New approach for high-power diode laser modules with homogenized intensity distribution

Bernd Köhler*, Florian Ahnepohl, Karsten Rotter, Jens Biesenbach

DILAS Diodenlaser GmbH, Galileo-Galilei-Str. 10, 55129 Mainz-Hechtsheim, Germany

ABSTRACT

In the last few years high-power diode laser modules with homogenized intensity distribution have found a growing number of applications, like annealing, hardening and surface illumination. The standard beam shaping concepts in such modules are using an optical waveguide or microoptical lens arrays for homogenization. For the generation of long lines with high aspect ratio these concepts have some significant drawbacks, especially if the line is composed of several sub-modules with shorter line segments. The homogeneity in the transition zone of these segments is always difficult to handle. In this paper we report on a new approach for homogenization of high-power diode laser modules by using linearly arranged fiber bundles to generate homogeneous lines. The main advantage of this concept is that scaling of line length is easily achieved by increasing the number of linearly arranged fibers. We present a detailed characterization of such a modular diode laser system with 3 kW output power and homogenization by means of a fiber bundle. The dimension of the homogenized line is 150 mm x 2.5 mm. In addition we present a number of different diode laser modules with homogenization by means of classical approaches, like microoptical cylindrical lens arrays. The output power of these modules ranges from 50 W to 11 kW with line dimensions from 3 mm x 50 µm up to two dimensional homogenized areas of 55 x 20 mm2. Keywords: High-power diode laser, beam shaping, homogenization, line module, high power density

1. INTRODUCTION In general high-power diode laser systems have become well-established laser sources for a number of different applications with output powers up to several kilowatt1. The only drawback of direct diode laser systems is the inhomogeneous intensity distribution, which is a consequence of the basic setup of such a diode laser system. However, if such diode laser systems are combined with homogenization optics these problems can be solved leading to very attractive homogenized laser devices. Beam homogenization techniques are well-established for laser systems based on excimer lasers. Typical applications are production of micromechanical structures and mask projection systems for lithography2. Recently an increasing demand for laser systems with a homogenized beam profile is observable. This includes systems with homogenization in one direction providing a line focus and systems that are homogenized in both axes leading to a rectangular or even square focus. These inquiries appear from different fields of applications like LCD-panel technology, printing industry, general illumination tasks, plastic welding, thermoplastic tape placement technology and other surface applications like hardening or thermal annealing. For many of these applications it is not essential that the wavelength of the laser beam has to be in the ultraviolet spectral range. Therefore diode lasers with wavelengths in the infrared spectral range are an attractive alternative for such laser systems. In contrast to excimer lasers diode laser systems offer a lot of advantages, like high wall-plug efficiency, high reliability, long lifetime, low maintenance, relatively low investment costs and a small footprint.

* b.koehler@dilas.de, tel. +49 (0)6131 9226 133; fax +49 (0)6131 9226 255; www.dilas.de

A number of different systems with homogenized intensity distribution based on high-power diode laser bars have been presented in the last few years3,4. One drawback of these systems is that the homogenization optics is always designed for a specific application and is therefore not easily scalable, e.g. with regard to line length.

2. BASIC CONCEPTS OF BEAM HOMOGENIZATION The basic goal of beam homogenization is to transform an arbitrary intensity distribution into a homogeneous intensity distribution that should be independent from fluctuations and inhomogeneities of the original laser beam. Optical parameters that define such a homogenization module are the homogeneity (mostly defined as peak to valley value), the shape of the intensity profile which can be Top-Hat or Gaussian, the edge-steepness and the dimension of the homogenized intensity distribution, which can be a homogenized line profile or a complete two-dimensional homogenized area. In addition, the total output power and the wavelength are important parameters for such a module. With regard to the mechanical setup the module should have a small footprint and consist of a small number of components. Furthermore it should be cost-effective and easy to align. Homogenization of a laser beam can be achieved by a number of basic concepts. One approach for transformation of a Gaussian beam to a Top-Hat beam is to rearrange parts of the laser beam by means of aspheric lenses5. However, the design of these aspheric lenses is very sensitive to the parameters of the incident beam. Therefore slight changes or fluctuations of the incident beam will lead to a significant deterioration of the resulting Top-Hat profile. Another very simple approach is homogenization by simple beam overlapping of several single laser sources placed next to each other. However, this method has a number of disadvantages that will be described in Sect. 2.1. More sophisticated solutions are homogenization by waveguides and microoptical arrays which will be described in Sect. 2.2 and 2.3., respectively. The new approach of homogenization by means of linearly arranged fiber bundles which is the main topic of this paper will be described in Sect. 2.4.

2.1 Homogenization by simple beam overlapping As indicated above a very simple approach of beam homogenization is to arrange a number of diode laser bars next to each other in the slow-axis direction. Fig. 1 shows the basic principle of such an arrangement for eight diode bars.

Fig. 1: Simple approach for beam homogenization by arranging diode laser bars next to each over. The diagrams below show the simulated intensity distribution for three different distances to the source. The simulation also shows the effect of one missing diode on the overall intensity distribution.

-80 -60 -40 -20 0 20 40 60 800,0

0,2

0,4

0,6

0,8

1,0

1,2 all diodes ; one missing diode

inte

nsity

[a.u

.]

position [mm]

-80 -60 -40 -20 0 20 40 60 800,0

0,2

0,4

0,6

0,8

1,0

1,2 all diodes ; one missing diode

inte

nsity

[a.u

.]

position [mm]-80 -60 -40 -20 0 20 40 60 80

0,0

0,2

0,4

0,6

0,8

1,0

1,2

all diodes ; one missing diode

inte

nsity

[a.u

.]

position [mm]

The slow-axis divergence of the laser bars causes a beam overlap between adjacent diode bars. This overlap increases with propagation of the beam leading to a homogenization of the overall beam. As a matter of course the degree of homogenization strongly depends on the distance to the diode module. The effect of homogenization is indicated in the results of the simulation in Fig. 1. One major drawback of this homogenization method is the sensitivity to intensity fluctuations of the diode bars. Differences in output power per bar, unequal degradation of individual bars or even a complete failure of one bar will have a significantly effect on the homogeneity. The effect of a complete failure on the intensity distribution is also shown in Fig. 1. A further disadvantage is the flattening of the edge area of the intensity distribution with increasing distance. For large distances this intensity distribution approaches the far-field angular distribution of the diode bar in slow-axis direction. However, despite these drawbacks the results of this approach are sufficient for a lot of applications. Due to the large lifetime of diode laser bars the effects of degradation or even failure are negligible for many practical applications.

2.2 Homogenization by means of a waveguide To overcome the drawbacks of the homogenization method mentioned above more sophisticated approaches have to be applied. One method is homogenization by means of an optical waveguide. Typically a waveguide is a rectangular glass volume with polished parallel side walls. The entrance and exit surfaces of the glass volume are also polished. Fig. 2 shows the basic principle of this homogenization method. The light of a diode laser source is focused into an optical waveguide. Inside the waveguide the light is guided by total internal reflection leading to a segmentation and mixing of the intensity distribution at the end of the waveguide. Finally the exit surface is imaged onto the work piece. The result of the homogenization depends on the number of reflections inside the waveguide, which is defined by the aspect ratio of the waveguide, the divergence of the incident beam and the refractive index of the waveguide. A detailed analysis of homogenization by means of an optical waveguides can be found in Ref. 6.

Fig. 2: Homogenization of a diode laser by means of an optical waveguide.

2.3 Homogenization by microoptical lens arrays An alternative approach for homogenization is performed with microoptical lens arrays. The basic setup for a homogenization system based on microoptical lens arrays is shown in Fig. 3. The light of a diode laser source is incident on a microoptical lens array, that cuts the beam into a number of individual beams (beamlets). The function of the field lens is to overlap all beamlets on the work piece leading to a homogenized intensity distribution. In comparison to the waveguide approach homogenization by means of microoptical lens arrays is more flexible and requires a reduced number of optical components. As a consequence the overall size of such a system can be significantly smaller. This is clarified by the comparison of the basic homogenization setups in Fig. 2 and Fig. 3. Fig. 4 shows the basic principle of beam homogenization with microoptical lens arrays. An inhomogeneous beam with beam width w is incident on a microoptical lens array. The lens array is characterized by the distance between two adjacent lens segments (pitch p) and the focal length fMA of an individual lens segment. The shape of the lens segments is depending on the application. Mostly cylindrical lens segments are used, but spherical or hexagonal shapes are also possible. Depending on the beam size d and the pitch p the incident beam is divided into a number of beamlets. Subsequently these beamlets are overlapped in the focal plane of a field lens with focal length ff. Homogenization in one direction is achieved with a setup as shown in Fig. 4. If homogenization is required in both axes a microlens array with spherical lenses can be applied. However, for different aspect ratios of the homogenized spot two microlens arrays with crossed cylindrical lenses have to be used.

work piece

waveguide

imaging optics diode source

Fig. 3: Homogenization of a diode laser by means of microoptical lens arrays.

Fig. 4: Principle of beam homogenization by means of microoptical lens arrays. The basic setup in Fig. 4 shows the principle of a non-imaging microlens array. One drawback of this simple setup is that an incident beam with a sufficient beam quality is required for good results with regard to homogeneity and edge steepness. An important parameter which defines the edge steepness is the ratio between the divergence of the incident beam θ0 and the divergence θMA which is generated by the microlens array. The divergence θMA is calculated by the parameters of the microlens array and is identical to the numerical aperture of the array. The formula for the numerical aperture of the microlens array is given by Equ. 1:

MAMA

MA NAfp

≈⎟⎟⎠

⎞⎜⎜⎝

⎛⋅

≈2

arctanθ (1)

A good edge steepness is realized if the divergence of the incident beam is much smaller than the numerical aperture of the lens array. In addition good homogeneity demands for a large number of beamlets to be overlapped in the focal plane of the field lens. The beam size DL of the homogenized beam is calculated from the parameters of the homogenization setup and is given by Equ. 2:

( )MA

fMAfL f

fpfD

⋅=⋅⋅≈ θtan2

(2)

Another drawback of using non-imaging microlens arrays is that diffraction effects might disturb the homogenized intensity distribution. To overcome these disadvantages of non-imaging microlens arrays a combination of two microlens arrays has to be used. The basic setup for such a system is shown in Fig. 3. Usually two identical microlens arrays with the same focal length are used. The distance d between the microlens arrays equals the focal length of the second array. Under these assumptions the size of the homogenized spot can also be calculated by Equ. (2). As well as for the non-imaging setup the first microlens array divides the beam into several beamlets. Afterwards the combination of the second array and the field lens produces images of each beamlet, which are overlapped on the workpiece. Therefore, such a setup is called an imaging microlens array. In comparison with non-imaging setups homogenizers based on imaging microlens arrays offer the advantages of better homogeneity and edge steepness. Furthermore the demand on beam quality of the incident beam is not so critical and diffraction effects are also reduced. Another important advantage is the possibility to adapt the size of the homogenized beam by changing the distance d between the two microoptical lens arrays. However, this can only be done for a small parameter range and at the cost of the edge steepness. In addition a setup based on imaging microlens arrays is more complicated because the two microlens arrays have to be aligned carefully. Typical transmission efficiencies for homogenizers based on microoptical lens arrays are above 95% for both non-imaging and imaging microlens arrays.

2.4 Homogenization by linearly arranged fiber bundles The homogenization concepts that have been described in Sect. 2.2 and 2.3 have in common that the optical systems are designed for a particular homogenized intensity distribution. For a different application a new optical design adapted to the demands of the new application is required. In addition, applications that ask for a line focus solution based on imaging optics get more complicated with increasing aspect ratio. The simple approach that has been described in Sect. 2.2 does not use imaging optics and is therefore in principle scalable to very long lines. However, that concept has two significant drawbacks. First, the power density is limited to the value that is given by the power density of the diode laser bar and secondly, the homogeneity of the line is very sensitive to power fluctuations of the diode laser bars as shown in Fig. 1. To overcome these drawbacks a new concept based on linearly arranged fiber bundles has been developed. The basic principle of such a system with a fiber bundle is shown in Fig. 5. A submodule consists of n diode laser bars, each of them combined with a fiber bundle where the number m of fibers per bar corresponds to the number of emitters per bar. Afterwards all fibers are arranged side by side in one line with minimized pitch between the fibers. Behind the fiber array the beam is collimated in one direction by means of a cylindrical lens. In the other direction the beam is not collimated and leaves the fiber with a divergence that corresponds to the numerical aperture of the fiber. Homogenization is achieved after a certain distance behind the fiber array due to beam overlapping of the individual beams. One important advantage of this concept is that the linear power density is significantly increased because the filling factor is maximized by the arrangement of the fibers with minimum lateral pitch. Compared to the concept of Sect. 2.1 this concept is also less sensitive to power variation of the diode bars, because the fibers of n individual bars are mixed in a fiber array of one submodule. In addition, a high depth of focus is given in the direction where the beam is collimated by a cylindrical lens. Scaling of the line length is very easy by simply placing submodules next to each other. Another advantage compared to a setup with microoptical lens arrays is that interference effects can be neglected. On the other hand edge steepness and homogeneity of the intensity profile is usually worse compared to other homogenization techniques (Sect. 2.2 and 2.3). Therefore this concept is especially attractive for long lines with high aspect ratio that are difficult to realize with conventional homogenization techniques.

Fig. 5: Schematical setup of a homogenized diode laser system with linearly arranged fiber bundles.

3. LAYOUT AND DESIGN OF THE LASER SYSTEM

In this section we describe the properties of a complete laser system that is based on several submodules with linear arranged fiber bundles for homogenization. The properties of the submodules will be described in Sect. 3.1.. The laser system was designed for the following target specifications :

• Line length : 150 mm • Line width : 2.5 mm • Homogeneity : better 10 % (peak to valley value) • Working distance : ≈ 100 mm • Depth of focus : > 5 mm • Linear power density : > 150 W/cm • Output power : > 3 kW • Wavelength : 1020 ± 10 nm • Modularity : scalable for increasing line length

3.1 Design and realization of a submodule The laser system is designed in the first instance for a line length of 150 mm, but in a next step the line length should be easily scalable from 150 mm to a line length of 500 mm or even 1000 mm. The definition of the submodule properties is a trade-off between modularity and number of modules which are needed for a specific line length. For the target specifications of the system described in this section we have determined the properties of one submodule as summarized in Table 1.

One submodemitters withsize of 110 / with a pitch oof 17.5 mm. 80 W/cm for The mechanilaser moduleroom is limidimension ofthermistor an

The output pfiber array atof the diode l

emitters per bar

(m)

ppe

19

d

dule consists h a total outpu150 µm and aof approximatThe dense athe individua

ical design of e and the proceited for a spef the processinnd photodiode

Fig. 6: Me

power of a subt a current of laser bars. It s

Fig. 7: O

ower er bar[W]

linear podensit[W/cm

80 80

diode bar

Table 1

of 6 diode laut power of 80a numerical aptely 155 µm.

arrangement oal bar up to abone submoduessing head wecific applicatng head is onle for temperatu

echanical design

bmodule is sho80 A. This co

should be rem

Output power of

owerty

m]diode bars

(n)6

1: Basic propert

aser bars mou0 W at a centerperture of 0.13Taking into a

of the fibers iout 217 W/cm

ule is shown inwhich holds thtion. The dimly about 16 x ure and power

n of a submodu

own in Fig. 7orresponds to arked that the

f a submodule a

s

power per submodule

with fiber arr[W]380

ties of a submod

unted on micrr wavelength 3. In the exit paccount the totincreases the

m at the exit pln Fig. 6. One e fiber array.

mension of th21 x 110 mmr control, resp

ule and detailed

as a functiona coupling ef

e fibers of the

as a function of

eray

fibers persubmodule

(n x m)114

dule for the kW

rochannel cooof 1020 nm. T

plane of the sutal number oflinear power

lane of the subimportant aspThis allows fo

he laser head m3, respectivelypectively.

view of proces

n of current. Tfficiency of 81fiber array ha

f current before

fiber core / cladding

[µm] fi110 / 150

submodule

W-system.

oled heatsinksThe individuaubmodule the f 114 fibers thr density by abmodule. pect of the desfor a very com

is about 220y. The proces

ssing head with

The maximum1 % when comve no antirefle

and after the fi

iber NAfiber p

[µm0.13 ≈ 15

s. Each bar cl fiber has a cfibers are line

his leads to an a factor of ab

sign is the sepmpact design if0 x 150 x 40 sing head has

fiber array.

m value is 380 mpared to the ection-coating

iber array.

itch]

emitting area[mm]

l

5 17.5

consists of 19ore / claddingearly arrangedemitting area

bout 2.7 from

paration of thef the availablemm3 and the

s an integrated

W behind theoutput power

g.

inear powerdensity

exit plane[W/cm]

217

9 g d a

m

e e e d

e r

The characteprocessing heright diagrama cylindrical described bechanging the

3.2 Properti Scaling of thspecificationthe intensity

Fig. 9: Ray-Trworking distan

eristic of the inead with the f

m shows a crolens with a fo

efore the dept working dista

Fig

es of the com

he line lengths as describedprofile is show

racing simulationce of 100 mm.

numo

subm

ntensity distrifiber bundle a

oss section of tocal length ofth of focus isance from 50

g. 8: Beam char

mplete kW sys

h and power d before. The wn in Fig. 9.

T

on of the comp

mberofodules

totnumof fib

8 91

ibution can beand the collimthe intensity df 5.8 mm and very high. Tmm to 100 m

acteristics of a

stem

is achieved bproperties of

Table 2: Proper

plete system wit

al ber

bers

totalpower

[W]2 3040

e seen in Fig. mating lens anddistribution fohas a width o

The beam widmm.

submodule with

by arranging f the complete

rties of the com

th 8 submodule

rworkingdistance

[mm]100

total system

8. The left pad being absoror the collimatof about 2.5 mdth increases

h 6 bars and 11

eight submode system are s

mplete kW-syste

es. The intensity

line length[mm]

lin

150

art of Fig. 8 srbed at a thermted direction.

mm at a workinonly from 1.

4 individual fib

dules in one ssummarized in

m

y distribution i

ne width[mm]

pode[W

2.5

shows the beamopile power The beam is ng distance o9 mm up to 2

bers.

system to fuln Table 2. A

s shown in two

ower ensity

W/cm]203

am leaving thedetector. Thecollimated byf 100 mm. As2.5 mm when

lfill the targetsimulation of

o directions at a

e e y s n

t f

a

The simulatioas the Top-Htarget specifimeasurementimprovementlead to a homFig. 10 showfor the kW-sonly 8 submo8 submodule80 A. The dim

Fig. 10: Desigsubmodules w

on shows thatHat region of tication. The cts have showt of the qualit

mogeneity of bws the design o

ystem. That podules are neees as a functiomension of th

gn of the proceswith fiber bundle

t the desired pthe intensity pcharacterizatiown that the hty of the fiberbetter than 10 of the processiprocessing heaeded. The lowon of the currehe complete pr

ssing head and aes and output po

parameters areprofile is abouon of the intenhomogeneity r bundles in c%. ing head withad is already p

wer right diagrent. A maximrocessing head

arrangement of ower of the com

e achieved witut 150 mm annsity distributis slightly a

combination w

h 11 submodulprepared for aram of Fig. 10

mum output pod is about 370

f diode laser submplete system w

th the current nd the line widtion for the coabove the tarwith individua

les and the arra longer line, 0 shows the ouower of more 0 x 170 x 65 m

bmodules (uppewith 8 submodu

design. The ldth is about 2omplete systerget value ofal power contr

rangement of but for the ta

utput power othan 3000 W

mm3 and the w

er part). Photo oules as a functio

ine length wh2.5 mm as reqm is not finisf 10 %. Howrol of each su

the diode lasearget line lengof the complet

is achieved aweight is about

of the processinon of current (lo

hich is definedquested by theshed yet. Firstwever, furtherubmodule will

er submodulesgth of 150 mme system withat a current oft 3.3 kg.

ng head with 11ower part).

d e t r l

s m h f

1

4.

In this sectiofirst system twill be exemdistribution. homogenizedx 50 µm, 600

4.1 Module w As describedAnother advathat directionaperture of 0combination small beam wFor realizingshows a simsimulation ispower of abobe further imwidth could also possibledirection of t

Fig. 11: Ray

4.2 Diode las In this sectiolaser unit of means of asp

. FURTHE

on further examthat will be d

mplarily descrIn addition tod intensity pro0 W with 150

with fiber arr

d in Sect. 2.4 antage is the gn is defined b0.13 leads towith the good

width. g such small bmulation for a

performed foout 400 W. Th

mproved by usialso be reduc

e by increasinthe line length

y-Tracing simu

ser module w

n we describethe module ispheric cylind

ER EXAMPL

mples of dioddescribed in Seribed in Sect. o the systems tofiles. Amongmm x 2 mm,

ray and addi

one importangood beam quby the parame a beam parad beam quality

beam widths sa line design or a system wihe power densing additional

ced below 100ng the numbe

h the number o

ulation of a line

with 1 kW out

e a diode lasers a vertical di

drical lenses. H

LES OF DIINTE

de laser moduect. 4.1 also u4.2 and Sect

that will be prg these are sys

600 W with 6

tional beam s

nt advantage uality in the dieters of the fibameter produy in the other

some additionwith dimensi

ith only one susity of the linl microoptical0 µm with appers of submoof submodules

with high pow

tput power an

r module withiode laser stacHomogenizat

ODE LASEENSITY DIS

ules with homuses an array t. 4.3 use micresented in thitems with an

60 mm x 2 mm

shaping optic

of the fiber airection perpenber. A fiber d

uct of only abdirection allo

nal optics is nions of 17 mubmodule ande is about 10l cylindrical lepropriate opti

odules. Howevs is limited to

er density based

nd homogeni

h 1 kW outputck with 12 dioion is realize

ER MODULSTRIBUTIO

ogenized inteof fibers for

crooptical lensis section we houtput power

m and 120 W w

cs

array concept ndicular to thediameter of 1bout 7 mm·mows the genera

necessary besim x 250 µm

d additional bekW/cm2. The

ens arrays instical elements.ver, because about 5.

d on one submo

ized field of 1

t power and a ode laser bars

ed by using tw

ES WITH HON

ensity distributhomogenizatis arrays for hhave already rof 120 W and

with 16 x 3 m

is the increase homogenize10 µm in com

mrad. The higation of a line

ides the simplat a working

eam shaping oe edge steepnetead of simple As a matter of the additi

odule with addi

12 x 12 mm2

homogenizeds collimated inwo microopti

HOMOGEN

tion will be pion. The otherhomogenizingrealized mored a line dimen

mm, respective

se of linear ped line. The bembination withgh linear powe with high as

le collimatingg distance ofoptics resultiness of the linee imaging optiof course pow

ional beam sh

itional beam sha

d field of 12 xn the fast-axiical cylindrica

NIZED

presented. Ther systems that

g the intensitye systems withnsion of 3 mmely.

power density.eam quality inh a numerical

wer density inspect ratio and

g lens. Fig. 11f 50 mm. Theg in an output

e profile couldics. The beamwer scaling ishaping in the

aping optics.

x 12 mm2. Thes direction byal lens arrays

e t y h

m

. n l n d

1 e t d

m s e

e y s

positioned directly behind the diode laser stack. A spherical field lens transforms the angular distribution into a homogenized area with a dimension of 12 x 12 mm2. The left part of Fig. 12 shows the mechanical design of the stack with homogenization optics. The working distance is about 60 mm. The output power of the system is shown in the center of Fig. 12 as a function of current. A maximum output power of 1100 W is achieved at a current of 95 A. The profile of the two dimensional homogenized area is shown in the right part of Fig. 12. The homogeneity is better than 10 %. The diode laser module was optimized for a hair removal application with a maximum duty cycle of 30 %.

Fig. 12: Design and measurement data of a diode laser module with 1 kW output power and a homogenized intensity distribution of 12 x 12 mm2.

4.3 System with 11 kW output power and line dimension of 55 x 3 mm2

Another system that has been realized in the past is a direct diode laser system with 11 kW output power at a single wavelength of 940 nm. This system was designed for customer specific large area treatment3. The initial optical setup yielded a homogenized intensity distribution with a dimension of 55 x 20 mm2. Because of the modular concept of the system a line focus could be easily realized by changing the homogenization module resulting in a line focus with a dimension of 55 x 3 mm. The homogeneity of the line was better than ± 5 % (peak to valley value) and the power density achieved a value of 67 MW/m2 (6.7 kW/cm2). Fig .13 shows the output power of the system as a function of current. A maximum power of 11 kW was achieved at a current of 60 A. The right part of Fig. 13 shows a measurement of the line profile. A more detailed analysis of the properties of the line is shown in Fig. 14.

Fig. 13: Output power as function of current for the 11 kW system (left) and measurement of the line profile (right).

Fig. 14: Cross section of the intensity profile in both directions for the 11 kW system.

5. SUMMARY AND CONCLUSION In conclusion, we have developed a new modular concept for the generation of homogeneous line foci based on diode laser bars with fiber arrays. This concept allows the generation of long lines with high aspect ratio and high power density. One of the main advantages of the concept is the scalability with regard to line length. As an example we have realized a system with 3 kW output power at a single wavelength of 1020 nm with a line geometry of 150 x 2.5 mm leading to a linear power density of more than 200 W/cm. The basic building block of the concept is a diode laser unit with 6 diode laser bars and an output power of about 400 W. The modular approach allows the customization of the total output power as well as the customization of intensity profile and focus dimensions. Although the building block is designed as a submodule for a system with long line focus, it is also an attractive light source for smaller line lengths with very small beam width in the direction perpendicular to the line direction. As an example we have shown a simulation for a system based on one submodule which produces a line geometry of about 17 mm x 250 µm with 400 W output power. In addition we have presented a couple of different modules based on classical homogenization techniques like homogenization by microoptical cylindrical lens arrays.. These systems have customized homogenization and imaging optics for homogenization in one or two directions resulting in line, rectangular or quadratic illumination profiles. The potential parameter range for the spot sizes extends from dimensions below 1 mm up to several hundreds of mm for each dimension with an output power range from 50 W up to 11 kW. We believe that in future the realized parameter range will be extended by further applications that will benefit from the advantages of high power diode laser systems with homogenized intensity distributions.

ACKNOWLEDGEMENT

A part of this work was sponsored by the German Federal Ministry of Education and Research (BMBF) within the german national funding initiative “Materials Processing with Brilliant Laser Beam Sources (MABRILAS)”.

REFERENCES

1. F. Bachmann; “Application adapted diode laser systems – A result of the German national research project

‘MDS’”; Proc. SPIE Vol. 4973, p. 68 (2003). 2. E.C. Harvey and P.T. Rumsby; “Fabrication techniques and their application to produce novel micromachined

structures and devices using excimer laser projection”; Proc. SPIE Vol. 3223, pp. 26-33, (June 1997). 3. B. Köhler, A. Noeske, T. Kindervater, A. Wessollek, T. Brand and J. Biesenbach; „11-kW direct diode laser

system with homogenited 55 x 20 mm2 Top-Hat intensity distribution”; Proc. SPIE Vol. 6456 (2007) 4. J. Meinschien, A. Bayer, P. Bruns, L. Aschke, V.N. Lissotschenko; “Improvements of high power diode laser line

generators open up new application fields”; Proc. SPIE Vol. 7198, (2009). 5. D. Shafer; “Gaussian to flat-top intensity distributing lens”; Optics & Laser Technology, Vol. 14, Issue 3, pp. 159-

160, (June 1982). 6. M. Traub, H. D. Hoffmann; H. D. Plum, K. Wieching, P. Loosen, R. Poprawe; “Homogenization of high power

diode laser beams for pumping and direct applications”; Proc. SPIE Vol. 6104, (Feb. 2006).

Copyright 2011 Society of Photo-Optical Instrumentation Engineers. Speaker: Dr. Bernd Köhler, DILAS Diodenlaser GmbH Paper Title: New Approach for High-Power Diode Laser Modules with Homogenized Intensity Distribution Session 5: High Power Laser Diodes Paper: 7918-29 of Conference 7918 Date: Monday, 24 January 2011 Time: 1:40 PM - 5:10 PM Copyright 2011, Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.