Frank Becker, Corinna Brettschneider, Peter Kallage and Wolfram Rath

Fiber Lasers A universal tool for industrial production Frank Becker, Corinna Brettschneider, Peter Kallage and Wolfram Rath Fiber coupled diode-laser pump modules Fiber lasers have already firmly established themselves within the industrial production sector and are synonymous with efficiency, precision and cost-effectiveness in a large number of laser material processing applications. They offer excellent single-mode or multi-mode beam qualities, which can be precisely adapted to the processing task and therefore represent a universal tool for industrial production. On the basis of selected application examples, the technology and the manifold application possibilities of the Rofin fiber lasers in the power range from 0.5 to 6 kw are reviewed. Modern efficient production would not be imaginable without lasers and laser processing. Continuous development of new applications and improved application performance on one hand and ongoing cost of ownership reduction of laser sources and systems on the other hand established the basis for a continuous grow of laser based technologies. The processing market of continuous cutting, welding and surface treatment, referred here as Macro applications, represent a large part of the total laser market for material processing. Within this market segment CO 2 and HR-FBG Output Fiber Yb-doped double-clad LMA fiber LR Pump Coupler LR-FBG Fig. 2 Fiber laser oscillator scheme, including twelve pumps and 6 1 pump coupler including signal feed through. adsorption cross section [pm 2 ] 2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25 0.00 absortion cross section emission cross section 800 850 900 950 1000 1050 1100 wavelength [nm] Fig. 1 Absorption and emission cross sections of a typical ytterbium-doped germanosilicate glass, as used for Yb-doped laser active fibers. fiber lasers are the dominating laser sources because of their high power, excellent beam quality, reliability and cost efficiency. The largest market segment is laser cutting of metals but also of wood, plastics, textiles and compounds. The second main application is laser welding. Due to keyhole formation, laser welding is highly efficient resulting in high welding speeds and low heat load on the welded part which results in low distortion. Examples are laser welding of powertrain components, injection systems, housings of sensors and airbag inflators but also parts of car bodies, tailored blanks, tubes and profiles from 0.1 mm to 15 mm weld penetration. These applications are driven by the well-defined, highly localised huge power density that is obtained by a small focus from lasers of medium and high beam quality. Fiber laser technology Fiber lasers are the latest technology of diode-pumped, solid-state lasers. With high wall plug efficiencies, beam delivery by a flexible fiber and application adapted beam qualities they captured a fast rising market share within the last five years. They profit from the development of large mode area, rear-earth doped double clad fibers of large cladding diameters, fiber integrated pump couplers and high-power laser diode pumps. These techniques enables the design of high-power fiber laser modules realized as integrated fiber design which makes the modules simple, robust and efficient, ideal for the use in lasers for industrial production. Power scaling is obtained by combining these modules by all-fiber combiners with high beam quality multi-mode output. Basic setup The laser active fibers allow the propagation of the pump power of poor beam quality within the first cladding. The inexpensive diode laser radiation is pumping the active material (Yb 3+ ) of the fiber core generating the fiber laser signal. The technique is associated with two principle benefits: Cooling becomes easy and efficient due to the high surface to volume ratio of a thin, long fiber and the generated laser power is conducted within the fiber core. The beam quality is predefined directly by the design of the fiber and therefore largely independent of the 34 Laser Technik Journal 1/2014 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Fiber Lasers output power. In this way it is easy to generate laser radiation with diffraction limited beam quality. The emitted beam is statistically polarized which ensures directional independent material processing. Physics Due to the design, diode-pumped Ybdoped fiber laser systems are characterized by low loss, low quantum defect (illustrated in Fig. 1), high amplification per cavity pass as well as high efficiency and low lasing threshold. Diode pumps The pump radiation of the inexpensive highly efficient diode lasers can be coupled into the laser-active fibers in different ways. To retain the advantages of an adjustment-free fiber-integrated design, fiber-coupled diode lasers have established themselves as standard for industrial fiber laser systems over the past decade. One distinguishes between fiber-coupled single emitters with low output power, multi-emitter modules with moderate output power as well as multi-bar systems with high output power. The effort for the pump module integration considerably depends on the number of required pump modules. Diode modules with high output power offer advantages here, however not only the pump output but also the brilliance of the pump source is decisive. By using tailored diode emitters, optimized for the necessary fiber coupling and an automated, inexpensive module manufacture, both requirements for a fiber laser pump source can be united, namely inexpensive diode laser modules whose fiber interface is, in addition, exactly matched to the fiber laser. Besides, due to their design these passive cooled pump modules have the same properties as fiber-coupled single emitters. With the help of innovative systems for pump fibers it is possible to couple highly efficient and cladding-free pump radiation with several hundred watt output into a fiber. Integrated design The concept of an integrated fiber laser design is completed by two additional techniques: Resonator mirrors and pump couplers Fiber integrated resonator mirrors are realized by diffractive structures that are scribed in the fiber core (Fiber Bragg Gratings, FBGs) which are used as high reflection mirror and partly transmissible out coupling mirror. Fiber-coupled diode lasers in combination with fiber integrated pump combiners have proven feasible in numerous fiber laser designs and retain the advantages of an adjustment-free fiber-integrated design. Driven by this development, fiber coupled diode lasers were established as standard for pumping of industrial fiber laser systems over the past decade. With a common pump combiner designed as fused tapered fiber bundle only simple, end-pumped systems can be set up which are subject to strong restrictions with regard to performance and scalability. To achieve a homogeneous, low inversion distribution within the active fiber with simultaneous high absorption and therewith conversion efficiency, a pump combiner with signal feed through (so-called pump coupler ) is necessary. Only the use of a pump coupler where the signal feed through provides lowest losses of signal power and beam quality with simultaneous very high pump light transmission of more than 98 %, allows the unique design of Rofin s high-power laser setup. Realization Using the above described individual components, a complete fiber-integrated laser is constructed, which can be pumped in both co and counter direction resulting in a dual side pumped fiber laser oscillator. The fiber laser series is based on such high efficient dual side pumped oscillator designs (Fig. 2). The design offers up to six pump ports per cavity side for fiber coupled diode modules with a 200 µm fiber core (NA 0.22). Actual products consist of these modules delivering 1.5 kw nominal output power at a beam quality BPP < 0.4 mm mrad at electrical optical efficiencies of app. 30 %. Lasers with linear polarized output can be set up by using a Yb-doped polarization maintaining large mode area (PLMA) fibers and suitable fiber Bragg gratings (FBG). 1 kw of linear polarized 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Laser Technik Journal 1/2014 35

Company ROFIN Hamburg, Germany Fig. 3 Comparison of cutting speeds using 50 µm and 100 µm fiber at 3 kw. speed [m/min] power could be generated at electrical optical efficiencies of app. 30 % comparable to statistical polarized lasers. Power scaling Power scaling of fiber lasers can be achieved by two different methods: scaling the power of the oscillator or several oscillators to a single fiber output. Both is realized with the company s fiber lasers. Starting deliveries in 2009 with 1 kw single-mode output power from a single fiber laser module power was scaled to 1.5 kw output power and delivered end of 2013. With more than 35 years of experience, ROFIN is a leading developer and manufacturer of lasers and laser-based technologies for industrial material processing applications. The company focuses on developing key innovative technologies and advanced production methods for a wide variety of industrial applications. In terms of fiber lasers, ROFIN is equipped with in-depth technology know-how and components due to specialized group members like Dilas, Corelase, Nufern, Optoskand, m2k Laser and PMB. Components like diodes, fibers and fiber beam deliveries, power supplies as well as high-power fiber optics are developed and manufactured by these specialists within the ROFIN group leading to high quality fiber lasers for industrial production. www.rofin.com 80 70 60 50 40 FL 030 cut stainless by N2 100 µm FL 030 cut stainless by N2 50 µm 30 20 10 0 0 2 4 6 8 10 12 thickness/penetration [mm] High-power all fiber combiner All fiber combiners allow a scaling of the fiber laser power by incoherent summation of single-mode fiber laser modules. Minimum losses and high multi-mode beam quality is obtained. Two, three and four single-mode input sources are combined all in fiber to up to 6 kw output. Output fibers of either 50 µm or 100 µm sizes are available to deliver the corresponding beam qualities. Fiber lasers for industrial applications Based on the components described above a series of industrial fiber lasers was launched by Rofin in 2009 and developed further to a complete laser family with stabilized output power from 500 W to 6000 W using two different cabinet concepts. The compact version with a small cabinet is ideally suited for easy integration into existing systems and the standard version as standalone solution offers a separated enclosure for beam management. Up to four fibers can be used with time or energy sharing including two channel interlocks per output and a multi-station fieldbus interface. Both versions have common integral features that help customers to realize their individual solutions: Fiber size and beam quality Direct laser with single-mode or multimode beam quality are using 20 µm, 50 µm or 100 µm output fibers. Larger fiber sizes can be adapted by either fiberfiber couplers or fiber-fiber switches enabling the user to adapt the beam quality to the process requirements between spot sizes of 20 µm up to several mm for surface treatment applications. Power control Power sensing and fast feedback control is an important feature to ensure reliable processing results. The output power is controlled internally even if the laser is used in 5 khz pulse mode. Back reflection robustness Reflected power from work pieces occurs with almost all processes. Power dissipation within the beam delivery system and sensors at different positions of the modular design protect the components from damage. Due to the high attenuation of reflected power and smart signal analysis, processing of brass, aluminum and copper is achieved reliably. Safety The fiber laser series is built for use with industrial laser systems which belong to the ISO machine safety standard EN/ISO 13849. All safety relevant components of the laser are certified by independent external institutions. Outstanding attention was taken to the two channel safety interlock of the power supplies. Opening the interlock shuts down the laser within 10 ms. Recovery of operation is achieved after 200 ms. Industrial interface The interface to systems is realized with the well proven structure of Rofin s macro lasers. Digital selectable pulse or ramping programs controlled by analogue or digital inputs help the user to realize the required power control for any cutting or welding application. Connected by either Digital I/O signals or fieldbus communication for up to four workstations the fiber laser series is easy to integrate into material processing systems. The design architecture enable observation, operation and remote control from different devices in a network. Possible devices are system controls, industrial handhelds or PCs. Scanner control An increasing number of industrial applications make use of fast and flexible beam deflection systems. These scanner systems can jump within a minimum time interval from one welding contour to the next which increases the productivity of a laser processing system enormously. The integrated scanner solution allows programming of scanner contours including laser processing parameters as power, processing speed and wobbling contours. System control is performed by the standard interface. Industrial applications Various industrial laser applications have been developed to standard processing techniques for production. 2D cutting of sheet metal and 3D cutting of tubes and profiles as well as transformed metal sheets by 5 axis systems or robots and trimming of hydro formed 36 Laser Technik Journal 1/2014 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Fiber Lasers Fig. 4 Stack of laser cut electric steel. The high accuracy of the individual cut sheets can be seen clearly. (Source: Stiefelmayer) profiles are some examples of laser cutting technologies, which correspond in total to app. 70 % of the macro laser market. Maximum sheet thicknesses or reachable cutting speeds are increasing as higher laser power levels are available. Laser beam welding is the second important market for industrial laser applications. Fiber lasers are universal welding tools for a large variety of applications starting from narrow welds of very thin battery foils up to heavy welds of construction elements. Even if brazing and surface processing are applications using low beam qualities but homogeneous intensity profiles fiber lasers can be adjusted to the requirements due to their high beam quality by choosing the right optical components to form the beam profile. High surface absorptivity at wavelengths around 1 µm and large process fiber sizes with top hat beam profiles allow the efficient use of fiber lasers for these processing techniques. The flexibility of fiber lasers is shown by the examples given in the next sections. They are arranged by beam quality for optimal performance. Single-mode applications With the introduction of high-power fiber lasers, fiber delivered lasers with fundamental transversal single-mode at high power were introduced into the market for the first time. The single-mode lasers are equipped with a 20 µm fiber of 8 m length delivering a beam quality of < 0.4 mm mrad up to 1.5 kw power. Spot sizes of less than 50 µm obtained with single-mode beam quality are well suited for applications like cutting of thin metal foils at very high speed, built up of fine structures by selective laser melting or remote cutting of metals and carbon fiber reinforced plastics. Fig. 5 Copper tube cut by 2 kw fiber laser. Multi-mode lasers of high beam quality The use of 50 µm or 100 µm fibers delivers beam qualities around 2 mm mrad or 4 mm mrad to the work piece. These beam qualities are used for cutting and for welding applications if small spot sizes or a long working distance (Scanner Welding) are required. As a result, a working distance of 0.6 to 1.5 m is possible. This gives a high robustness against spatter coming out of the welding process due to a higher live time of protection glasses coming along with less downtime for maintenance of the process head. Whenever material of thickness below 3 mm is to be cut the better beam quality can be transformed into higher cutting speeds (Fig. 3). Typical examples for cutting with high beam quality are the cutting of electrical steel for prototyping transformers or motors. Systems with high accuracy and highest acceleration are used to cut these parts economically. Fig. 7 Weld cross section of a fiber laser welded gear. Fig. 6 Welding of air conditioning cooling pipes (Aluminum with filler 100 µm fiber, (Source: TI Automotive). An example of precise cutting of electrical steel using a linear motor driven fiber laser cutting system is shown in Fig. 4. Due to the high acceleration of highspeed cuttings systems fast adaptations of laser parameters with varying cutting speed is required. This is achieved by transferring the actual cutting speed to the laser by an analog signal, which is transformed to adapted laser parameters in real time. Cutting of highly reflective materials Due to the wavelength and the robust laser design aluminum, copper and brass can be cut by fiber lasers reliably. Fig. 5 shows an example of a 2 mm copper tube cut by 2 kw fiber laser. The better absorption of the short wavelength provides higher cutting speeds for these highly reflective materials. Universal cutting systems are equipped with 100 µm fibers, which is a good compromise for cutting materials of a large range of thicknesses. Not only cutting but also welding is performed with these high beam qualities. Examples are the welding of injection nozzles at medium power levels and the welding of highly reflective materials of high thermal conductivity like aluminum or copper (Fig. 6). Heat sensitive parts are a working field for small spot sizes, too. Connecting electronic parts makes it necessary to keep the heat affected zone as small as possible, which is achieved by high welding speed and low energy input per unit length. The same technique minimizes the distortion. Applications for medium beam quality Medium beam qualities using 200 µm fibers are used for general welding applications i. e. in powertrain. A high penetration and low weld seam width 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Laser Technik Journal 1/2014 37

lead to almost now distortion which is an important point for gear box parts. Fig. 7 shows an example of a gear welded by a 3 kw fiber laser at 5 mm penetration. This beam quality is also able to be used with a 3D Scanner for scanning on-the-fly applications. The high positioning speed of the scanner when welding several seams in its working range keeps the on time of the laser high and increases the total production rate. Welding and surface processing at low beam quality 600 µm fibers are the general choice for welding to compensate tolerances of the work piece. Body-in-white components in the automotive industry are the typical area in which this high fiber diameter makes sense. Examples are overlap welding of galvanized steel for hang-on parts, doors, windows etc. The necessary gap between the sheets can easily be bridged by the relatively big melt pool. In the same way cosmetic weld seam can be done using the relatively low intensity of the laser in spot sizes realized with bigger fibers. Brazing can also be done using 600 µm or 800 µm fiber imaging it to a large spot size. Due to the flat top hat intensity profile efficient application results are obtained. The joint quality is very high and a rework of the seam surface is not necessary before painting the part, like it can be seen on many car roofs. Due to its low energy input the corrosion protection on the back side of the parts still is in good order. The zinc coating is typically remolten but not burned. An additional process step to protect the welding area is avoided this way. Summary Today, fiber lasers are universal processing tools for industrial production. They show a wider range of applicability than all other lasers before because of their wide range of power and beam qualities and due to their flexible beam switching, beam splitting and scanning possibilities. Furthermore, their efficiency, the wavelength and minimum maintenance requirements allow highly economical processes in a large variety of industries. On basis of fiber intergrated fiber-laser oscillators and -combiners Rofin s FL Series covers a power range from 0.5 to 6 kw for industrial laser processing. DOI: 10.1002/latj.201400021 Authors Frank Becker studied physical technologies at the University of Applied Science and Arts of Göttingen, Germany. Afterwards he worked for the Physikalisch Technische Bundesanstalt and for LIMO GmbH. He joined Rofin-Sinar Laser in 2002 and was involved in the R&D of diode and solid-state lasers. In 2007 he was appointed to project manager R&D for the fiber laser development group of Rofin-Sinar Laser, Hamburg. Corinna Brettschneider studied physical engineering at the University of Applied Sciences in Wedel, Germany, and joined Rofin for her diploma thesis in 1996. Since 2007, she is responsible for public relations and marketing. Peter Kallage studied mechanical engineering at the University of Hanover, Germany, and took his first job in 2005 as a researcher at the Hanover Laser Center, specializing in the field of joining technology. In 2007, he was promoted to Division Manager and became head of the Metal joining and cutting Group. Since October 2011, Peter Kallage is responsible for the applications lab at the headquarters of the Rofin Macro Division. Wolfram Rath studied Physics at Universities Heidelberg and Erlangen-Nürnberg with the degree of Dr. rer. nat. He was working with Siemens in CO 2 Laser and Excimer Laser development and applications before he became responsible for Rofin-Sinar s applications lab in Hamburg for Macro applications. Since 2011 he is responsible for the Product Management for all laser sources in the Rofin Macro. Corinna Brettschneider, ROFIN-SINAR Laser GmbH, Berzeliusstr. 87, 22113 Hamburg, Germany, Tel: +49-40-73363-4380, Fax: +49-40-73363-4138, E-mail: c.brettschneider@rofin-ham.de 38 Laser Technik Journal 1/2014 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim