###### Abstract
Deviations can be identified and compensated using feedback between temperature and processing parameters. Using this closed-loop control, overheating in corners and bubble formation can be prevented [12]. Furthermore, the temperature signal is linked to the vertical expansion of the weld seam, enabling indirect weld seam localization in the vertical direction [12,15]. However, misplacement of the seam or overheating cannot be clearly distinguished from one another as they both superimpose in the pyrometer signal, and no direct information about the weld seam geometry is given by pyrometry [18]. Figure 1: Sketch of the experimental setup for absorber-free laser transmission welding (**left**) and an exemplary weld seam photographed in polarized light (**right**) (adapted from [19]). In conclusion, the usability of absorber-free laser transmission welding is still limited, since the possibilities for process monitoring are still insufficient. We aim to improve this issue by introducing optical coherence tomography for 3D weld seam localization. Optical coherence tomography (OCT) is a technique that uses surface reflections and scattered light within a sample to generate 3D images of a geometry [21]. To enable OCT, we employ a spectral-domain OCT (SD-OCT) attached to a 2D scanner (Figure 2). In the OCT, the low coherence light of a superluminescent diode is divided into a sample and a reference path of an interferometric setup. The reflected light from the sample is collected and coupled back into the fiber. After combining it with the light from the reference path, the resulting interferogram is measured with a spectrometer. As the modulation of the interferogram contains the depth information of the sample, the depth-reflectivity profile \(R_{(2)}\) is extracted using a Fourier transformation of the output data [21]. By scanning in two directions, the one-dimensional data (A-scan, in the \(z\)-direction) is expanded volumetrically to obtain a three-dimensional reflectivity profile \(R_{(x,y,z)}\) (C-scan). Optical coherence tomography has excellent prerequisites for online monitoring of laser-based processes: data points can be acquired with several hundred kilohertz, and a scanner is usually already there for processing. OCT has already been used for non-destructive testing in different industrial applications of laser transmission welding. Usually, interfaces between different parts or materials are determined based on the abrupt change in optical properties. Since the upper joining material is partially transparent, a highly reflective surface for welding metal sheets to plastic [23] and internal structures with high differences in the refractive index like glass fibres or gas bubbles can be recognized [24; 25; 26; 27]. In transparent-absorbent laser transmission welding, OCT has already been used for measuring the seam width and for detecting gaps and internal pores [28; 29; 30; 31; 22]. If there are no errors, the interface between a properly welded transparent and absorbent partner is usually represented by an absent OCT signal [31]. However, because the absorbing partner contains additives, changing the optical properties compared to the transparent one, the interface of properly welded partners can be detected using an OCT with sufficient dynamic range [29]. Ultra-short pulsed laser welding of glass is similar to absorber-free laser transmission welding of polymers, as samples of an identical material or with very similar optical properties are joined. Weld seams in glass are localized via OCT by detecting gas-filled cavities [32] at the top and bottom of the seam [33]. In glass welding, therefore, there is an abrupt change in the refractive index at the interface from glass to gas. In conventional transparent-absorbent transmission welding as well as glass welding, there is always a boundary between different materials (polymer to gas, polymer to metal,  
Figure 2: Sketch of the scanner-integrated SD-OCT for volumetric weld seam analysis (adapted from [22]).