Since the first blue lasers made from GaN-based semiconductors reached power levels making them suitable for industrial applications a few years ago, users where asking for more power. Quickly, output powers for fiber-coupled diode lasers increased from several hundred watts in early 2018 [1] to 1000 W in 2019 [2] and even 2000 W in 2020 [3]. But lifetime has always been an object of concern since the blue diode-laser moved out of the TO-can to enter the high power world. As part of the research project “FoulLas”, which started in 2019 and is funded by the German Federal Ministry for Economic Affairs and Energy (BMWi), Laserline took on the task of developing a cw fiber-coupled diode-laser exceeding 2 kW blue laser power for fouling removal of vessels and submarine structures. Caused by stronger restrictions on the use of biocide containing coatings for ship hulls, new strategies against marine fouling moved into the focus of development activities. A new approach is to lethally damage the microorganisms on the subsea surfaces by laser irradiation to be washed away by the streaming water. Apart from that, they can no longer contribute to the spread of species. This paper covers concepts and possibilities of power increase beyond 2 kW for fiber-coupled lasers based on blue diodelaser bars. Results of a laser with more than 2 kW output power are presented. In addition, new findings on degradation processes and lifetime tests are reported. To tie in with the application, insights into the maritime application of fouling removal are given.
KEYWORDS: Thermal effects, Finite element methods, Wave propagation, Systems modeling, Thermal modeling, High power lasers, Laser applications, Dispersion
At high average power, ultra-short laser pulses (<1 ps) are not only affected by dispersive effects but also by thermallyinduced effects in the optical elements. In order to study those effects, we have developed a multi-physical simulation approach. It enables the analysis of the propagation of ultra-short pulses in the spatial and temporal regime against the background of the temperature-dependent refractive index profile, surface deformations, and dispersive effects. The simulation couples a thermo-mechanical finite element analysis and an optical analysis, using both ray-tracing and waveoptical methods. As case example, the influence of thermal and dispersive effects in a non-achromatic singlet lens and an achromatic doublet lens are analyzed and compared.
Thermo-optical simulation is an important extension of classical ray-tracing because many applications, especially in laser technology, have to deal with thermal effects. This paper discusses an approach for modeling thermally induced surface deformations of rotational symmetric optical systems: the discrete deformation data generated by Finite Element Analysis (FEA) are approximated using a global even polynomial which is then transferred to the ray-tracing. The implemented algorithm is validated by comparing approximated data to an analytic deformation function. Finally, the benefit of modeling the temperature dependent refractive index and the thermal deformation is demonstrated using the example of a plastic lens.
Semiconductors such as Si and GaAs are transparent to infrared laser radiation with wavelengths >1.2 μm. Focusing
laser light at the back surface of a semiconductor wafer enables a novel processing regime that utilizes this transparency.
However, in previous experiments with ultrashort laser pulses we have found that nonlinear absorption makes it
impossible to achieve sufficient optical intensity to induce material modification far below the front surface. Using a
recently developed Tm:fiber laser system producing pulses as short as 7 ns with peak powers exceeding 100 kW, we
have demonstrated it is possible to ablate the “backside” surface of 500-600 μm thick Si and GaAs wafers. We studied
laser-induced morphology changes at front and back surfaces of wafers and obtained modification thresholds for multipulse
irradiation and surface processing in trenches. A significantly higher back surface modification threshold in Si
compared to front surface is possibly attributed to nonlinear absorption and light propagation effects. This unique
processing regime has the potential to enable novel applications such as semiconductor welding for microelectronics,
photovoltaic, and consumer electronics.
Additive manufacturing, also known as 3D-printing, is a near-net shape manufacturing approach, delivering part
geometry that can be considerably affected by various process conditions, heat-induced distortions, solidified melt
droplets, partially fused powders, and surface modifications induced by the manufacturing tool motion and processing
strategy. High-repetition rate femtosecond and picosecond laser radiation was utilized to improve surface quality of
metal parts manufactured by laser additive techniques. Different laser scanning approaches were utilized to increase the
ablation efficiency and to reduce the surface roughness while preserving the initial part geometry. We studied post-processing
of 3D-shaped parts made of Nickel- and Titanium-base alloys by utilizing Selective Laser Melting (SLM) and
Laser Metal Deposition (LMD) as additive manufacturing techniques. Process parameters such as the pulse energy, the
number of layers and their spatial separation were varied. Surface processing in several layers was necessary to remove
the excessive material, such as individual powder particles, and to reduce the average surface roughness from asdeposited
22-45 μm to a few microns. Due to the ultrafast laser-processing regime and the small heat-affected zone
induced in materials, this novel integrated manufacturing approach can be used to post-process parts made of thermally
and mechanically sensitive materials, and to attain complex designed shapes with micrometer precision.
Utilizing the transparency of silicon at 2 μm, we are able to ablate the backside of 500-μm thick
silicon wafers without causing any damage to the front surface using a novel nanosecond
Tm:fiber laser system. We report on our high energy/high peak power nanosecond Tm:fiber
laser and provide an initial description of the effects of laser parameters such as pulse duration
and energy density on the ablation, and compare thresholds for front and backside machining.
The ability to selectively machine the backside of silicon wafers without disturbing the front
surface may lead to new processing techniques for advanced manufacturing in solar cell and
microelectronics industries.
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