## 1 Introduction
The Michelson interferometer setup can also be recognized in Figure 2, but instead of a bulk optic beam splitter, a fiber coupler is used, which serves the same purpose. The detailed functional principle of OCT, the most common signal processing steps and applications are described in detail in [5; 6; 7]. As a non-destructive testing method, OCT offers not only the advantage of non-contact and non-destructive inspection, but also a high spatial resolution and a fast measurement frequency. By using broadband light sources, it is possible to access in-depth information on (semi-)transparent components without the need for mechanical scanning in the z-direction. Figure 1: Schematic representation of a coaxial (**A**) and lateral (**B**) wire supplied system. Applications can be found for example in the characterization of optics [8], and in the quality inspection of polymers and ceramics [9; 10]. Since the axial and lateral resolution of OCT are not coupled, samples with high aspect ratios can also be examined. Thus, the high precision of surface defect detection even at large working distances, the high measurement frequency, and the possible integration into laser processes make OCT a promising tool for in-process control of various laser material processing procedures, with the possibility of adaptive control based on spatially and temporally resolved acquisition of the workpiece to be processed. In this context, coaxial OCT has already been demonstrated for different laser surface structuring applications, e.g., as a non-destructive testing of micromachined surfaces with a so-called ultrahigh-resolution (UHR)-OCT [11] or for a dynamic in-line monitoring of structured electrical steel sheets using an additional pair of galvo mirrors [12]. OCT has also been tested for in-situ quality control in additive welding, e.g., by detecting the roughness of metal surfaces in powder-based deposition processes [13] or investigations into melt pool morphology dynamics in SLM (selective laser melting) [14]. Besides OCT, other methods of coaxial process monitoring have also been used for inline process control of laser processes. Integrated optical systems such as high-speed cameras, near infrared cameras, photodiodes, or thermal detectors such as pyrometers were used to assess welding quality from melt pool dynamics [15; 16]. Although the data obtained by these methods can contribute to the understanding of the process, they do not provide direct information about the weld bead surface, which is essential for the quality of the final product. In-situ observation of the surface topography not only reveals qualitative characteristics, but also enables an extended understanding of topography deviations caused by varying process parameters. With the help of inline monitoring of the layered weld bead topography on the workpiece, process parameters can be adjusted based on the measurement data for stable process control. In this work, an advanced LMD-w system is presented and tested. The system contains an integrated, high-resolution SD-OCT for multidirectional in-situ monitoring of weld bead topography. An optical design developed for this application was used to overcome the challenges of coaxial wire guiding. It has been shown that the integrated OCT system can  
Figure 2: Schematic representation of the LMD-w system. It includes the coaxial processing head, the wire feeding system, and the integrated SD-OCT measuring system. (FC: Fiber coupler, DBS: Dichroic beam splitter, C: Collimator, OL: Objective Lens, M: Mirror, GM: Galvo mirrors, AL: Axicon Lens, P: Prism, W: Wire). detect shape deviations that would affect the quality and functionality of the workpiece. The use of OCT in the LMD-w process can eliminate the need for subsequent quality checks and significantly reduce process time and waste by adjusting process parameters during the process based on in-process measurement data.