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Organisationseinheit der BAM
Ultrashort pulse laser micromachining at repetition rates of several hundreds of kHz allows a cost-effective high precision manufacturing, which makes this method attractive for many industrial applications. Upon exceeding a critical laser intensity, hard X-ray radiation is generated as a side effect. Even if the emitted X-ray dose per laser pulse is low, for currently available high-repetition-rate laser systems the accumulated X-ray dose becomes significant and radiation safety must be considered. Influences of the processing parameters are discussed and radiation protection aspects will be outlined
The industrial use of ultrashort laser pulses has made considerable progress in recent years. The reasons for this lie in the availability of high average powers at pulse repetition rates in the several 100 kHz range. The advantages of using ultrashort laser pulses in terms of processing precision can thus be fully exploited. However, high laser intensities on the workpiece can also lead to the generation of unwanted X-rays. Even if the emitted X-ray dose per pulse is low, the accumulated X-ray dose can become significant for high-repetition-rate laser systems so that X-ray exposure safety limits must be considered. The X-ray emission during ultrashort pulse laser processing was investigated for a pulse duration of 925 fs at 1030 nm wavelength and 400 kHz repetition rate. Industrially relevant materials such as steel, aluminum and glass were treated. Tungsten served as reference. X-ray spectra were recorded, and X-ray dose measurements were performed for laser treatment in air. For laser intensities > 2 × 10^13 W/cm2, X-ray doses exceeding the regulatory exposure limits for members of the public were found. Suitable X-ray protection strategies are proposed.
Unwanted X-ray emission in ultrashort pulse laser processing: From metallic to biological materials
(2023)
Ultrashort laser pulses have become established in many industrial processes. Additionally, they are also an integral part of medical applications especially in ophthalmology and to some extent in dentistry. The availability of highly repetitive powerful laser sources and advanced laser beam control systems have favored these developments. However, the laser processing may be accompanied by the generation of unwanted X-rays. Small doses per laser pulse can accumulate to significant dose levels at high laser pulse repetition rates. Moreover, burst mode processing increases the X-ray dose rates compared to single pulse use and results in X-ray photon energies up to 40 keV for tungsten targets. For laser treatment of human teeth, clearly noticeable X-ray skin dose rates can be found. The paper summarizes the current state of the art in the field of undesired generation of X-ray radiation during ultrashort pulse laser processing in air.
The use of ultrashort laser pulses for material processing in air has many advantages. Due to the progressive development in the laser sector, average powers in the kW range with pulse repetition rates exceeding the MHz-level are available. The machining with high-intensity laser pulses can be accompanied by the generation of a near-surface electron plasma due to absorption and ionization of the material, a subsequent plasma heating by the laser pulse, and finally an interaction of “hot” plasma electrons with the workpiece leading to continuous and characteristic X-ray radiation. The amount of this unwanted X-ray radiation is determined by the laser parameters (pulse duration, intensity, wavelength, polarization), the workpiece (atomic number, surface preparation), and the laser process management (scanning or stationary regime, laser turning). The use of laser intensities above 10^13 W/cm^2 in combination with laser pulse repetition rates in the few 100 kHz-range can lead to X-ray dose rates exceeding the permitted X-ray limits for members of the public. Especially the materials tungsten and steel show significant X-ray emission. Recently, the current state of the art in the field of undesired generation of X-ray radiation during ultrashort pulse laser processing in air was reviewed. In this presentation, important aspects of the measured X-ray doses, X-ray spectra, and practical issues of radiation protection are discussed.
Der Vortrag gibt einen Überblick zu aktuellen Projektaktivitäten im BAM-Fachbereich 6.4 Technologien mit Nanowerkstoffen. Der Einsatz von Femtosekunden-Laserpulsen zur Mikro- und Nanostrukturierung, für Prozessschritte in der Photovoltaik und zur Reinigung und Dekontamination von Oberflächen wird beschrieben. Darüber hinaus werden Laser-Zerstörschwellenmessungen erläutert.
Im Überblicksvortrag werden Untersuchungen zur Laser-Mikro- und Nanostrukturierung mit Femtosekunden-Laserimpulsen und ausgewählte Anwendungen, z.B. in der Photovoltaik und Biomedizin, vorgestellt. Aktuelle Fragestellungen zur sicheren Prozessführung, wie die mögliche Erzeugung von Röntgenstrahlung beim Laserbearbeitungsprozess und unerwünschte Partikelemissionen, werden aufgegriffen.
The influence of paper type and state of degradation on laser cleaning of artifically soiled paper
(2009)
Ultrakurzpuls-Lasertechnik ist heute der Schlüssel für eine Vielzahl von Anwendungen in der industriellen Lasermaterialbearbeitung. Beim Einsatz hoher Laser-Intensitäten kann allerdings unerwünschte Röntgenstrahlung erzeugt werden. Die Röntgendosis und Energieverteilung ist von Laser-, Material- und Prozessparametern abhängig. Es können Röntgendosen über den erlaubten Grenzwerten für die Exposition des Menschen erreicht werden.
Die Erzeugung unerwünschter Röntgenstrahlung bei der Ultrakurzpulslaser-Bearbeitung von Cornea und Zahnmaterial wurde untersucht. Laser-induzierte Dosisleistungen über dem natürlichen Hintergrund wurden gemessen. Mögliche Strahlenexpositionen in realen Anwendungen am Menschen wurden im Online-Vortrag diskutiert. Im Rahmen des vom BfS geförderten Projektes haben die Hochschule Mittweida und die BAM gemeinsam Untersuchungen durchgeführt.
Die Laserbearbeitung von Materialien mit ultrakurzen Laserpulsen kann zu einer sekundären Emission gefährlicher Röntgenstrahlen führen. Dieser Effekt wurde bisher bei der Bearbeitung von technischen Materialien wie Metallen beobachtet. Die Röntgenemission bei der abtragenden Bearbeitung von biologischen Geweben ist noch weitgehend unerforscht. Der Vortrag präsentiert erste Untersuchungen und Ergebnisse des radiologischen Gefährdungspotentials bei der medizinischen Anwendung von Ultrakurzpulslasern am Menschen.
A promising technology in photovoltaics is based on micro-concentrator solar cells, where the photovoltaic active area is realized as an array of sub-millimeter sized thin-film solar cells. For copper-indium-gallium-diselenide (CIGSe), the solar cells can be arranged in the foci of a regular arrangement of micro-lenses to enhance their efficiency by light concentration, to allow a better heat dissipation and to save expensive raw material (indium). Different approaches to produce micro-sized precursors of CIGSe absorbers on molybdenum are presented using 30-fs laser pulses at 790 nm wavelength. On the one hand, a multi pulse surface structuring of the molybdenum or the underlying glass substrate and a subsequent physical vapor deposition (PVD) process were used for a site-selective aggregation of indium droplets. On the other hand, a single pulse laser-induced forward transfer (LIFT) was utilized to selectively deposit combined copper/indium/gallium precursor pixels on the molybdenum back contact of the solar cell. It was demonstrated that a postprocessing (selenization, isolation, contacting) of the laser-generated micro-sized precursors results in an array of working CIGSe solar cells with an efficiency of 2.8% for 1 sun illumination.
For pulse laser materials processing often optical step index and gradient index multimode fibers with core diameters ranging from 100 to 600 μm are used. The design of a high power fiber transmission system must take into account limitations resulting from both surface and volume damage effects. Especially, breakdown at the fiber end faces and self-focusing in the fiber volume critically influence the fiber performance. At least operation charts are desirable to select the appropriate fiber type for given laser parameters.
In industry-relevant studies the influence of fiber core diameter and end face preparation on laser-induced (surface) damage thresholds (LIDT) was investigated for frequently used all-silica fiber types (manufacturer LEONI). Experiments on preform material (initial fiber material) and compact specimens (models of the cladding and coating material) accompanied the tests performed in accordance with the relevant LIDT standards ISO 11254-1 and ISO 11254-2 for 1-on-1 and S-on-1 illumination conditions, respectively. The relation beam diameter vs. LIDT was investigated for fused silica fibers and preforms. Additionally, the laser-induced (bulk) damage threshold of fused silica preform material F300 (manufacturer Heraeus) in dependence on external mechanical stress simulating fiber bending was measured. All experiments were performed with 10-ns laser pulses at 1064 and 532 nm wavelength with a Gaussian beam profile.
Mikromaterialbearbeitung mit ultrakurzen Laserimpulsen: Vorzüge, Probleme, Sicherheitsaspekte
(2005)
Um Kulturgüter aus organischen Substanzen zu bewahren, wurden besonders in den 70er und 80er Jahren Biozide, wie z.B. das flüssige Holzschutzmittel Hylotox-59, eingesetzt. Es enthält Dichlordiphenyltrichlorethan (DDT), das im Verdacht steht, krebsauslösend zu sein. Durch „Ausblühen“ der Chlorverbindungen entstehen an der Holzoberfläche Kristallstrukturen. Eine Entfernung dieser Kristalle und damit eine Abreicherung von Chlor an der Oberfläche sind sehr erwünscht.
Eine Laserreinigung von ausgewählten Teilen musealer Holzobjekte aus dem Schlossmuseum Sondershausen wurde mit Impulsdauern im Femto- und Nanosekundenbereich durchgeführt. Der Chlornachweis vor und nach dem Lasereinsatz wurde mittels Röntgenfluoreszenzanalytik (RFA) erbracht. Chlorabreicherungen von 55-70% und 75% wurden beim Einsatz von ns-Impulsen bei 1064 nm Wellenlänge bzw. fs-Impulsen bei 800 nm Wellenlänge erreicht. Im fs-Fall verblieben keine kristallinen DDT-Rückstände auf den Probenoberflächen.
Laser-induzierte Zerstörung von optischen Komponenten bei Anwendung von Femtosekunden-Impulsen
(2008)
Laser-induced damage threshold (LIDT) testing of oxide and metal coatings with sub-30 fs pulses
(2007)
Die Verleihung des Deutschen Zukunftspreises an ein Team zum Thema "Ultrakurzpulslaser für die industrielle Massenfertigung - Produzieren mit Lichtblitzen" im Jahr 2013 belegt die industrielle Relevanz des Ultrakurzpulslasereinsatzes in der Materialbearbeitung. Zunehmend werden weitere Anwendungen z.B. in der Mess- und Medizintechnik erschlossen. Femtosekunden-Laserpulse weisen prinzipbedingt ein breites Spektrum auf und besitzen bereits bei moderaten Pulsenergien extreme Pulsspitzenleistungen, womit über „konventionelle“ Laserquellen hinausgehende Herausforderungen an den Laserschutz entstehen. Die in entsprechenden Laserschutzbrillen verwendeten Laserschutzfilter müssen eine große spektrale Absorptionsbandbreite aufweisen und dürfen nicht aufschalten, d.h. keine induzierte Transmission zeigen. Im Vortrag soll dargestellt werden, dass Laserschutzfilter auf Glasbasis, die über ein breites Absorptionsspektrum verfügen, einen ausreichenden Schutz gegenüber fs-Laserstrahlung bieten, wohingegen Laserschutzfilter auf Polymerbasis eine induzierte spektrale Transmission bei steigender Energiedichte aufweisen können, was zu einem drastischen Verlust der Schutzfunktion führt.
Der Vortrag gibt einen Überblick zu ultrakurzen Laserpulsen, der Ultrakurzpulslaser-Materialbearbeitung und der dabei ggf. auftretenden unerwünschten Röntgenstrahlung. Es wird die Laserbearbeitung von technischen und biologischen Materialien diskutiert. Schutzmöglichkeiten gegenüber der Röntgenstrahlung werden erörtert.
A promising technology in photovoltaics is based on micro-concentrator solar cells, where the photovoltaic active area is realized as an array of sub-millimeter sized thin-film solar cells. Different approaches to produce micro-sized precursors of CIGSe absorbers on molybdenum are presented using 30-fs laser pulses at 790 nm wavelength. On the one hand, a multi pulse surface structuring of the molybdenum or the underlying glass substrate and a subsequent physical vapor deposition (PVD) process were used for a site-selective aggregation of indium droplets. On the other hand, a single pulse laser-induced forward transfer (LIFT) was utilized to selectively deposit combined copper/indium/gallium precursor pixels on the molybdenum back contact of the solar cell. It was demonstrated that a postprocessing of the laser-generated micro-sized precursors results in an array of working CIGSe solar cells with an efficiency of 2.9% for 1 sun Illumination.
A bottom-up approach is presented for the production of arrays of indium islands on a molybdenum layer on glass using 30-fs laser pulses at 790 nm wavelength. The indium islands can serve as micro-sized precursors for indium compounds such as copper-indium-gallium-diselenide (CIGSe) used in photovoltaics. Molybdenum is the standard back contact material of CIGSe solar cells. Femtosecond laser ablation of glass and a subsequent deposition of a molybdenum film or direct laser processing of the molybdenum film both allow the preferential nucleation and growth of indium islands at the predefined locations in a following indium-based physical vapor deposition (PVD) process. A proper choice of laser and deposition parameters ensures the controlled growth of indium islands exclusively at the laser ablated spots. Based on a statistical analysis, these results are compared to the non-structured molybdenum surface, leading to randomly grown indium islands after PVD.
A bottom-up approach to produce arrays of indium islands on a molybdenum layer on glass using 30-fs laser pulses at 790 nm wavelength is presented. These islands can serve as micro-sized precursors for indium compounds such as copper-indium-gallium-diselenide (CIGSe) used in photovoltaics. Molybdenum is the standard back contact material of CIGSe solar cells. Femtosecond laser ablation of glass and a subsequent deposition of a molybdenum film or direct laser processing of the molybdenum film both allow the preferential nucleation and growth of indium islands at the predefined locations in a following indium-based physical vapor deposition (PVD) process. A proper choice of laser and deposition parameters ensures the controlled growth of indium islands exclusively at the laser ablated spots. Based on a statistical analysis, these results are compared to the non-structured molybdenum surface, leading to randomly grown indium islands after PVD.
Ultrashort laser pulses have found their way into industrial material processing. They take advantage of the fact that moderate laser fluences can produce high quality material removal without significant thermal influence. The growing availability of powerful, highly repeating laser sources and an advanced laser beam control have favored these developments. However, the laser-matter interaction can be accompanied by the production of X-rays. Small doses per laser pulse can accumulate to significant dose rates at high laser pulse repetition rates which may exceed the permitted X-ray limits for human exposure. Consequently, a proper radiation shielding must be considered in laser material processing. The paper summarizes the current state of the art in the field of undesired generation of X-ray radiation during ultrashort pulse laser material processing in air.