Laser wakefield accelerators can be an alternative to huge linear accelerators and cyclotrons. Electron bunches with 150-200 MeV energies are needed for Very High Energy Electron radiotherapy. Injection of electrons and their acceleration take place when the focused laser beam interacts with a gas plasma target. We utilise a combined laser micromachining technology with short-pulse and ultra-short-pulse lasers to manufacture complex gas nozzles in fused silica. TW-class lasers are able to accelerate electrons to high energies in a very short distance. A stable operation with electron energy around 3 MeV was demonstrated at a 1 kHz repetition rate. Flexibility in 3D carving within fused silica with lasers allows tailoring plasma targets to particular beams of ultra-high intensity lasers and achieving high energy of accelerated electrons with low energy spread and divergence. Electron energy above 100 MeV could be achieved using new kHz-class OPCPA lasers operating at pulse energy >50 mJ.
We present a new interferometric technique for gas jets density characterization employing a Wollaston shearing interferometer. The distinctive feature of this setup is the double pass of the probe beam through the gas target facilitated by a relay-imaging object arm that images the object on itself and preserves the spatial information. The double pass results in two-fold increase of sensitivity at the same time as the relay-imaging enables the characterization of gas jets with arbitrary gas density distribution by tomographic reconstruction. The capabilities of the double-pass Wollaston interferometer are demonstrated by tomographic density reconstruction of rotationally non-symmetric gas jets that are used as gas targets for the betatron X-ray source at ELI-Beamlines.
In this work, we will introduce the Bessel beam asymmetry control for glass dicing applications. Asymmetrical Bessel beam can be used to form modifications inside the glass with dominant crack propagation direction optimizing the dicing speed and sample separation forces. The Bessel beam asymmetry control will be demonstrated by applying axicon tilt operations and other approaches including the beam shaping with amplitude masks. Further, the Bessel beam dicing process will be compared to conventional processing techniques such as mechanical dicing (score and brake method), diamond saw and water jet cutting.
Optical elements are usually fabricated via conventional well-established processes - milling, grinding and polishing. However, these techniques cannot fully satisfy the growing demand for miniaturized optics with tailored properties. An alternative technology is a laser-based fabrication, including the ultrashort laser ablation and the subsequent CO2 laser polishing steps. Although this technique allows complex surface structuring, the fabricated optics require validation. In this contribution, we present the characterization of the laser-fabricated axicon from fused silica and comparison with the conventional element. We demonstrate that laser-fabricated axicon can generate the high-quality optical Bessel beam with a long non-diffractive length, which could be applied for 1 mm-thick glass intra-volume dicing. Furthermore, we demonstrate that the astigmatic aberrations, introduced via axicon tilt operation, allow generation of the asymmetrical intensity pattern, which could enhance the cleavability of modified glass sheets. The scribing process was optimized by the variation of processing parameters to minimize the force, required to separate modified glass sheets, using the fourpoint bending setup. Furthermore, the quality of the generated beam and volumetric scribing performance was compared to the conventional commercial oblate-tip axicon.
The intra-distance between laser-induced modifications is the limiting factor of glass dicing speed and efficiency. In this work, we present a novel method for the generation of directional cracks in the bulk of glass to enhance the dicing performance. The asymmetrical Bessel-like laser beams with a long non-diffractive length were formed by filtering their spectra of spatial frequencies. Experiments were carried out using the mJ-level laser source with sub-nanosecond pulse duration. The dicing process was optimised by varying processing parameters. The flexural strength of the modified material was measured using the four-point bending setup. Results showed that asymmetrical intensity distribution enables higher dicing speed, better cleavability and quality, compared to symmetrical Bessel-like laser beams.
Nowadays, conventional glass processing techniques, such as “score and brake” method, are being replaced by laser-based techniques. Precision, speed and quality makes laser glass processing a very attractive technique for industry. However, new laser-based techniques have to be validated in respect to conventional processing. For this we introduce comparative investigation of free glass processing techniques – rear side laser cutting, laser-based and mechanical dicing. Local weakening of the material and mechanical separation is a highly efficient two-step glass cutting approach. Material modification can be introduced by laser or mechanically. However, when complex shape cutting is required rear side laser cutting can offer much more flexibility. Glass is a brittle material, therefore generation of micro-cracks during processing is inevitable. Such side-effects can influence processed surface quality and material flexural strength.
Rear side glass cutting experiments were carried out by tightly focusing the laser beam on the sample back-surface. Nanosecond laser pulses with wavelength of 532nm were used. In the case of laser glass dicing process, Bessel beam was introduced to form elongated modifications in glass. High pulse energy sub-nanosecond laser at 1064 nm wavelength was introduced. For mechanical processing, the conventional “score and break” method was used without any additional post processing. In all cases surface chipping was introduced. There was no significant difference in terms of micro-crack size for rear side cutting and mechanical dicing techniques. However, sample resistance to mechanical load was higher for mechanical processing. In this work, in-depth investigation of these effects will be introduced.
The generation of intra-volume modifications and mechanical separation is a highly efficient two-step glass cutting approach. The high aspect ratio modifications, required for thick glass cutting, can be obtained by utilizing Bessel-like beams, which have a long non-diffractive length with a small quasi-propagation-invariant central core. Usually, laser-induced modification channels have to be tightly spaced to achieve a predictable glass separation. However, we demonstrated that cutting speed and glass cleavability may be significantly enhanced by introducing aberrations to the generated beam.
Glass cutting experiments were carried out using the fundamental frequency of the DPSS laser Atlantic HE (from Ekspla), which delivered 300 ps pulses of 2 mJ energy at 1 kHz repetition rate. The incident Gaussian beam was reshaped to the Bessel-Gaussian beam using a conical lens and a 4F optical demagnifying system. A 4-point bending setup was used to separate glass sheets and to evaluate the processing regimes. We have found that the used conical lens deviated from an ideal cone shape and had two often occurring manufacturing defects - the oblate-tip and elliptical cross-section. As a result, the generated beam had the on-axis intensity modulations and asymmetrical intensity distribution in the XY plane, which induced transverse cracks in the bulk of glass, which were extended along the major axis of an ellipse-shaped central core of the beam. Laser-induced transverse cracks in combination with high aspect ratio modifications were applied for fast cutting of the 1 mm-thick glass. Results were compared to glass cutting using the symmetrical Bessel-Gaussian beam.
Conventional processing tools of glass cannot fulfil the forever increasing industrial requirements for processing speed and quality. In the future these methods can be replaced by emerging laser-based techniques. While nowadays most of the research is dedicated for thin, especially chemically strengthened glass, used in electronic devices, there is still a need for a suitable processing technique for thick glasses. One of the most material-efficient and energy-efficient glass cutting techniques is to locally weaken the material along the cutting path by generating cracks or material modifications and then separate sheets by applying thermal or mechanical load. Such approach provides a clean cut with an infinitely thin kerf width without a need for post-processing. Bessel-Gaussian beams, commonly generated using a conical lens, have very appealing properties for processing of transparent materials, such as the long non-diffractive propagation length and self-reconstruction. However, due to manufacturing tolerances, the shape of an optical element deviates from an ideal cone and the intensity pattern is non-symmetrical and modulated along the beam propagation axis. We have found that such asymmetry leads to the significant elongation of laser-induced glass cracks along one dominant direction, which can be beneficial for fast glass cutting.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.