## 1 Introduction  
DED (Directed Energy Deposition) describes an additive manufacturing process in which thermal energy, usually supplied by a laser source, is used to melt the materials and deposit them layer by layer onto a substrate. Because of the good focusability of a laser source and the high degree of control over parameters like laser power, pulse duration, and wavelength, the laser as a processing tool offers the great advantage of precise energy deposition on the substrate. This makes the process suitable for manufacturing applications in a wide range of industries for example for tool making or tool repair [1]. Within the DED, mainly metal powders (e.g., in LPBF: laser powder bed fusion) or wires are used as feedstock material. The LMD-w process is considered particularly efficient and resource-saving due to the direct and complete deposition of the wire material used [2]. Unlike other processes, the LMD-w process is associated with less contamination and less health risks related to the use of metal powders, as well as a low-cost deposition [3]. To maintain precision during the entire process and to assess the quality of the welds, permanent process monitoring is required. Existing in-situ process monitoring is mainly limited to camera-based monitoring of the melt pool or indirect quality monitoring by means of temperature measurements. Ingeneral, it can be stated that in-process workpiece monitoring is indispensable, since the occurrence of inhomogeneities, pores, or cracks, which can result from process instabilities, strongly influence the mechanical properties of the final product. A direct correlation of the qualitative characteristics to the process parameters, the basis of adaptive process control, is only possible with a spatially and temporally resolved assessment of these characteristics. State-of-the-art wire processing heads use lateral feed of the process wire, which severely limits the flexibility of the system and does not allow for multidirectional applications. Asymmetric designs are also associated with more complex control systems and increased manufacturing times for the additive manufacturing of 3D components. For this reason, an LMD-w process with coaxial wire guidance was implemented. The difference between lateral wire guidance and a coaxial LMD-w process is illustrated in Figure 1. A measurement system based on low coherence interferometry was coaxially integrated into the process head, which allows for monitoring of the weld bead topography during the process. The measurement method is known as OCT; it originated in biomedicine and although it is still mainly used for retinal diagnostics, in recent years, OCT has become increasingly popular in industrial applications [4]. The basis of this method is an interferometer, with the Michelson interferometer-based set-up being the most commonly used. The beam of a low coherent light source (broadband spectrum) is split into two beam paths by a beam splitter; in the reference path, the light gets reflected by a mirror, while in the measuring path, the light gets reflected or backscattered from the surface of the sample or from deeper layers within a (semi-)transparent sample. Both reflected beams (from the reference path and measuring path) are recombined in the beam splitter. For a SD-OCT setup, as it is used in this work, the length of the reference mirror is fixed, and the depth information of the sample is encoded in the interference signal of the two recombined beams. The detection of the interference signal is realized by a spectrometer. The raw intensity signal detected by the spectrometer contains the information of the optical path length difference between sample path and reference path, encoded in the detected modulation signal. By performing a Fourier transformation on the detected intensity signal, the depth information of a single point measurement can be extracted.