Monthly Archives: October 2009

Texturing structures of Ti films by multiple femtosecond laser pulses

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By: Yongguang Huang

Femtosecond laser texturing has proved to be a particularly powerful method for creating a wide variety of surface structures on the metal films, such as a hollow microbump-nanojet sturcture and disk-column-nanodroplet structure on the gold films. The process of femtosecond laser texturing does not rely on material removal, but on hydrodynamic flow produced by inhomogeneous heating and film deformation. Femtosecond laser texturing the metal films demonstrates different behaviors depending on films property such as electron-phone coupling, electron thermal diffusion parameters and film thickness. In our work, we concentrated on the surface structures of the Ti films by cumulative femtosecond pulses texturing. With increasing laser average power, the pits structure, the bump structure and the crater structure were induced in order by cumulative pulses with the pulse energy of 0.2nJ/pulse to 2nJ/pulse from a femtosecond oscillator, as shown in Fig.1-2. It is worth noting that for forming the pits structure and the bump structure, the temperature of the Ti film is below than its melting point. And what force causes the film ablation and swelling? More detailed results and discussion will be presented at the conference.

Fig.1 Three-dimensional topography image by a non-contact three dimension surface profiler. The serial numbers a1,a2 to p correspond to the surface structure in the top. Their corresponding laser parameters and corresponding morphology curves are shown in Fig.2.

Fig.2 Left: Surface profiles of the surface structure of Ti films after laser irradiation 0.1s which were measured by a three dimension surface profiler. The correspond average power used are (a) 18mW, (b) 22mW, (c) 27mW (d) 32mW, (e) 36mW, ( f) 40mW, (g) 45mW, (h) 50mW, (i) 60mW, (j) 70mW, (k) 80 mW, (l) 90 mW, (m) 100mW, (n) 110mW, (o) 120mW, (p) 130mW.  The distance between the ticks on the ordinate axis corresponds to 100nm. Right: Atomic force microscope image of the crater structure (I), the bump structure (II) and the pits structure (III).

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

P206

Numerical Calculation of Laser Beam Path Influenced by High Temperature Gas Above Specimen During Laser Welding

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Beam deflection confronting again?  Take care of media bias during laser material processing.

By: Masami Mizutani and Seiji Katayama

Joining and Welding Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka, Japan

It has been generally accepted that the laser beams of about 1 mm in wavelength, such as fiber, disk and YAG lasers have smaller interaction to the media such as plume and plasma between the optics system and the specimen than CO2 laser beam of 10.6 mm.

However, we have recently found that weld bead formation is greatly affected by the state of the gas flow, the atmosphere, the ultrafine particles or the plasma plume above the specimen during fiber laser remote welding. The result showed that a laser beam tended to be deflected and focused on the far position without the positive elimination of the media above the specimen.    Meanwhile, mirage effects and heat haze effects are seen as natural phenomena by varied temperature of the air, which suggest that the air possessing the temperature gradient enable the ray to deflect.

Therefore, in order to understand what extent the temperature distribution of the media between the optics and the specimen affects the laser beam path, a theoretical calculation of the laser beam trajectory in the assumptive media, which possesses refractive index gradients led by temperature gradients, was carried out.  As one of the important results showing in the figure, the beam trajectories propagating through a high-temperature media below, show that the beam diameter at the original focal point (200 mm) is broadened and the concentration area of the beams shifts downward.  This result may interpret why the laser beam focused on the far position without the positive elimination of the media above the specimen during fiber laser remote welding, which has been experienced.

It should be emphasized that the trajectory of the laser beam, the wavelength of which is about 1 mm, is deflected to a certain extent, by the media possessing the refractive index gradient led by the thermal gradient, which is presumably attributed to the thermal and plume-jet-induced updraft, and the reheat of ultrafine particles.  Therefore, it is concluded that the beam profile of the laser, which has propagated through the gaseous matter possessing the thermal gradient, is no longer the same as originally intended.

Computing results of focused beams propagating from entrance to focal point of 200 mm, assumed for original rays without deflection to be focused at, showing differences in beam trajectories between with and without refractive index gradient.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

Paper 1403

Advanced Beam Steering In Helical Drilling

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By: Henrikki Pantsar1, Petri Laakso2, Mika Aikio2, Jouni Huopana2, Hans Herfurth1, Stefan Heinemann1

1 Fraunhofer USA, Inc. Center for Laser Technology, 46025 Port St, Plymouth, MI 48170

2VTT Technical Research Centre of Finland, Tuotantokatu 2, 53850 Lappeenranta, Finland

Helical laser drilling is a method for producing high quality holes with defined geometries in different materials among industries such as aerospace, medical device manufacturing and electronics. If the aspect ratio of the hole is small, drilling can be done using a fast scanner. However, a special drill head is needed for higher aspect ratio holes and improved precision. The drill head typically comprises wedges or a Dove prism to rotate the laser beam at high velocities. Using a pulsed laser, each pulse removes a portion of the material. Thermal effects and the thickness of the recast layer are significantly smaller than associated with single pulse or percussion drilling.

Combining a galvanometric mirrors together with rotating optics opens up possibilities for drilling and processing which cannot be accomplished with either of those devices separately. In addition to using the helical drill head for precision drilling and adjusting the hole diameter using the scanner mirror angles, it is possible to create non-circular geometric features by combining the movement of scanner mirrors and the rotational movement. Movement away from the origin along the x or y axis on the scanner’s Cartesian coordinate system is translated into the radial coordinate on the polar coordinate system and the angular coordinate is defined by the angle of the prism. In principle, the rotating prism creates a circular beam path which can be scanner at a rate up to 10,000 rpm. Using a sufficiently large prism, the helical drilling device can be stopped to engrave or mark the samples using the same optical setup. There is no need for removing the drill head.

VTT Technical Research Center of Finland has developed an add-on helical drill head which can be attached to typical galvanometric scanners. The head is based on a Dove prism which rotates at 5,000 rpm, creating 10,000 optical rotations per minute. Fraunhofer USA, Center for Laser Technology is currently using such a head to develop laser processes utilizing crystal and fiber based pulsed lasers for expanding the possibilities of helical drilling for industrial applications.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

C204

High Power Rate Femtosecond Lasers and Novel Dynamics during High Repetition Machining

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By: Andreas Tünnermann

Today, there is a strong need for advanced micro machining tools due to the increasing miniaturization of components and systems. Application examples include the drilling of fuel injection nozzles or the structuring of thin film solar cells. However, the fabrication of structures with micrometer or even nanometer precision is a difficult task. Since a few years, pulsed laser systems are replacing conventional tools for micro machining like edm-machines. However, especially the precise laser micro structuring of metals is typically limited by thermal and or mechanical damage in the surrounding. Here, the ability of ultrashort-pulse lasers to fabricate precise micro structures on solid targets is opening new perspectives. In the past years, the superior quality of ablated holes and patterns produced by femtosecond or picosecond laser pulses compared to nanosecond pulses has been demonstrated. Although the production of high quality and high aspect ratio holes in metals with ultrashort laser pulses is still an open field of research, it already has significant technological impact on industrial applications.

Despite of these benefits, the industrial use of ultrashort pulse lasers has been hindered by their complexity and the limited processing speed which does not allow for cost-effective manufacturing. These disadvantages can be overcome by the novel regeneratively amplified solid state or fiber laser sources, providing high average powers and repetition rates.

Recently, we demonstrated in our laboratories ultrafast fiber amplifier systems with 800-W-average-power and mJ-level-pulse energies. These sources are very promising for industrial micro machining applications because of their compactness, high average power and high repetition rates that enable a significant increase of the processing speed.

Systematic studies of the effect of high repetition rates and high average powers on the processing speed and on the morphology of the structures have been performed. At high repetition rates heat accumulation effects leading to melting and increased heat-affected zones have been observed. In addition, plasma shielding effects have been measured. However, by using the high-power ultrafast fiber laser systems with optimized process control we have been able to machine high quality melt-free, and high aspect ratio micro structures within a few tens of ms. These results clearly demonstrate, that high average power ultrafast fiber amplifiers will open new avenues for the micro manufacturing of solid materials – most recent results will be reported.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

Paper # M201

Ultra Short Pulse Laser Generated Surface Textures for Anti-Ice Applications in Aviation

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By: Gert-willem Römer, Daniel Arnaldo del Cerro, R.C.J. Sipkema, M.N.W. Groenendijk, A.J. Huis in ‘t Veld

In nature the morphology of surfaces is used to tune material properties to the highest possible level. Self cleaning surfaces for example, like that of the lotus leaf, amplify the hydrophobic properties of wax crystals by superimposing them on a rough, microstructured surface. A lotus leaf remains always clean thanks to this effect. Figure 1 shows the effect of this on a water droplet situated on a lotus leaf. The goal of this research is to apply this structure to the surface of a number of materials by ultrafast pulsed laser ablation. Continue reading →