# OCT Capillary Depth Measurement in Copper Micro Welding Using Green Lasers
## 3 Results
### Capillary Depth Measurement Using OCT
#### 3.2.4. Inline Measuring of the Melt Pool Width Using the OCT  
In the previous section, the OCT was used in a static configuration during the welding process. However, the scanning system provided with the LDD-600 allows to spacially modulate the OCT beam in any given shape during the welding process. This enables to determine multiple information about the welding process simultaneously. In the following section, the possibilities of the scanned approach of the OCT beam and the usage to determine melt pool width will be presented. By spacially modulating the OCT Beam perpendicular to the welding direction during the welding process, not only can the capillary depth be determined but also additional information, like the capillary width, melt pool width, and approximations, about the capillary form might be determined. In Figure 10a, an image of a weld seam is displayed, overlaid with an sinusoidal modulation path (white). The usage of this motion path allows the determination of multiple process information during the welding process, which are displayed exemplary in Figure 10b. The figure shows a cross section of the welding process for a full amplitude of the OCT beam motion path. The red line represents the melt pool evaluated from a cross section of the finished weld, while the black line represents the cross section from the evaluated depth of the OCT Signal. Additionally, the OCT Signal is colored according to the signal intensity. By evaluating the signal depth and intensity, weld characteristics, such as the melt pool width and the capillary width, can be extracted, in addition to the capillary depth. As a result of oscillating the OCT beam perpendicular to the welding process, the spatial resolution will be decreased. For a static configuration of the OCT beam, the resolution can be calculated by:  
\[r=\frac{v}{f_{measure}}, \tag{3}\]  
where \(v\) is the welding speed and \(f_{measure}\) is acquisition rate of the OCT System. For a scanned system, additional variables are needed to be taken into account. In the experiments, the welding process was overlaid with a sinusoidal modulation of the OCT beam perpendicular to the welding direction, which led to the movement pattern displayed in  
Figure 9: Comparison of the aspect ratios based on the capillary depth measurement (\(n=6\)). Figure 10a. The spatial resolution is now divided into the resolution along and perpendicular to the welding axis. The resolution along the weld direction is calculated by:  
\[r=\frac{v}{f_{mod}}, \tag{4}\]  
where \(v\) is the welding speed, and \(f_{mod}\) is the modulation frequency of the OCT beam. The resolution perpendicular to the welding direction is calculated by:  
\[r=\frac{2\cdot\pi\cdot f_{mod}\cdot Amp\cdot\sqrt{1-\frac{x^{2}}{Amp^{2}}}}{f_ {measure}}, \tag{5}\]  
where \(Amp\) is the Amplitude of the modulation, and \(x\) is the current distance from the middle axis. In the experiments, a modulation frequency of \(f_{mod}=2500\) Hz was used. In Figure 11a, the resulting resolution along the welding direction is displayed as a function of the modulation frequency. One can observe that, for a welding speed of \(v=500\,\frac{\mathrm{mm}}{\mathrm{s}}\) and the chosen modulation frequency, a resolution of \(100\,\mathrm{\SIUnitSymbolMicro m}\) can be achieved. In comparison to the static configuration, with a resolution of \(5\,\mathrm{\SIUnitSymbolMicro m}\) at \(v=500\,\frac{\mathrm{mm}}{\mathrm{s}}\), the resolution is significantly decreased. In Figure 11b, the resolution perpendicular to the welding direction as a function of the current distance from the middle axis for two suitable Amplitudes in the experiments is displayed. One can observe that, with increasing distance, the resolution increases, as well. As a result, the amplitude should be chosen as small as possible, to achieve the highest spatial resolution in the area of interest. In the following paragraph, the determination of the melt pool width is discussed.