## 3 Results and Discussion  
### Raw Data of the Sensor in Cutting Application  
To demonstrate the function of the sensor, the results of two exemplarily chosen cuts in 1 mm thick mild steel are discussed. For both cuts, a nominal velocity of 100 mm/s, a gas pressure of 8 bar (nitrogen), and a nozzle to workpiece distance of 1 mm are chosen. In the first cut, the laser power is set to 1.4 kW, which results in a complete cut and, to highlight the different signal signature, for the second sample, 1.0 kW is used resulting in a cutting interruption on both long straight lines of the sample as shown in Figure 3.  
The voltage signals of both cuts for the Si and InGaAs diode are shown in Figure 4 for a specifically chosen scenario, including the piercing, a waiting period, an acceleration phase of the drives, the movement along the geometry depicted in Figure 1, and, finally, the laser switch off. Please note, the sensor signals look similar for a series of cuts when using constant laser parameters and no differences are observed between cutting stainless steel and mild steel.  
During the time frame shown in Figure 4, the laser is switched on after 0.1 s. The immediately-detected sharp signal rise of both photodiodes marks the piercing. For the next 1.1 s, the laser remains switched on, yet the linear drives remain at the piercing position. During this phase, both photodiodes show an almost constant signal of about 2 V. After 1.2 s, the drives start to accelerate, which is indicated by a rise and jitter of both diode signals. The rounding in the cut geometry can be seen at about 2 s, being  
Figure 3: Picture of the complete cut (**top**) and incomplete cut (**bottom**) with the cut interruption on both straight lines.  
Figure 2: Design of the sensor on top of the cutting head. The dot and dash line show the laser path and the dash line the thermal radiation.  
expressed by a lower voltage from the InGaAs diode. The sharp 90-degree turn of the laser system at the end of the cut contour is associated with a negative peak in the sensor signal of the InGaAs diode shortly before second 3. The laser is switched off after 3 s, which leads to a sharp negative step of the Si diode signal and a slow decrease of the InGaAs signal. The latter is caused by the cooling process of the workpiece, with the main contributing intensities of the thermal radiation shifting towards a longer wavelength (the maximum shift according to [PERSON]'s law). It is worthwhile to stress that the signal behavior shown in Figure 4 (top), in general, reveals the described trend, independent of the employed process parameters for a complete cut.  
The temporal evolution of the signals of both photodiodes for an incomplete cut (Figure 4, bottom) shows, at first glance, a rather similar behavior as the complete cut. However, a detailed comparison yields remarkable differences for both photodiodes, particularly at the beginning and the end of the cutting interruption (highlighted by two grey shaded rectangular in Figure 4, bottom). These differences are observed consistently whenever a cutting interruption is enforced by changing process parameters, such as a decrease in laser power or an increase of velocity.