Ultrashort pulse lasers: Becoming a fully digitised production technology
Dr Arnold Gillner, of Fraunhofer ILT, discusses the growing application range of ultrashort pulse lasers in industrial manufacturing
Ultrashort pulse lasers present a class of laser beam sources for industrial applications that offer outstanding features with respect to intensity, photon density and pulse duration.
Operating in either the picosecond or femtosecond domain, the energy from USP lasers can be concentrated into a material with ultra-high precision in the range of a few hundred nanometres during processing. Moreover, in ablation applications, due to the absorbed energy being removed via the ablated material, in addition to the very low thermal penetration that ultrashort pulses offer, thermal damage can be avoided when processing materials using USP lasers.
From micro to macro
Due to the power scaling developments currently ongoing for USP lasers in the kilowatt range – for example the Fraunhofer cluster of excellence ‘Advanced Photon Sources’ aims to develop a generation of USP lasers with average powers ranging up to 20kW over the next three years – it is possible that these benefits could be brought to macro processing applications in the future. This would open up large markets for the technology outside the micro processing field, where many USP lasers are currently used.
These macro processing applications could include using high-power USP lasers to process fibre-reinforced composite materials at large scales without thermal influence, enabling large surfaces to be provided with microstructures that minimise friction.
One such material is carbon fibre, a high-strength composite playing an increasingly important role in the automotive and aerospace industries. While processing carbon fibre using mechanical means leads to rapid tool wear, due to the high strength of the material, this is not an issue when using USP lasers, as they are contact- and therefore wear-free.
Pyramidal structure created in tool steel using a picosecond laser. (Image: Fraunhofer ILT)
Under optimum conditions, however, USP lasers enable the almost non-thermal processing of carbon fibre, since despite the high thermal conductivity of the material, thermal input is minimised by the short duration of the pulses. This enables the resin matrix to remain intact and the structure of the processed part to be maintained.
As an example, using a pulse duration of 15ps, pulse energy of 30µJ and a pulse repetition rate of 1MHz, a volume ablation rate of 80mm³/min can be currently achieved when processing carbon fibre, with the heat-affected zone being almost zero. Even higher ablation rates over larger areas can be expected in the future, with the increases in power that are coming.
Other macro applications for USP lasers can be found in the generation of functional surfaces. High-intensity and non-linear absorption effects can be achieved when using USP lasers, causing self-organising processes to occur in ablation that lead to micro and nanoscale structures being produced in materials. These ultra-small structures can be used to establish superhydrophobicity or micro optical functionalities on a surface. These surfaces can be used to join polymer components to metal parts in lightweight design in the automotive industry.
Wielding higher power
Using increasingly higher-power USP lasers with high repetition rates in the megahertz region could cause thermal issues, such as overheating, melt production and low ablation quality if certain parameter sets and fluence ranges aren’t considered. High ablation quality can only be achieved when the processing fluence is close to the ablation threshold, which, going forward will require new processing strategies and innovative system components.
As recently discussed at the fifth Ultra Short Pulsed Laser Workshop at the Fraunhofer Institute for Laser Technology in Aachen, in order to use high-power USP lasers, the most important developments that need to take place have to be carried out in the field of system technology. Controlled laser fluence on the surface of the workpiece, fast beam manipulation and modulation, as well as flexible beam design, are all necessary to take advantage of the new developments of USP lasers.
Besides using polygon scanners to achieve ultra-high speed scanning, multiple laser beams provide the best and most versatile high-power ablation solution. With switchable single beams out of a spacial light modulator or a diffractive optical beam splitter, high ablation rates can be achieved, while maintaining the high processing quality of USP laser ablation. With this approach, a next step to an all-optical manufacturing system can be provided.
In addition to necessary developments of system components, new laser systems based on fibre lasers and dedicated amplifiers will enable the scaling of the average power and pulse energy of USP lasers. The high average power of the InnoSlab amplifier principle, which is now available, will enable the advantages of these systems to be transferred to macro processing. Other system designs, such as disc lasers with very low phase distortions and low nonlinear effects at high fluences, will allow even further power and energy scaling – which will then result in new application developments.
Taking the tools out of processing
The ultimate goal for USP lasers is to use them as a fully flexible digital tool, as an alternative to conventional tool-based processing, with the technology having already proven its advantages across a number of sectors:
- Display industry: cutting flexible substrates of OLED-Displays, the drilling and cutting of glass, and the structuring of thin films;
- Automotive industry: machining hydraulic and fuel injection components;
- Printing industry: engraving cylinders for embossing and printing;
- Medical sector: processing cardiovascular stents;
- Aircraft construction: machining carbon fibre composites;
- Tool manufacturing: generating embossing and injection moulding tools;
- Photovoltaics: structuring conductive and dielectric layers of thin-film solar cells and crystalline Si solar cells; and
- Electronics industry: drilling and cutting printed circuit boards.
In low-energy and low-power traditional laser processing, the amount of deposited energy is determined by linear absorption of the laser energy. In USP laser processing, however, very high photon densities can be generated in the focal spot of a processing optic, which enables nonlinear absorption mechanisms to occur. This means that even transparent materials can be processed at wavelengths where the materials do not show any absorption.
Display technology has therefore become one of the fastest growing fields of application for high-power USP lasers, aided by the progressive market penetration of smartphones and tablet computers. Using nonlinear absorption mechanisms and specially-designed processing principles – such as filament cutting or elongated focus geometries – USP laser energy can be concentrated and deposited into a very thin line in transparent materials. This causes thermal and mechanical stress in the materials, and leads to separation of the parts to be cut.
A structure for print preparation generated using an ultrashort pulse laser. (Image: Fraunhofer ILT)
USP lasers are especially being used for the drilling and cutting of hardened display glasses with hardness depths greater than 40µm. The openings in displays for camera lenses, loudspeakers and microphones, as well as the operating units, are mainly cut this way. In principle, however, the USP laser is also suitable for the format cutting of display glasses, although even higher cutting speeds must be achieved. In the future, the range of applications for USP laser cutting systems could be expanded if sapphire, which cannot be cut using conventional cutting methods, is used as the display glass of devices for weight reasons.
A new field of USP laser processing in the display industry is the separation of polymer-based OLED displays. For smartphones and tablets, OLED displays are manufactured on polymer films over large areas, for productivity reasons. These large multiple-display sheets have to be separated into single small displays without any thermal damage, discoloration or recondensation of the ablated material being caused. Therefore, USP lasers operating in the UV wavelength are used, which provide low absorption depth, as well as low thermal influence of less than 50µm.
USP laser technology is also gaining ground in metal processing, where using pulse durations of 10ps and shorter enables superior processing quality compared to when longer pulses are used.
The technology has a long history of metalworking. One of the first applications explored using USP lasers was the drilling of injection nozzles for diesel injection technology. Even though this application –which requires drilling depths of up to 1mm and bore diameters of 70µm – has not yet established itself, the drilling of injection nozzles using USP lasers for gasoline injection technology has been industrially introduced. Here, the wall thickness is only 0.3mm and the bore diameters range from 70 to 200µm. Compared to conventional eroding processes, higher qualities and processing speeds can be achieved here using USP lasers. New optical set-ups, such as helical drilling optics, can also be used to achieve high-precision bore holes with arbitrary geometries.
In the manufacturing of injection moulding tools for medical products and micro optical components, picosecond and femtosecond lasers are also being used, due to their high precision and the high flexibility of processing they enable, by removing the need for additional tools.
In addition, in the printing, engraving and embossing industry – for example, in security printing technology, printing electronic circuits, and embossing light guide foils and scatter foils for display production – the high precision and resolution of USP lasers is enabling new applications.
New high-power USP laser systems with high repetition rates are now being used to achieve metal ablation rates of more than 20mm³/min, with lateral resolutions below 3µm and depth resolutions below 100nm.
Dr Arnold Gillner is the department manager for ablation and joining at Fraunhofer ILT and managing director of the Fraunhofer Group for Light & Surfaces