Investigation on Subsurface Damage Patterns in Ultrashort Pulse Laser Machining of Glass using Optical Coherence Tomography

. 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.


Motivation
The optics industry has relied primarily on conventional mechanical processes, which have a well-established set of characteristics.Conventional manufacturing of brittle materials typically involves an iterative process of grinding and polishing with bound and loose abrasives of diminishing size to achieve a specified shape and an optical surface.During this process, the generation and intersection of small cracks leads to chipping and material removal, while surface roughness and crack depth are reduced iteratively.However, even after the final polishing, subsurface damage (SSD) may still be present in the form of cracks [1], which decrease material strength, optical quality, and laser-induced damage threshold (LIDT), leading to restricted applications and limited lifetime of the optics.Therefore, during the polishing step, the material must be removed below the zone of the SSD to expose the undamaged bulk material underneath.The larger the SSD, the more complex the polishing process.For standard optical components, conventional processes are often well established and achieve reasonable qualities with economic process chains.Especially for freeform optics, this is not yet the case and the research on suitable technologies, process chains and process parameters is ongoing.
One objective of this conference contribution is to promote the application of high-resolution optical coherence tomography (OCT) as ideal non-destructive characterization method for ultrashort pulse (USP) laser processing.Using the example of areal USP laser ablation in brittle materials, it is shown that surface and subsurface features can be analysed regarding depth, morphology and distribution.USP laser ablation is a precise, sub-aperture machining method and therefore well-suited for the manufacturing of freeform optical components.As in conventional manufacturing of freeforms using mechanical abrasives, Subsurface Damage generated in USP machining remains a challenge.Removal and damaging mechanisms of areal USP machining are not yet fully understood [2] and established.Based on ultrashort pulse durations using femtosecond (fs) and picosecond (ps) lasers, a quasi-athermal removal process with a minimal heat affected zone is achieved.Material removal is induced by material ionization and ablation on a micron scale with small interaction zones.Non-linear absorption effects in focussed pulses play a significant role for energy disposure in the machining area.Damage in USP machining can be related to thermal effects, coulombexplosion shock-waves as well as electron damage.
Current measurement methods for SSD in conventional optics manufacturing are mainly destructive [3].For the characterization of USP processes, single focus spots are observed using pump-probe experiments or by microscopy on the side edge of a glass plate.OCT overcomes dimensional limitations with high sensitivity, high-resolution 3D measurements.

Materials and Methods
The experiments were carried out on Schott N-BK7 (BK) and Corning HPFS7890 (FS) fused silica substrates of size 40 x 40 x 10 mm³ with polished initial surfaces.The surfaces were machined using USP on the entire area with N = 25 passes.For characterization using OCT, a field of view of 1,3 mm² x 1,3 mm² in the centre of the machined area was captured per parameter set.Two picosecond lasers were selected as energy sources for the comparison of material processing with infrared (IR), visible (VIS) and ultraviolet (UV) light.Most parameter combinations have been executed using a 1030 nm wavelength A200 laser from Amphos with a variable spatial light modulator.The other parameter combinations were carried out with a Lumera Hyper Rapid 25 laser with wavelengths λ of 532 nm and 355 nm using second-and third harmonic generation.Different beam profiles were realized using the spatial light modulator for IR as well as a diffractive optical element for VIS.A total of 14 parameter sets have been machined on both FS and BK.For each beam profile, one parameter combination with lower and another one with higher accumulated fluence   were realized by adjusting the line-  and pulse spacing   , see Table 1.Scan speed   and pulse energy   have been set accordingly.The OCT system used as well as the evaluation methods developed for Subsurface Damage are described elsewhere [4].Overall, an isotropic resolution of 1 µm is achieved with a minimum detectable reflectivity of −88 dB.Based on sagittal projections of the registered 3D volume, the defect classification system in Table 2   Seven defect types were differentiated.It is shown that FS mainly exhibits deep radial cracks.In BK, morphologies seem to be more parameter-dependent, showing inclined cracks with different shapes.

Results and Conclusion
Using OCT, defect morphologies in areal USP machining were analysed.Material-dependent differences between FS and BK have been shown.As seen in Fig. 2, crack depths for the materials and parameter sets span between 32 µm (BK10) and 441 µm (FS11) using an SSD area threshold of 1%, exceeding previously mentioned damage depths from previous studies [5] by one order of magnitude.The results emphasize the need for further research to gain a better understanding of damage mechanisms in USP laser machining and to optimize areal USP laser machining in regard to SSD.
has been developed.Corresponding OCT visualisations are shown in Fig 1, revealing differences in defect morphology between the glasses FS and BK.

Table 1 .
Investigated parameter sets for the areal USP laser processing of glasses.

Table 2 .
Categories (cat.) for defects in USP laser processing.