## 1 Introduction  
Fiber laser cutting is an effective and well-established method for processing different types of metals in industry [1; 2]. Despite this broad use in industry and comprehensive scientific studies on fiber laser cutting [3], still, a demand exists to provide a reliable monitoring system for in situ quality control, especially for aluminum. In addition to everyday disturbance variables, e.g., unclean optics, thermal lens effects, and variation in material properties, the high fluid dynamics of the melt pool and complex laser material interaction within the kerf challenge the successful monitoring and detection of the cut quality [4]. During laser fusion cutting, the material on the kerf front is kept continuously molten by the focused laser radiation and blown out of the kerf by an inert gas jet [5]. Specifically, the behavior of the melt flow dynamic on the cutting front has a strong influence on the cut quality [6], with a stable, fast, and smooth melt flow being preferable and leading to an overall better cut quality [7]. Common quality losses for laser fusion cutting are the formation of interrupted striation patterns on the kerf wall, adhesion of burr on the bottom edge, and cut interruptions. Striations on the kerf wall are considered to be linked to the boundary layer between gas flow and melt film [8]. Cuts with interrupted striations show higher fluctuations of the cut front and a local increase in the melt film, resulting in an increase in the absorbed irradiance [9]. For higher feed rates, the angle of the kerf increases, resulting in a larger absorbing area irradiated by the laser [10]. Excessively high temperatures in the kerf cause local vaporization, in turn disturbing the melt flow and resulting in a deterioration of the quality with interrupted striation patterns [11] and the formation of burr [12]. Burr forms when parts of the melt adhere to the lower cut edge. The main reason for this is the momentum of the melt flow, which is not sufficient to overcome the adhesion force [13]. Furthermore, with improper parameters, such as low laser power and assisted gas pressure or too high a feed rate and nozzle-to-workpiece distance, cutting interruption may occur [14]. As a result, the workpiece cannot be detached from the sheet panel, hampering atomized pick-and-place machines, necessitating rework, or increasing waste. Therefore, the key condition for any quality controlling monitoring system is the capability to spot the process zone spatially and temporally resolved, which necessitates an imaging sensor to observe the whole kerr with a high sampling rate due to the dynamics of the melt. With respect to the increasing demand for resource efficiency, cutting interruptions are undesirable, and autonomously operating cutting machines with quality monitoring are highly demanded. In contrast to laser welding systems or CO\({}_{2}\) laser-based cutting systems, for which various sensor concepts have already been reported [15; 16], quality monitoring setups for laser fusion cutting using fiber lasers, especially for non-steel materials, are comparatively rarely reported or in industrial use, despite the similar or comparable monitoring and sensor concepts. Systems for quality monitoring based on image sensors have been reported for CO\({}_{2}\) laser cutting with external illumination [17] and without external illumination [18; 19]. In recordings with external illumination, a band-pass filter or equivalent is used in order to record only the illuminating wavelength for the purpose of imaging the surface and structure of the kerr. In contrast, sensor systems without external illumination record the thermal process radiation from the process zone. Coaxial monitoring approaches for near-infrared laser fusion cutting demonstrate a correlation between the obtained image and the cut quality [20; 21].