects the timing information by up to 25 ns to account for space dependent electron drift time. In addition, this waveform analysis is used for particle identification, e.g., to distinguish between x-rays and MeV electrons in our experiment. [ABSTRACT FROM AUTHOR]

A method for determining the complete higher-order scattering matrix of an acoustic discontinuity is developed. The method is demonstrated for a right-angled waveguide bend, and the magnitude and phase of the reflection and transmission coefficients are extracted precisely. The procedure is straightforward and based on the solutions to the Helmholtz equation by the finite element method (FEM). The consistency of the scattering coefficients found by this method is verified by their properties of symmetry, and their accuracy is established by the conservation of energy. The reliability of the new technique is further proved by means of an arbitrary sound source and by comparing the direct FEM response to the reflection matrix calculation. Some features of the scattering matrix as a function of frequency are surprising, such as the steps and reversion of the phase evolution or the complete loss of transmission of the incoming wave. The methodology detailed in this paper can be extended to other multiport junctions, such as T-junctions or size discontinuities in ducts. [ABSTRACT FROM AUTHOR]