, LASER

High Power Diode Lasers

for Industrial ApplicationsWolfgang Horn

Fig.1 Customized laser module for simultaneous spot welding.

Wolfgang Horn,DILA5 Diodenlaser

GmbH, Galileo-Gali­lei-5traBe 10,55129

Mainz, Germany

New wavelengths, high bright­ness and increasing reliabilityare breaking new ground for diodelasers. The higher brightness allowsdirect fiber coupling of several100 W in 200 fim and 400 fim fibersand therefore i.e. the pumping offiber lasers. Such high power fibercoupled laser diodes combined with

fast beam deflection units (galvoscanners) have been implementedin industrial processes and used

for quasi-simultaneous welding ofpolymers, heat treatment in the submillimeter range or selective sol­dering in solar cell production. Be­sides pumping of solid state lasers,high power diode lasers becomemore and more a competitive toolfor many applications in materialprocessing.

Reliability

High power diode lasers mainlyconsist of a diode laser bar, a heatsink on which the bar is mounted

and some micro optics. Most im­portant for a proper function ofthe laser diode is an effective heat

removal by the heat sink. In mostlaser systems two different types ofheat sinks are used.

Micro-channel heat sinks with

DI-water coolingDI (de-ionized) water cooledmicro-channel heat sinks have themost effective heat removal and can

therefore operate on a high powerlevel. Manufacturing very compactmulti kilowatt laser modules bystacking the laser diodes is a bigadvantage for this technology.

On the other hand the coolingcircuit has to fulfill high demandsfor water quality. Purity, conduc­tivity, flow rate and temperatureneed to be set within a certain

range to guarantee a maximum

lifetime. For industrial production

regular maintenance and stockingof consumables like particle filtersand DI-cartridges has to be takeninto account. But even with perfectwater conditions the flow insidethe micro channels causes wearand therefore the lifetime of a laser

diode is in most cases limited by thelifetime of heat sink.

Massive copper heat sinks for

conduction coolingConduction cooled diode modulescombined with thermal electrical

chillers (TEe) don't need any waterat all. For higher laser power theTEC gets less efficient and has tobe replaced by a water cooled plate.Compared to micro channel heatsinks the heat removal and there­

fore the maximum laser power islimited. Direct stacking of conduc­tion cooled diodes is not possible.

The demands for the cooling cir­cuit and the integration of a conduc­tion cooled laser diode into a turn­

key system are low. The temperatureof the diode is measured inside theheat sink. Additional flow or tem­

perature sensors are not needed.

The lifetime is mainly limited by thesemiconductor material because theheat sink is free of wear. Conduction

cooled diodes are mostly used forfiber coupling where the size of thelaser module is not as important asfor direct beam applications.

But for some customized so­

lutions it can be advantageous touse the direct beam. Figure 1 showsa laser module which is used for

simultaneous welding of 10weldingspots. Four of these modules areintegrated in a production line forsyringes. The module consists of10 conduction cooled laser diodesmounted on a common water

cooled plate. [1]Besides all other aspects reliabi­

lity and availability are most impor­tant issues for production. Duringthe last years diode lasers improvedto fulfill the demands of industrialstandards. Micro-channel and con­duction cooled diodes both show

lifetimes of more than 14,000 hours

cw operation (Fig. 2) and 16 millioncycles in pulsed mode (Fig. 3) with

minimal degradation. Byextrapo­lating these data the estimated chiplifetime is more than 40,000 hours.

Depending on the operating condi­tions, such diode lasers have alrea­

dy proven to run for several years inproduction.

Higher Output Power and

New Wavelength

New semiconductor material de­

livers more laser power per bar.InGaAIAs based chip material withemission at a wavelength of 808 nmhas shown up to 120W laser power.

16 Physics' Best January 2008 © 2008 Wiley-VCH Verlag GmbH &Co. KGaA,Weinheim

Fig.2 Lifetime tests with Dj-water (pink) and conduction

cooled (blue) single diode laser bars in cw operation.

oo 2000 4000 6000 8000 10000 12000 14000 16000

Time [hours]

product range, starting from singlebars up to full featured, stand alonelaser systems.

The main applications in mate­rials processing in the power rangeup to several hundred Watts aresoldering and polymer welding.Especially in polymer welding thediode laser has technical advantagescompared with other laser typesand technologies like ultrasonicwelding. For typical widths of thewelding seams of more than 1mmthe homogeneous energy distributi­on in the focal plane with a big laserspot, compared with other lasers,leads to excellent results. The usageof galvo scanners makes the highlyflexible diode lasers even more

flexible and allows quasi simulta­neous welding of closed contoursoften needed for sealing all typesof housings used in medical devicemanufacturing and automotiveproduction. As part of the ROFIN

group, DILAS can offer galvo so­lutions with an advanced software

package used for marking systemsand world wide support.

High power diode laser are alsoused as a flexible and precise tool

IIII

II

I IiI

! II

II

iI

II

I

i

I

I IIII60beam quality, the coupling into

smaller fiber diameters with higherlaser power opened new optionsfor material processing. Due to theadvantages for industrial produc­tion most of the systems today areequipped with fiber coupled diodelasers. Even if the laser module

needs to be exchanged, an easy on­site replacement can be done withina short time. The possibility of aplug and play diode exchange is in­creasing the availability of the lasersystem for production. Because ofthe independence oflaser sourceand processing head new teachingof the process is not necessary: theprocessing head stays inside thelaser cell. Using conduction cooleddiode laser modules the system canrun nearly maintenance free. Nocomplex and costly cooling unit isneeded. Up to 100 W laser outputcan be generated using an air cooledPeltier-chiller without any water.

A turn-key diode laser systemfeatures the laser module and all

its supply and control units in a 19"rack mountable chassis. The set

up gets completed by a fiber, beamshaping and/or focusing opticswith complementary features forprocessing control and surveil­lance. For example the integratedpyrometer has been established as ahelpful tool for many applications.Turn key systems become morecompact and easy to integrate.Figure 5 shows the new DILAS turn

key diode laser system family: TheCOMPACT series which fulfilsall demands of a state-of-the-art

production tool. It will be intro­duced on the LASER 2007 show in

Munich and completes the DILAS

New Applications Arising

Compared with diodes used todaythe efficiency increased from about55 % (60 W output power) to morethan 62 % (100 W output power) [2].

With InGaAs based semicon­

ductor material at 940 nm opticalpower of up to 170 W can be achie­

ved. Such highly efficient chips al­low the increase of output power ofdiode stacks and modules without

any other design changes. The on­going lifetime tests show promisingresults. With the good beam qualityavailable, the laser radiation can be

coupled into 200 11mand 400 11mfibers. Fiber coupled diode lasersare not only used for materialsprocessing but also for pumping ofsolid state lasers and especially forfiber lasers.

With higher laser power also fornon-standard wavelengths a largenumber of different applicationscan be addressed (Table 1). Just tomention InGaAs on InP substrates

for wavelengths> 1300 nm and(AlGaIn)(AsSb) on GaSb substratesfor wavelengths> 1800 nm. In the­

se wavelength ranges, high powermeans 10... 20 W per laser diode bar.

Examples are direct medical oraesthetic applications like photo­dynamic therapy and hair removal.Printing industry and defence tech­nology take also advantage of de­dicated wavelengths. Table 1 gives anoverview about typical applicationfor different wavelengths.

Several years ago most integratorsused direct diode lasers in their

production lines. With increasing

7014'

6012"1max, efficiency: 63.52 %

::;-50

10

~40~ 80~d: 606" 30

20

~O I

20

060

0 20400

24681012141618 Current (A)MShot 70

60 ~

50 [

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40 ~.

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"~l,~!f!f.k!.7:'~m1080 100 120

Fig.3 Lifetime tests with single diode laser bars in pulsed ope­ration (1,5Hz with 50 % duty cycle from 0 to nominal power).

Fig.4 Lifetime tests with high output power (micro channelheat sink).

© 2008 Wiley-VCH Verlag GmbH & Co. KGaA.Weinheim Physics' Best January 2008 17

LASER

~

• DILAS ...••• •

Fig.5 Turn key diode laser system for

production: DILAS COMPACT.

in solar cell manufacturing. Mostsilicon solar cells are interconnected

with pre tinned copper ribbons to

strings by conventional soldering.The soldered strings are then lami­nated into modules. The handling of

these long and fragile strings is diffi­cult and needs complex equipment.Using In-Laminate Laser Soldering(ILLS) technique developed by theInstitute for Solar Energy ResearchHameln (IFSH) the very thin solarcells are interconnected to a module

by placing cell and ribbon on theirfinal position onto the laminate. Thelaser beam is soldering cell with rib­bon and the handling of the stringscan be avoided completely.

The achieved electrical contactresistance is below 0.1 m [2 cm2 and

the peel forces exceed lON/em. Asknown from electronic productionthe laser is well suited for the useof lead free solder. To increase the

process stability the soldering isdone with a closed loop tempera­ture control. The pyrometer is inte­grated in the processing head andaligned in the optical path of thelaser beam. Compared to standardsoldering methods laser solderingis contactless and produces nomechanical stress. The localized

heating minimizes the thermalstress for the solar cells which have

a thickness below 200 fim. The ILLStechnology allows producing thin­ner and therefore cheaper siliconwafers. [3]

Summary and Outlook

Diode laser bars and modules have

become a reliable and long living

product. The conduction cooleddiode lasers can be operated witha simple cooling device. New semi­conductors allow high efficientfiber coupling in small fibers. Allthese attributes put the availability

of diode laser systems in an indus­trial production chain on a veryhigh level.

The joint research initiativeBRIOLAS of the Federal Ministryof Education and Research (BMBF)

in line with the actual research pro­

gram "Optical Technologies for the21st Century" is following the incre­asing demands from the market toimprove brilliance and reliability ofhigh power diode lasers. With newtechnologies the field of technicallyfeasible and economically reasona­

ble applications will be expanded.

References[1] W Horn, Flexible kundenspezifische

Losungen zum KunststoffschweiGen,6. Workshop Anwendung von

Hochleistungsdiodenlasern, Dresden,Germany (2006)

[2] New generation oflaser bars in the

starting blocks: World record efficiencyfor 808 nm achieved in the laboratory,

www.briolas.de. April 2006

[3] M. Gast, M. Kontges, R. Brendel, In­Laminate Laser Soldering - A GentleMethod to assemble and interconnect

silicon solar cells to modules, 21st Euro­

pean Photovoltaic Solar Energy Confer­

ence, Dresden, Germany (2006)

Tab. 1:Diode laser applicationsWavelength

Application Industry

630 - 635 nm

Medical Photodynamic Therapy Medical

652 n m

Medical Photodynamic Therapy Medical

668 nm

Medical Photodynamic Therapy Medical

670 nm

Cr'·: LiSAF - fs-Laser DPSSL

689 nm

age-related macular degeneration Medical

730 n m

Medical Photodynamic Therapy Medical

780 nm,f1A «1 nm

Diode Pumped Gas Laser (Rubidium Vapor)Defense

785 n m

TM'·: YAG => 2 Ilm DPSSL

792 f 797 nm

NdH :YLF DPSSL

795 nm, f1 A < 1 nm

Rb'· f Xe'39f - pumping Instrumentation

805 f 808 nm

Nd:YAG DPSSL

810 + 10 nm

hair removal, etc. Medical, Material Processing

830 nm

Pre-Press, Computer to plate (CTP), Direct on press (DOP)Printing

852 nm, f1A« 1 nm

Diode Pumped Gas Laser (Caesium Vapor)Defense

868 - 885 nm

Nd'· : XXX (various host crystals) DPSSL

901 nm

Vb'· : SFAB DPSSL

905 nm

Rangefinder direct Instrumentation

915 nm

Vb: Glas, Fiber Laser, Medical DPSSL, Medical

940 nm

Vb'· :YAG, Disk DPSSL

968 nm r:d < 1 nm

YbH : YAG, Disk DPSSL

973 - 976 nm

Vb'· : Glass, Fiber Laser DPSSL

980 ± 10 nm

Medical Medical, Material Processing

1064 nm

Medical Medical

1330 - 1380 nm

Medicai Medical

1450 - 1470 nm

Acne, Turbulence Detection Er 3+ pumpingMedical, various others

1530 nm

Rangefinder Defense

1700 nm

Missile Defense Defense

1850 nm

Turbulence Detection, Plastic weldingDefense, Material processing

18 Physics' Best January 2008 © 2008 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim