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aito Alahautala, Erkki Lassila, and Rolf Hernberg

A multiplexing method based on narrow diode-laser arrays �DLAs� in an axially symmetric configurationis described. The use of submillimeter narrow DLAs improves beam quality considerably in the slow-axis direction compared with typical 1-cm-wide DLAs. The axially symmetric geometry is advantageousfor efficient spatial, wavelength, and polarization multiplexing. With narrow DLAs and an axiallysymmetric geometry, a small circular focus with more than 10-kW�mm2 average power density ispossible. Theoretical calculations and preliminary experimental results are presented. © 2004 OpticalSociety of America

OCIS codes: 060.4230, 140.2010, 140.3300, 140.3390.

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. Introduction

iode-laser devices have proved to be competitive inuch materials-processing applications in which theiream quality is sufficient. However, the beam qual-ty of direct diode-laser light sources is not yet goodnough for cutting and keyhole welding metals.his is caused mainly by the large width ��1 cm� of

he traditional high-power DLAs, which seriouslyimits the beam quality in the slow-axis direction.

Brightness B is a relevant quantity in the determi-ation of beam quality. It is defined as

B �P

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here P is the optical power of the light source, A ishe light-emitting area, � is the solid angle in whichight is emitted, and Q is the beam parameter productBPP� of an axially symmetric beam. However, themission of a diode laser is elliptical and not rotation-lly symmetric. Therefore it is practical to define Qn the following way:

Q � �QF QS�1�2, (2)

The authors are with Tampere University of Technology, Insti-ute of Physics, Optics Laboratory, P.O. Box 692, FIN-33101 Tam-ere, Finland. E. Lassila’s e-mail address is erkki.lassila@tut.fi.Received 3 September 2003; revised manuscript received 30 De-

ember 2003; accepted 30 January 2004.0003-6935�04�132760-07$15.00�0© 2004 Optical Society of America

760 APPLIED OPTICS � Vol. 43, No. 13 � 1 May 2004

here QF and QS are the BPPs in fast- and slow-axisirections, respectively. QF is defined as the productf the beam waist wF and the beam-diverging half-ngle �F.1The focusability of a beam can be determined from

rightness B, which is inversely proportional to thequare of Q. Therefore Q is an important parametern determining the quality of a laser beam. Theigher the brightness �or the lower the BPP� theigher power densities are obtainable. The beamuality of a traditional diode-laser array �DLA� can beorders of magnitude lower in the slow-axis direction

han in the fast-axis direction. Therefore the treat-ent of the slow-axis direction is critical in designingigh-power DLA systems.The optical power of a single edge-emitting diode-

aser emitter is restricted to a few watts. To obtainhe power levels required in materials processing, its necessary to use a great many emitters. This issually accomplished by stacking DLAs on top of onenother. One DLA contains typically 20–40 emit-ers side by side so that the effective width of the lightource is �1 cm �Fig. 1�. Along with the increasedower, however, the beam quality is deteriorated ow-ng to the non-light-emitting space between the emit-ers. If, for example, twenty 0.15-mm emitters aresed in a 1-cm-wide DLA instead of one such emitter,he light power increases approximately 20 times,ut at the same time QS increases more than 60imes. In other words, in this case use of a wide DLAeduces brightness to approximately one-third of thealue of a single emitter.The effect of DLA stacking on QF is basically the

ame for wide and narrow DLAs. Deterioration of the

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eam quality in the fast-axis direction can be mini-ized by inserting collimation optics for each DLA.Several attempts have been made to improve the

eam quality in the slow-axis direction by opticaleans. This has been done at the expense of the

ast-axis direction so that the emission can be focuseds efficiently as possible. Improved QS is usuallyccomplished by optically stacking the emissions ofhe emitters in the DLA on top of one another by somerism or mirror arrangement after fast-axis collima-ion �FAC�.2–8 The problem with this kind of opticaltacking is that the complexity and the size of theevice increase, which usually increases the overall Qnd makes it difficult to utilize different multiplexingethods �polarization, wavelength, and spatial mul-

iplexing� simultaneously and efficiently. A differ-nt approach is to use narrow DLAs containing onlyne or a few emitters �Fig. 1�b��.

. Principle

or a 0.15-mm narrow DLA QF is typically smallerhan 1 mm mrad and QS is approximately 20 mm rad. Therefore the beam quality is similar in both

irections if 10–20 such DLAs are stacked. Such atack provides approximately 25–50-W cw lightower if efficiently cooled. Narrow DLAs can betacked, e.g., as shown in Fig. 2.To increase the power level and the power density

n the focus, different multiplexing methods can betilized. In polarization multiplexing �PM� twoeams of the same wavelength and with perpendic-lar plane polarizations are combined by using a po-

arization beam combiner �PBC�. Efficient PM isossible with edge-emitting diode lasers since themission is originally plane polarized. In wave-ength multiplexing �WM� two beams of differentavelengths are combined by using suitable beam

ombiners, e.g., dichroic mirrors. In spatial multi-lexing �SM� beams from separate light sources arepatially combined into the same location. All mul-iplexing methods increase the power density in the

ocus. However, SM also increases the effective nu-erical aperture of the focused beam.Utilization of axially symmetric configuration and

arrow DLA stacks provides an excellent basis forfficient and simultaneous use of all the multiplexingechniques mentioned �Fig. 3�.9

The beam emitted by the left-hand side laser stackf wavelength n propagates through a polarizationotator �e.g., �2 waveplate� into a compensating el-ment. The compensating element equalizes, e.g.,ath-length differences between laser stacks. Fromhe compensating element the beam continueshrough a beam reflector �prism or mirror� into aBC. In the PBC the two different polarizations ofavelength n are combined, after which the com-ined beam is reflected into an axial beam. Allavelengths are treated in a similar way.WM is accomplished with dichroic mirrors that

ransmit all axial beams coming from the left but re-ect all radial beams into the axial direction. Slow-xis collimation �SAC� is made by using a singleylindrical lens or a cylindrical lens stack as shown inig. 3. The lens stack maximizes the beam quality bytilizing the fact that there is more room for the beamo expand before the collimation in the outer periphery.n this way the beam is always collimated with as large

lens as possible, which partially compensates forberration differences between paraxial and non-araxial beams and minimizes system overall magni-cation. The magnification is inversely proportionalo the size of the SAC lens. For the structure shownn Fig. 3 the width of the SAC lens can be increasedrom �15 mm �inner periphery� to �22.5 mm �outereriphery�. This means that the size of the focus pro-uced by the outermost SAC lens is approximatelywo-thirds of the size of the focus produced by thennermost SAC lens �ignoring aberration effects�. Af-er SAC the beam is focused with focusing optics �FO�.ocusing optics can be either a reflective element suchs a suitably shaped parabolic reflector or a refractiveens. SM is made efficiently with the axially symmet-ic sector structure shown in Fig. 3.

ig. 2. Laser stack made of narrow DLAs with FAC. The draw-ng symbols of the laser stack are also shown. The symbols aresed in Figs. 3, 4, and 7. FAC lenses can be made considerablyhorter if desired. The size of the cooling mount can be adjustedo that sufficient heat transfer from the DLA to coolant is provided.

1 May 2004 � Vol. 43, No. 13 � APPLIED OPTICS 2761

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By utilizing the sectionally modular structurehown in Fig. 3, it is possible to combine the emissionsrom hundreds of narrow DLAs into a single beam.his provides new possibilities for generating powerensities high enough for metals processing. The de-ice generates a circular focus in contrast to the rect-ngular focus of a typical non-fiber-coupled directiode-laser device. Furthermore the optical axis isree for other processing needs such as monitoring,lowing, suction, or material feeding.An apparent drawback for the above-mentioned ap-

roach is that a larger number of narrow DLAs will beeeded for a certain power level as compared with wideLAs. However, the size of a narrow DLA is similar

o that of typical electronics components, which pro-ides new possibilities for automatized stacking by us-ng conventional modern techniques for mountinglectronics. A narrow emitting area also gives morereedom in the positioning of FAC lenses or lens arraysnd diminishes the effects of various undesirable phe-omena such as deformations induced by differences

n thermal expansion coefficients �smiling�.Another advantageous aspect of narrow DLAs is in

heir cooling. DLAs are usually mounted on heatinks that absorb the heat produced by the DLAs.sually the heat sinks are designed to be as thin asossible to increase the brightness of the laser stack.wide DLA is essentially a linear heat source, the heat

f which is confined to a one-dimensional planar flown the cooling plate. A narrow DLA is closer to a pointource, which will cool more efficiently by two-imensional heat transfer, when coupled to a wideooling plate, as shown in Fig. 2. Also the fact thathe heat produced by a narrow DLA is only a fractionf the heat produced by a wide DLA makes coolingasier. Wide DLAs are traditionally micro-channel-ooled. With narrow DLAs it is possible to manageithout microchannels, if desired, by using larger cool-nt channels. The more efficient cooling also makest possible to increase brightness or lifetime.

762 APPLIED OPTICS � Vol. 43, No. 13 � 1 May 2004

. Modeling and Experiments

o evaluate the performance of the described concept,e modeled by computer ray tracing two designsased on an axially symmetric architecture with 10ector modules. The calculated quantities wereorking distance �WD�, focus diameter �FD�, and av-

rage power density �APD�.The relatively simple design A, shown in Fig. 4�a�,

ncluded two laser wavelengths and 0.635-mm-wideight sources. To validate the ray-tracing modelingor this configuration, one sector module was simu-ated experimentally.

The more elaborate high-performance design B,hown in Fig. 4�b�, had three wavelengths, 0.2-mm-ide light sources, and cylindrical SAC lens stacks.his configuration was subject to ray-tracing analysisnly. For reasons of finance and time, it was not pos-ible at this stage to build a practical device of thisesign.

. Modeling: Design A

et us assume that each stack contains 10 narrowLAs with 0.635-mm-wide light-emitting areas. Ifach bar produces effectively �after transmission andoupling losses� 7-W light power, the total cw opticalower of the device is 2 10 10 7 W � 1.4 kW.f the efficiency of WM were 90%, the total outputould be 1.9 10 10 7 W � 1.33 kW.The height of the stacked laser beam in the fast-

xis direction after FAC was considered to be 15 mm.he focal length of the cylindrical SAC lens was 50m. Calculations were performed with three focus-

ng lenses. The aspherical form was chosen for theocusing lens with a 51-mm focal length. The focus-ng lenses with 78- and 159-mm focal lengths werepherical. The diameter of all focusing lenses was5 mm. The FD was obtained as the Gaussian im-ge size, which was calculated by the OSLO Lightrogram. The modeled optical system layout wasimilar to that of design B, which is explained in more

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etail below. The only exception was that a singlelow-axis lens was used in design A instead of theens stack used in design B. Calculated results aren Table 1.

The results are reasonable in the sense that theize of the FD increases approximately linearly withD. Since APD is inversely proportional to the

quare of the FD, the size of the FD is of major im-ortance when high-power densities are required.n this respect the power and brightness of the lightource should be maximized and the magnificationinimized. The brightness could be increased by

sing narrower DLAs and by denser packaging.he magnification could be decreased by using SAC

enses with longer focal length. The structurehown in Fig. 4�a� has room for SAC lenses with aocal length of 97 mm instead of the lens with a

ig. 4. Cross-sectional view of �a� an axially symmetric laserevice design with two wavelengths and �b� an improved designith three wavelengths. The desirable orientation of FO dependsn the lens; e.g., for spherical lenses the orientation in �a� mayeduce spherical aberration compared with the orientation in �b�.

Table 1. Design A: Calculated Results

Lens WD �mm� FD �mm� APD �kW�mm2�

1 51 0.82 2.392 78 1.16 1.193 159 2.20 0.33

Note: FD, 1�e2 value of the focus diameter; APD, average powerensity inside focus �assuming a Gaussian beam profile, the opticalower inside focus is 1.26 kW�.

0-mm focal length used in the experiment. Thisecreases magnification and FD by �50% and there-ore increases APD by �400%. Furthermore thePD can be increased by accommodating more sec-

ors and by adding new wavelengths and�or polar-zations to the structure. The effect of a multilayerAC lens stack �Fig. 4�b�� is dealt with below in thealculations of high-performance design B.

Note that the power densities given above areverage values over the entire 1�e2 focus area, andhe maximum intensity values would be noticeablyigher. For example, assuming a Gaussian beamrofile, the average power density over the areaimited by the FWHM radius is �1.7 times greaterhan the corresponding 1�e2 average power densityalue.

. Experimental Validation: Design A

he experimental setup in Fig. 5 was built in order toxamine the properties of design A. The custom-ade DLA contained four 0.125-mm emitters spaced

.045 mm from one another. Thus the effectiveidth of the light source was 0.635 mm in the slow-xis direction. The emission wavelength was 808m. The DLA was manufactured by Coherent Tut-ore, Inc. The DLA was operated in pulsed modeince cooling was not arranged. The DLA was colli-ated with a microlens in the fast-axis direction andith a cylindrical lens in the slow-axis direction.he focal length of the SAC lens was 50 mm. TheLA and the microlens were mounted on a copperount. A filter was placed after the SAC lens in

rder to decrease the intensity of the focused lightppropriately.The light source was scanned vertically in steps ofmm so that the total vertical displacement from the

ptical axis varied from �10 to 10 mm. In thisay it was possible to simulate the performance of a

ystem with multiple off-axis DLA stacks in an axi-lly symmetric geometry by using only a single DLA.owever, note that in reality the size of the focus forsystem with an arbitrary number of sectors would

robably be somewhat larger owing to, e.g., the posi-ioning inaccuracies of DLAs and optics.

The intensity distribution in the focus on the opti-al axis was recorded with a high-resolution CCDrray for each vertical DLA position �11 differentositions�. The representation of the focus wasormed by calculating an average image from theeasured 11 images.

Fig. 5. Experimental setup.

1 May 2004 � Vol. 43, No. 13 � APPLIED OPTICS 2763

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The measurement procedure was repeated forhree different focusing lenses �FO� with workingistances of 50, 75, and 150 mm. The lens with a0-mm WD was an aspherical condenser lens, andhe lenses with 75- and 150-mm WDs were stan-ard plano–convex lenses. All lenses were com-ercial.The average images and the corresponding inten-

ity distributions of the foci are shown in Fig. 6. Theumerical results calculated from the experimental

ntensity distributions are in Tables 2 and 3.As can be seen from Fig. 6 the intensity distribu-

ions in the slow-axis direction are rather irregular.his was due to the geometry of the DLA with fourmitters and the differences in the emissions of themitters. Smoother distributions could have beenbtained with a single-emitter diode laser.One of the main results of the preliminary experi-ental measurements is that the fast-axis FD was

lways considerably smaller than the slow-axis FD.his implies that the diameter of the focus formed byn axially symmetric device is restricted by the widthf the light source in the slow-axis direction if oneses DLAs with an effective emitting width of the

764 APPLIED OPTICS � Vol. 43, No. 13 � 1 May 2004

rder of 0.5 mm. The good focusability of a laserystem based on narrow DLAs and axial symmetry iserified by the fact that the vertical position of theocus stays relatively unchanged although the DLA isoved vertically.In the experiments the effective width of the light-

mitting area was 0.635 mm in the slow-axis direction.f, e.g., 0.15-mm-wide light sources had been used, oneould roughly estimate that the slow-axis FDs wouldecrease approximately to one quarter of the presentalues. In this case the FDs in both fast and slowirections would have a similar size. This provideshe most compact and uniform illumination in focus.

A comparison of the theoretical and experimental�e2 FD values presented in Tables 1 and 2 showseviations not greater than 5%. This level ofgreement makes us confident in the capability ofhe modeling calculations for predicting the perfor-ance of this type of device with a precision suffi-

ient for the present analysis.

. Modeling: Design B

he high-performance design B was modeled for aonfiguration with each stack containing 12 narrow

ig. 6. �a� Measured images of foci presented in an inverted gray scale and corresponding intensity distributions in �b� the slow-axis and

Table 2. Experimental Slow-Axis Focus Diameters

Lens WD �mm� 1�e2 FD �mm� FWHM FD �mm�

1 50 0.78 0.552 75 1.10 0.693 50 2.18 1.23

Table 3. Experimental Fast-Axis Focus Diameters

Lens WD �mm� 1�e2 FD �mm� FWHM FD �mm�

1 50 0.22 0.132 75 0.22 0.143 150 0.36 0.19

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LAs with a 0.2-mm emitter width. If each DLAroduces effectively �after transmission and couplingosses� 2.8 W of optical cw power and if the efficiencyf WM is estimated to be approximately 90%, theotal cw light output of the device will be approxi-ately 2.75 10 12 2.8 W � 924 W.The height of the stacked laser beam in the fast-

xis direction after the FAC was considered to be 15m. The SAC was accomplished with a lens stack

ontaining four lenses �Fig. 7� filling the sector aper-ure as completely as possible �Fig. 3�. The SAC lensidths were 15, 17.5, 20, and 22.5 mm increasing

oward the sector periphery. The focal length of thennermost SAC lens was 97 mm, and the focal lengthf the outermost SAC lens was 150 mm as shown inig. 7. The working distance of the system was 102m, and the diameter of the focusing lens was 75m. Both the SAC lenses and the focusing lensere corrected for spherical aberration. The FDas obtained as the sum of the Gaussian image sizend the point-spread function.The FD of the Gaussian image for the improved

onfiguration varied from 0.14 �a 22.5-mm-wide SACens� to 0.21 mm �a 15-mm-wide SAC lens� in the slow-xis direction. It is common knowledge that the fast-xis direction can be collimated at an accuracy of therder of 2 mrad, which results in an even smaller FDhan the FD in the slow-axis direction. Owing to theffect of aberrations, the overall 1�e2 FD is estimatedo increase to a value of �0.25 mm according to theoint-spread function analysis. The effects of posi-ioning inaccuracies, component imperfections, andispersion in the optical system are estimated to en-arge the FD to �0.3 mm. Therefore the calculatedPD in the 1�e2 FD would be �13 kW�mm2. Theractical issues and their effects on the system perfor-ance will be investigated in more detail in the future.The results of modeling the high-performance de-

ign B are in Fig. 8. The present results are sur-ounded by a dashed ellipse. For comparison, theorresponding performance data for several commer-ial kilowatt-level direct diode-laser devices are in-luded in Fig. 8. The comparison indicates that theresent concept has substantial potential in terms of

oth power density and focus diameter. Experimen-al verification, still pending, gives a more precisealidation.Note that advances in diode-laser technology will

mprove the performance of direct diode-laser devicesn the future. For example, the beam quality in thelow-axis direction can be considerably improvedhen tapered diode lasers are used.10 Moreover cwower levels of 4–6 W from 0.1-mm emitters haveecently become commercially available.11,12 Thisould increase the brightness by a factor of 4 com-ared with the brightness of the DLAs used in thealculations for design B.

. Conclusions

novel architecture for a high-power diode-laser de-ice based on stacked narrow DLAs arranged in sectorodules around a symmetry axis has been outlined.he concept has been analyzed by means of ray-tracingodeling, and the modeling has been experimentally

alidated for a sector module of simple design.

ig. 7. Modeling of design B. For clarity only three sectors are drawn. Mirrors and compensating elements are assumed to be ideal,nd they are marked by white boxes for simplicity.

ig. 8. Performance of design B �indicated by the ellipse� com-ared with kilowatt-level commercial diode-laser devices. TheD of all presented devices is approximately 100 mm; 3 p

ndicates the multiplexing of three wavelengths with polarizationultiplexing. �The efficiency of WM was estimated to be 90% and

he efficiency of PM, 80%�. The values for the Laserline devicesre FWHM values. Brightness B values are in kW�mm2�sr, andhey are approximative for the commercial devices.

1 May 2004 � Vol. 43, No. 13 � APPLIED OPTICS 2765

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The method results in improved beam quality andherefore higher power densities than currentlyvailable. This may provide new applications forigh-power direct diode-laser devices, e.g., in the fieldf optical metals processing.A device based on narrow DLAs and axial symme-

ry also has several other attractive features. First,he optical axis is free for, e.g., monitoring, blowing,aterial feeding, or suction. Second, the focus is

ircular. This may be particularly desirable in cer-ain applications such as fiber coupling. Also theooling of narrow DLAs is easier to realize than theooling of wide DLAs.

Computer ray-tracing calculations for a device de-ign with 0.2-mm light sources indicated that by uti-izing 10 sectors, three wavelengths �no polarization

ultiplexing�, and a suitable four-layer slow-axis col-imation lens stacks, it is possible to obtain an effec-ively circular focus with �0.3 mm 1�e2 diameter andore than 10-kW�mm2 average power density at a

02-mm working distance. These values, if realized,ill enable substantial improvement compared with

he performance of the present commercial directiode-laser devices. The modeling was experimen-ally validated and found to be within a 5% agree-ent for a simpler design based on the same concept.

t is up to future experimental verification of theresent design to pinpoint the actual performanceore precisely.

The helpful assistance of Hannu Sipponen in per-orming the empirical experiments is gratefully ac-nowledged.

766 APPLIED OPTICS � Vol. 43, No. 13 � 1 May 2004

eferences1. R. Diehl, ed., High-Power Diode Lasers: Fundamentals, Tech-

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8. V. Krause and C. Ullmann, “Laser optics and diode laser,” U.S.patent 5,986,794 �16 November 1999�.

9. T. Alahautala and E. Lassila, “A method and a laser device forproducing high optical power density,” International patentapplication WO 03�097858 �27 November 2003�.

0. M. Kelemen and M. Mikulla, “High-power high-brightnessridge-waveguide tapered diode lasers,” �28 August 2003�,http:��www.iaf.fhg.de�ipdetetd�frames_e.htm.

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