Spatial and spectral emission characteristics and efficiency of high-power diode laser (HPDL) based pump sources
enable and define the performance of the fundamental solid state laser concepts like disk, fiber and slab lasers.
HPDL are also established as a versatile tool for direct materials processing substituting other laser types like CO2 lasers
and lamp pumped solid state lasers and are starting to substitute even some of the diode pumped solid state lasers. Both,
pumping and direct applications will benefit from the further improvement of the brightness and control of the output
spectrum of HPDL.
While edge emitting diodes are already established, a new generation of vertical emitting diode lasers (VCSELs) made
significant progress and provides easy scalable output power in the kW range. Beneficial properties are simplified beam
shaping, flexible control of the temporal and spatial emission, compact design and low current operation. Other
characteristics like efficiency and brightness of VCSELs are still lagging behind the edge emitter performance.
Examples of direct applications like surface treatment, soldering, welding, additive manufacturing, cutting and their
requirements on the HPDL performance are presented. Furthermore, an overview on process requirements and available
as well as perspective performance of laser sources is derived.
Joining fiber reinforced polymers is an important topic for lightweight construction. Since classical laser transmission welding techniques for polymers have been studied and established in industry for many years joint-strengths within the range of the base material can be achieved. Until now these processes are only used for unfilled and short glass fiber-reinforced thermoplastics using laser absorbing and laser transparent matrices. This knowledge is now transferred to joining long glass fiber reinforced PA6 with high fiber contents without any adhesive additives. As the polymer matrix and glass fibers increase the scattering of the laser beam inside the material, their optical properties, changing with material thickness and fiber content, influence the welding process and require high power lasers. In this article the influence of these material properties (fiber content, material thickness) and the welding parameters like joining speed, laser power and clamping pressure are researched and discussed in detail. The process is also investigated regarding its limitations. Additionally the gap bridging ability of the process is shown in relation to material properties and joining speed.
So far, the main approach to weld absorber-free thermoplastics is exploiting their intrinsic absorption by choosing a
proper wavelength of the laser. In order to melt the joining partners spatially restricted at the interface usually optics with
a high numerical aperture are used. However, practice shows that the heat affected zone (HAZ) extends over a large area
along the beam axis regardless of the optics used. Without clamping or convective cooling thermally induced expansion
of the material can cause blowholes or deformation of the irradiated surface. To reduce the thermal stress on the part
surface a dynamic beam superposition is investigated with the laser beam performing a precession movement.
Within the plastic industry laser transmission welding ranks among the most important joining techniques and opens up
new application areas continuously. So far, a big disadvantage of the process was the fact that the joining partners need
different optical properties. Since thermoplastics are transparent for the radiation of conventional beam sources (800-
1100 nm) the absorbance of one of the joining partners has to be enhanced by adding an infrared absorber (IR-absorber).
Until recently, welding of absorber-free parts has not been possible.
New diode lasers provide a broad variety of wavelengths which allows exploiting intrinsic absorption bands of
thermoplastics. The use of a proper wavelength in combination with special optics enables laser welding of two optically
identical polymer parts without absorbers which can be utilized in a large number of applications primarily in the
medical and food industry, where the use of absorbers usually entails costly and time-consuming authorization processes.
In this paper some aspects of the process are considered as the influence of the focal position, which is crucial when both
joining partners have equal optical properties. After a theoretical consideration, an evaluation is carried out based on
welding trials with polycarbonate (PC). Further aspects such as gap bridging capability and the influence of thickness of
the upper joining partner are investigated as well.