Ultrafast laser processing of transparent materials supported by in-situ diagnostics

For the development of industrial NIR ultrafast laser processing of transparent materials, the absorption inside the bulk material has to be controlled. Applications we aim for are front and rear side ablation, drilling and inscription of modifications for cleaving and selective laser etching of glass and sapphire in sheet geometry. We applied pump probe technology and in situ stress birefringence microscopy for fundamental studies on the influence of energy and duration (100 fs – 20 ps), temporal and spatial spacing, focusing and beam shaping of the laser pulses. Applying pump probe technique we are able to visualize differences of spatio-temporal build up of absorption, self focusing, shock wave generation for standard, multispot and beam shaped focusing. Incubation effects and disturbance of beam propagation due to modifications or ablation can be observed. In-situ imaging of stress birefringence gained insight in transient build up of stress with and without translation. The results achieved so far, demonstrate that transient stress has to be taken into account in scaling the laser machining throughput of brittle materials. Furthermore it points out that transient stress birefringence is a good indicator for accumulation effects, supporting tailored processing strategies.Cutting results achieved for selective laser etching by single pass laser modification exemplifies the benefits of process development supported by in situ diagnostics.

Throughput scaling by spatial beam shaping and dynamic focusing

With availability of high power ultra short pulsed lasers, one prerequisite towards throughput scaling demanded for industrial ultrafast laser processing was recently achieved. We will present different scaling approaches for ultrafast machining, including raster and vector based concepts. The main attention is on beam shaping for enlarged, tailored processed volume per pulse. Some aspects on vector based machining using beam shaping are discussed. With engraving of steel and full thickness modification of transparent materials, two different approaches for throughput scaling by confined interaction volume, avoiding detrimental heat accumulation, are exemplified. In Contrast, welding of transparent materials based on nonlinear absorption benefits from ultra short pulse processing in heat accumulation regime. Results on in-situ stress birefringence microscopy demonstrate the complex interplay of processing parameters on heat accumulation. With respect to process development, the potential of in-in-situ diagnostics, extended to high power ultrafast lasers and diagnostics allowing for multi-scale resolution in space and time is addressed.

SINGLE PASS CLEAVING OF ULTRAFAST LASER MODIFIED C-SHAPED GLASS EDGES

We report on single-pass laser cleaving of transparent materials with C-shaped edges that exhibit 45-deg tangential angles to the surface. A holographic 3D beam splitter distributes several foci along the desired edge contours, including C-shaped edges. Single-pass, full thickness laser modifications are achieved requiring single-side access to the workpiece only without inclining the optical head. After having induced laser modifications with feed rates in the order of 100 mm/s actual separation is performed using a selective etching strategy.

Detecting Distance between Surfaces of Large Transparent Material Based on Low-Cost TOF Sensor and Deep Convolutional Neural Network

Detecting distance between surfaces of transparent materials with large area and thickness has always been a difficult problem in the field of industry. In this paper, a method based on low-cost TOF continuous-wave modulation and deep convolutional neural network technology is proposed. The distance detection between transparent material surfaces is converted to the problem of solving the intersection of the optical path and the transparent material’s front and rear surfaces. On this basis, the Gray code encoding and decoding operations are combined to achieve distance detection between surfaces. The problem of holes and detail loss of depth maps generated by low-resolution TOF depth sensors have been also effectively solved. The entire system is simple and can achieve thickness detection on the full surface area. Besides, it can detect large transparent materials with a thickness of over 30 mm, which far exceeds the existing optical thickness detection system for transparent materials.

Derivation of Equations for a Size Distribution of Spherical Particles in Non-Transparent Materials

This paper presents a new proposition on how to derive mathematical formulas that describe an unknown Probability Density Function (PDF3) of the spherical radii (r3) of particles randomly placed in non-transparent materials. We have presented two attempts here, both of which are based on data collected from a random planar cross-section passed through space containing three-dimensional nodules. The first attempt uses a Probability Density Function (PDF2) the form of which is experimentally obtained on the basis of a set containing two-dimensional radii (r2). These radii are produced by an intersection of the space by a random plane. In turn, the second solution also uses an experimentally obtained Probability Density Function (PDF1). But the form of PDF1 has been created on the basis of a set containing chord lengths collected from a cross-section.The most important finding presented in this paper is the conclusion that if the PDF1 has proportional scopes, the PDF3 must have a constant value in these scopes. This fact allows stating that there are no nodules in the sample space that have particular radii belonging to the proportional ranges the PDF1.