Lasers are a powerful class of production processes that are limited by their specific optical boundary conditions in the generation of structures with high aspect ratios. Guiding the laser beam via a water jet as an optical waveguide enables much greater ablation depths with small lateral structure sizes. This book describes an inline measurement system that enables depth measurement and control of material removal in water jet based laser micro machining.

The economical production of high aspect ratio moulds for micro injection moulding applications has high demands on depth accuracy. This thesis develops the coaxially integration of an inline measurement system to generate geometric depth information about the workpiece directly during a water jet based laser process. This development is being pursued with the aim of transferring the advantages of the Laser-MicroJet as a pure laser cutting process to complete 3D machining capabilities by means of a measurement-based feedback loop. The hypothesis that the inline integration of an optical measuring system based on low-coherence interferometry enables depth measurement capabilities during water jet guided laser ablation and thus enables the micro structuring of high-precision injection moulds is pursuit. The process monitoring and control of the Laser-MicroJet has special requirements on the measurement technology, which results from the very differentiated characteristics of the process. These are that optical technology is required, as the geometry must be measured directly from the laser processing zone, i.e. in the water jet. Furthermore, a dimensional measurement is required to be able to measure the geometry directly. The technology must also be able to scan a sufficient axial range along the water jet and achieve both a temporal resolution and a depth resolution in the order of magnitude of the laser ablation. Given these requirements, this thesis investigates low-coherence interferometry by developing and evaluating a measurement system for the water jet guided laser micro structuring process. The investigation takes place on two levels. A theoretical analysis is carried out by means of a physical model, which is continued alongside the development of a hardware demonstrator of the measurement system. Finally, the sensor is integrated into the production process, first in the laboratory and then in the production environment. The validation of the inline measurement system is carried out using the parameters of sensitivity, measurement range and measurement resolution. The evaluation of the measurement system shows that measurements in the water jet are possible in view of the requirements for optical integration, direct geometric measurement data collection, measurement range in the water jet and resolution.