## 1 Introduction  
Saving energy is becoming increasingly important. Especially in the area of generation and transformation of electrical energy, a considerable loss is to be expected. In the case of classic electrical sheet, several layers of steel and insulation materials are stacked. Electrical steel is used in a wide range of applications, from transformers and sensors to motors and generators [1; 2; 3; 4]. One of the most important aspects of electric steel is the energy density. Unfortunately, a high energy density also leads to an increased energy loss. At the same time, noise generation is also increased due to, among other things, long-term delamination of the steel stack [5; 6; 7]. One way to improve these factors is to laser-texture the steel sheets. Laser micro structuring is an excellent solution due to its capacity to create functional surface structures on a micrometer scale [8; 9; 10; 11; 12; 13; 14; 15]. The aim of the Horizon 2020 research project ESSIAL is, among other things, to improve the magnetic properties, as described above, by laser structuring of the steel sheets. The structures being considered are on grain-oriented (GO) electrical steel with the spatially width in the range of 25-500 \(\upmu\)m and 1-25 \(\upmu\)m depth range, as well as structures on grain non-oriented (GNO) in the range of 10-25 \(\upmu\)m in width and 1-50 \(\upmu\)m in depth [16; 17; 18]. To ensure that the target parameters of the process are met, a monitoring system is required that also minimizes the influence on the process and the machine itself and can be used to constantly observe the process and to avoid production rejects and to make production even more resource-efficient [19; 20]. The low coherence interferometry (LCI) allows here the measurement of the ablation and thus the over surface structureswithout the need to remove the workpiece from the machine and at the same time uses the same beam path as the existing laser structuring process. LCI is an extension of classical optical coherence tomography (OCT) [21]. OCT was developed for ophthalmology in the early 1990s and has already been used in technical applications for several years [22; 23; 24; 25; 26]. While OCT provides high resolution imaging results for tomographic images, using the LCI method allows the measurement of the surface distance and thus the derivation of the topology of the surface on the workpiece [27; 28]. The optical components of a laser machine are designed for a specific wavelength range, and the wavelength of the process laser is located in this range. The energy of the process laser is capable of destroying the components of the measuring system, so that the two systems must be separated in a certain way. A beam splitter can be used to achieve this, but the measurement wavelength must still be different from the wavelength of the process laser. This results in a non-linear offset between the spot positions of the two beams, so that simultaneous measurement and processing of the same position is no longer possible. The distance between the two spots increases as the beam moves further away from the center of the objective lens. In principle, this can be compensated by a color-corrected objective lens, but not completely or the choice of these objectives are very limited [29; 30]. For this to be done, it is necessary to develop a method that allows this non-linear offset to be compensated adaptively during the process, thus allowing direct measurement of the material ablation. It must also be investigated how such compensation can be implemented and controlled. The strategy and the development of a system that meets these requirements is described in the following sections. The goal of the ESSIAL research project is to integrate a measuring system into the laser process. For this purpose, a laboratory system will be developed and then scaled up step by step to complete machine integration.