Subsurface damage (SSD) induced during conventional manufacturing of optics contributes mainly to a reduction in the performance and quality of optics. In this paper, we propose the application of full-field optical coherence tomography (FF-OCT) as a high-resolution and nondestructive method for evaluation of SSD in optical substrates. Both ground and polished surfaces can be successfully imaged, providing a path to control SSD throughout the entire optics manufacturing process chain. Full tomograms are acquired for qualitative and quantitative analyses of both surface and SSD. The main requirements for the detection of SSD are addressed. Data processing allows the removal of low-intensity image errors and the automatic evaluation of SSD depths. OCT scans are carried out on destructively referenced glass samples and compared to existing predictive models, validating the obtained results. Finally, intensity projection methods and depth maps are applied to characterize crack morphologies. The experiments highlight differences in crack characteristics between optical glasses SF6 and HPFS7980 and illustrate that wet etching can enhance three-dimensional imaging of SSD with FF-OCT.

In recent years, the demand for highly integrated and lightweight components has been rising sharply, especially in plastics processing. One strategy for weight-saving solutions is the development of conductive tracks and layouts directly on the polymer housing parts in order to be able to dispense with the system integration of additional printed circuit boards (PCB). This can be conducted very advantageously and flexibly with laser-based processes for functionalizing polymer surfaces. In this work, a three-step laser-based process for subsequent selective metallization is presented. Conventional injection molded components without special additives serve as the initial substrate. The Laser-Based Selective Activation (LSA) uses picosecond laser pulses to activate the plastic surface to subsequently deposit palladium. The focus is on determining the amount of deposited palladium in correlation to the laser and scan parameters. For the first time, the dependence of the metallization result on the accumulated laser fluence (F_acc) is described. The treated polymer parts are characterized using optical and scanning electron microscopy as well as a contact-type profilometer.

In order to meet the increasing requirements of freeform production, a new process chain is presented that combines grinding with thermally induced plasma jet polishing. This is demonstrated by an example of a self-designed Alvarez lens geometry made of fused silica glass. A 3-stage grinding process is applied, comprising coarse pre-shaping, fine freeform shape generation, and ultra-fine first surface smoothing. Grinding is performed using a 5-axis computerized numerical controlled machining process with spherical diamond grinding tools. After grinding, the surface quality is sufficient for subsequent plasma jet polishing, in terms of shape accuracy, mid-spatial frequency errors, and roughness. The surface shape is retained during polishing while simultaneously achieving a high surface quality and meeting the roughness requirements for optical applications. The requisite improvements to the grinding processes, as well as a parameter optimization for plasma jet polishing are presented. Moreover, the application of a convolution function, which describes plasma jet polishing theoretically, helps to reduce the number of samples necessary to optimize the process chain with respect to favorable transfer points, thereby perfectly linking the processes. The novel process chain offers a versatile and efficient approach for the fabrication of optical freeform surfaces with a final roughness of Sq < 0.5 nm (white light interferometer, 50×).

Material removal rates as well as surface and subsurface quality are key aspects for the industrial application of ultrashort pulse (USP) laser machining. However, revealing so-called subsurface damage (SSD) is challenging. The presented study visualizes and quantifies subsurface damage patterns in areal USP laser ablation of fused silica (FS) and glass N-BK7 (BK). For the first time, using high-resolution optical coherence tomography (OCT) as non-destructive and three-dimensional (3D) evaluation method, SSD morphologies of areal laser machining induced damages are analysed. Influences of laser wavelength, beam geometry and processed material are investigated. Discovered differences of damage morphologies and depth in FS and BK point out the relevance of selecting suitable process parameters. Based on the evaluation of volumetric OCT data, the authors were able to quantify damage morphologies using the surface texture ratio as well as power spectral density functions. One important finding for the quantification and comparability of damage depths in USP laser processing is the influence of applicable evaluation thresholds. In comparison to area thresholds of 0.001% being applicable to OCT measurements, more lenient thresholds of e.g. 1% commonly applied in destructive SSD measurement methods in average result in a reduction of measured damage depths by a factor of ~ 2. This potentially leads to an underestimation of damage depths depending on methods on thresholds used. The presented measurement and evaluation methods as well as gained process insights are important assets for the future optimization of low-damage USP laser micromachining of brittle materials. Moreover, the general applicability and relevance of OCT-based morphological damage analysis in laser material processing is shown.

Ultrashort pulse (USP) laser ablation is gaining popularity as a novel manufacturing technique for brittle materials, enabling the creation of complex freeform shapes that are challenging to produce with conventional optics manufacturing techniques. Freeforms have revolutionized optics manufacturing by providing designers with increased degrees of freedom using non-rotational symmetric components. However, this evolution presents new challenges for manufacturing processes, calling for innovative solutions such as USP ablation. To ensure the industrial viability of areal USP laser machining, it is crucial to not only consider material removal rates but also surface quality and subsurface damage (SSD). Especially for optical applications, harsh quality requirements must be met. This study investigates the SSD patterns of fused silica (FS) and borosilicate glass N-BK7 (BK) processed under different laser wavelengths, beam geometries and processing parameters using high-resolution optical coherence tomography (OCT). It is shown that OCT as non-destructive and 3D evaluation method is well-suited for analysing USP processes. The discovered differences in defect morphology between FS and BK emphasize the importance of selecting appropriate processes and process parameters when working with different materials. Compared to previous studies, for the parameter sets analysed here using OCT, much higher defects depths of up to 441 µm were revealed.

Mechanical cracks induced during grinding of brittle materials known as subsurface damage (SSD) reduce mechanical and optical properties of optical components. A characterisation of SSD is needed to guarantee a good quality and to optimize individual processes and processing chains. Current research focuses on non-destructive methods such as optical coherence tomography (OCT) to evaluate SSD depth and distribution and to replace currently established, but time-consuming and labour-intensive destructive methods. Yet the imaging of SSD remains challenging, even with high-resolution OCT providing a high sensitivity. The presented work proposes a combined measurement approach of enhanced SSD imaging by using a potassium hydroxide (KOH) wet etching process prior to OCT measurement. An etching process using 30% KOH at 80°C is applied and resulting etching rates are analysed. It is shown by an iterative etching experiment on optical glass SF6 that the KOH etching process enhances OCT signals of SSD under the surface, revealing up to 2.4-times deeper maximum SSD depths using an identical measurement setup.