Diode laser modules of highest brilliance for materials processing

Alexander Knitsch*a, Axel Luftb, Tobias Großc, Detlev Ristauc, Peter Loosena, Reinhart Poprawea

aFraunhofer Institute for Laser Technology, Aachen, GermanybThyssenKrupp Nothelfer GmbH, Lockweiler, Germany

cLaserzentrum Hannover e.V., Hannover, Germany

ABSTRACT

Beam quality and output power of mostly 2-dimensional stacked diode laser systems are insufficient for the demands ofmaterials processing. To increase the output power at almost constant beam-quality, superimposition of diode laser barsof different wavelengths as well as polarization-multiplexing of s- and p-polarized laser beams is possible. Differenttechniques for wavelength-multiplexing have been developed. The so-called multi-filter concept of a spanned coatedetalon with edge-filters has turned out best. The concept features a modular design, simple adjustment and easy add-onof more wavelengths. Concerning the polarization-multiplexing we take advantage of the almost linear polarized diodelaser bars. Ordinary used beam splitter cubes with a cemented structure are less qualified for high radiance. Hence thebeam combination is achieved with beam displacers made of a birefringent crystal (YVO4) which provide hightransmittance and convenient adaptation. Finally an experimental set-up with 8 diode laser bars of 4 differentwavelengths, i.e. 8-times beam superimposition, is realized. The set-up called multiplexer obtains a radiance of about4 x 106 W cm-2 sr-1 and outnumbers all other comparable high power diode laser systems.

Keywords: Wavelength-multiplexing, polarization-multiplexing, high power diode laser, multi-filter, multiplexer,beam displacer, birefringent crystal

1. INTRODUCTION

The application of high power diode lasers for materials processing was long time limited to diode-pumped solid statelasers due to insufficient output power and poor beam-quality. Further progress in output power and heat-sinktechniques leads to direct materials processing of diode laser systems recently. In this regard a simple 2-dimensionalstacked diode laser system with suitable beam shaping reaches a radiance of up to 104 W cm-2 sr-1 covering a wide rangeof application including hard- and soft-soldering, hardening, sintering and heat conductivity welding of metals1 and inparticular laser welding of polymers2. To reach the application range of CO2- and solid state lasers (e.g. welding andcutting of metals) a radiance of at least 106 W cm-2 sr-1 is necessary. Thereupon the superimposition of several differentwavelengths as well as the combination of s- and p-polarized beams allows a scaling of output power at almost constantbeam quality, i.e. an increase of radiance.

The techniques of wavelength- and polarization-multiplexing are subject-matter of this paper, whereas section 2 dealswith polarization-multiplexing and wavelength-multiplexing is treated in section 3. Based on the results of these twoparagraphs we demonstrate an experimental set-up (section 4) which unites the most appropriate concepts, respectively.

* knitsch@ilt.fhg.de; http://www.ilt.fhg.de; Fraunhofer Institute for Laser Technology, Steinbachstrasse 15,52074 Aachen, Germany; phone +49 241 8906 414; fax +49 241 8906 121

On the one hand we use a beam displacer made of a birefringent crystal (YVO4), on the other hand we have developed amulti-filter element where a plano-parallel plate is spanned coated with edge-filters. Altogether the so-calledmultiplexer, superimposing 8 diode laser bars, obtains a radiance of some 106 W cm-2 sr-1 whereas the value is limited tothe availability of high power diode laser bars of different wavelengths.

2. POLARIZATION-MULTIPLEXING

By the use of almost linear polarized diode laser bars the beams of a diode laser system consisting of s- and p-polarizedlaser bars can be superimposed. The so-called polarization-multiplexing leads to nearly double output power at constantbeam-quality.

Generally the beam combination is achieved by using polarization beam-splitter cubes because of a convenient,protected form and easy adjustment. For high-power applications, particularly for higher radiance (up to106 W cm-2 sr-1), the damage threshold of the polarization beam-splitter cubes with their cemented structure must beconsidered. Alternatively the polarization-multiplexing can be realized with a beam displacer made of a birefringentcrystal.

The difference between the beam-splitter cube and the beam displacer rests on the functional principle. Alternatinghigh- and low-index dielectric layers are deposited on the hypotenuses of two right-angle prisms, which are thencemented together to form the beam-splitter cube. Therefore the s- and p-polarized beams combine at right angles toeach other.3 The function principle of the beam displacer is based on birefringence of the used crystal. In case ofuniaxial crystals the refraction index perpendicular to the optical axis (ordinary index, no) differs from the refractionindex parallel to the optical axis (extraordinary index, ne), so that the index-ellipsoid of the refraction index is given by

( ) )1(,sincos1

2

2

2

2

2eo nnn

θθθ

+=

where θ defines the angle between beam propagation and optical axis (O.A.).4

Hence for a s- and p-polarized beam there is a beam displacement or beam combination, respectively (fig. 1).

Fig. 1: Beam walk-off in birefringent crystals.4

The so-called walk-off angle ρ can be calculated by the following equation

( ))2(,2sin

11

2tan

22

2

θθρ

−=

eo nn

n

whereas θ is given by

)3(arctano

e

n

n=θ

and n2(θ) from equation (1).

The beam displacement d and accordingly the beam pitch for polarization-multiplexing is related to the walk-off angleand the crystal length z by

)4(.tanz

d=ρ

Normally the walk-off angle obtains less than one degree (e.g. for SiO2), but for high birefringent crystals such asTitanium Dioxide (TiO2), Calcite (CaCO3) and Yttrium Orthovanadate (YVO4) the walk-off angle reaches up to 6°.Whereas all the latter crystals roughly have the same characteristics, YVO4 features good temperature stability, a widetransparency range (400-5000 nm) and easy crystal growth and fabrication.5

The crystal dimensions for polarization-multiplexing of high power diode lasers are given by the refractive indices (no

and ne) of YVO4, beam dimensions (fast- and slow-axis) and desired beam displacement, e.g. d = 1 mm. Concerning thehigher beam divergence in slow-axis after fast-axis and slow-axis collimation (FAC and SAC) the superimposition isrealized in fast-axis direction. Calculated from equation (1)-(4) this leads to crystal dimensions of (HxWxL)3x14x10 mm3 (fig. 2).

Fig. 2: Polarization-multiplexer (YVO4) with AR (800-980 nm).

3. WAVELENGTH-MULTIPLEXING

The beams of diode laser bars of different wavelengths can be superimposed by various methods such as dielectricedge-filters, diffraction, prisms or other optical components. With regard to efficiency, adjustment and compactdimensions dielectric edge-filters, preferably made by ion beam sputtering (IBS)6,7, are generally used for wavelength-multiplexing. According to polarization-multiplexing the superimposition is also realized in fast-axis direction.Additionally for wavelength-multiplexing, the polarization direction must be considered. The slope of the edge-filters issteeper for s-polarized than for p-polarized beams.7 If necessary, the polarization direction can be rotated by retardationplates.

In principle there are two basic concepts of wavelength-multiplexing, namely superimposition by bandpass or band-stopfilters.7 Where the latter concept leads to higher efficiency it is more difficult to adjust. To reduce the expense ofadjustment but keep a high efficiency we have developed the concept of a multi-filter (fig. 3).

Fig. 3: Function principle of the multi-filter concept for n different wavelengths.

By the use of a plano-parallel plate (etalon), which is spanned coated by edge-filters (EF), AR- and HR-coatings, thebeam is guided inside the etalon. Each additional beam is combined with the others at the given pitch. AssumingHR = AR = REF for reflection rates R and TEF = T for transmittance, the efficiency ηMF of the multi-filter is given by

)5(.1

12

)1(22

−

−+=−

R

R

n

RT

n

R nn

MFη

With typical values of R = 99% and T = 95% the efficiency of the multi-filter is shown in comparison with otherconcepts (fig. 4).

Fig. 4: Comparison of various multiplex-concepts.

Multi-filter

HR

HR

HR

AR

EF1

EF3

EF2

AR

λ2

λ1

λ4

λ3

λn EF(n-1)

HR

Diode laser stack

75

80

85

90

95

100

1 2 3 4 5 6 7 8 9 10

Number of different wavelengths n

E

ffic

ienc

y

[%

]

Band-stop filter

Bandpass filter

Multi-filter

η

The advantage of the multi-filter concept is the compact and modular design with actually two elements, namely thediode laser stack and the multi-filter. This allows an easy adjustment of only the two components. The lay out designfor the multi-filter depends on the beam pitch s of the diode laser stack, the angle of incidence α and the refractionindex n of the substrate (fig. 5).

Fig. 5: Lay out design for the multi-filter concept.

Hence the thickness d of the etalon is given by

( ))6(.

sinarcsintancos2

=

n

sd

αα

Even though a complete compensation of the lengths of path of the different beams is not achievable7, the angle ofincidence can be chosen so that at given beam pitch and refraction index the thickness of the etalon remains almostconstant for ± 2°-rotations. Due to a high sensitivity of edge-filters regarding the angle of incidence a chance for poweroptimization is given within this range of adjustment. Furthermore coating faults can be adjusted simply by rotation.Altogether the multi-filter (fig. 6) features a high efficiency, modular design, simple adjustment with poweroptimization and easy add-on of more wavelengths.

Fig. 6: Multi-filter for superimposition of 4 different wavelengths.

ns

d

α

4. EXPERIMENTAL SET-UP

On the basis of the results of section 2 and 3 an experimental set-up of 4 different wavelengths, i.e. 8-timesmultiplexing, is realized. The limited numbers depend on the availability of different wavelengths on the market.Concerning the slope of the edge-filters a superimposition of 8 different wavelengths in the range of 780 – 980 nm, i.e.16-times multiplexing, is possible.

Two diode laser stacks each consisting of 4 diode laser bars of different wavelengths (778, 804, 902 and 970 nm) arebuilt up with fast- and slow-axis collimation (fig. 7).

Fig. 7: Four wavelengths diode laser stack.

For wavelength-multiplexing the multi-filter is designed as described in chapter 3 (cp. fig. 6). Due to a pitch of thediode laser stack of s = 3.45 mm and a refraction index n = 1.452 of the substrate, the angle of incidence of α = 50°leads to the largest range of adjustment (cp. chapter 3). The corresponding thickness of the multi-filter can be calculatedfrom equation (6) to d = 4.32 mm. The spanned coatings are made by IBS-process using a template technique. Toprovide high slope efficiency both multi-filters are coated for s-polarized beams only. Hence the polarization directionof the superimposed beam of either stack is rotated at 90° by the use of a zero-order broadband retardation plate (λ/2)with a central wavelength of 865 nm. The polarization-multiplexing is realized utilizing a beam displacer made ofYVO4 (cp. chapter 2), whereas the s- and p-polarized beams are parallel arranged with a beam pitch of 1 mm by usingan adjustable folding mirror. The results of wavelength- and polarization-multiplexing are best shown in fig. 8 and 9where a CCD-image of the beams of the diode laser stack and the superimposed beam are displayed, respectively.

Fig. 8: Four wavelengths diode laser stack (CCD-image). Fig. 9: Superimposition of 8 diode laser bars (CCD-image).

Housing

Assembled diode laser baron heat sink

FAC

SACPower supply

The beam quality almost maintains after wavelength- and polarization-multiplexing whereas the beam parameterproduct (BPP) in fast-and slow-axis amount to BPPf.a. = 1.5 mm mrad and BPPs.a. = 210 mm mrad. Due to poweroptimization by rotation of the multi-filter (cp. chapter 3) it actually succeeded to obtain the calculated efficiency ofηMF = 92%. With an optical output power of 190 W (@50 A) a radiance of about 4 x 106 W cm-2 sr-1 is achieved.

The entire set-up for wavelength- and polarization-multiplexing (fig. 10), the so-called multiplexer, also features anadapted step mirror for beam shaping8, so that the superimposed beam can be coupled into a fiber with a core diameterof 320 µm (N.A. = 0.2).

Fig. 10: Experimental set-up (multiplexer) for wavelength- and polarization-multiplexing.

5. CONCLUSION

We have demonstrated a high power diode laser system, the so-called multiplexer, with a radiance of about4 x 106 W cm-2 sr-1. By comparison of various wavelength-and polarization-multiplexing concepts, the multi-filter andthe beam displacer emphasize themselves particularly for superimposition of many different wavelengths. The beamdisplacer made of birefringent YVO4-crystal features a good temperature stability, high transparency and the capabilityof high power applications. The advantages of the multi-filter concept are the compact and modular design inconjunction with diode laser stacks as well as simple adjustment and easy add-on of more wavelengths. Theexperimental proved power optimization by small rotations round the angle of incidence (± 2°) leads to a highefficiency of the multi-filter. Assumed development of high power diode laser bars concerning output power, beamquality and wavelengths-variety, the multiplexer can be expanded to a radiance of up to 107 W cm-2 sr-1.

ACKNOWLEDGEMENTS

This work was sponsored by the German Bundesministerium für Bildung und Forschung (BMBF), ReferenceNo. 13 N 7290/5.

Housing

Beam displacer

Step mirror for beam shaping

Folding mirror(adjustable)

Multi-filter

4 λ diode laser stack Broadband retardation plate (λ/2)

REFERENCES

1. F. Bachmann, “Hochleistungs-Diodenlaser für die Materialbearbeitung”, Physikalische Blätter, 3, 63-67, 20012. U.A. Russek, G. Otto, M. Poggel, “Verbindliche Nähte”, Laser-Praxis, 1, 14-16, 20013. W.G. Driscoll, W. Vaughan, Handbook of Optics, McGraw-Hill, New York, 19784. D. Meschede, Optik, Licht und Laser, Teubner, Stuttgart, 19995. http://www.castech-us.com/yvo4.htm6. H. Niederwald, “Dünne Schichten für optische Anwendungen”, Schichtkunde – Schnittstelle zwischen Verfahren

und Anwendung, VDI, Düsseldorf, 19917. A. Luft, Hochleistungs-Diodenlaserstapel hoher Strahldichte, Shaker, Aachen, 20018. K. Du, M. Baumann, B. Ehlers, H.G. Treusch, P. Loosen, “Fiber coupling technique with micro step-mirrors for

high-power diode laser bars”, OSA TOPS, 10, 1997