Micromachining of transparent materials with high-power lasers demands new approaches to utilize most of the laser power without reduction of the quality. The usually exploited Gaussian beams become absolute when specific beams can be utilized to do the set task. For instance, the Bessel beams can be used to drill very high aspect ratio holes with a single laser pulse, which increases the speed of manufacturing by many orders. In our research, we have even more versatility in the beam shapes as we use the beam shaping techniques which withstand high-power laser pulses. Therefore, higher-order, vectorial Bessel beams, and flattop beams were used to micromachine transparent materials and results will be shown.
We study the properties of glasses and crystals after an ultraviolet (UV) laser ablation process using 300 fs duration pulses, centered at 206 nm wavelength at the repetition rate of 50 kHz. We show that linear light absorption allows fabricating grooves with a surface roughness in the order of tens of nanometers and even sub-10 nm with low heat affected zones (HAZ) without any post-processing. We produced a single scan laser ablated microchannels’ depth, surface roughness and full width at half maximum maps based on the experimental results. These maps show relations with an overlap of laser pulses and a pulse energy and are useful when considering the working regime for specific applications. Our results depict the minimum channel bottom roughness of 8 nm in Sapphire after one scan. This value represents several times smaller roughness when compared to the nanosecond UV laser ablation. We have also studied a multi shot ablation and by evaluating our results using a scanning electron microscope (SEM) we noticed nanogratings on the walls of the craters of soda-lime and BK7 glasses with a period of 300 nm, that is bigger than the laser wavelength (206 nm). We also show that a precise surface micro patterning can be achieved with our experimental setup. Finally, our theoretical fluence distribution optimization provides a precise fabrication parameters estimate for a small surface roughness condition.
Surface roughness is an important parameter that has a great influence on various material properties. It determines the rate of corrosion, wettability, as well as optical properties of different materials. Low roughness (< 100 nm Ra) surfaces are difficult to achieve even with fs pulses, therefore investigation of the theoretical intricacies is of major interest when engraving transparent materials. In our study, we numerically investigate the evolution of the surface roughness when it is scanned with a UV femtosecond laser beam and compare the numerical results to the experimentally acquired values. The study contains a single scan as well as multiple scans (up to 10 scans) on the surface. We found that in the case of a single scan the dominant surface roughness determining factor is the overlap of the pulses in x and y directions. It was found, that parameters such as pulse overlap, laser-scanner synchronization, and initial beam profile strongly influence the resulting surface roughness in a nonlinear manner. In the case of a multi-scanned surface, we determined that the resulting surface roughness can be minimized by introducing rotation of every following layer at a certain angle with respect to the previous one. The angle for minimized surface roughness highly depended on system configuration. The investigated theoretical model is in good relation to the experimentally acquired results and provides valuable information when optimizing the process for minimal-roughness micromachining.
We analyze the concept of using customized spatially variable waveplates for beam tailoring towards enhanced various glass microwelding process. These elements work as precise flat optical elements that have very high diffraction efficiency (<90%), high optical damage threshold, and can be freely customized for specific needs that transform the spatial intensity profile into tailored beam shape. In this work, we investigate custom made flat-top beam and "C" shaped beams for deep microwelding purposes. By using numerical simulations and experimental research we compare the performance of such beams and demonstrate thin glass deep microwelding capabilities using custom beam shaping elements.
During the last decade the zeroth order Bessel-Gauss laser beam has found many uses in the transparent material processing. The high aspect ratio channels can be created that slice through various thin transparent materials and increase the efficiency of cutting. However, the generation of high-quality Bessel-Gauss beam remains a challenge due to imperfection of glass axicon manufacturing, i.e. rounded tip, not smooth surface etc. These imperfections generate intensity modulation along propagation axis or even modify transversal central core intensity distribution, that results in worsening of micro-machining quality. The diffractive optical element (DOE) is a great alternative that do not suffer from previously mentioned problems. In this study we show the possibility of generating high quality Bessel-type beams with geometric phase optical elements (GPOEs) (manufactured by Workshop of Photonics). These elements act as precise flat DOEs that have very high diffraction efficiency (>90%), high optical damage threshold and can be freely customized for specific needs. Therefore, with the use of high-power laser they can be applied to process transparent materials. In this work, controllable phase shifts are implemented in axicon phase masks to create unique and fanciful Bessel-type beams as well as asymmetric core beams for thin glass modification/cutting application. Using numerical simulations and experimental data we compare performance of GPOEs and demonstrate thin glass processing using powerful laser with reshaped intensity distribution by GPOE.
A well-known and already having many material processing applications zero-order Bessel beam makes a great base for improvements to have even broader applicability. In our work, we analyze vector Bessel beams (VBB), which can be generated with high efficiency and quality via the use of Geometric Phase Optical Elements. The beam transverse polarization distribution enables to change intensity distribution easily, i.e. a polarizer in front of the beam will generate invariant over the propagation multi-peak ring- the shaped structure which could be very beneficial to modify material at multiple sights with a single laser shot. We analyze higher-order VBB generated modifications in thin glass and analyze the applicability for etching of large diameter holes.
Beam profile engineering, where a desired optical intensity distribution is generated by phase shifting and/or amplitude changing elements, is a promising approach in various laser-related applications. For example, vector geometrical phase elements enable various flat special optical elements such as top-hat converters. We present a study on engineering efficient top-hat converters inscribed in the glass by femtosecond laser pulses. We start with an amplitude encoded top hat converter and demonstrate how its efficiency can be further increased by introduction of phase masks and by the polarization of the incident beam. Experimental verification of the concept is also presented.
Nondiffracting beams are known for their long line of focus, which has various applications in laser materials processing. Zeroth order Bessel beam is usually generated with an axicon and has a distinct circular spatial spectra. The nature of higher order Bessel beams, elliptical and parabolic nondiffracting beams is also conical and their spatial spectra have their own azimuthal modulation. We study numerically and verify experimentally generation of vortical Bessel beams, their superpositions along with elliptical and parabolical beams using an axicon. Laser induced modifications in glasses for various durations and beam powers using generated pulsed beams are analyzed.
Zeroth order Bessel beams are widely used in laser micromachining of transparent materials. The small diameter of central core and elongated focus enables to generate high aspect ratio voids. The simplest way to generate this beam is to induce a conical shape phase with an axicon. However, the quality of the axicon tip is very crucial to generate smooth Bessel beams since it is known that a blunt axicon tip induces large intensity modulation in propagation direction. Alternative Bessel beam generation method is to use a Diffractive Optical Elements (DOEs) that do not suffer from previously mentioned problem. In this work we demonstrate generation of a zeroth order Bessel beam with Geometric Phase Optical Elements (GPOEs) (manufactured by Workshop of Photonics) acting as a diffractive beam shaping element. Having absolute control of induced beam phase, we have modified mask phase so that half of it had additional phase shift or spatial transposition resulting in creation of fanciful induced beam phase patterns. With the use of laser beam propagation numerical modeling we show that these new phase masks can create various beam transverse intensity patterns such as asymmetrical central core, generation of multiple peaks or even large rings that are highly demanded for various laser micromachining applications. We have chosen couple of most perspective beam shapes and manufactured GPOEs to generate them. The experimentally generated beams were compared to numerical simulations. As the GPOEs are able to work with high power pulses we have also investigated induced transparent material modifications.
Long focal lines with transverse spot sizes as small as a few wavelengths are called optical needles. A zeroth order Bessel beam being a good example is widely used in such applications as laser micromachining. In practice Bessel beam generated with an axicon has a peak in axial intensity distribution and is not only due to aberrations caused by planar dielectric material interface. Here, we investigate optical needles with controlled axial intensity distribution via intensity modulation of the incoming beam. We have chosen to generate constant axial intensity Bessel beam and propose spatial transmission mask to do so. Experimental verification is presented using diffractive optics elements based on Pancharatnam-Berry phase. We demonstrate a flattening of the axial intensity profile of the Bessel beam without the alteration to the optical needle diameter.
Non-diffracting Bessel beams and its modified versions are widely used in industry for transparent material micro processing purposes - cutting, drilling etc., due to generation of high aspect ratio micro voids. More and more applications of such beams involve manipulation of their transverse intensity profile to create unique tools for novel micro processing applications, for example, asymmetric and multi-peak transverse profiles create directional strain and crack in modified area for glass cutting, while other intensity patters may be used to create complex structures in multiphoton polymerization applications. In this work we demonstrate experimental generation of higher order vector Bessel beams which are notable for their ring-shaped transverse intensity profile together with multi-peak transverse polarization components, where ring diameter and number of peaks in separate polarization components depends on beams order. These unique beams were realized using axicon together with higher order s-plates - spatially variant waveplates based on femtosecond laser written nano gratings in fused silica glass substrates. Induced nanogratings withstands high intensity laser radiation without changing its spatial structure which allows us to use nanograting based elements for ultra-short high-power pulsed laser beam shaping. Generated higher order vector Bessel beams and their separate polarization components were used to inscribe modifications in transparent materials and to investigate beam`s applicability for ultra-fast laser micro processing purposes.
We present numerical and experimental investigation of the effect that the input pulse chirp has on the energy transfer from 5 μJ fs-pulses at 800 nm to water. The chirp is seen to control efficiently the energy transfer and the geometrical properties of the excited plasma volumes. Agreement between simulations and experiments is obtained via a parametric study, the details of which are presented here. These results may find applications in the control of underwater bubble and sound wave formation, and also in laser surgery involving aqueous media.
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