Peter Loosen

High power diode lasers : technology and applications

1Motivation and introduction 2High power diode laser technlogy and characteristics 2.1Principles of diode operation 2.2Manufacturing technology 2.3Chip characterization methods and operation 2.4Broad area emitters and arrays 2.5High brightness emitters and arrays 3Packaging of diode laser bars 3.1General aspects 3.2Mounting of diode laser bars 3.3Cooling 3.3.1Introduction 3.3.2 Conduction cooling 3.3.3Micro-channel heatsinks 3.3.4Heatspreaders 3.4Expansion-matched packages 3.5Mounting of micro-optics 4Stacking and incoherent superposition 4.1 Introduction and Survey 4.2 Beam collimation 4.3 Techniques for Beam Combination 4.4 Stacking Techniques 4.5 Beam symmetrization and fiber coupling 4.6 Beam-Quality limits and comparison to coherent coupling 5 Laser systems: beam characteristics, metrology and standards 5.1 Introdiuction 5.2 Theoretical background of beam propagation 5.2.1 Preliminaries 5.2.2 Temporal integration, coherence 5.2.3 Wigner distribution 5.2.4 Propagation of the Wigner distribution through linear optical systems 5.3 Density distribution 5.3.1 Power density distribution in the far field 5.3.2 Width of a power density distribution in a transverse plane 5.4 Propagation of the beam width 5.4.1 Theoretical background 5.4.2 Beam classification 5.4.3 Propagation of the beam width of stigmatic and simple astigmatic beams 5.5. Measurement of the beam power 5.6 Measurement of the power density distribution and the beam propagation ratio, M^2 5.6.1 Camera systems 5.6.2 Mechanical scanning devices 5.6.3 Measuring beam caustics 5.6.4 Power density in the far field measurement 5.6.5 Evaluating the widths of ameasured power density distribution 5.6.6 Determination of the beam propagation ration, M^2 5.7 Beam positional stability 5.8 Wavefront of a laser 5.9 Lifetime 5.10 Table of international standards related to laser metrology 5.11 References 6 Diode laser systems 6.1 Introduction 6.2 Multi purpose laser systems 6.2.1 Optical cutting plotter with 100 W 6.2.2 Free space propagation systems in the kW range 6.2.3 Fibre coupled system in the kW range 6.2.4 High brightness system with 100 W 6.2.5 High brightness kW system 6.3 Modular diode laser systems 6.3.1 Soldering laser, integrated into a gripping tool - pick and join 6.3.2 Individually addressable intensity line 6.3.3 Line modules for contour adapted plastics welding 6.3.4 Diode laser line cutter 6.3.5 Annular diode laser tool 6.3.6 Process controlling modular diode laser system for transformation hardening 6.3.7 Ring shaped laser for laser assisted machining 6.4 List of symbols 6.5 References 7 Applications 7.1 Joining technologies 7.1.1 Introduction 7.1.2 Metal welding 7.1.3 Brazing 7.1.4 Soldering 7.1.5 Laser Beam Welding of Thermoplastics 7.1.6 References 7.2 Cutting and laser assisted machining technologies 7.2.1 Introduction 7.2.2 Precision cutting with the optical cutting plotter 7.2.3 Single-shot cutting-to-length 7.2.4 Oxygen cutting with annular beam 7.2.5 Laser asisted machining 7.3 Surface treatment 7.3.1 Introduction 7.3.2 Hardening 7.3.2.1 Process principles and equipment 7.3.2.1.1 Differences between / principles of hardening and remelting 7.3.2.1.2 Absorption depending on angle of incidence, material, surface roughness,

Beam shaping and fiber coupling of high-power diode laser arrays

A technique for coupling the radiation of a high-power diode laser bar into one multimode fiber with high efficiency, easy alignment requirements and low manufacturing costs is demonstrated using a single fiber with 400 micrometers core diameter. The principal item of the fiber-coupling system is a pair of micro step-mirrors--a novel design for beam shaping. The overall efficiency from diode-laser to fiber is 71% with 20 W cw laser power through the fiber. Polarization and wavelength multiplexing renders the system scaleable to higher output power which makes it highly suitable for material processing and pumping of lasers.

Diode laser modules of highest brilliance for materials processing

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

Compact fiber-coupled high-power diode-laser unit

We developed a compact fiber-coupled high-power diode-laser unit with optical output power up to 40 W cw, coupled into a multimode fiber with 600 micrometers core diameter and NA 0.22. This diode-laser unit is suitable for pumping solid-sate or fiber lasers as well as for material processing. Essential part is a novel beam-shaping system with compact size, high flexibility and low alignment requirements, which uses a pair of micro step-mirrors. The whole unit fits into a housing of approximately 110 X 100 X 90 mm.

Optimization of microchannel heat sinks for high-power diode lasers in copper technology

Basis of the developments discussed in the presentation are 10 mm GaAs diode laser bars mounted on copper micro channel heat sinks. Optimizing the micro channel heat sinks leads to decreased thermal resistance and decreased pressure drop. In the presentation the steps to ten times reduced pressure drop and optical power output of the diode lasers of over 100 Watts will be described.

Development of simultaneous laser welding process applied to thermoplastic polymers

Transmission laser welding of thermoplastic polymers offers innovative solutions to overcome limitations of conventional joining technologies. Different process strategies provide to flexibility, short process time and high quality weld seams. However, even if laser beam welding of thermoplastics becomes widely used in industry, important developments are necessary with regard to the assembly process and the development of specific polymers. Generally, the assembly is done by contour welding or quasi-simultaneous welding (using scanning heads). Simultaneous welding can be another interesting alternative as well the possibility to achieve direct simultaneous welding while using an arrangement of compact and modular set up of power diode laser. This process shows important advantages as the short process time, no dynamic motion system and increased gap bridging capability. Main constraints are the control of the beam intensity distribution and the lack of flexibility concerning the seam geometry. A specific optical system to allow simultaneous welding using a conventional fibre coupled laser was developed. This beam shaping optic assures an homogeneous intensity distribution and can easily be exchanged when changing the geometry of the seam.An example of electronic packaging using this innovative process is presented in this article.Transmission laser welding of thermoplastic polymers offers innovative solutions to overcome limitations of conventional joining technologies. Different process strategies provide to flexibility, short process time and high quality weld seams. However, even if laser beam welding of thermoplastics becomes widely used in industry, important developments are necessary with regard to the assembly process and the development of specific polymers. Generally, the assembly is done by contour welding or quasi-simultaneous welding (using scanning heads). Simultaneous welding can be another interesting alternative as well the possibility to achieve direct simultaneous welding while using an arrangement of compact and modular set up of power diode laser. This process shows important advantages as the short process time, no dynamic motion system and increased gap bridging capability. Main constraints are the control of the beam intensity distribution and the lack of flexibility concerning the seam geometry. A specific...

Diode laser systems for cutting applications of thin materials

For cutting applications of sheet thicknesses up to 1u2005mm a compact diode laser system of high brilliance has been developed. By the combination of three different wavelengths as well as two polarization states six linear polarized diode laser bars are superimposed nearly without loss in beam quality. The maximum optical output power amounts to 155u2005W at a beam parameter product of about 22u2005mm mrad. The wavelength- and polarization-multiplexing is realized with one cemented optical component consisting of edge filters, a λ/4-retardation plate and a polarization beam splitter. The linear shaped beam of the six superimposed diode laser bars is symmetrized by micro step mirror beam shaping technology. Limited by manufacturing tolerances and particularly the mounting process of the diode laser bars, the system reaches the limits of incoherent beam multiplexing of six broad area diode laser bars. Combined with beam expanding and focussing optics, cutting experiments on different metallic and organic materials with sheet thicknesses ranging from 0.1 to 1u2005mm have been realized.For cutting applications of sheet thicknesses up to 1u2005mm a compact diode laser system of high brilliance has been developed. By the combination of three different wavelengths as well as two polarization states six linear polarized diode laser bars are superimposed nearly without loss in beam quality. The maximum optical output power amounts to 155u2005W at a beam parameter product of about 22u2005mm mrad. The wavelength- and polarization-multiplexing is realized with one cemented optical component consisting of edge filters, a λ/4-retardation plate and a polarization beam splitter. The linear shaped beam of the six superimposed diode laser bars is symmetrized by micro step mirror beam shaping technology. Limited by manufacturing tolerances and particularly the mounting process of the diode laser bars, the system reaches the limits of incoherent beam multiplexing of six broad area diode laser bars. Combined with beam expanding and focussing optics, cutting experiments on different metallic and organic materials wi...

267-W cw AlGaAs/GaInAs diode laser bars

High-power 980 nm-diode laser bars have been fabricated in the AlGaAs/GaInAs material system. The bars are 1 cm wide and comprise 25 broad area lasers with 200 micrometer aperture and 2 mm resonator length. Hence, the fill factor is 50%. To reduce the power density at the facet, we used an LOC structure with low modal gain, which also helps to prevent filamentation. The measured threshold current was 14 A and a record output power of 267 W cw was achieved at 333 A with an electro-optical conversion efficiency of 40%. With less thermal load, at 150 W output power the conversion efficiency was as high as 50% and the corresponding slope efficiency was 0.9 W/A. Microchannel copper heat sinks with a thermal resistance of less than 0.29 K/W were used for mounting the bars. The coolant temperature was set for all measurements to 22 degrees Celsius and the flux was 0.9 l/min. Additionally, the top electrode of the p-side down mounted bars was cooled by a second heat sink, which was pressed gently on the top electrode.

NEODYMIUM:YAG 30-W CW LASER SIDE PUMPED BY THREE DIODE LASER BARS

A theoretical calculation of pump power deposition in a direct water-cooled Nd:YAG laser rod, side pumped by three diode laser bars is presented. The pumping cavity design provides a nearly uniform pump profile. More than 30-W cw output power with optical-to-optical efficiencies of more than 30% are obtained.

Design and industrial applications of high-power laser diodes

Developments concerning high-power diode lasers, optical systems for beam shaping and superposition and complete systems for direct materials processing are discussed, along with examples of their industrial applications.