We have investigated high-power diode laser bars from 808 nm to 980 nm. The presentation is focussed upon the development of suitable laser bars for improved beam quality at increased output power. For better beam shaping structures with reduced fill factor of 30% were developed. They were operated in continuous wave mode at power levels of up to 60W. Moreover industrial applications require lifetimes of more than 10,000 hours. We present data yielding an extrapolated lifetime of up to 100,000 hours at 40W with 60% wall-plug efficiency at 980nm cw.

Novel waveguide structures are presented that facilitate high power, single lateral mode output in narrow stripe semiconductor lasers. Flared tapered waveguide lasers, fabricated by a metal-organic chemical vapor deposition (MOCVD) selective area epitaxy (SAE), are shown to attain output powers of 650mW with stable single lateral mode beam properties. Novel integrated mode filters, which induce mode selective lateral radiation loss via curvature, or frustration of the index guide, are shown to increase the threshold for the 1st order mode to prevent it from attaining threshold. The addition these unique mode filters, which do not increase the fabrication complexity, extends the range of single lateral mode operation in narrow stripe devices.

The main challenge to address single emitters in a high-power diode-laser-bar is the thermal and electrical management to avoid crosstalking. Especially p-side up assembly leads to increasing thermal influence of neighbouring emitters due to the low thermal conductivity of GaAs. Electro-magnetic fields inside and outside the laser-bar, for example caused by high frequency modulation (10 MHz) at a high current (up to 1 A), induce voltages into neighbouring electric circuits, hence the output power of neighbouring emitters can be affected.

In order to achieve a thermally stable diode laser system based on high power diode laser bars, actively cooled heatsinks in form of micro channel heat sinks (MCHS) are used to face the power loss density of 10⁶ W/m² while requiring a minimum device volume. At identical junction temperature, passively cooled diode lasers are usually lower in power and the device volume is much higher due to the heat flux spreading design of passive heatsinks. However, as a matter of principle, the cooling with MCHS sinks requires a sealing between the heat sink itself and the system around. This sealing is usually achieved by o-rings, what can never avoid the transfer of vapor from the cooling system into the vicinity of the diode laser. Extreme requirements on availability, which lead to corresponding lifetime requirements, like in telecom applications, already require passively cooled diode lasers without any water in the inner system boundaries. For applications not requiring the extreme compact design volume of actively cooled diode lasers but requiring extreme lifetime or a minimum outlay on the periphery, we started looking into passively cooled diode laser stacks. To achieve a minimized temperature rise in the junction, we already developed a new copper-based heat sink, spreading the power loss in an optimized manner. Based on this heatsink, we started developing a heat exchanger with a low thermal resistance while keeping the water out of the inner system boundaries. The thermal resistance is low enough to run up to 12 passively cooled diode lasers on a low ambient temperature with a minimum of periphery requirements.

YAG lasers have been the only practical choice for high power fiber coupled lasers in the kilowatt and above range. In recent years, however, high power diode laser systems have made inroads into the high power fiber coupled laser arena. In the past, fiber coupled high power diode laser systems have suffered from either sub-par power levels or poor beam quality (i.e. large fiber diameters with large Numerical Apertures). This paper presents a high brightness high power fiber coupled diode laser system that rivals some YAG systems. This system will deliver 1000W of power to the work piece through a 1000 micron 0.20 NA (100 mm-mrad) fiber at a single wavelength. With a wide spectrum of commercially available wavelengths, this system can be optimized for material processing or laser pumping applications (being ideally suited to the end pumping of high power fiber lasers).

The use of diode laser bars has traditionally been limited in many applications because of their poor beam quality in the plane of the junction (slow direction). Spectral Beam Combination (SBC) corrects this defect with minimal impact on efficiency. Aculight has demonstrated efficient SBC of nearly 400 single transverse mode semiconductor lasers. The semiconductor lasers were packaged in two adjacent 1-cm wide, 200 emitter bars. The M² of a single 1-cm wide diode bar is typically greater than 1000. Using SBC we have measured an output M² of less than 1.5. The output bandwidth of the device is ~10 nm; and it is this spectral width that allows combination with the result that each of the 200 lasers deliver power through the same output aperture. The efficiency of this process was measured at >70% when high quality optical components were employed. Efficiency is defined as the SBC output power normalized to the power provided by the diode bars without SBC. With single emitters we have achieved SBC efficiencies of >80%.

Two applications emerge as drivers for higher brightness fiber-coupled diode lasers: advanced solid-state pumping schemes and materials processing. In contrast to the well-established side-pumping schemes of laser rods, advanced pumping schemes for today's solid-state lasers make use of the high brightness of the pump sources to increase the performance and efficiency of the solid-state laser. Materials processing applications such as metal welding and cutting are commonly served with solid-state lasers or CO₂ lasers. Lately, the increased lifespan, reduced systems costs and increased brightness of fiber-coupled diode laser systems make them a new alternative. In this work, a diode laser system is described that yields 250 wats in a 600 micrometer spot with a numerical aperture 0.2 of the focused beam, corresponding to a F/# of 2.4. The system is based on a single 15 bar stack that operates in cw-mode. For brightness enhancement, it incorporates a measure to increase the fill factor of the emitting aperture and polarization multiplexing. The brightness in the focus spot is 10⁵W/cm² with a F# of 2.4 focusing optic and 3x10⁵W/cm² with a high speed of F/# of 1.4. To achieve the required symmetry for fiber coupling, the system incorporates a beam transformer that assimilates the beam quality along the two main axes of the beam profile. A monolithic design is chosen to reduce alignment tolerances and to increase ruggedness.

For high power laser applications where optical fibers are used the main problem is to keep the power losses as small as possible. To fullfill this demand it is important to utilize the power from the laser efficently and avoid damages on the fiber due to high temperature caused by radition outside the fiber core. Thererfor the design has to be made in a robust way that protect the fiber from damages caused by the high power. Depending on the beam quality from the laser two different kind of fibers have to be used, a fiber with polymer cladding for high NA and silica cladded fibers for lower NA. In both cases the core is made of fused silica. Since there is always some radiation not keept within the fibre core the cooling is of great importance. The radiation can be caused by scattering light from the beam itself or from back reflection in the application. Different kind of design used to keep the power losses as small as possible as well as design of efficient cooling systems will be discussed in this paper. Experimental results from our development of optical fibers will be present.

In order to optimize the soldering process of laserbars onto heatsinks with Indium solder, several investigations have been made. First the growth of Indium oxide film is examined. With this knowledge four different reduction materials are selected. Formic acid as a wet chemical reduction, a plasma activated Hydrogen/Argon gas, a gas enriched with formic acid, and a protective layer of Gold were investigated and compared for an optimized reduction of the oxide film of the Indium solder. A cross section of the solder interface after the soldering process is made in order to see the distribution of the metals. High diffusion of the solder with its contact partners is a sign of a good connection. Enough pure Indium has to be available after the soldering process in order to use its creek properties to reduce the mechanical stress in the laserbar.

The National German research project "MDS", which stands for the German equivalent of "Modular Diode Laser Beam Tools", is now in its fifth year. The aim of the project is the development of new diode laser technology as well as novel applications, especially taylored to the special features of high power diode lasers. The project has a strong vertical structure: All necessary technologies for a successful performance of the project, i.e. chip-technology, laser-system-technology and applications are represented in the project. The aspect of laser systems can be divided into two fields: one is of course improving the beam quality and power, i.e. brilliance of conventional diode laser systems; this can be reached by further development of the semiconductors the other is using the individual bars and assembling them in a special way, which is adapted to the application. That's what is meant by the word "modular". This paper will mainly focus on the special systems and their applications and illuminate the actual status as well as the hurdles, which had to be overcome during the project.

Characterized by high conversion efficiency, small size, light weight and a long lifetime, high power diode lasers are currently being developed for application to various types of metal fabrication, such as welding. In this report, a 4kW high power direct diode laser was used to weld aluminum alloys, which are the focus of increasing attention from the automobile industry because of their light weight, high formability and easy recyclability. The applicability of a direct diode laser to aluminum alloy bead-on plate, butt and lap-fillet welding was studied under various welding conditions. A sound bead without cracks was successfully obtained when 1 mm thick aluminum alloy was welded by bead-on welding at a speed of 12m/min. Moreover, the bead cross section was heat conduction welding type rather than the keyhole welding type of conventional laser welding. Investigation of the welding phenomena with a high-speed video camera showed no spattering or laser plasma, so there was no problem with laser plasma damaging the focusing lens despite the diode laser's short focusing distance.

Heat Treatment is one of the most promising application of multi kilowatt high power diode lasers. Providing a sufficient beam quality HPDL's have the advantage of their high efficiency comparing to Nd:YAG-lasers. Application of scanning mirror optics for multi kilowatt lasers is well known at CO₂- or Nd:YAG-lasers. Fraunhofer IWS has developed a special driver unit, which generates automatically an optimized scanning function to provide a stress adapted intensity profile. Know the application of this technology at multi kilowatt high power diode lasers has been implemented successfully. Using 2.5 kW diode laser power hardening tracks of 30 mm in width and a penetration of about 1 mm are possible. Applying the temperature guide laser power controller LompocPro additionally, stress adapted hardening of edges with varying cross sections became possible. Besides hardening this system allows heat treatment with a rectangular beam of 5 x 85 mm². Some applications show the performance of this technology.

The high power diode laser is a new industrial tool. It has several advantages and disadvantages compared to the conventional industrially used CO₂ and Nd:YAG laser. The most promising areas of application of diode laser have been considered to be thin sheet welding and hardening. Quite a few feasibility studies of the use of diode laser have been carried out in Finland. So far there has been some application in which diode laser is the most suitable laser. Typically, the HPDL is integrated to an industrial robot. The welding of stainless steel housing, car door lock and catalytic converters are typical examples of applications in which diode laser has technological as well as economical advantages over the conventional laser and welding techniques. The welding of these products requires good control over the heat input, short through put time and low investment. The weld cross-section of a diode laser weld is, because of conduction limited welding process, more suitable for these applications than the keyhole welding. Hardening of a large gear wheel presents also a good example of an application in which the diode laser makes it possible to economically produce structures that have not earlier been possible. Hardening requires a special form of heat delivery in order to ensure evenly hardened zone and acceptable quality. The application was performed with two high power diode lasers. The case studies of these four applications are presented and discussed in details in this paper.

High power diode laser (HPDL) is the newest laser tool for industrial manufacturing. The most promising areas of application of HPDL are thin sheet welding and hardening. The HPDL has several advantages and disadvantages compared to lasers CO₂ and Nd:YAG lasers currently used for welding. There is quite a few industrial applications in which diode laser is the most suitable laser. A typical industrial installation consists of a HPDL, an industrial robot, work piece manipulation and safety enclosures. The HPDL welding process is at this moment conduction limited and has therefore different parameters than the keyhole welding. In this study the basic HPDL welding parameters and the effect of the parameters on the welding process, weld quality and efficiency are examined. Joint types tested are butt joint and fillet lap joint. The parameters tested are beam intensity, welding speed, spot size, beam impingement angle. The materials tested are common carbon steel and stainless steel. By the experiments carried out it can be seen that all of these parameters have an effect on the weld quality and the absorption of the laser power during welding. The higher the beam intensity is the shorter also the throughput time is. However, in case of fillet joint the maximum welding speed and best visual out look are achieved with totally different set of parameters. Based on these experiments it can, however, be seen that reliable welding parameters can be established for the welding of various industrial products. The beam quality of the diode laser is not optimum for high speed keyhole welding but it is a flexible tool to be used for different joint types.

Solid solution strengthened cobalt based Stellite 21 hardfacing layers produced by off-axis and newly developed HPDL multi-feeder cladding techniques were compared. The properties studied include thickness of the heat-affected zone (HAZ), dilution, microstructure, microhardness and powder catchment efficiency. Both off-axis and multi-feeder cladding techniques were found to give pore and crack-free coating layers metallurgically bonded to the substrate. Energy dispersive spectroscopy (EDS) studies indicated that only a limited amount of dilution took place in all the coatings. Microhardness values (HV1) were observed to be in the range of 360 - 387 HV1 in as-laser-clad condition. Due to more stable cladding process and use of two side feeders, which produced more homogenous powder cloud along the laser beam axis and more accurately focused powder stream to the rectangular laser beam spot, multi-feeder cladding technique was more efficient in terms of powder catchment efficiency, which was at a level of 62 - 65 %. Both powder-feeding techniques produced relatively high heat input into the workpiece. Multi-feeder cladding technique produced smoother and less oxidised surface quality than off-axis technique, because shielding gas flow was implemented differently.

High reliable diode laser pump modules are essential for free-space optical telecommunications. Besides the reliability, low mass and small dimensions, radiation-hardness and low power-consumption are requirements to be met for space applications. A diode laser module suited for pumping Nd:YAG lasers for optical intersatellite links has been developed. The module consists of two diode laser bars overlaid by a polarization beam splitter to increase the system’s reliability. Each diode laser bar consists of six emitters. If the integrated photodiode detects the failure of one bar, the second, substitute bar is switched on and can fully sustain all module functions. To equalize the beam quality of the diode laser bar, a pair of micro step mirrors is used for each bar. The laser beam is focussed on the entrance of a d=200 μm, NA=0.22 fiber. Both the coupling efficiency and the accuracy of the mounting of the diode laser components have been analyzed by raytracing. Passive cooling has been chosen because liquid chilling systems are unsuitable for space applications. To evaluate the effects of different heat sink materials and to predict the temperature drop over the module, a 3D finite element analysis for the steady-state temperature distribution of the module has been performed. The optical output power of the module described above amounts to 2,8 W with one bar operating derated to 0.5x maximum current, and the whole unit fits in a housing of 78 x 50 x 24 mm. Further developments will lead to a more compact design and a smaller fiber diameter.

Performance degradation and lifetimes of high power diode arrays are important issues for laser manufacturers and end users. To fully understand these issues long term testing and failure analysis of arrays is required. To perform this testing we have set up an automated lifetime experiment to examine the characteristics of high power arrays over time. Subsequent material analysis of the arrays will uncover failure mechanisms.

Wide-range wavelength tunability is demonstrated for commercial high-power laser diodes emitting at 980 nm, 830 nm, and at 808 nm. High pressure shifts the emission wavelength of the lasers due to the increase of bandgaps in the active layers with the rate of about 10 meV per kbar. For the 980 nm InGaAs/GaAs laser the threshold currents and the differential efficiencies remain constant with pressure which allows for the constant operating current and the emitted power in the full tuning range. For 830 nm and 808 nm GaAs/AlGaAs lasers there is an increase of threshold currents with pressure related to the leakage through X minima in the conduction band of AlGaAs. This limits the tuning range unless we operate the laser at lower temperature. We designed the pressure cell with Peltier cooling allowing for independent control of temperature down to 0 Celsius and pressure up to 20 kbar. The laser beam passes through the sapphire window or through the multi-mode fiber. Our device allows for the tuning of 980 nm laser down to 840 nm, 830 nm laser down to 745 nm, and 808 nm laser down to 720 nm. We were able to keep the output power fixed in the full tuning range: 300 mW for the 980 nm laser and 400 mW for the 830 nm and 808 nm lasers.

A lumped-parameter mathematical model has been developed for the laser soldering process and validated experimentally by measuring the temperature of solder preforms placed on a lead on a printed circuit board when heated with laser. This model results in four first-ordered nonlinear differential equations, which can be solved with MATLAB. Further, a comparison has been made between diode laser, and Soft Beam (xenon-arc white light) soldering with respect to the solder joint quality, cycle time and input optical energy requirements. In addition, scanning electron microscopic studies have been done to compare the microstructure of the solder and the formation of intermetallics, when the solder is melted with laser, Soft Beam and a conventional soldering iron. It is found that the diode laser soldered joint has a thinner intermetallic and finer grain size resulting in a higher mechanical strength.