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JOURNAL DE PHYSIQUE IVColloque C4, supplkment au Journal de Physique 111, Volume 4, avril1994

Recent developments in new laser systems used for material processing

E. BEYER, V. KRAUSE and F LOOSEN

Fraunhofer-Institut fu asertechnik, Aachen, Germ any

1. Introduction

In the development of high power laser systems for material processing

some general trends can be pointed out. First, there is a trend to developlow cost laser systems by high volume production lots and all by cheapercomponents like turbine blowers, switch-mode power supplies etc. Fur ther-more, new laser concepts e. g. diffusion cooled C02-lasers are able to redu-ce the costs. Another very important trend is that a higher beam power isrequested by the customers. Welding applications which have been donetwo or thr ee years ago with a 5 kW laser system should be done now with10 kW beam power. One reason is that the production speed i s very impor-tant and influences the production costs. Working with higher processingspeeds requires a more stable beam power and position. This leads to clo-sed control loops to control process parameters like beam power. As multi-kilowatt Nd:YAG-laser systems are on the market now, there is a strongtrend to use robot systems with fibers, even if th e C02-laser with similarpower is cheaper th an the Nd:YAG-laser). The whole system including la-ser source, beam guiding and robot (gantry) could be cheaper when using aNd:YAG-laser with a fiber for the beam guiding. In addition to these trendsconcerning C02- and Nd:YAG-lasers, high power diode laser systems arebeing developed a t t he moment. Diode laser systems can be used for pump-ing Nd:YAG-lasers or for direct applications in mate ria l processing. In th e

last case they will be used mainly for hardening, soldering, melting surfa-ces and maybe spot welding in the next years.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1994403

http://www.edpsciences.org/http://dx.doi.org/10.1051/jp4:1994403http://dx.doi.org/10.1051/jp4:1994403http://www.edpsciences.org/

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C4-14 JOUR NA L DE PHY SIQUE IV

| L a s e r D i o d e s [ Nd :YA G | C Q 2Effic iency 30 - 60 % 1 - 3 % 5 - 1 0 %W ave leng th 0.78 - 0.83 um 1.064\im 10 .6 jamPowe r mu lti-kW up to 3 kW up to 20 kWLifetim e 20.000 -10 0.0 00 h 10.000 h ~ 10.000 hM ainten anc e ma inten anc e f ree each 200 h ( lamps) each 500 hPrice /W att 200 - 400 DM /Wa tt 300 - 1000 DM /Wa tt 200 - 400 DM /W attFib er Deliv ery possible possible not possibleVo ltage ~ up to 100 V up to 1000 V up to 10 kVWat t s pe rLasing Volume 1000 W att /cm3 50 Wat t / cm3 1 Wat t / cm3

Table 1 Compa rison of Laser Beam Sources

Table 1 shows a comparison of laser diodes, Nd:YAG- and C02- lasers . Impo rta nt is th e very good overall efficiency of th e diodes . A prob lem at th emoment are the high wat ts per las ing volume. This leads to a heat ing ofthe diodes themselves. Therefore, the most important work which is carriedout at the moment is to increase the cooling efficiency. If the price of thediodes is reduced as we could expect from fig. 1, it will be about $ 10 perwat t beam power in the next years . The change f rom C02 - or Nd:YAG-lasers to diode lasers can be compared with the development of the transistor in the electrical industry and the change from valves in TV or radio tot ransis tor technology.

2 . D i o d e l a s e r s y s t e m s

To generate high power laser diode radiation, there are mainly three diffe

re nt way s possible: f irst stac king of diodes to a closed packed 2-dime nsionalarray, second lens multiplexing and third f iber multiplexing. Fig. 2 and 3demonstrate the different possibil i t ies.

To conduct the diode laser radiation into a fiber, micro-optics have to beused. These micro-optics can be corrected micro-optics with 3-4 elements,grin lenses, spherical optics or asperical optics. The total power containedin the special angle of radiation is demonstrated in f ig. 4. The exampleshows the diode, the numerical aperture of 0.7 and a cylindrical grin lens

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with a diameter of 1.5 mm. The dotted-broken line indicates th at e. g. un-der a n divergence angle of 23' with a cylindrical grin lens only 22% of thetotal beam power is contained. This is also the maximum beam power

which can be focussed into a hundred micrometer fiber diameter. If a hig-her percentage of beam power should be coupled into a fiber of hundredmicrometer diameter, t he micro-lens has to change from quarz to a high in-dex materia l an d the best possibility is to use aspherical optics.

Fig. 5 shows an arrangement of fiber coupling to conduct the radia tion of adiode array into a single fiber. Cylindrical and elyptical micro-lenses areused to paralize th e different beams and t he single beams ar e focussed intothe fiber with a bigger lens.

When using a lot of fibers (fiber bundle) and for each diode array (focussedinto one fiber) a separate power supply, the laser diode offers a totally newpossibility in laser technology. By focussing the laser light coming out ofthe different fibers to the workpiece, it is possible to shape the integralbeam profile by controlling and changing the output power of the differentdiode units. This allows a controlled beam profile adapted to the processwhich opens new possibilities in the case of hardening, remelting, solderingetc. A scheme is demonstrated in fig. 6.

Fig. 7 shows the different laser applications as a function of th e power den-

sity which is necessary and the interaction time for the process. The laserdiode intensity is limited a t th e moment to l o 6 W per cm2 and this meansall applications below this power density can be done with laser diodes infuture.

Another application for laser diodes is the pumped Nd:YAG-laser. The ad-vantage i s higher overall efficiency and, resulting from this, t he heat inputinto the Nd:YAG-crystal is lower and this leads to a better beam quality.However, this good beam quality will be reduced again if the beam guidingfiber is used. This i s demonstrated in fig. 8, where the beam quality a t theentrance into th e fiber and t he ou tput quality of the beam a t the end of thefiber is plotted. With lower fiber diameters higher output beam qual ity can

be reached. I t is indicated th at a s trong bended fiber will reduce the qualityof th e outcoming beam, too. 2 and 2,5 kW Nd:YAG-lasers with a beam qua-lity allowing a deep welding process are on the market now.

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C4-16 JOURNAL DE PHYSIQUE IV

Fig. 9 demonstrates a 3 kW Nd:YAG-laser system which is comerciallyavailable and i s built up by six 500 W systems which ar e pulsed. By combi-ning the six laser heads with two fast beam multiplexers it becomes possi-

ble to bring the radiation of beams into one fiber, one after another , so thatat least 3 kW continuously come out of the fiber. The effect is th at the 3 kWsystem has a beam quality of a 500 W Nd:YAG-laser system which is defi-nitely higher th an t he quality of a conventional 2 kW system.

The costs and th e volume of C02-lasers have dramatically been reduced i nth e la st years. In the las t five years there was a reduction of costs of morethan a factor 2. Fig. 10 shows the laser volume per kilowatt output poweras a function of th e years . The development of diffusion cooled lase r sy-stems will be responsible for a reduction of volume and costs which will behigher than indicated in fig. 10.

Fig. 11 demonstrates th at in principle two possibilities ar e being developed

a t t he moment. These are a nul ar and slab geometries. Therfore, th e beamhas to be guided between the two plates or the two tubes which are theelectrodes.

The first commercially available diffusion cooled laser system in the kilo-watt range was presented by Rofin Sinar at the Laser Conference 1993.Fig. 1 2 shows a cost comparison of a n axial-flow DC-excited C02-laser in

the kilowatt range and a diffusion cooled RF-excited laser. The size of thediffusion cooled laser can be approximately th e half of the axial-flow DC-excited laser. The costs can be approximately 30% lower including the moreexpensive RF power supply.

The u se of switch-mode power supplies for DC-excited transverse flow sy-stems can reduce the price of the systems by approximately 20% to 30%and , in addition to that , leads to the possibility to pulse the systems withfor example 2 or 3 kHz.

Fig. 1 3 shows a totally controlled laser system. In a lot of applications likecutting, welding and drilling the process can be controlled by adapting th ebeam power in th e range of a 10 th of a millisecond. This can be easily doneby Nd:YAG-lasers and laser diodes but i t is more difficult when using C 02 -

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laser s. Therefore, a RF-excited COZ-laser ha s a very big advantage compa-

red with DC-excited lase r systems due to the possibility to adapt the outpu tpower to the interaction phenomena of the process in a very short time.

5. Conclusion

The direct application of high power laser diodes will find a market in the

field of surface treatment in the near future. Diode pumped Nd:YAG-laserswill find a market if the price for laser diodes decreases. The advantagesar e higher efficiency and bette r beam quality of th e Nd:YAG-laser. We can

expect that the market for C02-la sers will develop to smaller and cheapersystems with higher beam power, which offers the possibility for a totally

controlled process.

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JOURNAL DE PHYSIQUE IV

84 86 88 90 92 94

Year

84 86 88 90 92 94

Year- - -- .Fig. 1 Laser D iode Power and P rice Development

1 . Stacking to a c los e packed2-dimensional array

2. L ens multiplexing

3. Fiber multiplexing

1 . Stacking to a clos e packed2-dimension al array

heat sinks

laserdiodebars

liquid

2. Lens multiplexing

collimationcoupling

opticsoptics

t

diodes

3. Fiber m ultiplexing

coupling

laserdiodes

direct coupling

Fig. 2 Laser Diode Com bination Fig. 3 Laser Diode Combination

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JOURNAL DE PHYSIQUE IV

-hG Patent

Fig. 6 Diode Arran gem ent to Control the Beam Profile (Fh GP aten t)

10

. . . .. ..%

Interaction time I s

Fig. 7 Laser Materials Processing

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Nd:YAG - Laser

- traight Fiber-

10.' 10

Fig. 8 Fiber Transmission-Beam Quality

Fig. 9 Combination of Six Laser Heads wi th Tw o Fast Beam Multiplexers

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JOURNAL DE PHYSIQUE IV

1 I I

l o 2 g-

VIPL - -[I/kWl E

l o 0 n-

0i

-0

1 o - ~ I I I1960 1980 2000

Year

Fig. 0 Development o f laser power and laser volume

Fig. I I Comparison: Anular-Slab Geometry

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100% 70%

22% -Gas Circulation

19% 7%

19% - ower Supply - 35%27% -Mechanics,Optics

15%

13% -Control & supply 13%

Axial-Flow Diffusion-CooledDC-Excitation RF-Excitation

C0,-Laser: P, = 2 kW(Lot size apr. 50)

Fig. 12 Cost Comparison

laser beam bsrm beammaw workplecesource handling amp(ng Interas(lon bndllng

J J II PROCESS COMMANDS SAFETY PRECAUTIONS

Fig. 13 On-line Control in Laser Beam Machining