Direct diode lasers and their advantages for materials processing and other applications

Direct diode lasers and their advantages for materials processing and other applications Haro Fritsche a *, Fabio Ferrario a, Ralf Koch a, Bastian Krusche a, Ulrich Pahl a, Silke Pflueger b Andreas Grohe a and Wolfgang Gries a Florian Eibl c, Stefanie Kohl d, Michael Dobler d a DirectPhotonics Industries, Max-Planck-Str. 3, 12489 Berlin, Germany b DirectPhotonics Inc., 18035 Redwood Drive, Los Gatos, CA 95033, USA c Fraunhofer-Institute for Laser Technology ILT, Steinbachstr. 15, 52074 Aachen, Germany d Institute of Photonic Technologies (LPT), Friedrich-Alexander-Universität Erlangen-Nürnberg, Konrad-Zuse-Str. 3/5, 90152 Erlangen, Germany ABSTRACT The brightness of diode lasers is improving continuously and has recently started to approach the level of some solid state lasers. The main technology drivers over the last decade were improvements of the diode laser output power and divergence, enhanced optical stacking techniques and system design, and most recently dense spectral combining. Power densities at the work piece exceed 1 MW/cm 2 with commercially available industrial focus optics. These power densities are sufficient for cutting and welding as well as ablation. Single emitter based diode laser systems further offer the advantage of fast current modulation due their lower drive current compared to diode bars. Direct diode lasers may not be able to compete with other technologies as fiber or CO 2 -lasers in terms of maximum power or beam quality. But diode lasers offer a range of features that are not possible to implement in a classical laser. We present an overview of those features that will make the direct diode laser a very valuable addition in the near future, especially for the materials processing market. As the brightness of diode lasers is constantly improving, BPP of less than 5mm*mrad have been reported with multikw output power. Especially single emitter-based diode lasers further offer the advantage of very fast current modulation due to their low drive current and therefore low drive voltage. State of the art diode drivers are already demonstrated with pulse durations of <10µs and repetition rates can be adjusted continuously from several khz up to cw mode while addressing power levels from 0-100%. By combining trigger signals with analog modulations nearly any kind of pulse form can be realized. Diode lasers also offer a wide, adaptable range of wavelengths, and wavelength stabilization. We report a line width of less than 0.1nm while the wavelength stability is in the range of MHz which is comparable to solid state lasers. In terms of applications, especially our (broad) wavelength combining technology for power scaling opens the window to new processes of cutting or welding and process control. Fast power modulation through direct current control allows pulses of several microseconds with hundreds of watts average power. Spot sizes of less than 100 µm are obtained at the work piece. Such a diode system allows materials processing with a pulse parameter range that is hardly addressed by any other laser system. High productivity material ablation with cost effective lasers is enabled. The wide variety of wavelengths, high brightness, fast power modulation and high efficiency of diode lasers results in a strong pull of existing markets, but also spurs the development of a wide variety of new applications. Keywords: High power diode laser, high brightness diode laser, short pulses, development of diode lasers, application of diode lasers, cutting, welding, brazing, surface treatment, ablation * contact: Haro.Fritsche@Directphotonics.com 1

1. BACKGROUND High power diode lasers find an increasing number of applications in materials processing and pumping of solid state lasers as their brightness increases. Beyond improvements in the design of the diodes themselves - for minimum slow axis divergence, highest power from a given size aperture and improved wall plug efficiency - optical and spectral stacking are deployed to scale power and brightness. Diode lasers are mainly used for pumping solid state lasers and serve the applications of plastic welding, brazing, surface treatment, cladding and heat conduction welding, to name the major markets. Keyhole welding has been barely accessible for diode systems due to the limited power and beam quality, and cutting remained out of reach for commercially available direct diode lasers. With improvements of the beam quality to the range of 2.5 to 7.5 mm*mrad and kilowatts of output power those new laser can now address these sophisticated applications. Keyhole welding requires a beam quality in the range of 10 to 20 mm*mrad with power levels in the range from 1 kw up to 6 kw and more. Cutting requires a beam quality of less than 10 mm*mrad and power levels in the range of 2 kw to 6 kw, specifically about 5 mm*mrad and 2 to 3 kw for thin gauge cutting up to several millimeters, and about 7 mm*mrad for thicker gauge materials. Key markets and their requirements for beam quality and power are summarized in Figure 1. Also shown is the performance limit of representative industrial direct diode laser systems and their development over the last two decades. The lines indicate the best combination of beam quality and output power available at the specific time. Markets below the lines and past the end points of the lines were not accessible at the given time. 1,000" Hardening) BPP#[mm*mrad]# 100" 10" 2013+ Medical) Devices)&) ) )Consumer) Product) Brazing) (Keyhole) Welding) ) ) Cutting) Cladding) 1" 10" 100" 1,000" 10,000" Power#[W]# Figure 1: Performance parameter beam parameter product and output power for the main markets and Diode lasers are about 50% efficient resulting in compact, mobile systems with 40% wall plug efficiency (WPE). CO 2 lasers typically show 10% WPE and are commonly used in 2D cutting machines due to their low cost. Disk lasers have about 25% WPE and fiber lasers 30% WPE. Both laser types are fiber delivered and commonly used in high-speed 2D cutting machines and welding applications. Together with direct power modulation new applications can be addressed. Multiple processes can be served in one system, such as cleaning, cladding and protection of weld seams. Hand guided tools complement the mobile systems enabling laser processing of large complex structures in the field. New developments also include localized processing of larger areas with an ensemble of diode lasers equipped with fast electronic switching and replace scanning systems, for example in marking. 2. ADVANCEMENT OF DIRECT DIODE LASERS Many applications did not only demand higher output power, but also higher beam quality. Especially the pumping of fiber lasers spurred new system designs requiring bright diode lasers to operate at a single stable wavelength and to be cooled either by conduction or with clean tap water. Both single emitters and mini bars represent viable approaches. Single emitters further eliminate the need for optics symmetrizing the beam of the diode lasers prior to launching into the fiber. The aperture size is chosen to fit the beam quality of the fiber. Multiple emitters are stacked in the orthogonal axis 2

until a symmetric beam quality is achieved in both axes, thus maximizing the power launched into the fiber. A volume Bragg grating in the output beam stabilizes the wavelength for pumping applications and enables dense spectral combining for power scaling at constant beam quality. Figure 3: Multiple single emitter module optically stacking 8 diodes in fast axis for launching into a 100 µm fiber, 0.15 NA. The module delivers > 80 W of optical power.. The DirectPhotonics design evolves around a monolithic slow axis collimator (SAC) array that collimates the individual single emitters and simultaneously stacks them on top of each other. Therefore the single emitters are mounted to a monolithic staircase-like heatsink in a reflow process. The precision machined SAC is passively aligned and mounted to the common heatsink. Each single emitter is subsequently collimated in fast axis with a pointing error of less than 0.1 mrad in both axes (Figure 1). This design allows full collimation in both axes with a minimum pointing error through only one active alignment step minimizing manufacturing costs. The FAC alignment is automated ensuring consistent, high quality production. The output beam of such a module consists of a stack of beams of the individual diodes in fast axis. The number of stacked emitters is determined by the beam quality required for the system. Typically eight diodes are stacked to couple into a 100 µm fiber with 0.15 NA. The optical efficiency for collimation and stacking is more than 95% and the far field is inscribed into the accepting aperture of the fiber maximizing fiber coupling efficiency, which is typically 90% for uncoated fibers. 140 W of output power is measured from a device with a 100 µm fiber with 0.2 NA. Polarization multiplexing and dense spectral combining are subsequently deployed for power scaling. 500W are available from a 100 µm fiber with 0.15 NA [4]. Dense spectral combining enables even further power scaling and 2 kw from a 100 µm fiber, 0.1 NA are demonstrated [5], but with broad linewidth. Minibar based devices deliver 600 W from a 200 µm fiber with 0.2 NA at a single wavelength and 4 kw from a 400 µm fiber. 0.12 NA with multiple wavelengths [6]. Fig. 4 Beam profile of far field (left) and near field (right) of an individual channel resulting in 4.2 mm*mrad beam parameter product in free space 3

Dense spectral combining enables beam qualities of better than 8 mm*mrad, but the power is currently limited to 2 kw. Fig. 5 spectrum of free running coarse 500 W-Building-Block compared with VBG stabilized Building Block. 3. APPLICATIONS Cutting and Welding Heat conduction welding with diode lasers was established over the last 8 years for seam welding of furniture, appliances and tubes, to name the major applications. Keyhole welding became possible, once multiple kilowatts could be delivered through a fiber of less than 300 µm. Thin mild steel can be welded with several meters per minute (Figure 5) [8] and especially aluminum welding benefits from the short wavelength of the diodes in the range of 900 nm and is now established in the automotive body-in-white production. Cutting is currently limited to thin gauge material, preferably mild and stainless steel. As seen in Fig. 5 the broad spectrum of a DirectProcess 900 500W building block as unstabilized and wavelength stabilized version. In the unstabilized version the effective band width is about 80 nm (and double with higher power) which will result in a higher depth of focus for smoother cutting edge and also for better power absorption in materials like aluminum where the absorption is not very high around 1 µm. Through the broad band width the absorption is spread up over the whole wavelength range and the energy deposition is advantageous and speed up weldings which allows to replace point weldings by welding lines even in automated manufacturings Selective Laser Melting Selevtive Laser Melting (SLM) is a relatively new technology where fine metal powder is melted line by line with moderate powers (some hundred Watts) to generate 3D printed structures with resolutions in the range of few hundred microns The general approach is to scan the surface with a single spot and solidify the powder line by line which results in very long process times. Fraunhofer ILT designed a new system based on more cost-efficient diode lasers featuring an array of multiple spots for massive parallel melting of structures which will reduce the build time significantly, even for complex structures. Diode lasers are a benefit here because fibers can deliver the laser light directly to the process without the risk of destroying crucial parts of the lasers due to the fact that the transport fibers of diode lasers are cheap to replace. In addition the remote control of diode lasers is easy to synchronize with the process head s movement and power control is possible from 0-100% within very short times. 4

Fig. 6 principle of standard SLM (left) and new Fraunhofer ILT array exposure concept with an array of spots. Laser Pumping One of the versatile benefits of the DirectProcess Family is the possibility of selectable wavelength, which makes the very same system a good choice for material processing as mentioned above, but also for use as a pumping module for laser pumping. Fig. 7 left: spectrum of Ytterbium[16], with its broad absorption spectrum > 900 nm, right: very narrow absorption lines or Er:YAG [17], both spectrums are addressable with the DirectDiode Family without change of Design (middle). Ytterbium is well known for being pumped at 976 nm, as there is a very strong absorption line, but pumping at lower wavelength is possible. Especially suitable for the DirectProcess family as it has some emission lines which will not change very much by current fluctuation, as the current in the system is very low compared to most diode laser systems. But also special pumping applications can be addressed with diode laser systems as the resonant pumping process of Er:YAG where up to 7 lines can be pumped simultaneously. Those absorption lines can currently only be addressed by VBG stabilized diode lasers, which will increase the output power of the Er:YAG systems directly without post amplification. 5

4. CONCLUSION Diode laser show 40% electro-optical efficiency, which is the highest among all laser systems and leads to very compact systems. Diode lasers also show exceptional reliability of 30,000 hours to 100,000 hours. Together with the straightforward system design diode lasers have the lowest cost of ownership among all industrial lasers. Low cost, reliability and compact size lead to a strong market pull spurring a wide variety of applications. Diode lasers today mainly serve niche applications and enable new applications, such as mobile laser systems. As their beam quality recently increased, diode laser experience a strong pull from the global welding and cutting market. DirectDiode laser systems will address a wide varsity market from classical material processing over new materials especially when mixing uncommon wavelength up new laser pumping applications. The Beam quality of those systems began to reach a quality to compete with classical laser applications but also to fill a gap between the fiber lasers and the CO 2 lasers. 5. REFERENCES 1. Volodin, B.L. et al., Volume Bragg Gratings (TM) A New Platform Technology for WDM Applications. White paper available at www.pd-ld.com/pdf/vbg PAPER.pdf. Copyright PD-LD (2003). 2. Robin K. Huang*, Bien Chann, John D. Glenn, Ultra-high brightness, wavelength-stabilized, kw-class fiber coupled diode laser, Proceedings Photonics West (2011). 3. Krause, V. Koester, A., Koenig, H., Strauss, U., Brilliant high-power diode lasers based on broad area lasers, Proceedings Photonics West (2008). 4. Heinemann, S., Fritsche, H., Kruschke, B., Schmidt, T., Gries, W., Compact High Brightness Diode Laser Emitting 500W from a 100 µm Fiber, Proceedings Photonics West (2012). 5. Robin K. Huang*, Chann, B., Burgess, J., Kaiman, M., Overman, R., Glen, J., Tayebati, P., Direct diode laser with comparable beam quality to fiber, CO 2, and solid state lasers, Proceedings Photonics West (2012). 6. Koehler, B., Segref, A., Wolf, P., Unger, A., Kissel, H., Biesenbach, J., Multi-kW high brightness fiber coupled diode laser, Proceedings Photonics West (2012). 7. Friedmann, P., Gilly, J., Schleife, J., Giesin, C., Mortitz, S., Herbstritt, M., Fatscher, M., Kelemen, T., High efficiency frequency stabilized tapered amplifiers with improved brightness, Proc. SPIE 7918 (February 21, 2011) 8. Holzer, M.. Metger, J., Diodenlaser fuer Industrielle Anwendungen, Diode Laser Workshop, Dresden (March 2012) 9. Arthur, J., Laser Applications for Maintenance and Sustainment Solutions, Air Force Material Command, https://tdksc.ksc.nasa.gov/servlet/dm.web.fetch/teermarthurlaserapplications.pdf?gid=102457 10. Zediker, M., company website, http://www.foroenergy.com/applications#tab-applications-drilling 11. Nowotny, S., Brueckner, F., Hillig, H., Leyens C., Beyer, E., Hochleistungs-Laser-Auftragsschweissen durch Energiequellenkombination, Diode Laser Workshop, Dresden (March 2012) 12. Herfurth, H., Multibeam LAM for Efficient Part Repair and Manufacture, CTMA Symposium (March 2012) 13. Krause, V., company website, http://laserline.de/download-english/laserline_afpt_casestudy.pdf 14. Meier, O., Mobile Materialbearbeitung mit Hochleistungs-Diodenlasern, Diode Laser Workshop, Dresden (March 2012) 15. Jensen, S., company website, http://www.visotekinc.com/diode-laser-products-a-services/laser-processingtools/metalpass 16. Xu, J.,Tu, C., Wang, Y., He, J., Multi-wavelength continuous-wave laser operation of Yb:Ca 3 Gd 2 (BO 3 ) 4 dirordered crystal, Optical Material, Volume 33, 11( Sept. 2011), P1766-1769 17. Fritsche, H., Lux, O., Schuett, C., Heinemann, S., Dziedzina, M., Gries, W., Eichler, H.J., "Increased efficiency of Er:YAG lasers at 1645 nm using narrow bandwidth diode lasers and dual-wavelength resonant pumping", Proceedings of SPIE Vol. 8959, 895907 (2014) 6