This PDF file contains the front matter associated with SPIE Proceedings Volume 7918, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Diode laser modules based on arrays of single emitters offer a number of advantages over bar-based solutions including enhanced reliability, higher brightness, and lower cost per bright watt. This approach has enabled a rapid proliferation of commercially available high-brightness fiber-coupled diode laser modules. Incorporating ever-greater numbers of emitters within a single module offers a direct path for power scaling while simultaneously maintaining high brightness and minimizing overall cost. While reports of long lifetimes for single emitter diode laser technology are widespread, the complex relationship between the standalone chip reliability and package-induced failure modes, as well as the impact of built-in redundancy offered by multiple emitters, are not often discussed. In this work, we present our approach to the modeling of fiber-coupled laser systems based on single-emitter laser diodes.

State-of-the-art broad-area InGaAs-AlGaAs strained quantum well (QW) lasers show an optical output power of over 20 W and a power conversion efficiency of over 70% under CW operation. Unlike broad-area (Al)GaAs QW lasers, broad-area InGaAs strained QW lasers show two failure types including facet catastrophic optical damage (COD) and bulk failure. Optimization of facet passivation processes has led to significant reduction in occurrence of facet COD (or COMD), but bulk failure (or COBD) has received little attention although it is crucial to understand degradation processes responsible for COBD and then develop COBD-free lasers for high reliability applications including potential satellite systems. Our group recently proposed a model for the COBD process and this paper further investigates the root causes of COBD in the broad-area lasers. We performed accelerated life-tests of MOCVD-grown broad-area strained InGaAs-AlGaAs single QW lasers at ~975 nm, which predominantly yielded catastrophic bulk failures. We employed various non-destructive techniques to study pre- and post-stressed lasers. Time resolved electroluminescence (TR-EL) was employed to observe formation and progression of dark spots and dark lines through windowed n-contacts during entire life-tests that eventually led to COBD. Deep level transient spectroscopy (DLTS) was employed to investigate trap characteristics in degraded devices at different stages of degradation to study the role that non-radiative recombination centers (NRCs) play in COBD processes. Time resolved photoluminescence (TR-PL) was employed to measure carrier lifetimes from both undamaged and damaged active areas to find correlation between dark line defects in degraded lasers and non-radiative recombination processes.

System designers and end users of diode pumped solid state lasers often require knowledge of the operability limits of QCW laser diode pump sources and their predicted reliability performance as a function of operating conditions. Accelerated ageing at elevated temperatures, duty cycles and/or currents allows extended lifetime testing of diode stacks to be executed on compressed timescales with high confidence. We present a novel, time-efficient technique for the determination of accelerated lifetime test conditions using degradation rate data, rather than the traditionally used failures against time data. To assess the effect of thermally accelerated ageing, 4 groups of 4 stacks each were operated for 60 million pulses at different temperature stress levels by varying the pulse repetition rate from 100Hz to 250Hz. The measured power degradation rates fitted to an Arrhenius type model, result in activation energy of 0.47- 0.74eV, apparently indicating two thermally activated degradation modes with different activation energies. Similarly, for current accelerated ageing, another 4 groups of 4 stacks were tested at operation currents from 120A to 150A. The optical power degradation rates due to current stress follow a power law behavior with a current acceleration factor of 1.7. The obtained acceleration parameters allowed considerable reduction of the lifetime test duration, which would have otherwise taken an unacceptably long time under nominal operating conditions. The successful results of the accelerated lifetime have been a major milestone enabling qualification of SCD stacks as pump sources for the laser altimeter in ESA Bepi-Colombo space mission. The presented reliability analysis allows life test qualification programs to be accelerated for generic QCW stacks and their lifetime to be predicted in various operating environments.

We report on the high-power high-temperature long-pulse performance of the 8XX-nm diode laser bars and arrays, which were recently developed at Lasertel Inc. for diode laser pumping within high-temperature (130 °C) environment without any cooling. Since certain energy in each pulse is required, the diode laser bars have to provide both high peak power and a nice pulse shape at 130 °C. Optimizing the epi-structure of the diode laser, the laser cavity and the distribution of waste heat, we demonstrate over 40-millisecond long-pulse operation of the 8XX-nm CS bars at 130 °C and 100 A. Pumping the bar with 5-Hz frequency 15-millisecond rectangular current pulses, we generate over 60 W peak power at 100 A and 130 °C. During the pulse duration, the pulse shape of the CS bars is well-maintained and the power almost linearly decays with a rate of 1.9% peak power per millisecond at 130 °C and 100 A. Regardless of the pulse shape, this laser bar can lase at very high temperature and output pulse can last for 8 ms/2ms at 170 °C/180 °C (both driven by 60 A current pulses with 5-Hz frequency, 10 millisecond pulse width), respectively. To the best of our knowledge, this is the highest operating temperature for a long-pulse 8XX-nm laser bar. Under the condition of 130 °C and 100 A, the laser bars do not show any degradation after 310,000 10-millisecond current pulse shots. The performance of stack arrays at 130 °C and 100 A are also presented. The development of reliable high-temperature diode laser bar paves the way for diode laser long-pulse pumping within a high-temperature environment without any cooling.

Improved performance and reliability of 9xx nm single emitter laser diodes are presented. To date, over 15,000 hours of accelerated multi-cell lifetest reliability data has been collected, with drive currents from 14A to 18A and junction temperatures ranging from 60°C to 110°C. Out of 208 devices, 14 failures have been observed so far. Using established accelerated lifetest analysis techniques, the effects of temperature and power acceleration are assessed. The Mean Time to Failure (MTTF) is determined to be >30 years, for use condition 10W and junction temperature 353K (80°C), with 90% statistical confidence.

Optical metrology system reliability during a prolonged space mission is often limited by the reliability of pump laser diodes. We developed a metrology laser pump module architecture that meets NASA SIM Lite instrument optical power and reliability requirements by combining the outputs of multiple single-mode pump diodes in a low-loss, high port count fiber coupler. We describe Monte-Carlo simulations used to calculate the reliability of the laser pump module and introduce a combined laser farm aging parameter that serves as a load-sharing optimization metric. Employing these tools, we select pump module architecture, operating conditions, biasing approach and perform parameter sensitivity studies to investigate the robustness of the obtained solution.

Northrop Grumman Cutting Edge Optronics (NGCEO) has recently developed high-power laser diode arrays specifically for long-life operation in quasi-CW applications. These arrays feature a new epitaxial wafer design that utilizes a large optical cavity and are packaged using AuSn solder and CTE-matched heat sinks. This work focuses on life test matrix of multiple epitaxial structures, multiple wavelengths, and multiple drive currents. Particular emphasis is given to the 80x and 88x wavelength bands running at 100-300 Watts per bar. Reliable operating points are identified for various applications including range finding (product lifetimes less than 1 billion shots) and industrial machining (product lifetimes greater than 20 billion shots). In addition to life test data, a summary of performance data for each epitaxial structure and each bar design is also presented.

The Beam Parameter Product (BPP) of a passive, lossless system is a constant and cannot be improved upon but the beams may be reshaped for enhanced coupling performance. The function of the optical designer of fiber coupled diode lasers is to preserve the brightness of the diode sources while maximizing the coupling efficiency. In coupling diode laser power into fiber output, the symmetrical geometry of the fiber core makes it highly desirable to have symmetrical BPPs at the fiber input surface, but this is not always practical. It is therefore desirable to be able to know the 'diagonal' (fiber) BPP, using the BPPs of the fast and slow axes, before detailed design and simulation processes. A commonly used expression for this purpose, i.e. the square root of the sum of the squares of the BPPs in the fast and slow axes, has been found to consistently under-predict the fiber BPP (i.e. better beam quality is predicted than is actually achievable in practice). In this paper, using a simplified model, we provide the proof of the proper calculation of the diagonal (i.e. the fiber) BPP using BPPs of the fast and slow axes as input. Using the same simplified model, we also offer the proof that the fiber BPP can be shown to have a minimum (optimal) value for given diode BPPs and this optimized condition can be obtained before any detailed design and simulation are carried out. Measured and simulated data confirms satisfactory correlation between the BPPs of the diode and the predicted fiber BPP.

Integrating volume holographic gratings into micro-optical components such as cylindrical fast-axis collimation lenses (VHG-FAC) for diode lasers constitutes a promising concept for wavelength stabilization by forming an external cavity laser. Compared to standard wavelength stabilization configurations the integrated element reduces the alignment complexity and is furthermore insensitive to the smile-error of diode laser bars. In order to configure and optimize these components the diffraction of the divergent field distribution of a broad area semiconductor laser must be calculated. The present paper presents the extension of the coupled-mode theory in order to calculate the spectral distribution of the diffracted field and the coupling efficiency within the external cavity. The model was extended to three-dimensional space and supplemented to include surface effects, polarization dependency and wave-optical propagation. The asymmetric spectral distribution emitted by an external cavity laser with VBG-FAC is tracked back to the feedback of highly divergent radiation diffracted at the holographic grating. Power losses due to the coupling efficiency within the cavity are also calculated for various field distributions and compared with experimental data. In summary the mathematical model allows to estimate the minimum spectral width and the losses using a VHG-FAC in an external cavity. Thus the injection locking concept using the VHG-FAC can be compared to the spectral characteristics and estimated power losses of standard wavelength stabilization configurations, e.g. the alignment of the grating in the collimated beam.

Semiconductor laser diodes with a tapered gain region provide a beam quality near to the diffraction limit combined with high output power. They can be configured as laser with a high-reflectivity coating on the rear facet as well as amplifier with an antireflection coating on both facets. In amplifier configuration they can be used in external cavity or MOPA - configuration with the advantage of a precisely tunable wavelength. Today amplifiers are commercially established with an optical output-power of 1-2W in a wide range of applications such as non linear spectroscopy and frequency doubling for blue-green outputs. Especially for the pumping of doped fiber amplifiers and materials processing higher power is requested. By extension of the resonator length from 2000μm up to 5000μm the output power of the amplifiers has been increased up to 10W at 976nm with beam quality near diffraction limit up to 8W. A wall-plug efficiency of more than 55% was reached in continuous wave operation. Improvements of the epitaxy structure and the mounting technology are essential for realizing longer resonators. Therefore an improved InGaAs/AlGaAs single quantum well vertical structure with low internal losses was grown by molecular beam epitaxy. The lateral design of the investigated devices consists of a ridge-waveguide section and a taper section with 4° taper angle. Due to the 4° taper angle the devices provide a small lateral far-field angle < 11° (95%).

In this paper, we utilize the concept of the Wigner distribution function (WDF) on distributed-Bragg-reflector taper lasers (DBR-TPL). The WDF allows the derivation of the phase and the intensity distribution just as well as the spatial coherence properties of the laser beam. For a given single-mode fiber the coupling efficiency for a given beam and optical system can be obtained by means of a simple overlap integral. Simultaneously, this approach delivers the corresponding beam forming requirements to meet the optimum coupling condition. We found a good agreement between the measured coupling efficiencies of the DBR-TPL into a single-mode fiber under varying coupling conditions and the corresponding efficiencies derived from the measured WDF by simulating the same coupling conditions.

Spectrally-narrowed semiconductor laser diodes utilizing external volume gratings can be used to improve TEM00 power scaling and power conversion efficiency in diode-pumped solid state and fiber lasers. This approach is particularly attractive for pumping the narrow upper laser level of Nd:YAG DPSS lasers at 885 nm and the 1532 nm absorption band of Er:YAG DPSS lasers. While it is often believed that the use of such external gratings to wavelength lock diode lasers lead to unavoidable losses in power and efficiency, nLIGHT's proprietary laser designs and external volume grating integration techniques have eliminated these losses in our wavelength-locked diode laser products, enabling a broad range of spectrally locked laser diodes for pumping DPSS as well as fiber laser systems.

We report on a new series of fiber coupled diode laser modules exceeding 1.2kW of single wavelength optical power from a 400um / 0.2NA fiber. The units are constructed from passively cooled laser bars as opposed to other comparably powered, commercially available modules that use micro-channel heat-sinks. Micro-channel heat sinks require cooling water to meet demanding specifications and are therefore prone to failures due to contamination and increase the overall cost to operate and maintain the laser. Dilas' new series of high power fiber coupled diode lasers are designed to eliminate micro channel coolers and their associated failure mechanisms. Low-smile soldering processes were developed to maximize the brightness available from each diode laser bar. The diode laser brightness is optimally conserved using Dilas' recently developed propriety laser bar stacking geometry and optics. A total of 24 bars are coupled into a single fiber core using a polarization multiplexing scheme. The modular design permits further power scaling through wavelength multiplexing. Other customer critical features such as industrial grade fibers, pilot beams, fiber interlocks and power monitoring are standard features on these modules. The optical design and the beam parameter calculations will be presented to explain the inherit design trade offs. Results for single and dual wavelengths modules will be presented.

A record, 940W, CW output-power has been achieved for a single, 1cm-wide, 5mm cavity-length, 77% fill-factor, 940nm, laser-diode bar operated at 900A and 20°C heat-sink temperature. The slope efficiency below 400A is 1.2W/A and the peak power-conversion efficiency is 70%. The laser bar was attached to a novel EPIC (Enhanced Performance Impingement Cooler) heat-sink which has a heat removal capacity exceeding 3kW/cm². Constant current operation at 580A (~600W), 20°C over a period of 100hrs was also demonstrated. These record results are, in large part, due to the record low thermal resistance of 0.060K/W, about a third that of micro-channel coolers.

We report on the development of a novel active heat sink for high-power laser diodes offering unparalleled capacity in high-heat flux handling and temperature control. The heat sink employs convective heat transfer by a liquid metal flowing at high speed inside a miniature sealed flow loop. Liquid metal flow in the loop is maintained electromagnetically without any moving parts. Thermal conductance of the heat sink is electronically adjustable, allowing for precise control of diode temperature and the laser light wavelength. This paper presents the principles and challenges of liquid metal cooling, and data from testing at high heat flux and high heat loads.

Modules consisting of multiple single emitters pose demanding challenges on assembly, production capabilities and cost. A fiber coupled module has been produced delivering 100 W optical power from a 105 μm, NA 0.15 fiber. The module consists of two times six vertically stacked single emitters combined by polarization multiplexing. Special attention was paid to the development of a very robust low cost pigtail. The developed semiautomatic highly accurate process enables assembly times way faster than possible in a purely manual procedure. The achievable yield in combination with low material costs proofs the excellent potential for the manufacturing of cost efficient laser modules.

A record, 250W, CW output-power has been achieved for a single, 1cm-wide, 3.5mm cavity-length, 20% fill-factor, 976nm, laser-diode bar operated at 20°C. The remarkable laser-bar performance was in part the result of a novel EPIC (Enhanced Performance Impingement Cooler) heat-sink with a thermal resistance of 0.16K/W. The superb thermal management resulted in record brightness for a laser bar, i.e. a slow-axis divergence of 10° (95% power containment angle) was achieved at 200W output-power. A coupling efficiency of ~74% into a 200μm core, 0.22NA fiber was achieved.

Multiple Single Emitter (MSE) modules allow highest power and highest brightness fiber coupled diode lasers based on standard broad area diodes. 12 single emitters, each rated at 11W, can be stacked in fast axis and yield more than 100W in a fully collimated beam with a beam quality of 7mm*mrad in both axes. Optical transfer efficiencies of >88% from diode facet to after the fiber are achieved resulting in efficient and compact fiber coupled modules. Volume Bragg Gratings (VBG) stabilize the wavelength over a tuning range of >10nm and narrow the linewidth of individual diodes to less than 2nm. The brightness is scaled by polarization multiplexing and optical stacking is deployed for larger fibers resulting in 700W delivered from a 200μm fiber, 0.2 NA. Wall plug efficiencies of 35% are achieved. The challenge of MSE fiber coupled diode lasers lies in high precision, high yield manufacturing and not so much the optical design of the device, since only collimating lenses and a focusing optic are used. However, a large number of individual components must be handled and consistently aligned with high precision. The 100W module comprises 12 single emitters and the 700W/200μm/0.2 laser comprises 120 single emitters with 85% optical fill factor. Pointing tolerances and collimation errors of all emitters cannot exceed 10% of the spot size to realize the benefits of highest brightness from single emitters compared to bars. The two major assembly processes of MSE fiber coupled diode lasers are the precision diode reflow process and the accurate 5 axis alignment of the fast axis collimation lens (FAC). The reflow process enables positioning of 12 single emitter diodes on submounts within +/-5μm on a common heatsink. Special image processing software performs automated precision alignment and fixation of the FAC with a consistent accuracy of better 0.3um and 0.12mrad. It is also deployed for automated alignment of the external VBG. Wavelength stabilization in an external resonator aims to maximize the locking range and to minimize the drop of output power as well as linewidth. Front facet reflectivity of the diode laser, reflectivity of the volume Bragg grating (VBG) and different resonator designs are investigated.

High-brightness fiber coupled laser modules are presented with output powers of more than 75W and electro optical efficiencies of more than 45%. An ongoing lifetime test shows nearly 3000h reliable operation. A wavelength stabilization using external gratings is shown with stable wavelength locking over a large temperature range. To combine the output powers of the fiber coupled modules fiber combiners were used and powers in the 400 and 1000W range were achieved for output fibers of 200 and 400μm, respectively.

New solid-state laser devices, especially fiber laser systems, require increasingly higher optical pump power provided by fiber-coupled diode laser modules. In particular for defense technology, robust but lightweight high-power diode laser sources with high brightness are needed. We have developed a novel diode laser device combining high power, high brightness, wavelength stabilization and low weight, which becomes more and more important for a multitude of applications. Heart of the device is a specially tailored laser bar, which epitaxial and lateral structure is designed such that only standard fast- and slow-axis collimator lenses are required to couple the beam into a 200 μm fiber with numerical aperture of 0.22. In this paper we present a detailed characterization of the new diode laser device with up to 775 W of optical power coupled into a 200 μm, NA 0.22 fiber. One important feature of the device is a lightweight design due to a special housing optimized for low weight. In addition we present results of a diode laser device with 675 W of optical output power and improved spectral quality, which is ensured over a wide range of temperature and current by means of volume holographic gratings for wavelength stabilization. For this device an overall efficiency of more than 42.5 % has been achieved. Furthermore we present a compact diode laser source with 230 W of optical power coupled into a 200 μm, NA 0.22 fiber. This diode laser device is optimized with regard to highest efficiency and yields an overall electro-optical efficiency of more than 50 %.

High power (HP) laser diodes with apertures around 100um pump solid state and fiber lasers, used for material processing. The necessity for the second stage lasers originates from the well-known limitation of brightness of laser diodes with the aperture increase due to appearance of multiple lateral modes. For the first time we report suppression of lateral modes of 100um wide laser diodes by digital planar holograms. Digital planar hologram narrows spectrum of laser diodes, similar to simple gratings, used in DFB and DBR lasers.

Single-mode-emitting high-power diode laser arrays (SM-HPDLA) are available industrially with more than 50 W emission power per bar. Based on this platform an expandable prototype solution is realized for fiber coupling of a stacked array with more than 100 W to an optical fiber with diameter of 200 micron and NA of 0.11. Advanced methods of controlled assembly of micro-optics by infrared laser-soldering have been developed therefore. We present a compact and scalable concept with scalability on 2 internal and 2 external factors. Internal factors are the increasing beam quality and power stability of high-power single-mode-emitting arrays and the improved assembly accuracy for diode bar and micro-optics. External factors are the interlaced coupling of stacked beam emission from the stacked array and the further option to use optimized polarisation coupling with several diode laser stacks.

We demonstrate monolithic distributed-Bragg-reflector tapered diode lasers having an output power up to 12 W, a small spectral width of below ▵λ<10 pm and a beam quality close to the diffraction limit. This results in a brightness close to 1 GWcm-2sr-1. Due to these excellent electro-optical characteristics we achieved visible laser light up to P=1.8 W in a single-path second harmonic generation experiment. This allowed us to develop compact Watt-class (P=1.1 W) visible laser modules having an excellent beam quality (M²<3) with a narrow spectrum (▵λ<30 pm). The entire device is integrated on a micro-optical bench with a volume below 20 cm³. In another application we demonstrate for the first time a femtosecond gigahertz SESAM-modelocked Yb:KGW laser. Such a laser system benefits from the small spectral emission and the focusability of the developed diode laser. A record peak power of 3.9 kW was achived. At the repetition rate of 1 GHz, 281 fs pulses with an average output power of 1.1 W were generated. This Yb:KGW laser has a high potential for stable frequency comb generation.

High-power single emitters and laser bars find several growing industrial applications such as materials processing. A steady increase in efficiency and output power is needed to conquer new markets. The development of high-end diode lasers usually starts with the development of the epitaxial structure. Therefore, simulations have been performed before the optimized layer structures have been tested using broad area (BA) lasers. Single emitter laser diodes are needed for fiber coupling with a minimal loss of light. We developed such emitters with high output powers at a small far field angle showing output powers of more than 20 W around 940 nm. The high-quality laser bars available at JENOPTIK have been improved and extended to new wavelength ranges. At the lower end, laser bars have been developed around and below 800 nm. At the higher end, design parameters have been optimized for 1060 nm emission wavelength.

Semiconductor lasers with emission in the range 790 - 880 nm are in use for a variety of application resulting in different laser designs to fulfill requirements in output power, operation temperature and lifetimes. The output power is limited by self heating and catastrophic optical mirror damage at the laser facet (COMD). Now we present data on bars fabricated with our new facet technology, which enables us to double the maximum facet load. We present q-cw laser bar with 80% fill factor with increased power level to 350W in long term operation at 200μs and 100Hz. The COMD limit of the bar is as high as 680W. Using Quantel's optimized packaging stacks with 11 bars of 5mm widths are tested at up to 120A resulting over 66% power conversion efficiency at 1600W output power. Laser bars for continuous wave operation like 50% fill factor bars had an COMD limit of approx. 250W with conventional facet technology, the value is equivalent to 10W per 200μm emitter (conditions: 200μs). The new facet technology pushes the facet stability to 24W/emitter. The new process and an improved design enable us to shift continuous wave operation at 808nm from 100W to 150W/bar with lifetimes of several thousand hours at 30°C using DILAS mounting technology. Higher power is possible depending on lifetime requirements. The power conversion efficiency of the improved devices is as high as 62% at 200W cw. The next limitation of 8xxnm lasers is high temperature operation: Values of 60-80°C are common for consumer applications of single emitters. Therefore Osram developed a new epitaxial design which reduced the generation of bulk defects. The corresponding Osram single emitters operate at junction temperatures up to 95°C, a value which corresponds to 80°C heat sink temperature for lasers soldered on C-mount or 65°C case temperature for lasers mounted in TO can. Current densities of the single emitter broad area lasers are as high as 1.4kA/cm² at 850nm emission wavelength.

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

We report results of a spatially-multiplexed broad area laser diode platform designed for efficient pumping of fiber lasers or direct-diode systems. Optical output power in excess of 100W from a 105μm core, 0.15NA fiber is demonstrated with high coupling efficiency. The compact form factor and low thermal resistance enable tight packing densities needed for kW-class fiber laser systems. Broad area laser diodes have been optimized to reduce near- and far-field performance and prevent blooming without sacrificing other electro-optic parameters. With proper lens optimization this produces ~5% increase in coupling / wall plug efficiency for our design. In addition to performance characteristics, an update on long term reliability testing of 9XX nm broad area laser diode is provided that continues to show no wear out under high acceleration. Under nominal operating conditions of 12W ex-facet power at 25C, the diode mean time to failure (MTTF) is forecast to be ~ 480 kh.

Advanced solid state laser architectures place increasingly demanding requirements on high-brightness, low-cost QCW laser diode pump sources, with custom apertures both for side and end rod pumping configurations. To meet this need, a new series of scaleable pump sources at 808nm and 940nm was developed. The stacks, available in multiple output formats, allow for custom aperture filling by varying both the length and quantity of stacked laser bars. For these products, we developed next-generation laser bars based on improved epitaxial wafer designs delivering power densities of 20W/mm of emission aperture. With >200W of peak QCW power available from a full-length 1cm bar, we have demonstrated power scaling to over 2kW in 10-bar stacks with 55% wall plug efficiency. We also present the design and performance of several stack configurations using full-length and reduced-length (mini) bars that demonstrate the versatility of both the bar and packaging designs. We illustrate how the ROBUST HEAD packaging technology developed at SCD is capable of accommodating variable bar length, pitch and quantity for custom rod pumping geometries. The excellent all-around performance of the stacks is supported by reliability data in line with the previously reported 20 Gshot space-grade qualification of SCD's stacks.

High power diode laser line generators are nowadays industrial standard for applications like plastic processing, vision inspection and drying. With increased beam quality, especially peak intensity and homogeneity, they also enable new applications like hardening, annealing or cutting of various materials. All of these applications have in common that simultaneous processing is limited by the scalability of the generated line length without changing process relevant parameters of the line like working distance, peak intensity, homogeneity and depth of focus. Therefore, a patent pending beam shaping concept is presented that enables the interconnection of an arbitrary number of nearly free selectable laser sources to generate scalable laser lines with outstanding beam parameters. System design, experimental setup and results of a laser line generator are shown. It is based on a stitching concept consisting of ten fibre coupled high power diode lasers, which generates a 200mm long and 2mm wide laser line with a homogeneity level of 97% p-v over a depth of focus of +/- 5 mm with an overall output power of up to 4.2 kW. The concept is discussed regarding industrial applications and the options for even higher beam quality, especially the capability of generating lines with increased power densities up to several kW/cm² and a line length of several meters.

The ever increasing demand for high-power, high-reliability operation of single emitters at 9xx nm wavelengths requires the development of laser diodes with improved facet regions immune to both catastrophic and wear-out failure modes. In our study, we have evaluated several laser facet definition technologies in application to 90 micron aperture single emitters in asymmetric design (In)GaAs/AlGaAs based material emitting at 915, 925 and 980nm. A common epitaxy and emitter design makes for a straightforward comparison of the facet technologies investigated. Our study corroborates a clear trend of increasing difficulty in obtaining reliable laser operation from 980nm down to 915nm. At 980nm, one can employ dielectric facet passivation with a pre-clean cycle delivering a device lifetime in excess of 3,000 hours at increasing current steps. At 925nm, quantum-well intermixing can be used to define non-absorbing mirrors giving good device reliability, albeit with a large efficiency penalty. Vacuum cleaved emitters have delivered excellent reliability at 915nm, and can be expected to perform just as well at 925 and 980nm. Epitaxial regrowth of laser facets is under development and has yet to demonstrate an appreciable reliability improvement. Only a weak correlation between start-of-life catastrophic optical mirror damage (COMD) levels and reliability was established. The optimized facet design has delivered maximum powers in excess of 19 MW/sq.cm (rollover limited) and product-grade 980nm single emitters with a slope efficiency of >1 W/A and a peak efficiency of >60%. The devices have accumulated over 1,500 hours of CW operation at 11W. A fiber-coupled device emits 10W ex-fiber with 47% efficiency.

TeraDiode has produced a fiber-coupled direct diode laser with a power level of 1,040 W from a 200 μm core diameter, 0.18 numerical aperture (NA) output fiber at a single center wavelength. This was achieved with a novel beam combining and shaping technique using COTS diode lasers. The fiber-coupled output corresponds to a Beam Parameter Product (BPP) of 18 mm-mrad and is the lowest BPP kW-class direct diode laser yet reported. The laser has been used to demonstrate laser cutting and welding of steel sheet metal up to 6.65 mm thick. Further advances of these ultra-bright lasers are also projected.

In wavelength region of red color, luminous efficacy rapidly increases as wavelength shortens. In that sense, red laser diode (LD) with shorter wavelength is required for display applications. Experimental results for short wavelength limitation in AlGaInP LDs are shown and discussed in this paper. Broad area LDs with 625, 630, and 638 nm are successfully fabricated. Operation current versus output power (P-I) characteristics and its temperature dependence of 625 nm LD are inferior to that of 630 and 638 nm ones. The main reason might be carrier leakage, and the results indicate that additional countermeasures to carrier leakage should be adopted to realize a 625 nm LD with the same temperature characteristics as 630 and 638 nm LDs. Conversion efficiencies from input electrical power to luminous flux output of the LDs are also studied. 625 nm LD has low efficiency, though brightness of 625 nm light is 1.7 times of 638nm one with the same output power. And 630 nm LD shows better conversion efficiency at high luminous flux region than 638 nm one, though the P-I characteristics of 630 nm is worse than that of 638 nm one. The tendency is inverted at low flux region, indicating that the lasing wavelength of red LD for laser display should be chosen carefully.

We report on experimental and theoretical investigations concerning the influence of the ridge etching depth and the corresponding effective refractive index step on the electro-optical properties of ridge waveguide diode lasers emitting near 635 nm. We observe a suppression of higher order lateral modes for larger steps in effective index leading to a more homogeneous far field, as required e.g. in "flying spot" display applications. With the proper design choice, a total optical output power of P > 200 mW at 638 nm and 15°C and a beam quality M² < 2 could be achieved.

Semiconductor lasers play an important role in many applications. Depending on the wavelength of the emitted laser light in the blue (e.g. 405-445 nm), red (~ 650 nm), near infrared (780 - 1070 nm) and e.g. the eye-safe wavelength region around 1500 nm a manifold of applications exist. Due to their increasing power and brightness single emitter devices are becoming increasingly widely used for the assembly and packaging of high power diode lasers. In the near infrared typical emitter widths are 50, 90 (100) and 200 μm with power levels available > 15 W. Also larger stripes are available - up to 1000 μm - with power levels > 25W. For highest power laser devices not only the power of the emitter is important - but of equal importance is the subsequent optics to collect all the emitted power while maintaining the brightness of the source. High NA acylindrical micro-lenses very well account for the strong asymmetric emitter characteristics of the fast and slow axis and thus, result in best collimation and coupling efficiencies in contrast to spherical lenses. LIMO's cost-effective micro-optics wafer technology is most suited for such acylindrical optics. It allows the manufacture of different materials to cover wavelengths ranges from the UV to the NIR, e.g. 380 - 2000 nm. Since both sides of a wafer can be structured with crossed cylindrical lenses one single monolithic optical element simultaneously shapes the fast and slow axis of the emitted light. Additionally, mechanical reference planes can be integrated in such monolithic optics for precise and simple integration. Application examples for collimation and fiber coupling optics in the near infrared as well as focussing/pump optics in the blue wavelength range are shown.

High power diode laser bars are interesting in many applications such as solid state laser pumping, material processing, laser trapping, laser cooling and second harmonic generation. Often, the free running laser bars emit a broad spectrum of the order of several nanometres which limit their scope in wavelength specific applications and hence, it is vital to stabilize the emission spectrum of these devices. In our experiment, we describe the wavelength narrowing of a 12 element 980 nm tapered diode laser bar using a simple Littman configuration. The tapered laser bar which suffered from a big smile has been "smile corrected" using individual phase masks for each emitter. The external cavity consists of the laser bar, both fast and slow axis micro collimators, smile correcting phase mask, 6.5x beam expanding lens combination, a 1200 lines/mm reflecting grating with 85% efficiency in the first order, a slow axis focusing cylindrical lens of 40 mm focal length and an output coupler which is 10% reflective. In the free running mode, the laser emission spectrum was 5.5 nm wide at an operating current of 30A. The output power was measured to be in excess of 12W. Under the external cavity operation, the wavelength spread of the laser could be limited to 0.04 nm with an output power in excess of 8 W at an operating current of 30A. The spectrum was found to be tuneable in a range of 16 nm.

We demonstrate coherent polarization locking of multimode beams from four broad area emitters in a diode bar. The beams are overlapped into single output by using walk-off crystals and waveplates while their phases are locked via polarization discrimination. Coherent locking of multimode beams enabled power scaling of coherent diode output while retaining beam quality of single emitter. We obtained power of 7.2 W with M² of 1.5 x 11.5 from a 980 nm diode laser. This corresponds to brightness improvement of more than an order of magnitude.