###### Abstract  
Quality and reliability are of the utmost importance for manufacturing in the optical and medical industries. Absorber-free laser transmission welding enables the precise joining of identical polymers without additives or adhesives and is well-suited to meet the demands of the aforementioned industries. To attain sufficient absorption of laser energy without absorbent additives, thulium fiber lasers, which emit in the polymers' intrinsic absorption spectrum, are used. Focusing the laser beam with a high numerical aperture provides significant intensity gradients inside the workpiece and enables selective fusing of the internal joining zone without affecting the surface of the device. Because seam size and position are crucial, the high-quality requirements demand internal well seam monitoring. In this work, we propose a novel method to determine weld seam location and size using optical coherence tomography. Changes in optical material properties because of melting and re-solidification during welding allow for weld seam differentiation from the injection-model base material. Automatic processing of the optical coherence tomography data enables the identification and measurement of the weld seam geometry. The results from our technique are consistent with microscopic images of microtome sections and demonstrate that weld seam localization in polyamide 6 is possible with an accuracy better than a tenth of a millimeter. laser transmission welding; transparent polymers; optical coherence tomography; process monitoring; image processing +
Footnote †: journal: _Sciencedence_  
Footnote †: _Scienced_
clamped in an overlap. A laser beam with the wavelength in the polymers' intrinsic absorption spectrum between 1.6 \(\upmu\)m and 2 \(\upmu\)m is used. Focusing the beam with a comparably high numerical aperture (NA) provides significant intensity gradients inside the specimen, which enables the selective fusing of the joining zone [1,13,14]. The joining of two 1 mm thick partners with a 0.05 mm to 0.6 mm wide weld seam, for example, is possible without affecting the surface of the upper joining partner [11,14]. The main challenge in absorber-free welding is the precise generation of the weld seam in the horizontal and vertical direction along the desired trajectory. If the seam extends too far vertically to the upper surface, a visible and tactile bulge occurs. An overly short seam insufficiently penetrating both joining partners causes instability and inadequate tightness [1,10,12]. Nevertheless, horizontal alignment is also important, since cells or drugs can temporarily linger and lump in remaining gaps if a fluidic channel is not sealed exactly at its edge. This creates an unacceptable risk to the patient using such a device and requires process control [12,18]. In addition to integral tests of the complete product (i.e., burst pressure, leakage and drop-down tests) [4,20], a separate weld seam analysis is necessary to validate the welding process. Conventionally, thin sections of the weld seam area are prepared by grinding or cutting the specimen with a microtome. Extracted sections are viewed in a transmitted light microscope with polarized light. The weld seam becomes visible in the 2D image due to the change in optical properties (i.e., birefringence) during plasticization and solidification (Figure 1, right) [5]. A three-dimensional (volumetric) examination is possible via computer tomography (CT), enabling the detection of geometric errors (i.e., gaps and bubbles) and melt blowouts into fluidic channels by quantifying the density change between the polymer and combustion gases or surrounding air. Unfortunately, the density difference of the base and the weld seam material is usually too small for weld seam detection via CT. A major disadvantage of the methods discussed above is that they are useful for detecting errors only in post-processing. In contrast, pyrometry enables online (in-situ) temperature monitoring during processing.