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In Situ Measurement of Plasma and Shock Wave See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/287557215 Measurement of plasma and shock-wave dynamics and properties inside holes during laser drilling of metals Article · January 2008 CITATIONS READS 0 31 4 authors, including: Martin Hermans LightFab GmbH 32 PUBLICATIONS 342 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Research Campus Femto Digital Photonic Production View project All content following this page was uploaded by Martin Hermans on 08 March 2019. The user has requested enhancement of the downloaded file. Measurement Science and Technology Related content - On-line depth measurement for laser- In situ measurement of plasma and shock wave drilled holes based on the intensity of plasma emission Chao-Ching Ho, Chih-Mu Chiu, Yuan-Jen properties inside laser-drilled metal holes Chang et al. - Optodynamic observation of double laser- To cite this article: Mihael Brajdic et al 2008 Meas. Sci. Technol. 19 105703 induced breakdown at the water surface Darja Horvat, Rok Petkovšek and Janez Možina - Development of in situ time-resolved Raman spectroscopy facility for dynamic View the article online for updates and enhancements. shock loading in materials S. Chaurasia, V. Rastogi, U. Rao et al. Recent citations - Dynamic laser piercing of thick section metals Jetro Pocorni et al - Investigation of the piercing process in laser cutting of stainless steel Jetro Pocorni et al - Stability evaluation of short-circuiting gas metal arc welding based on ensemble empirical mode decomposition Yong Huang et al This content was downloaded from IP address 134.61.138.223 on 08/03/2019 at 11:48 IOP PUBLISHING MEASUREMENT SCIENCE AND TECHNOLOGY Meas. Sci. Technol. 19 (2008) 105703 (8pp) doi:10.1088/0957-0233/19/10/105703 In situ measurement of plasma and shock wave properties inside laser-drilled metal holes Mihael Brajdic, Martin Hermans, Alexander Horn and Ingomar Kelbassa Lehrstuhl fuer Lasertechnik, RWTH Aachen University, Steinbachstrasse 15, 52074 Aachen, Germany E-mail: [email protected] Received 10 April 2008, in final form 20 July 2008 Published 27 August 2008 Online at stacks.iop.org/MST/19/105703 Abstract High-speed imaging of shock wave and plasma dynamics is a commonly used diagnostic method for monitoring processes during laser material treatment. It is used for processes such as laser ablation, cutting, keyhole welding and drilling. Diagnosis of laser drilling is typically adopted above the material surface because lateral process monitoring with optical diagnostic methods inside the laser-drilled hole is not possible due to the hole walls. A novel method is presented to investigate plasma and shock wave properties during the laser drilling inside a confined environment such as a laser-drilled hole. With a novel sample preparation and the use of high-speed imaging combined with spectroscopy, a time and spatial resolved monitoring of plasma and shock wave dynamics is realized. Optical emission of plasma and shock waves during drilling of stainless steel with ns-pulsed laser radiation is monitored and analysed. Spatial distributions and velocities of shock waves and of plasma are determined inside the holes. Spectroscopy is accomplished during the expansion of the plasma inside the drilled hole allowing for the determination of electron densities. Keywords: laser drilling, shock wave, spectroscopy (Some figures in this article are in colour only in the electronic version) 1. Introduction A novel method for time and spatial resolved monitoring of plasma and shock wave dynamics by photography and Laser drilling is influenced by plasma formation and dynamics spectroscopy of the optical emission in the confined space of a such as plasma shielding or plasma-supported ablation [1]. blind hole in stainless steel is presented. These investigations Understanding plasma/laser beam interaction and the resulting allow for the determination of shock wave and plasma plasma dynamics during laser drilling is therefore a matter of velocities and of plasma properties such as the electron density interest. These interactions are not yet investigated inside distribution. No similar investigations performed before in holes during the drilling of metals. Process understanding metals are known to the authors. is based either on ex situ investigations such as destructive testing methods based on metallography and microscopy 2. Experimental details [2, 3] or on nondestructive testing methods based on in situ lateral investigations of plasma dynamics, which are limited to 2.1. Sample preparation the expansion into the half-space above the irradiated surface A specific sample preparation with prepared mockup holes of the metal [4, 5]. Investigations on laser drilling have been including a transparent material as a window is used performed with transparent materials in order to detect the (figure 1): in metal foils with thicknesses comparable to optical emission inside the holes [6, 7]. The different physical the diameters of holes typically drilled (about 80 µm) with properties of these materials in comparison to metals yield the used laser system grooves with the same dimension as different processes during drilling. the thickness are cut with a length comparable to typical 0957-0233/08/105703+08$30.00 1 © 2008 IOP Publishing Ltd Printed in the UK Meas. Sci. Technol. 19 (2008) 105703 M Brajdic et al the transmissivity of the cover slips significantly. For measurement of velocities and determination of electron densities by spectroscopy, the slight change in transmissivity during the experiments (about 50 pulses have been used for each mockup) is not relevant. 2.2. Optical emission photography: monitoring system The measurement system to monitor the optical emission of the expanding plasma consists of the described mockups, a microscope objective for imaging the plasma, an intensified CCD camera (iCCD) as well as a pulse/delay generator for synchronizing the laser system and the camera Figure 1. Stainless steel sample with mockup holes. Hole (figure 4). The pulse/delay generator (Quantum Composers × × dimensions: 80 µm 80 µm 400 µm. 9500+) provides pulse widths of 10 ns–1000 s, delay times of 0–1000 s at an accuracy of 1 ns and an RMS-jitter of <250 ps. depths of the holes (about 80 µm, figure 2, left). The The used iCCD camera (4Picos, Stanford Computer Optics, grooves are generated in the metal foils by structuring Inc.) takes 8-bit grey scale pictures and provides exposure using femtosecond laser radiation (THALES Concerto). The times of 200 ps–80 s with a jitter less than 20 ps. The CCD chip structured grooves exhibit nearly melt-free geometries due allows for a resolution of 752 × 582 pixels and a pixel size of to the use of femtosecond laser radiation. The structuring 8.4 µm × 8.2 µm. The electrons, generated by the interaction parameters are λ = 800 nm,tP = 80 fs,fRep = 1 kHz and of photons with the photo cathode, are multiplied by a micro- Pav = 80 mW. Both sides of the metal foil are covered channel plate (MCP). The resulting signal amplification is with optical transparent material (cover slips) and used as adjustable by the voltage of the MCP. The camera offers transparent walls (window) (figure 2, right). The used cover a signal output of the MCP gate, indicating the exposure slips consist of D263 borosilicate glass with a thickness time. of ≈150 µm and a transmission ratio of ≈90% for the The laser radiation is provided by a diode pumped solid investigated wavelengths. Guiding laser radiation between state (DPSS) Nd:YLF laser (Vitro VIR 0.5) delivering laser the cover slips to the end of the groove, these mockups can radiation with pulse durations of 5–9 ns, resulting in a focus be used as predrilled holes with a diameter corresponding to diameter of 15 µm and maximum intensities of ∼1011 Wcm−2 the foil thickness and a depth corresponding to the length with the used optics. The radiation is focused into the mockup of the grooves. The generated plasma will expand along holes. The optical emission of the expanding plasma, induced this cavity, confined from two sides by the metal substrate by a single pulse of the laser radiation, is imaged by a single and from the other two sides by the transparent window frame of the iCCD camera at a predefined delay time after (figure 3). The propagation of the laser-induced plasma the incident pulse. The delay time between the laser pulse inside the mockup is assumed to be approximately comparable and camera exposure is increased stepwise (t 1ns) with the plasma dynamics in a conventionally laser-drilled by adjustment of the pulse generator. Exposure times and hole. Each mockup can be used about 100 pulses gain of the iCCD camera are adapted to the intensity of the long until deposition of melt, vapour and plasma reduces optical emission. This experimental setup allows time and Figure 2. Schemes of sample preparation. Left: groove structuring in metal foil with femtosecond laser radiation; right: covering of the structured metal foil with optical transparent material. 2 Meas. Sci. Technol. 19 (2008) 105703 M Brajdic et al Figure 3. Scheme of monitoring optical emission of the plasma through the optical transparent material during drilling. Figure 4. Scheme of experimental setup for lateral emission photography. spatial resolved measurement of the expanding plasma in this Monitoring the optical emission with the described setup confined environment. at a temporal resolution of 1 ns is achieved. A spatial resolution The laser beam is divided by a beam splitter, allowing for of 0.57 µmpixel−1 is achieved using a microscope objective the monitoring and recording of the pulse with a high-speed with a magnification of 50× allowing the imaging of the ≈ photo diode (Thorlabs SV2-FC) with a bandwidth of 2 GHz plasma over the complete hole length of 400 µm whereas with and rise and fall times of <150 ps.
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