This PDF file contains the front matter associated with SPIE Proceedings Volume 6876, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

We demonstrate a technique for collimating conduction-cooled QCW diode laser stacks that achieves very high brightness in a compact and robust package. First we collimate the bars in the fast-axis using a pre-aligned array of Doric GRIN cylindrical lenses, where each lens is oriented to correct the gross positioning errors of each bar. We then measure the residual beam errors using a proprietary wavefront mapping system, and fabricate a refractive wavefront correction phaseplate to effectively flatten the wavefront. The type of lens used exhibits particularly low aberration in the presence of misalignment, ensuring that the resultant wavefront can be effectively corrected. We applied the technique to two 0.5 mm pitch, 12-bar stacks operating at 1.2 kW. By this method, we repeatably obtained a 10-fold increase in stack brightness, reducing fast-axis beam divergence for the entire stack to below 0.3°, close to the theoretical limit. The result is an extremely compact, high brightness source optimised for side-pumping thin slab lasers.

We report a novel coolant recirculation loop for high-power laser diodes which allows operating diode bar heat exchangers at their design coolant Reynolds number while consuming as little as 20% of their nominal coolant supply inflow. While operating in a single-phase regime, the new concept uses low coolant flow-rates comparable to proposed evaporative coolers. Unlike evaporative coolers, the new concept is compatible with many standard microchannel or impingement-type heat exchanger designs. The theory, design, and applications are presented.

High power water cooled diode lasers find increasing demand in biomedical, cosmetic and industrial applications, where very high brightness and power are required. The high brightness is achieved either by increasing the power of each bar or by reducing the emitting area of the stacks. Two new products will be presented: Horizontal CW stacks with output power as high as 1kW using 80 W bars with emitting area width as low as 50 μm; Vertical QCW stacks with output power as high as 1.2kW using 120 W bars. Heat removal from high power laser stacks often requires microchannel coolers operated with finely filtered deionized (DI) water. However, for certain industrial applications the reliability of this cooling method is widely considered insufficient due to leakage failures caused the highly corrosive DI water. Two solutions to the above problem will be discussed. A microchannel cooler-based package, which vastly reduces the corrosion problem, and a novel high-power laser diode stack that completely eliminates it. The latter solution is especially effective for pulsed applications in high duty cycle range.

We present kW QCW vertical and horizontal arrays composed of 200W bars (peak power) at 8xxnm wavelength. We also present an unique Bar-on-Submount design using the electrically insulating submounts, which can provide a platform for simple and flexible horizontal array construction. The p-n junction temperature of the arrays under QCW operation is modeled with FEA software, as well as measured in this research. Updated reliability test results for these kW arrays will be also reported. As the examples, we present the performance of the vertical arrays with > 57% Wall-Plug-Efficiency and the horizontal arrays with < 23 degree fast axis divergence (FWHM), both with 808nm wavelength. The available wavelength for such arrays ranges from 780nm to beyond 1 um. Coherent also have the capability to produce the array with wide and relatively uniform spectrum for athermal pumping of solid-state lasers, by integrating diode lasers bars with different wavelength into single array.

The power consumption of semiconductor diode laser bars has continually increased in recent years while the heat transfer area for rejecting the associated thermal energy has decreased. As a result, the generated heat fluxes have become more intense making the thermal management of the laser systems more complicated. A common solution to this problem is to use the microchannel cooler, a small liquid enhanced heat sink capable of rejecting heat fluxes higher than those of finned air sinks of comparable size. The objective of this study is to improve and enhance heat transfer through an existing microchannel cooler using the computational fluid dynamics technique. A commercial software package is used to simulate fluid flow and heat transfer through the existing microchannel cooler, as well as to improve its designs. Three alternate microchannel designs are explored, all with hydraulic diameters on the order of 300 microns. The resulting temperature profiles within the microchannel cooler are analyzed for the three designs, and both the heat transfer and pressure drop performances are compared. The optimal microchannel cooler is found to have a thermal resistance of about 0.07°C-cm²/W and a pressure drop of less than half of a bar.

Here we present characteristic performance of laser-diode devices employing a novel CTE-matched heatsink technology (where CTE is Coefficient of Thermal Expansion). Design variants of the composite-copper platforms include form-fit-compatible versions of production CS (for standard 1-cm-wide bars) and CT (for single-emitter devices and mini-bars) assemblies. Both employ single-step AuSn bonding and offer superior thermal performance to that of current production standards. These attributes are critical to reliability at high powers in both CW and hard-pulse (e.g., 1sec on/1sec off) operation. The superior thermal performance of the composite-copper CS device has been verified in CW testing of bars where 85W is typically obtained at 95A (compared to 76W from production-standard, indium-bonded, solid-copper CS devices). This result is especially significant as alternative CTE-matched bar platforms (e.g., those employing a sub-mount bonded to a solid copper heatsink) typically compromise the effective thermal resistance in order to achieve the CTE match (and often require two-step bonding). The close CTE match of the composite-copper CS results in relatively narrow, single-peaked spectra. Initial step stress tests of eight devices in hard-pulse operation up to 80A has been completed with no observed failures. Six of these devices have subsequently been operated in hard-pulse mode at 55A for >4000 with no failures. The CT variant of the composite-copper heatsink is predicted to offer a reduction in thermal resistance of nearly 30% for a 5-emitter mini-bar (500-μm pitch). In first-article testing, the maximum achievable CW power increased from 20W (standard CuW CT) to 24W (composite-copper CT). As with the CS devices, the composite-copper CT assemblies exhibited characteristically narrower spectral profiles.

A next-generation microchannel cooler has been developed for packaging laser diode arrays that eliminates many of the problems associated with typical copper-based cooling designs. The coolers are built on well-established Low-Temperature Cofired Ceramic technology and provide excellent thermal performance. They do not require the use of deionized water. This work highlights the strengths of the new cooler technology. The results of a long-term, high-flow-rate test which demonstrates the excellent erosion resistance of these coolers are presented. Three devices have been tested for 2500 hours at a flow rate of 0.25 GPM and show minimal signs of erosion. This data is compared to a similar test conducted with copper coolers. Several design parameters are also addressed for the ceramic coolers. The available form and fit characteristics are addressed, as is the custom-configurable nature of the devices.

High power diode laser bars require packages with a high cooling efficiency and long-term stability. Due to the increasing output power of the diode laser bars the thermal resistance of the packaging becomes even more important. It is the key information about the cooling efficiency of a package and in particular of the heat sink. Besides the heat sink the thermal resistance depends also on the solder interface, packaging process, and bar structure such as fill factor and resonator length. This work presents a thermal comparison of different packaging types and laser bar designs. Different package types are experimentally measured and analyzed by numerical calculations to obtain information about the influence of the different parameters: Conductively cooled and water cooled copper heat sinks as well as a new type of expansion matched micro-channel heat sink made out of Cu-AlN sandwich are investigated. In addition to the different packages, laser bars with different resonator lengths are mounted and analyzed regarding their thermal behavior; the dependency of the thermal resistance on the resonator length is a particular interest of the investigation. In parallel to the experiments thermal simulations of the same packages and laser bar geometries are performed. The boundary conditions chosen in the simulations are comparable to the experimental values and the same parameters are varied. The relations between theoretical and experimental results are presented. The analysis shows the influencing factors, so that the optimum package can be chosen for a specific application.

Diode-pumped solid-state lasers are gaining acceptance as the desired laser source for materials processing as well as a host of new applications that are expanding rapidly. Because of this, the performance, stability and lifetime of the diode-pump source face unprecedented scrutiny. Increasing the lifetime of the diode, while increasing power, remains a primary focus of the industry. One lifetime limiting issue is that of a voltage potential in the water cooling channels which can cause cooler degradation and lower efficiency over time. Studies have been carried out that explore different cooling approaches based on passive schemes where insulation layers are present to shield the voltage from the water channels. However, with the introduction of insulation layers, a reduction of the deployable power from that of microchannel coolers is seen. This report explores the effects of passive cooling approaches on the power and divergence of 1 cm AuSn/CuW mounted bars with fill factors ranging from 10% to 50%. It is shown that a 150 W array can be realized on a passive cooler and multiplexed to give a 1600 W stack. Thermal modeling is presented along with life-test data for passively cooled devices.

Micro-lens arrays are widely used for beam shaping, especially beam homogenization of various laser sources. Monolithic arrays of cylindrical lenslets made of glass, semiconductors or crystals provide great advantages in laser applications, e.g. high efficiency, intensity stability and very low absorption. However, up to now, mainly symmetrical micro-lens surfaces are utilized in most applications due to design and manufacturing restrictions. The manufacture and application benefits of asymmetrical cylindrical-like micro-lens surfaces are enabled by LIMO's unique production technology. The asymmetrical shape is defined by uneven-polynomial terms and/or an asymmetrical cut-off from an even polynomial surface. Advantages of asymmetrical micro-lenses are off-axis light propagation, the correction of aberration effects or intensity profile deformations when the illuminated surfaces are not orthogonal to the optical axis. Additionally, the opportunities in simultaneous illumination from numerous light sources to one target are extended by just geometrical arrangement without the need for collinear beam alignment. First application results of such micro-lens arrays are presented for beam shaping of high power diode lasers. The generation of a homogeneous light field by a 100 W laser with tilted illumination at an angle of 35° is shown. A multi-kW line generator based on the superposition of over 50 diode laser bars under different illumination angles is demonstrated as well. Thus, laser material processing like plastics welding, soldering or annealing becomes much more convenient and less demanding regarding beam steering.

Bars with high and low filling factors serve the different schemes for beam transformation and fiber coupling. We report on highly efficient 8xx bars for operation in excess of 100 W and reliable broad-area single-emitter lasers (BASE) with 90 um aperture being capable to deliver in excess of 10 W from a 105 um core fiber. For 9xx bars we present solutions with power levels per device ranging from 60 W to 300 W corresponding to linear power levels beyond 8.5 W per 100 um stripe width indicating convergence of BAR and BASE devices. Life test results for these devices will be shown and high brightness fiber coupled solutions will be discussed.

We present record output power levels (a few hundred Watts) in continuous-wave (CW) and quasi-CW (QCW) from 2D vertical-cavity surface-emitting laser (VCSEL) arrays, corresponding to power densities exceeding 1kW/cm² in CW and 3.5kW/cm² in QCW. These VCSEL arrays emit around 975nm with narrow spectral width (<1nm) and excellent wavelength stability (<0.07nm/K). Peak power conversion efficiency of properly designed arrays exceeds 50%. Additional features of these arrays include emission in a circular, low-diverging beam, and reliable high-temperature operation. These arrays can also be operated reliably in short pulses (<200nsec) at many times their roll-over CW current, making them useful for high-energy applications. VCSEL arrays with 2.2kW peak output power operating under 100nsec pulse-width have been demonstrated.

GaSb based diode laser both as single emitters and as arrays, emitting between 1.9 and 2.2 μm, have a huge potential especially for materials processing, medical applications and as optical pump sources for solid state laser systems emitting in the 2-4 μm wavelength range. Determined by the absorption characteristics of thermoplastic materials at wavelengths around 2 μm, the light emitted by the diode laser will be absorbed by the material itself and can thus be used for marking and welding without the addition of e.g. colour pigments. We will present results on different (AlGaIn)(AsSb) quantum-well diode laser single emitters and linear laser arrays, the latter consisting of 20 emitters on a 1 cm long bar, emitting at different wavelengths between 1.9 and 2.2 μm. To improve on the typically poor fast axis beam divergence of diode lasers emitting at these wavelengths, we abandoned the broadened waveguide concept and changed over to a new waveguide design which features a rather narrow waveguide core. This results in a remarkable reduction in fast axis beam divergence to 43° FWHM for the new waveguide design. Electro-optical and thermal behaviour and the wavelength tunability by current and temperature have been carefully investigated in detail. For single emitters cw output powers of 2 W have been demonstrated. For diode laser arrays mounted on actively cooled heat sinks, more than 20 W in continuous-wave mode have been achieved at a heat sink temperature of 20 °C resulting in wall-plug efficiencies of more than 26%.

The strong increasing laser market has ongoing demands to reduce the costs of diode laser pumped systems. For that reason JENOPTIK Diode Lab GmbH (JDL) optimized the bar brilliance (small vertical far field divergence) and bar efficiency (higher optical power operation) with respect to the pump applications. High efficiency reduces the costs for mounting and cooling and high brilliance increases the coupling efficiency. Both are carefully adjusted in the 9xx nm - high power diode laser bars for pump applications in disc- and fiber lasers. Based on low loss waveguide structures high brilliance bars with 19° fast axis beam divergence (FWHM) with 58 % maximum efficiency and 27° fast axis beam divergence (FWHM) with 62 % maximum efficiency are developed. Mounted on conductive cooled heat sinks high power operation with lifetime > 20.000 hours at 120 W output power level (50 % filling factor bars) and 80W (20 % filling factor bars) is demonstrated. 808nm bars used as pump sources for Nd:YAG solid state lasers are still dominating in the market. With respect to the demands on high reliability at high power operation current results of a 100 W high power life time test are showing more than 9000 hour operation time for passively cooled packaged high efficiency 50 % filling factor bars. Measurement of the COMD-level after this hard pulse life time test demonstrates very high power levels with no significant droop in COMD-power level. This confirms the high facet stability of JDL's facet technology. New high power diode laser bars with wavelength of 825 nm and 885 nm are still under development and first results are presented.

Leveraging improvements to device structures and cooling technologies, ultra-high-power bars have been integrated into multi-bar stacks to obtain CW power densities in excess of 2.8 kW/cm² near 960 nm with spectral widths of <4nm FWHM. These characteristics promise to enable cost-effective solutions for a variety of applications that demand very high spatial and/or spectral brightness. Using updated device designs, mini-bar variants have been employed to derive CW powers of several tens of Watts near 940 nm on traditional single-emitter platforms. For example, >37 W CW have been obtained from 5-emitter devices on standard CuW CT heatsinks with AuSn solder. Near 808 nm, a PCE of 65% with a slope efficiency of 1.29 W/A has been demonstrated with a 20%-fill-factor, 2-mm-cavity-length bar.

Tapered diode lasers combine high output power and a beam quality near to the diffraction limit resulting in very high brightness. Therefore, they are finding use in a wide range of applications today, such as pumping of rare-earth-doped fibre amplifiers, tunable frequency doubling of diode lasers for blue-green outputs, and non linear spectroscopy. Due to increasing brightness and lifetime tapered lasers even become attractive for material processing and for telecom applications like pumping of Er-doped fiber amplifiers or raman amplifiers. In order to further enhance the brightness of tapered diode lasers the output power has to be increased while simultaneously the beam quality has to be kept near the diffraction limit. For this purpose we have grown low modal gain, single quantum well InGaAs/AlGaAs devices emitting at 976 nm by molecular beam epitaxy. The lateral design of the investigated laser diodes consists of a tapered section and a ridge-waveguide section. Since it has been shown by previous simulations and experiments that longer tapered sections allow higher output power with unchanged beam quality, we use tapered section lengths of 2000 μm, 3000 μm and 4000 μm. The beam quality parameter M² and output powers as well as the nearfields of the different structures were carefully investigated. For longer devices we reach an optical output power of more than 10 W per single emitter in continuous wave mode (cw) without any distinct thermal rollover.

Higher reliability and power efficiency achieved with low-demanding cooling make single emitter diodes a more effective pump source than monolithic laser diode arrays. Continuously improving performance and increasing brightness of single emitter pumps are accompanied with a steady reduction of cost of pumping (dollar-per-watt). Performance advantages do not compromise reliability of the pumps. These features ensure that single emitter diodes are the most effective solution even for multi-kWatt systems pumping. Here we report on a recent progress in single-mode and multi-mode edge-emitting diodes.

For the pumping of solid state lasers with high peak power pulses up to the TW range QCW diode laser stacks with pulse lengths between 200μs and 2ms are used. To realize long-term stable pump modules we already presented high power, high brightness 100W QCW diode laser bars [1] having a lateral aperture of 1.7mm only, a length of 4mm and a vertical divergence of 14° FWHM. Based on these we have developed a mounting scheme for stacks with > 1kW output power using these new kind of diode lasers. Due to the geometric dimensions of the chip we successfully realized a stack with a passive cooling scheme on both sides. Furthermore, we only used expansion matched materials such as CuW and Al2O3 ceramics, as well as AuSn solder processes for fixing the parts together. As a result the stack is very insensitive against environmental influences. Due to the small vertical divergence we were able to use fast axis collimators with large focal lengths, which relax the lens adjustment tolerances. At the conference we will present results for diode laser stacks with an output power of more than 1kW at duty-cycles up to 10% and an efficiency of about 50%. The beam parameter product for such diode laser stacks result in < 50mm•mrad for the vertical direction and in < 75mm•mrad for the lateral direction. These beam parameter values enable the coupling of the pump module to an optical fiber having a 1.2mm core diameter and a NA of 0.22. Furthermore, the low vertical fill factor of the stack radiation allows the combination of two stacks by beam deflection mirrors without significantly degrading beam quality, hence doubling the power coupled into the same fiber.

Based on a well established technology for continuous-wave (cw) diode lasers, further development and optimization lead to high performance laser bars for quasi-continuous-wave (qcw) operation suitable for pumping applications. Mounted on standard heat sinks, these 808nm laser bars exhibit more than 300W (400W) qcw output power with 50% (75%) filling factors. Reliability tests of these bars are running at >200W. Several GShots at 2, 4 and 10% duty cycle (d.c.) were already achieved. With this high performance qcw laser bars, passively cooled laser stacks were developed and tested using a new design compatible to high power operation. Thermal expansion matched materials and hard solder techniques allow reliable operation, even under rough environmental conditions. Output powers of 2.5kW (>300W per bar) were demonstrated from a stack with 8 bars. After environmental tests (vibration and thermal cycles), an ongoing life test exhibits more than 2.5GShots with 1.6kW (~200W per bar) at 4% duty cycle.

A high-power and short-wavelength GaInP/AlGaInP quantum-well laser diode array was designed and fabricated. Because a conduction band offset of this material system is small, a carrier leakage from an active layer is an important limiting factor of the maximum light output. In this work, long cavity length of 1.5 mm, high front facet reflectivity of 18% and AlInP cladding layers were adopted to reduce the leakage. An evaluation test of the fabricated array was performed under CW operation. At 15°C, high light output of 12W was obtained with injection current of 16A. The lasing wavelength was 643.3 nm. Moreover, high wall-plug efficiency of 34% was achieved. These excellent characteristics are considered to be due to the effective suppression of the carrier leakage.

High-brightness 650 nm tapered lasers with output powers up to 1 W and nearly diffraction limited beam quality at 500 mW were realized. The vertical structure is based on an InGaP single quantum well (SQW) embedded in AlGaInP waveguide layers and n-AlInP and p-AlGaAs cladding layers. The tapered structure consists of a 750 μm long ridge waveguide section and a 1.25 mm long flared section. Taper angles of 2°, 3° and 4° were manufactured. At 15°C, the devices achieve 1 W at an operating current below 2 A in CW operation. The conversion efficiency is about 20%. At 500 mW output power a nearly diffraction limited beam quality with a beam propagation ratio of about 1.5 was measured. The reliability was studied in a long-term test for five tapered diodes at 250 mW over 1,000 h and than at 500 mW over 2,000 h. All diodes survived this test. The beam quality remains nearly stable over the complete reliability test.

As GaAs based laser diode reliability improves, the optimum architecture for diode pumped configurations is continually re-examined. For such assessments, e.g. bars vs. single emitters, it is important to have a metric for module reliability which enables comparisons that are the most relevant to the ultimate system reliability. We introduce the concept of mean time between emitter failures (MTBEF) as a method for characterizing and specifying the reliability of multi-emitter pumps for ensemble applications. Appropriate conditions for an MTBEF model, and the impact of incremental changes of certain conditions on the robustness of the model are described. In the limit of independent random failures of individual emitters as the dominant failure mechanism it is shown that an ensemble of multi-emitter modules can be modeled to behave like an ensemble of single emitter modules. The impact of thermal acceleration due to failed emitters warming other emitters on a shared heat-sink is considered. Data taken from SP built multi-emitter devices bonded with AuSn on CTE matched heat-sinks is compared with the MTBEF model with and without correction for the thermal acceleration effect.

Thermo-mechanical stress occurring during the packaging process and during operation limits the reliability of high-power diode laser bars. The stress is caused by the mismatch of the thermal expansion coefficients between the heat sink and laser bar material. A soft solder layer can partially reduce the stress by relaxation. A convenient approach for reducing the stress is the matching of the thermal expansion of the heat sink to the laser bar material. The disadvantage of most expansion-matched heat sinks is a higher thermal resistance so that the device temperature increases and the lifetime decreases. For the development of thermal and strain optimized diode laser packages an analysis of both the thermal and strain distribution is reasonable. In this work the strain is analyzed by electroluminescence using the correlation between stress and the polarization properties of the laser bar radiation. This method allows a qualitative emitter resolved strain mapping along the slow-axis. Because of the correlation between temperature and wavelength a thermal analysis of mounted laser bars can be done by an emitter resolved spectral mapping. Irregularities in the thermal contact between laser bar and heat sink such as defects in the solder layer become visible by irregular emitter spectra. The work shows examples for the optimization of the package. The analysis of the thermal and strain distribution shows the advantages and disadvantages of the particular approaches, like variations of solder thickness or expansion matched packages.

We demonstrate very high reliability level on 980-1060nm high-power single-mode lasers through multi-cell tests. First, we show how our chip design and technology enables high reliability levels. Then, we aged 758 devices during 9500 hours among 6 cells with high current (0.8A-1.2A) and high submount temperature (65°C-105°C) for the reliability demonstration. Sudden catastrophic failure is the main degradation mechanism observed. A statistical failure rate model gives an Arrhenius thermal activation energy of 0.51eV and a power law forward current acceleration factor of 5.9. For high-power submarine applications (360mW pump module output optical power), this model exhibits a failure rate as low as 9 FIT at 13°C, while ultra-high power terrestrial modules (600mW) lie below 220 FIT at 25°C. Wear-out phenomena is observed only for very high current level without any reliability impact under 1.1A. For the 1060nm chip, step-stress tests were performed and a set of devices were aged during more than 2000 hours in different stress conditions. First results are in accordance with 980nm product with more than 100khours estimated MTTF. These reliability and performance features of 980-1060nm laser diodes will make high-power single-mode emitters the best choice for a number of telecommunication and industrial applications in the next few years.

Improvements of laser diode bar efficiency and mounting technology have boosted output powers of passively cooled diode lasers beyond the 100W cw limit. After an introduction about reliablity statements and reliability assessment, the performance increase by technology improvements is documented in current-step failure discrimination tests. Electro-optical parameters of improved diode lasers are subsequently presented in detail as well as the results of lifetime tests at different powers and in different operation modes - steady-state and repetitive/intermittent ("hard pulse") cw operation.

Multi-mode InGaAs strained quantum-well single emitters at 920-980nm have become indispensable as pump lasers for various applications. Most previous reports on these lasers have been focused on their performance characteristics with limited reports on failure mode analysis although understanding the physics of failure is crucial in developing a proper physics-based lifetime model. Our group recently reported accelerated lifetest results with accumulated test hours of over 6000 hours and identified catastrophic sudden failures as dominant failure modes. In this paper we report our investigation of catastrophically degraded high power multi-mode single emitters with various destructive and non-destructive techniques including high-resolution TEM. The lasers studied were broad-area strained InGaAs single QW lasers at 940-980nm with typical CW output powers of over 6W at an injection current of 7A with a wall plug efficiency of ~60%. An EBIC technique is employed to identify location of dark line defects in degraded lasers with different amounts of drop in optical output power. A FIB technique is then employed to prepare TEM samples from the DLD areas allowing cross-sectional HR-TEM analysis. This is to investigate generation and growth of defects and dislocations in multi-mode lasers catastrophically degraded under different accelerated stress conditions. Finally, we report our results on deep traps associated with degraded devices as well as with pristine devices using DLTS technique.

Reliability tests for 650 nm broad area lasers and bars with GaInP quantum wells embedded in AlGaInP waveguide layers and n-AlInP and p-AlGaAs cladding layers will be presented. Reliable operation of broad area lasers with 100 μm stripe width at 1.0 W output power over 10,000 h and of 5 mm wide bars with ten 100 μm wide emitters (filling factor 20%) at 8 W over 4,000 h will be reported. 6 mm wide bars with twelve 60 μm wide emitters (filling factor 12%) at 7 W showed a mean time to failure of 3,750 h.

So far, diode laser systems could not compete against CO₂-lasers or DPSSL in industrial applications like marking or cutting due to their lower brightness. Recent developments in high-brightness diode laser bars and beam forming systems with micro-optics have led to new direct diode laser applications. LIMO presents 400W output from a 200μm core fibre with an NA of 0.22 at one wavelength. This is achieved via the combination of newly designed laser diode bars on passive heat sinks coupled with optimized micro-optical beam shaping. The laser is water cooled with a housing size of 375mm x 265mm x 70mm. The applications for such diode laser modules are mainly in direct marking, cutting and welding of metals and other materials, but improved pumping of fibre lasers and amplifiers is also possible. The small spot size leads to extremely high intensities and therefore high welding speeds in cw operation. For comparison: The M² of the fibre output is 70, which gives a comparable beam parameter product (22mm*mrad) to that of a CO₂ laser with a M² of 7 because of the wavelength difference. Many metals have a good absorption within the wavelength range of the laser diodes (NIR, 808nm to 980nm), which permits the cutting of thin sheets of aluminium or steel with a 200W version of this laser. First welding tests show reduced splatters and pores owing to the optimized process behaviour in cw operation with short wavelengths. The availability of a top-hat profile proves itself to be advantageous compared to the traditional Gaussian beam profiles of fibre, solid-state and gas lasers in that the laser energy is evenly distributed over the working area. For the future, we can announce an increase of the output power up to 1200W out of a 200μm fibre (0.22 NA). This will be achieved by further sophistication and optimisation of the coupling technique and the coupling of three wavelengths. The beam parameter product will then remain at 22mm*mrad with a power density of 3.8 MW/cm² if focussed to a 200µm spot. This leads to excellent laser cutting results with extremely small cutting kerfs down to 200μm and very plane cutting edges. Process speeds rise up to more than 10m/min i.e. for thin sheets of stainless steel or titanium.

We describe the performance of diode laser bars mounted on conductive and water cooled platforms using low smile processes. Total smile of <1μm is readily achieved on both In and AuSn based platforms. Combined with environmentally robust lensing, these mounts form the basis of multiple, high-brightness products. Free-space-coupled devices utilizing conductively-cooled bars delivering 100W from a 200μm, 0.22NA fiber at 976nm have been developed for pumping fiber lasers, as well as for materials processing. Additionally, line generators for graphics and materials processing applications have been produced. Starting from single bars mounted on water-cooled packages that do not require de-ionized or pH-controlled water, these line generators deliver over 80W of power into a line with an aspect ratio of 600:1, and have a BPP of <2mm-mrad in the direction orthogonal to the line.

A 1.8kW diode laser source made up of 100 W diode bars is designed for fibre delivery, using two new techniques to enhance the delivered brightness and equalise the beam-parameter product. Custom corrective phase plates in laser-cut silica are attached permanently to the two stacks of ten bars, correcting bar smile and restoring a factor of 2.5 in lost brightness. The two units are beam-compacted and polarisation coupled to a single array beam. As a final step, a novel confocal beam-slicer produces five segments from the slow-axis beam profile and stacks the segments in the fast-axis direction.

Fiber-coupled diode lasers have become an established source for many industrial applications due to their high wall-plug efficiency, minimal maintenance and cost per watt. To decrease system size and cost for cooler and driver, high coupling efficiencies have become more and more important. Recent developments in broad area laser diode bars (BALB) and beam shaping systems with micro-optical components are leading to new highly efficient fiber coupling. We present newly developed high power diode laser modules which are performing at outstanding efficiencies with smallest package design. The combination of recently designed laser diode bars on passive heat sinks and optimized micro-optics results in laser modules with up to 60W out of a 200μm fiber with a 0.22 NA and > 50% electro-optical efficiency out of the fiber core, based on only one laser diode bar. The applications for such laser diode modules range from pumping of fiber lasers and amplifiers, over materials processing to medical applications. The presentation of the technology will show a path to scale high-brightness laser systems to higher power levels and efficiencies. The combination of different coupling techniques will allow laser modules with 100W out of 100μm fiber core up to 1.6kW out of 400μm fiber core with electro-optical efficiencies of > 45%.

In this work, we present the development of a suite of high brightness, fiber-coupled diode lasers at Coherent Direct Diode Systems. Experimental results of high coupling efficiency into 50-200 μm, 0.2 numerical aperture (NA) fibers will be presented. In addition to the high-brightness laser diode bar with single mode emitters, various configurations of diode laser bars with multimode, broad area emitters will be utilized to achieve similar level of brightness. The enabling technology in these products is supported by key developments in micro and macro optics, diode laser packaging, and system architecture.

Driven by the potential of the fiber laser market, the development of high brightness pump sources has been pushed during the last years. The main approaches to reach the targets of this market had been the direct coupling of single emitters (SE) on the one hand and the beam shaping of bars and stacks on the other hand, which often causes higher cost per watt. Meanwhile the power of single emitters with 100μm emitter size for direct coupling increased dramatically, which also pushed a new generation of wide stripe emitters or multi emitters (ME) of up to 1000μm emitter size respectively "minibars" with apertures of 3 to 5mm. The advantage of this emitter type compared to traditional bars is it's scalability to power levels of 40W to 60W combined with a small aperture which gives advantages when coupling into a fiber. We show concepts using this multiple single emitters for fiber coupled systems of 25W up to 40W out of a 100μm fiber NA 0.22 with a reasonable optical efficiency. Taking into account a further efficiency optimization and an increase in power of these devices in the near future, the EUR/W ratio pushed by the fiber laser manufacturer will further decrease. Results will be shown as well for higher power pump sources. Additional state of the art tapered fiber bundles for photonic crystal fibers are used to combine 7 (19) pump sources to output powers of 100W (370W) out of a 130μm (250μm) fiber NA 0.6 with nominal 20W per port. Improving those TFB's in the near future and utilizing 40W per pump leg, an output power of even 750W out of 250μm fiber NA 0.6 will be possible. Combined Counter- and Co-Propagated pumping of the fiber will then lead to the first 1kW fiber laser oscillator.

In the last decades, diode laser systems conquered the spectral range step-by-step from conventional gas lasers, wherever they can match or outperform in optical specifications. Although highly anticipated in the ultraviolet wavelength range, for instance in high-resolution lithography, biological and medical fluorescence applications or holography, cw single frequency operation of sufficient power has been a challenge for diode or other solid state laser systems. Currently this scope is still dominated by the HeCd gas laser, emitting at 325 nm with powers of up to 100 mW. In this paper we present a diode laser system emitting at 325 nm offering the same output power by efficient second harmonic generation (SHG) of a master oscillator power amplifier (MOPA) at 650 nm. For the master oscillator a ridge waveguide diode is anti-reflection coated and used in an external cavity diode laser (ECDL) with grating feedback in Littrow configuration. This setup features a MHz line width (coherence length of 100m), a coarse tuning range from 649 nm to 657 nm and a mode hope free tuning of 20 GHz. In a second step, we use a tapered amplifier to boost the output from the ECDL to a level of 400 mW, for powering an efficient second harmonic generation process in an enhancement cavity. Faraday isolators on both ends of the amplifier stage prevent back reflection and stabilize the single mode operation of the system. Together with astigmatism compensation this yields to a high spatial quality (M²<1.5) of the amplified beam. The frequency doubling is achieved by using a four mirror bow-tie enhancement resonator fitted with a Beta-Barium Borate (BBO) crystal. The cavity length is actively locked to the laser frequency using the Pound-Drever-Hall method. With this set-up, stable and reliable laser operation is achieved. After a few minutes warm-up time, fixed frequency and tunable UV output power of more than 100 mW could be generated, opening this important wavelength range for future product development.

Here we present details of the design and performance of a family of compact, fiber-coupled, multi-bar, laser-diode stacks. The highest-power variant employs a pair of 6-bar stacks and a removable 400-μm, 0.22 NA fiber to deliver >400 W at 50 A. The overall power conversion efficiency (PCE) near 976-nm exceeds 40% at 400 W in CW operation with an uncoated delivery fiber. The brightest variant reaches a power density near 800-kW/cm² at 976-nm through a 200-μm, 0.22 NA fiber. Module variants have been built and characterized at multiple wavelengths between 780-nm and 980-nm. Applications for such modules include pumping of active fibers, pumping of rubidium vapor and direct material processing.

We have developed a single-emitter multi-mode laser-diode-pump platform for high efficiency, brightness and high reliability in a small form factor. This next-generation package is scalable to higher optical power and offers a low-cost solution for industrial applications, such as fiber lasers, graphic arts and medical. The pump modules employ high coupling efficiency, >90%, high power-conversion efficiency, >50%, and low thermal resistance, 2.2°C/W, in an electrically-isolated package. Output powers as high as 18W have been demonstrated, with reliable operation at 10W CW into 105μm core fiber. Qualification results are presented for 0.15NA and 0.22NA fiber designs.

Miniaturized optical systems that couple light from broad area or trapezoid diode laser bars with cw powers up to 100 W into fibers with core diameters between 50 μm and 100 μm have been developed and assembled on smart Direct-Copper-Bond (DCB) system platforms that incorporate active cooling structures as well as hermetic housing facilities. The approach for a fast and flexible joining of the optical elements by a flux-free applied solder is to jet liquid solder spheres onto joining geometries, thus enabling for creating complex shaped solder joint geometries with high accuracies.

Pumping fiber lasers is the driving force for the development of high brightness, mid power, passively cooled, fiber coupled diode lasers. We compare concepts for providing 50 W in a 100 micron fiber at the optimum fiber laser pump wavelength of 976 nm. The set up is experimentally demonstrated and compared to the optical analysis. Three basic diode laser concepts are included into this comparison: single emitters, high density emitter arrays and low density emitter arrays on bars. Low density stacking in the horizontal direction with increasing the filling factor by a microlens array is the first concept. For this concept two diode bars with low filling factor are fast and slow axis collimated. Beam transformation, shaping and focusing are similar to the second concept. In the second concept a diode laser array with high filling factor is regarded. An 800 μm diode laser bar consists of an array of four or five emitters. Two bars are polarization coupled and collimated with single lenses. Beam symmetrization is performed by the well known step mirror. A simple anamorphotic optic enables beam shaping and fiber coupling. The third one, single emitters, represents optical beam combining of laser diodes that are high density stacked in the vertical direction. Five emitters are placed in an optical stack, each one collimated with its own lens. Two optical stacks are polarization coupled and focused on the fiber end. The three concepts are compared in terms of power efficiency and complexity, and the results of prototype systems are presented.

Within the German national research project "Briolas" Osram Semiconductors and Laserline GmbH cooperated in the subproject "Brilasi" which focused mainly on Brilliant Laser Diodes for Industrial Applications. The project was finished in December 2007 and lasted more than three years. Laserline and Osram are investigating the performance of broad area diode lasers with a bar width from 1.0 to 10.0 mm in high brilliance diode laser beam sources. Within the program different fibre coupled laser sources are built up: 1. Fibre coupled diode laser beam source with a Beam Parameter Product (BBP) of 40 mmxmrad built from diode laser bars with 10 mm bar width. 2. Fibre coupled diode laser beam source with a BBP of 20 mmxmrad built from Mini-Bars with a bar width of 3.0 mm and 8 emitters. These Different solutions are characterized regarding the electro optical performance. The laser output characteristics are determined for the diode laser device as well as the complete beam source. Lifetime tests are conducted to determine the long term stability of the prototypes and the different chip material.

High power semiconductor lasers are commonly used as efficient pump sources for solid state lasers or multiplexing applications. Common wavelengths are 808nm e.g. for pumping of Nd:YAG and 9xx nm for pumping of disk or fiber lasers. Together with these wavelengths 880nm can be used as 3rd or 4th wavelength for multiplexing in direct material processing lasers. These industrial lasers are typically operating with some kW laser output power. For scaling to higher powers up to several kW, management of waste energy and power supply is gaining more and more importance. High efficient and reliable diode sources are vital to build systems with very good overall performance. The German framework project "BRILASI" had the target to develop basic technologies of next generation brilliant high power diode lasers for industrial applications. In this paper we present laser bars which combine industrial standards with highest efficiencies at 808, 880, 940 and 980nm and power range above 100W/bar. Room temperature efficiencies of 70% were demonstrated at wavelengths above 900nm and power levels of 130W. For 808nm, we reached efficiencies up to 62% at 20°C. For high temperature operation, we will show laser structures of 808nm optimized for 50°C.

This paper discusses the use of diffractive optical elements (DOEs) and micro-optics fabricated by precise pressing in glass for beam shaping of high-power diode lasers. The DOEs are used to diffract the light into the point of interest and to improve the laser beam quality. We have realized circular, flat-top and multi-beam intensity profiles. The highest measured diffraction efficiency was higher than 95 %. The new established fabrication process has potential for mass production of DOEs. SCHOTT's precision glass molding process guarantees a very constant quality over the complete production chain.

In the scope of the project TRUST, more than 20 aging tests have been performed on about 300 high-power laser diode bars with a variation of the mounting technology, current load, operation temperature, and operation mode. Our main goals were the improvement of the reliability and the determination of acceleration parameters for aging tests. We present selected results of long-time aging tests, acceleration factors, and thermal activation energies for high-power 808 nm laser diodes. Due to the increasing demand for higher output powers, we focus mainly on gold-tin mounted laser bars and show their great potential in comparison to the standard indium packaging technology.

Within the project TRUST a total of about 600 actively cooled high power laser diode bars is analyzed. These devices are packaged by various project partners and by applying different packaging technologies. A number of analytical tools is applied to the devices, among others strain profiling by photocurrent spectroscopy. We present selected results such as the evolution of packaging-induced strains when advancing the technology from indium- to AuSn-soldering. The thermal properties of all devices are screened before the aging experiments by using thermal imaging. This involves monitoring of complete thermal profiles along each bar as well as the identification of "hot" emitters. These statistics turns out to be batch-specific and sensitive to the soldering technology used.

Thermal imaging is demonstrated as an attractive alternative for standard temperature measurements in diode lasers. It allows for the determination of time resolved temperature distributions in arbitrary materials of laser devices. Because of the partial mid-infrared transparency of the semiconductor materials involved, several issues complicate the thermal imaging approach. We analyze these detrimental effects for the case of GaAs based high-power diode lasers and demonstrate how to circumvent them. This leads to a deeper insight into the composite thermal emission signal from diode lasers and eventually to an accurate determination of absolute temperatures of semiconductor diode lasers.

In the last few years an increasing demand for high-brightness diode laser sources is observable, which is mainly driven by applications for fiber laser pumping and materials processing. A number of different approaches have been investigated in the past for the realization of such systems. In this paper we compare different concepts for high-brightness, high-power diode laser modules that are based on the new generation of tapered diode laser bars and new developments in broad area diode laser bars, respectively. One of the main advantages of tapered diode laser bars is the good beam quality in the slow-axis direction, which allows the design of high-power laser systems with a symmetric beam profile without the necessity of using sophisticated beam shaping systems. Such laser modules with multiple bars aiming for kilowatt output power can be realized with different incoherent coupling principles, including spatial multiplexing, polarization multiplexing and wavelength multiplexing. On the other hand, modules with a single or only a few tapered diode laser bars aim for very high brightness suitable for fiber coupling with fiber diameters down to 50 μm with a numerical aperture (NA) of 0.22. In this paper we present a detailed characterization of the new generation of tapered diode laser bars, including typical electro-optical data, measurements of beam quality and lifetime data. Tapered diode laser bars typically suffer from a broad spectrum which is extremely obstructive for pumping applications with small absorption bandwidths. To overcome this disadvantage we used volume bragg gratings (VBG) to improve the spectral quality of tapered diode laser bars. In addition to further improve the brightness of such diode laser systems we investigated external phaseplates to correct for smile and lens aberrations.

High-power laser bars with emission in the red spectral range (635 - 660 nm) are of great interest for several applications such as display and projection solutions or pumping of Cr:LiSAF solid state lasers. Another field of application is a medical use of red lasers. The German funded project ROLAS combines medical and technical aspects of photodynamic therapy (PDT). One special approach under investigation is a laser bar based multi-port PDT system: To allow the optimum treatment of widespread, complex-shaped tumors a PDT laser system with 8 independently operable fiber outputs is designed, based on two laser bars with 652 nm emission and independently addressable emitters. The necessity of individually addressable emitters leads to a more complex and allows a thermally less optimized package design. In combination with conductive cooling - which is a must for most medical applications - the possibilities for low-temperature operation of the laser bars are severely constricted. Especially for high-power laser bars in the 635-660 nm range operation under the expected unfavourable thermal conditions constitutes an additional challenge: These devices by principle exhibit a strong temperature dependence of their performance due to the comparably weak carrier confinement in the InGaAlP material system. In this paper, based on detailed measurements, an analysis of the temperature dependence of the laser bar performance is carried out and the consequences for mounting and application of the laser bars are shown. The measurements illustrate the significant progress that has been achieved during the last two years in terms of temperature stability by applying specific design measures.

Laser modules for single mode fiber (SMF) coupling of frequency stabilized diode lasers are so far mainly realized with ridge lasers due to their good beam quality. Tapered lasers are beam sources with a beam quality which is nearly as good as that of a ridge laser but with a higher optical output power. Therefore they have the potential for a higher SMF-coupled power than ridge lasers. It will be shown how the radiation of a tapered laser or amplifier can be frequency stabilized and coupled into a SMF in a compactly build module. To couple a tapered laser different coupling systems, using cylindrical lenses either crossed or in combination with rotational lenses are possible. The advantages, problems and coupling results of those systems will be illustrated. For many applications it is necessary to stabilize the frequency of the laser. This can be achieved for example by a fiber bragg grating, written in the SMF in which the laser is coupled or by a volume holographic grating, applied to a lens in the coupling system. Another possibility is the use of a tapered amplifier, which is stabilized by a fiber bragg grating on the backside of the amplifier.

We present recent advances in high power multimode and single semiconductor lasers. We review high power operation with increased spectral brightness using on-chip internal gratings and increased spatial brightness at wavelengths from the near infrared to the eye-safe regime. New high power, high brightness fiber coupled semiconductor lasers are described.

High power, high brightness, single emitter laser diodes with different apertures from 5 μm to 1000 μm are reported on, in the wavelength range from 780 nm to 1060 nm. On going progress at Axcel Photonics for both single-mode and multi-mode laser diodes will be presented. These diode lasers show high slope efficiency, low threshold current and low voltage, etc. Laser diodes with different emitting apertures at 5μm, 50 μm, 90 μm, 200 μm, 400 μm, 1000 μm, are reported on and discussed in detail. The reliability data for different sized emitters is presented. These results demonstrated that Axcel's technologies enable laser diodes made from Al based material grown on GaAs substrates, which can reliably operate at high brightness and high power in the near infrared-wavelength range under wide range of emitting apertures. These laser diodes are suitable for a wide variety of applications including medical, material processing, graphics, pumping solid-state lasers and fiber lasers.

Coupling to the fiber Bragg grating (FBG) is a well-established technique of emission wavelength stabilization in single mode pump lasers used for instance in Er-doped fiber amplifiers (see e.g. Ref. 1). The output power of these devices usually does not exceed 1 W due to limited heat transfer from narrow active stripe of the single mode laser diode. To satisfy increasing demand for wavelength-stabilized pump lasers with higher output power we attempted to extend FBG stabilization scheme to high power multimode broad area lasers. This scheme brings usual advantages of FBG stabilization such as environmental wavelength stability, compactness and low cost. Development work has been carried out using our reliable high power pump laser with output aperture of ~ 50 - 100 μm. The emission wavelength of the free running laser was around 960 nm at room temperature. The fiber gratings with reflection maximum at about 975 nm were written in a commercially available multimode fiber. In initial experiments the laser was coupled with discrete optics to the multimode fiber containing FBG. Introduction of the spectrally selective optical feedback locked the laser emission to the Bragg wavelength. The laser emission remained locked to this wavelength up to a maximum drive current of 8 A within a heat sink temperature range of 40°C in this experiment. The overall spectral width of stabilized laser emission facilitates effective and stable pumping into absorption lines as narrow as 5 nm FWHM. Similar results were obtained on the Bookham commercial pump modules with FBG in the output fiber. The modules emitted up 4 W of wavelength-stabilized power from the output fiber with 50 μm core diameter.

More and more applications, like tunable frequency doubling of diode lasers for blue-green outputs, non linear spectroscopy, or pump laser sources for fiber lasers necessitate diffraction-limited tunable narrow linewidths and high output powers in the multiwatt regime. For these applications, tapered lasers based on a tapered amplifier with gain-guided design can be used in an external cavity set up to guarantee both - frequency stabilization and tunability. We have realized frequency stabilized high-power ridge-waveguide tapered diode lasers with more than 4W of cw output power. These low modal gain, single quantum well InGaAs/AlGaAs devices emitting between 920nm and 1064nm were grown by molecular beam epitaxy. Tapered single emitters consist of an index-guided ridge section and a gain-guided taper section with an overall length of 3.5mm. The taper angle was 6°. With a high-reflectivity coating on the rear facet and an antireflection coating on the front facet more than 10W of output power have been demonstrated. To optimize the beam quality at higher output power the two different sections have been operated by different operation currents. For this purpose the tapered diodes have been mounted p-side down on structured submounts. For wavelength tunability and frequency stabilization the tapered diodes, provided with AR coatings on both facets, have been used in external cavity setup in Littrow configuration. The influence of the different operation currents on the electrooptical and beam characteristics has been carefully investigated in detail. Within this operation mode a nearly diffraction limited behavior up to 5W has been established.

Operation and performance are described for tunable 50mW distributed feedback semiconductor laser modules with feedback frequency stabilization. Frequency stability resulting in average phase noise reduction of over 35 dB is achieved, comparable to the performance of single-frequency fiber lasers, but with tunability and greatly reduced sensitivity to microphonic disturbance. The noise performance holds up well in and following harsh environments. These advantages make this laser module a good low-cost alternative to fiber lasers for applications involving interferometric sensors and interferometric measurement techniques, such as seismic signal detection for hydrocarbon exploration and earth movement analysis, underwater acoustic signal sensing, perimeter security, intrusion detection, and others currently being developed.

Current laser systems based on high-power laser diode bars need active cooling either water cooling or the use of thermo-electric coolers to ensure an adequate operating temperature for a reasonable lifetime. Here is a solution with a bonded fin heat sink and forced ventilation introduced, a diode laser bar with an improved efficiency and a low thermal resistance as well as an optical system for a highly efficient fibre coupling. With this system it is possible to couple 25 Watt continuous wave power from a single laser diode bar on a passive heat sink into a fibre with 200 μm core diameter. The basis for this performance is a heat sink with an exceptionally low thermal resistance. Several new features are introduced to reach a low overall gradient between the laser diode temperature and the ambient temperature. In addition, it does geometrically fit to the layout of the optical design. Shape and aspect ratio of both heat sink and housing of the laser system are matched to each other. Another feature is the use of hard-soldered or pressed bars to achieve a thermo-mechanically stable performance. The long-term thermal characteristic was tested. The operation temperature comes to saturation after about 30 minutes. Therefore it can be used for continuous wave operation at 25 Watt output power. At a quasi continuous operation at 70 percent duty cycle a peak power of 30 Watt out of the fibre is possible. From this technology results a compact fibre coupled laser system what is simple to drive compared with current high power laser systems, because there is no need to control the operating temperature. This gives way for more compact driver solutions. Fields of application are laser marking systems and material processing, where a simple driver system is requested. Also medical applications need this requirement and a compact cooling too so that mobile integrated solutions become possible. Further developments allow multiple laser diode systems for specific industrial applications demanding more power. Our measurements show the potential for direct air-cooled laser systems with 100 Watt power out of the fibre.