This PDF file contains the front matter associated with SPIE Proceedings Volume 6456, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

As diode pumped solid state lasers gain more market share, the performance, stability and lifetime of the diode pump source faces unprecedented scrutiny. Lifetimes of diode pumps in excess of 35,000 hrs are sought with no intervention or maintenance from the end user. One lifetime and power limiting phenomena for arrays is that of solder creep typical with traditional mounting using soft solders such as Indium. Harder solders such as Gold/Tin on Copper-Tungsten submounts provide a more robust and stable mounting system for long term high power pump sources. Furthermore, beam multiplexing of laser bars require tight wavelength and polarization purity which are affected by mounting induced strain. In this investigation, high power 940 nm laser bars, operating in the 100 to 200 W power range, were mounted using AuSn/CuW and In soldering schemes. The differences in thermal and strain characteristics are investigated through the examination of the emitter wavelength, nearfield measurements, polarization and smile. The measurements are correlated with finite element modeling to predict the 3-dimensional thermal distributions within the laser bars.

In semiconductor lasers, key parameters such as threshold current, efficiency, wavelength, and lifetime are closely related to temperature. These dependencies are especially important for high-power lasers, in which device heating is the main cause of decreased performance and failure. Heat sources such as non-radiative recombination in the active region typically cause the temperature to be highly peaked within the device, potentially leading to large refractive index variation with bias. Here we apply high-resolution charge-coupled device (CCD) thermoreflectance to generate two dimensional (2D) maps of the facet temperatures of a high power laser with 500 nm spatial resolution. The device under test is a slab-coupled optical waveguide laser (SCOWL) which has a large single mode and high power output. These characteristics favor direct butt-coupling the light generated from the laser diode into a single mode optical fiber. From the high spatial resolution temperature map, we can calculate the non-radiative recombination power and the optical mode size by thermal circuit and finite-element model (FEM) respectively. Due to the thermal lensing effect at high bias, the size of the optical mode will decrease and hence the coupling efficiency between the laser diode and the single mode fiber increases. At I=10I_th, we found that the optical mode size has 20% decrease and the coupling efficiency has 10% increase when comparing to I=2I_th. This suggests SCOWL is very suitable fr optical communication system.

We present the reliability of high-power laser diodes utilizing hard solder (AuSn) on a conduction-cooled package (HCCP). We present results of 50 W hard-pulse operation at 8xx nm and demonstrate a reliability of MTTF > 27 khrs (90% CL), which is an order of magnitude improvement over traditional packaging. We also present results at 9xx nm with a reliability of MTTF >17 khrs (90% CL) at 75 W. We discuss finite element analysis (FEA) modeling and time dependent temperature measurements combined with experimental life-test data to quantify true hard-pulse operation. We also discuss FEA and measured stress profiles across laser bars comparing soft and hard solder packaging.

In recent years record performance characteristics from multi-mode InGaAs strained quantum well single emitters at 920-980nm have been reported including a maximum CW optical output power of ~20W and a power conversion efficiency of ~75%. These excellent performance characteristics are only possible through combined optimization of laser structure design, chip fabrication processes, and packaging. Whereas broad area multi-mode single emitters likely have sufficient reliability for industrial uses, reliability of these lasers still remains a concern for communications applications including deployment in potential space satellite systems where high reliability is required. Most of previous reports on these lasers have been focused on their performance characteristics with very limited reports on failure mode analysis although understanding the physics of failure is crucial in developing a proper lifetime model for these lasers. We thus report on the reliability and failure mode analysis of high power multi-mode single emitters. The lasers studied were broad area strained InGaAs single QW lasers at 940-980nm with typical aperture widths of around 100&mgr;m. At an injection current of 7A typical CW output powers were over 6W at 25°C with a wall plug efficiency of ~60%. First, various lasing characteristics were measured including spatial and thermal characteristics that are critical to understanding performance and reliability of these devices. ACC burn-in tests with different stress conditions were performed on these devices until their failure. We report accelerated lifetest results with over 5000 accumulated test hours. Finally, we report failure mode investigation results of the degraded lasers.

The degradation behavior of broad-area laser diodes and bars emitting at 650 nm under constant power operation is investigated. In addition to the increase in operation current the temperature of the laser facets was monitored using Raman spectroscopy. The formation of defects was studied using photocurrent spectroscopy while cathodoluminescence provided insight into the position of extended non-radiative defects at different stages of degradation. Although the facet does not show any visible alteration even for failed devices, its immediate vicinity appears to be the starting point of the observed gradual degradation effects. At the same time the local facet temperature is increased. The observed aging behavior is compared to the known degradation scenarios for devices emitting at 808 nm. In both cases there is a clear correlation between packaging-induced strain and observed degradation effects as demonstrated by the results obtained for bars. For the red devices a correlation between optical load and facet temperature exists which proves that here facet heating is indeed caused by re-absorption processes. Furthermore, the gradual degradation process is not accompanied by the creation of dark bands along 100 directions as observed earlier for 808 nm devices. The observed gradual degradation of the 650 nm devices is primarily accompanied by the formation of deep-level point defects, followed by the creation of macroscopic areas of reduced luminescence intensity. Packaging induced strains become important when gradual bar degradation is monitored at early stages.

There are strong demands at the market to increase power and reliability for 808 nm diode laser bars. Responding to this JENOPTIK Diode Lab GmbH developed high performance 808 nm diode laser bars in the AlGaAs/GaAs material system with special emphasis to high power operation and long term stability. Optimization of the epitaxy structure and improvements in the diode laser bar design results in very high slope efficiency of >1.2 W/A, low threshold current and small beam divergence in slow axis direction. Including low serial resistance the overall wall plug efficiency is up to 65% for our 20%, 30% and 50% filling factor 10 mm diode laser bars. With the JENOPTIK Diode Lab cleaving and coating technique the maximum output power is 205 W in CW operation and 377 W in QCW operation (200 &mgr;s, 2% duty cycle) for bars with 50% filling factor. These bars mounted on micro channel cooled package are showing a very high reliability of >15.000 h. Mounted on conductive cooled package high power operation at 100 W is demonstrated for more than 5000h.

High power diode lasers play an important role in laser and systems technology. Over the past years the necessity arose to improve power and beam quality (i.e. brilliance) of the high power diode lasers. This is not only for building pump sources but also to provide appropriate light sources for various direct applications of high power diode lasers. The funding initiative "BRIOLAS" launched by the German Federal Ministry of Education and Research (BMBF) is the answer to this demand. In ten projects diode technology, manufacturing technology, quality, pump applications, materials processing applications, medical systems and applications and also applications in the display technology are intensively investigated; the results shall be basis for new applications, for new products and for new ideas in the field of high power diode laser technology. The individual projects are vertically structured and in each project the entire technology chain necessary for the final success is represented by the participants. The first project has been launched end of 2004 in frame BMBF's funding program "Optical Technologies", the last one was started just recently. The BMBF is supporting the BRIOLAS-initiative with about 30 M€. This article provides a general overview over the target and the actual status of the initiative.

In order to meet the ever increasing demands of many high power laser diode customers, Nuvonyx has worked to improve a number of key metrics of the diode laser package. The most often challenged specifications are power per bar, efficiency, and reliability in both hard pulse and constant current mode. In response to these requests, Nuvonyx has worked to offer commercial component devices in excess of 100 and 150 watts per bar package in multiple wavelengths. The packages are routinely combined to form single stacks that generate greater than 3.5 kilowatts each and two-dimensional arrays which produce light in excess of 10 kilowatts. These parts all demonstrate predicted lifetimes in excess of 10,000 hours. The micro-channel cooled heat sink has also been improved by closer matching the coefficient of thermal expansion of the cooler to the laser diode bar, which allows for harder solders such as gold-tin to be employed. All of this work has helped to meet the specifications of the most demanding laser diode customers.

808 nm tapered lasers have been investigated under current pulsing conditions without thermal load. The pulse length was 100 ns. The lasers are based on a super-large optical cavity structure with a very small vertical divergence angle of 18° (FWHM). The output power, beam quality and spectral behavior of the lasers were measured. Resonator geometries with different ridge waveguide lengths and taper angles were used for the optimisation of output power and beam quality. As a result, 27 W output power has been achieved. Nearly diffraction-limited beam quality up to 9 W has been obtained with an optimised lateral geometry. At even higher power, spectral broadening and beam quality degradation of the lasers were observed.

High power laser diodes and diode arrays emitting at the wavelength of 808nm are widely used for pumping neodymium (Nd+) doped solid state lasers and fiber lasers, medical surgery, dental treatment and material processing. In general, the power is limited by catastrophic optical mirror damage (COMD) and heat dissipation. In this paper we demonstrate 29W CW output power at 808 nm from a 400 &mgr;m single emitter with 2mm cavity length. The device thermally rolls over due to the excess heat. The L-I curve rolls over at 29.5W, the laser is still alive, and we can repeat the test again and again without catastrophic optical mirror-damage (COMD). The device consists of an InAlGaAs/AlGaAs/GaAs, optimized special graded-index separated-confinement heterostructure (GRINSCH) broad waveguide (BW), single quantum well (SQW) and barriers between waveguide and cladding layers. A weak temperature dependence characteristic, which is desirable for high power and reliable operation, is obtained from these devices.

Ongoing optimization of epitaxial designs, MOCVD growth processes, and device engineering at Spectra-Physics has yielded significant improvement in both power conversion efficiency (PCE) and reliable power, without compromising manufacturability in a high-volume production environment. Maximum PCE of 72.2% was measured at 25 °C for 976- nm single-emitter devices with 3-mm cavity length. 928 W continuous-wave (CW) output power has been demonstrated from a high-efficiency (65% maximum PCE) single laser bar with 5-mm cavity length and 77% fill factor. Eight-element laser bars (976 nm) with 100&mgr;m-wide emitters have been operated at >148 W CW, corresponding to linear power densities at the facet >185 mW/&mgr;m. Ongoing life-testing, in combination with stepped stress tests, indicate rates of random failure and wear-out are well below those of earlier device designs. For operation near 800 nm, the design has been optimized for high-power, high-temperature applications. The highest PCE for water-cooled stacks was 54.7% at 35°C coolant temperature.

We present recent advances in high power semiconductor lasers including increased spectral brightness using on-chip internal gratings and increased spatial brightness at wavelengths from the near infrared to the eye-safe regime.

Diode lasers supply high power densities at wavelengths from 635-nm to 2000-nm, with different applications enabled by providing this power at different wavelengths. As the range of available wavelengths broadens, many novel medical and atmospheric applications are enabled. Traditional quantum well lasers provide high performance in the range 635- nm to 1100-nm range for GaAs-based devices and 1280-nm to 2000-nm for InP, leaving a notable gap in the 1100 to 1280-nm range. There are many important medical and sensing applications in this range and quantum dots produced using Stranski-Krastanow self-organized MBE growth on GaAs substrates provide an alternative high performance solution. We present results confirming broad area quantum dot lasers can deliver high optical powers of 16-W per emitter and high power conversion efficiency of 35% in this wavelength range. In addition, there are growing applications for high power sources in wavelengths > 1500-nm. We present a brief review of our current performance status in this wavelength range, both with conventional quantum wells in the 1500-nm to 2500-nm range and MOCVD grown quantum cascade lasers for wavelengths > 4000-nm. At each wavelength, we review the designs that deliver this performance, prospects for increased performance and the potential for further broadening the availability of novel wavelengths for high power applications.

We report present advantages of high power 9xxnm diode laser bars for pumping of disc laser and especially for pumping fibre lasers and amplifiers. The strong demand for reduce system costs needs to have a good compromise in improved diode laser power, conversion efficiency, reliability and beam quality leading to simplified system designs. Basis of the new generation for the 9xxnm laser diode bars at JENOPTIK Diode Lab is a low loss wave guide AlGaAs - structure with low vertical far field angle of 27° (FWHM). Recently we demonstrate an output power in excess of 500W in CW operation from a diode laser bar with 50% filling factor and 3.0mm cavity length. This record was possible due to high power conversion efficiency of >68 %, optimised facet coating technology and an excellent active cooling. New results on conductive cooled high brightness laser bars of 20% filling factor with special emphasis to the needs of high efficiency fibre coupling will be presented. Lifetime tests under long pulse conditions have demonstrated a very high reliability for 120 W laser bars with 50 % filling factor and for 60 W laser bars with 20 % and 30 % filling factor.

Laser diodes and bars with high efficiency, power, and reliability are critical for a wide variety of applications including direct material processing and pumping high power and efficient fiber lasers and solid state lasers. We present progress towards the 80% power conversion efficiency goal of the DARPA Super High Efficiency Diode Sources (SHEDS) program. Currently using JDSU SHEDS technology, laser bars and stacks respectively achieve as high as 76% and 73% total power conversion efficiency at room temperature. Single-emitter laser diodes incorporating SHEDS technology achieve up to 19W peak CW power from a 100um wide aperture.

High-power diode lasers operating at 808 nm and consisting of a multiple ridge-waveguide structure have been fabricated. Lasers with this structure show a more stable far and near field pattern in comparison to conventional single stripe broad area lasers. A reliable continuous wave operation at room temperature over 8000 h at 8 W and 800 h at 10 W has been achieved with 200 &mgr;m stripe width devices.

We describe the performance and reliability of high power vertical diode stacks based on silicon monolithic microchannel coolers (SiMMs) operating at >1000W/cm² CW at 808 and 940nm. The monolithic nature of these stacks makes them inherently robust and compact. Typical emitting dimensions for a 10-bar stack are ~8.8mm × 10mm with CW output power up to 1.5kW. Originally developed at Lawrence Livermore National Laboratory and now actively being developed for commercial applications at Coherent, this technology offers several advantages over current copper-based micro-channel coolers. These devices do not require use of DI water, strict monitoring and control of the pH level, careful control of the water velocity, or sealed cooling systems. The need for hydrostatic seals is also drastically reduced. A typical ten bar stack requires only 2 o-ring seals, compared to 20 such seals for a similar stack using copper microchannel cooling. Mature and readily available wet etching technology allows for cost effective batch fabrication of the sub-mount structure while achieving repeatable high precision components based on photolithographic fabrication processes.

Diode laser systems are well established for applications which demand high continuous wave (cw) power. These applications are material processing like cutting and welding of metals as well as polymers where diode laser systems are less expensive and more compact than solid state lasers. Even though the optical output power and the beam quality of diode lasers are increasing steadily, the use of these sources is generally limited to cw applications. For processes during which ablating of material is demanded, however, conventional diode lasers are inferior compared to pulsed solid state lasers as diode lasers suffer from the absence of optical intracavity q-switching. Some examples of these applications are coating removal and marking. To overcome this drawback, we have developed several diode laser systems that use high peak-current drivers and thereby allow to operate the diode lasers at currents up to 500 A. The pulse source was tested with fiber coupled single emitters, conventional diode lasers and customized AR-coated diode laser bars. With the new diode laser driver, a peak output power of 250 W can be achieved with pulse durations of approx. 100 ns. Polarization coupling of two bars increases the power by a factor of two. Thereby an output power of 500 W can be demonstrated. These systems reach an intensity of 27 MW/cm² per diode laser bar which is sufficient for ablating processes. We will demonstrate the design of the prototype system as well as results of marking and coating removal experiments with the system.

We demonstrate 940nm diode lasers with more than 100W QCW output power having an aperture width 5 to 10 times smaller than commonly used 10mm bars. We used a super-large vertical waveguide structure to reduce the facet load. The waveguide design results in a very small vertical divergence of only 14° FWHM (24° including 95% of power). The threshold current of a device with 1mm wide aperture is about 8A and the slope efficiency is above 70%. The lateral far field width is below 10°, including 95% of power, and the wall plug efficiency is around 50% at 100W output power.

In this paper we report the development of high power high brightness semiconductor laser chips using a combination of quantum well intermixing (QWI) and novel laser designs including laterally unconfined non-absorbing mirrors (LUNAM). We demonstrate both multi-mode and single-mode lasers with increased power and brightness and reliability performance for the wavelengths of 980 nm, 940 nm, 830 nm and 808 nm.

Focused development under the DARPA SHEDs program has lead to extremely high power conversion efficiency in the 9xx-nm wavelength band, leading to bars with efficiency in excess of 74%. We review progress in advancing efficiency and detail the route to > 85% at room temperature. The 9xx-nm wavelength band is commercially used for pumping Ytterbium-doped solid-state crystals and fiber lasers - only one of many diode laser markets. Fortunately, the lessons learned under SHEDs are transferable to other wavelengths. We report breakthrough efficiency results in the 8xx-nm band, for example showing 71% power conversion efficiency from 790-nm bars at powers > 100-W for CW and QCW packaging and testing. These wavelengths are required for pumping Neodymium-doped crystals, as used in the majority of fielded high power Diode Pumped Solid-State Laser systems. High efficiency is delivered using low voltage SHEDs designs, in combination with work to optimize the performance of the quantum well.

The new packaging technology from JENOPTIK Laserdiode GmbH and the new chip technology from JENOPTIK Diode Lab GmbH increases the output power, the quality and durability of new broad area lasers. Tests with different pulse widths and duty cycles have been conducted. A maximum linear power density of 213mW/&mgr;m has been found for 808nm and 980nm laser, limited by thermal rollover. The tests were performed for duty cycles from 0.1% to 5% and pulse widths of 50&mgr;s and 100&mgr;m. Over 32W output power was reached for 150&mgr;m emitter at a 0.1 % duty cycle and 50&mgr;s pulse length. With the new diode laser technology 10mm bars with a 44% filling factor were produced. These laser bars, mounted on micro channel coolers, reached a maximum output power of 1000W. To our knowledge this is the highest power reported up to now for 980nm material with 100&mgr;s pulses and 0.1% duty cycle.

In comparison with other laser systems diode lasers are characterized by a unique overall efficiency, a small footprint and high reliability. However, one major drawback of direct diode laser systems is the inhomogeneous intensity distribution in the far field. Furthermore the output power of current commercially available systems is limited to about 6 kW. We report on a diode laser system with 11 kW output power at a single wavelength of 940 nm aiming for customer specific large area treatment. To the best of our knowledge this is the highest output power reported so far for a direct diode laser system. In addition to the high output power the intensity distribution of the laser beam is homogenized in both axes leading to a 55 x 20 mm² Top-Hat intensity profile at a working distance of 400 mm. Homogeneity of the intensity distribution is better than 90%. The intensity in the focal plane is 1 kW/cm². We will present a detailed characterization of the laser system, including measurements of power, power stability and intensity distribution of the homogenized laser beam. In addition we will compare the experimental data with the results of non-sequential raytracing simulations.

There are many advantages of delivering optical power from high power laser diode arrays through an optical fiber. However, most high power diode lasers require complicated optical trains in order to couple their light into a fiber because of the poor beam quality. Recently, Nuvonyx has reported implementations of a single spatial mode, high brightness laser diode bar technology that exhibits much improved beam quality. Optical power from the high brightness laser diode arrays is coupled into a 400 &mgr;m core fiber with a 0.22 numerical aperture (NA) at power density levels exceeding 1.4 MW/cm². These systems are suitable as standalone industrial direct diode laser systems or as multikilowatt fiber laser pump sources. In addition, Nuvonyx presents experimental results of further development in its fiber coupled diode laser systems program including techniques for wavelength stabilization and spectral beam combination.

We present new developed high power diode laser modules which are performing at outstanding brightness and their applications. The combination of recently designed laser diode bars on passive heat sinks and optimized micro-optics results to laser modules up to 50W out of a 100μm fibre with a 0.22 NA at one single wavelength based on broad area laser bars (BALB) and up to 50W out of 50μm fibre with a 0.22 NA based on single-mode emitter array laser (SEAL) bars. The fibre coupled systems are based on diode lasers with a collimated beam of superior beam data, namely < 10 mm x 10 mm beam diameter (FW1/e²) and < 2mrad x 2mrad divergence (FW1/e²). Such free beam diode lasers deliver 30 W or 60 W output power. The applications for such laser diode modules varies from direct marking, cutting and welding of metals and other materials up to pumping of fibre lasers and amplifiers. Marking speed with up to 30mm/s on stainless steel was observed with 20W laser power and 50&mgr;m fibre with a conventional marking setup. Cutting speed of about 1m/min of 0.2mm Kovar sheet was shown with a diode laser module with 50W laser power from a 100&mgr;m fibre.

A 40W one-cm laser diode array was coupled to a .22NA 200micron diameter optical fiber, using a compact monolithic 3-D waveguide based device, with 50% coupling efficiency. No auxiliary optical elements were used between the array and the coupler, or between the coupler and the optical fiber.

High brightness becomes more and more important in diode laser applications for fiber laser pumping and materials processing. For OEM customers fiber coupled devices have great advantages over direct beam modules: the fiber exit is a standardized interface, beam guiding is easy with nearly unlimited flexibility. In addition to the transport function the fiber serves as homogenizer: the beam profile of the laser radiation emitted from a fiber is symmetrical with highly repeatable beam quality and pointing stability. However, efficient fiber coupling requires an adaption of the slow-axis beam quality to the fiber requirements. Diode laser systems based on standard 10mm bars usually employ beam transformation systems to rearrange the highly asymmetrical beam of the laser bar or laser stack. These beam transformation systems (prism arrays, lens arrays, fiber bundles etc.) are expensive and become inefficient with increasing complexity. This is especially true for high power devices with small fiber diameters. On the other hand, systems based on single emitters are claimed to have good potential in cost reduction. Brightness of the inevitable fiber bundles, though, is limited due to inherent fill-factor losses. At DILAS a novel diode laser device has been developed combining the advantages of diode bars and single emitters: high brightness at high reliability with single emitter cost structure. Heart of the device is a specially tailored laser bar (T-Bar), which epitaxial and lateral structure was designed such that only standard fast- and slow-axis collimator lenses are required to couple the beam into a 200&mgr;m fiber. Up to 30 of these T-Bars of one wavelength can be combined to reach a total of > 500W ex fiber in the first step. Going to a power level of today's single emitter diodes even 1kW ex 200&mgr;m fiber can be expected.

The common wavelength regime for high-power diode laser modules is the range between 800 nm and 1000 nm. However, there are also many applications that demand for a wavelength of around 2 &mgr;m. This wavelength range is extremely interesting for applications such as the processing of plastics, medical applications as well as environmental analytics. The interest in lasers with this wavelength is based on the special absorption characteristics of different types of material: Numerous plastics possess an intrinsic absorption around 2 &mgr;m, so that the use of additives is no longer necessary. This is of great value especially for medical-technical products, where additives require a separate approval. Furthermore the longer wavelength allows the processing of plastics which are clear and transparent at the visible. In addition, water, which is an essential element of biologic soft tissue, absorbs radiation at the wavelength about 2 &mgr;m very efficiently. As radiation of this wavelength can be guided by glass fibers, this wavelength may be very helpful for laser surgery. Currently available lasers at the spectral range about 2 &mgr;m are solid-state lasers based on Ho- and Tmdoped crystals. These systems suffer from high purchase costs as well as size and weight. In contrast to this, diode lasers can be built more compact, are much cheaper and more efficient. For this background, GaSb based high-power laser diodes for the wavelength regime of 1.9 - 2.3 &mgr;m are developed at the Fraunhofer Institute for Solid State Physics (IAF). At the Fraunhofer Institute for Laser Technology (ILT), fiber-coupled laser diode modules based on these laser bars are designed and realized. A first module prototype uses two laser bars with a wavelength of 1.9 &mgr;m to provide an output power of approx. 15 W from a 600 &mgr;m, NA 0.22 fiber. The module setup as well as the characteristics of the laser bars at 1.9 &mgr;m wavelength are described in this paper.

Although many artificial light sources like high-pressure sodium lamp, metal halide lamp, fluorescent lamp and so on are commonly used in horticulture, they are not widely applied because of the disadvantages of unreasonable spectra, high cost and complex control. Recently new light sources of light-emitting diode (LED) and laser diode (LD) are becoming more and more popular in the field of display and illumination with the improvement of material and manufacturing, long life-span and increasingly low cost. A new type of super-bright red LD (BL650, central wavelength is 650 nm) was selected to make up of the supplemental lighting panel, on which LDs were distributed with regular hexagon array. Drive circuit was designed to power it and adjust light intensity. System performance including temperature rise and light intensity distribution under different vertical/horizontal distances were tested. Photosynthesis of sweet pepper and eggplant leaf under LD was measured with LI-6400 to show the supplemental lighting effects. The results show that LD system can supply the maximum light intensity of 180 &mgr;mol/m² •s at the distance of 50 mm below the panel and the temperature rise is little within 1 °C. Net photosynthetic rate became faster when LD system increased light intensity. Compared with sunlight and LED supplemental lighting system, LD's promotion on photosynthesis is in the middle. Thus it is feasible for LD light source to supplement light for vegetable crops. Further study would focus on the integration of LD and other artificial light sources.

Experimental results on RW and BA DFB lasers emitting at 785 nm suitable for Raman spectroscopy are presented. Optical spectra of the RW DFB laser reveal single mode operation with a side-mode suppression ratio of more than 45 dB at optical output powers up to 163 mW. A reliable operation of more than 8800 h of these devices is demonstrated. Within a spectral width of 0.6 nm, more than 99.9% of 1.1 W optical power of the BA DFB laser emitting at 785 nm are included. Raman measurements with RW and BA DFB lasers as excitation light sources and polystyrene as a test sample are presented. At an output power of 1.1 W of the BA device the integration time for the Raman measurement could be reduced to 50 ms.

An air-cooled THS of ceramics solder-mounts a distributed array of LDs and the ICs for an advanced electronics. This high power, compact, hermetic and self-cooled hybrid engine hosts also the coupling optics behind a window. Inside, many well spaced Laser Diodes / LDs emit a regular field of rays; next, a mating array of optical elements collectively reshapes each beam. After crossing the window, a large aspheric mirror collects and focuses the beams. Its reasonably tight focus and consistent powers can weld directly many materials or perform other industrial tasks. The system can also integrate more CCD and OE sensors to properly automate each welding etc sequence. On current coolers, the LDs face substantial optical limits, a complex assembly and many practical restrictions. The THS handles large powers avoiding water; a standard Pick and Placer can assemble nearly raw LD dices; a larger spacing gets around the optical constraint and many LDs can work concurrently, to behave as a true laser.

High optical output power in the multi-kW range from a fiber coupled diode laser can reach the beam quality of lamp pumped solid state lasers. Direct diode laser application as deep penetration metal welding becomes feasible. Polarization and wavelength multiplexing are established techniques to scale the optical power of diode lasers at almost constant beam quality. By use of volume diffraction gratings in an external cavity laser it is possible to constrict the spectral bandwidth of diode lasers and to reduce the wavelength shift related to temperature or current injection. Due to the stabilization of the wavelength multiplexing of diode laser beams at small distance of the center wavelengths can be realized. The development of a fiber coupled diode laser is presented. The set up consists of twelve modules which serve for an average optical power of 1.5 kW. Each module utilizes dense wavelength multiplexing of two diode laser bars with a center wavelength spacing of 3 nm. The diode laser bars are wavelength stabilized at center wavelengths of 908 nm, 911 nm, 975 nm and 978 nm. The spectral bandwidth of all diode laser bars is within 1 nm in the full power range. Stable operation at an average power of 136 W at 908/911 nm and 115 W at 975/978 nm with a wavelength shift less than 0.1 nm is achieved by the modules. Further coarse wavelength and polarization multiplexing and beam transformation enable fiber coupling to a 600 &mgr;m fiber with a numerical aperture of 0.175 (95% power inclusion).

Volume holographic gratings (VHG) provide the capability of narrowing and stabilizing the wavelength of semiconductor lasers by forming an external cavity laser (ECL). The standard configuration of these ECL's is to use a collimating lens followed by the VHG to provide feedback to the resonator and lock the wavelength. In this configuration both elements have to be carefully aligned with tolerances in the sub-µm and mrad range. The present paper presents a fast-axis collimation lens (FAC) with integrated VHG for locking a laser diode bar. Besides the advantage of having only a single element, the integrated element is also less sensitive to alignment tolerances with respect to the locking due to the large divergence angle of the uncollimated array compared to a collimated array. Using a standard AR coated array with 19 emitters an output power of 67.4 W was achieved. The spectral bandwidth was within 1 nm over the whole power range. Due to high stability requirements in this application, glass was chosen as the VHG material. Though the refractive index is low compared to standard FAC lenses, the design and manufacturing process of the lens still guarantees a diffraction limited collimated beam.

A novel phase locked array design, based on direct reflection feedback among adjacent cavities using an external grating, is analyzed and proposed. As a result of both longitudinal and transverse wavenumber selection, caused respectively by the narrow grating reflection bandwidth and by the array geometry, only one among the free running cavity eigenmodes can couple into a phase-locked, collective array eigenmode. The coupled array mode is experiencing the high reflectivity of the grating and surpasses the low gain of the free running modes, experiencing only a much lower reflectivity from the cavity edge mirror (anti-reflective coating). Thus phase locking and single mode operation can be concurrently achieved.

We present a novel approach to achieving both wavelength stabilization and wavelength agility in high-power two-dimensional stacks of high-power laser diodes. This approach utilizes volume Bragg gratings® with Bragg period that varies as a function of position within the clear aperture of the element according to a periodic function with period equal to the spacing between the laser diode bars within the stack. The Bragg period varies linearly within each period so that translation of the volume Bragg grating element results in simultaneous tuning of the wavelength of all the bars in the stack. As a result, the wavelength of the stack is adjustable, stable and the emission line is narrowed to < 0.5 nm. This kind of laser diode stacks is particularly suitable for pumping of gaseous media with very narrow absorption lines, e.g. atomic vapors of rubidium, cesium, potassium etc.

Northrop Grumman / Cutting Edge Optronics has developed three designs for microchannel-cooled laser diode arrays in which the coolant is electrically isolated from the current path. As a result, these arrays do not require the use of deionized water. The thermal performance of two of these designs is presented and, in one case, shown to far exceed the performance of standard copper microchannel-cooled packages. Also presented is a microchannel cooler made from ceramic material. This design leverages existing technology to create a low-cost, high-performance alternative to copper-based microchannel coolers. This approach offers the greatest promise for future development due to the vast assortment of existing capabilities that have already been developed for similar ceramic structures used in the electronics industry.

We describe the performance and reliability of multi-bar diode stacks assembled with hard solder attachment of the laser diode bar to the conduction-cooled package substrate. The primary stack package design is based on a modular platform that makes use of common piece parts to incorporate anywhere from 2-7 bars, operating at peak powers of 80W/bar to 200W/bar. In assembling monolithic type diode stack packages, it is typical to use a soft solder material such as indium for P-side bar attachment into the package. Due to its low melting point and low yield stress, indium can provide a solder joint that transfers low stress to the laser bar. However, during CW and QCW operation, indium is prone to migration that can cause device failure due to a number of well-known mechanisms. This shortcoming of soft-solder bar attachment can limit the number of shots the stack delivers over its operating life. By replacing the soft solder typically used for P-side attachment with a hard solder, it is possible to greatly reduce or eliminate certain failure modes, thereby increasing the operating life of the part. We demonstrate lifetime of > 1E9 shots at 80 W/bar, 250 us/40 Hz pulses, and 50C package operating temperature.

Micro-channel heatsink assemblies made from bonding multi-layered etched metal sheets are commercially available and are often used for removing the high waste heat loads generated by the operation of diode-laser bars. Typically, a diode-laser bar is bonded onto a micro-channel (also known as mini-channel) heatsink then stacked in an array to create compact high power diode-laser sources for a multitude of applications. Under normal operation, the diode-laser waste heat is removed by passing coolant (typically de-ionized water) through the channels of the heatsink. Because of this, the heatsink internal structure, including path length and overall channel size, is dictated by the liquid coolant properties. Due to the material characteristics of these conductive heatsinks, and the necessary electrically serial stacking geometry, there are several restrictions imparted on the coolant liquid to maintain performance and lifetime. Such systems require carefully monitored and conductive limited de-ionized water, as well as require stable pH levels, and suitable particle filtration. These required coolant systems are either stand alone, or heat exchangers are typically costly and heavy restricting certain applications where minimal weight to power ratios are desired. In this paper, we will baseline the existing water cooled Spectra-Physics Monsoon^TM heatsink technology utilizing compressed air, and demonstrate a novel modular stackable heatsink concept for use with gaseous fluids that, in some applications may replace the existing commercially available water-cooled heatsink technology. We will explain the various benefits of utilizing air while maintaining mechanical form factors and packing densities. We will also show thermal-fluid modeling results and predictions as well as operational performance curves for efficiency and power and compare these data to the existing commercially available technology.

The field of applications for diode laser bars is growing continuously. The reasons for this are the growing width of available wavelengths and the increasing optical output power. In parallel to this the requirements for packaging for the high power diode laser bars increase and are more manifold. Expansion matched, non corrosive, non erosive, low thermal resistance and high thermal conductivity are some of the keywords for the packaging in the near future. Depending on the thermal power density, two different types of heat sinks are used: active and passive. The active heat sinks can further be subdivided in micro- or macro-channel heat sinks. The development of macro-channel heat sinks was necessary because of the limited lifetime of the common micro-channel heatsink. The bigger channels reduce especially erosion and corrosion effects. By taking the increasing resonator length of the laser bars into account the cooling performance of the macro-channel heatsink will be sufficient for many applications. In cases of high thermal power densities there are still no alternatives to micro-channel heat sinks. New material combinations shall minimize the erosion and corrosion effects. New raw materials such as diamond composite materials with a higher thermal conductivity than copper and matched thermal expansion will find their working field at first in the passively cooling of laser bars. The next generation of active heat sinks will also be partly made out of the high performance materials. The point of time for this improvement depends on machining behavior, availability and price of the raw material.

In this paper we report on quasi-continuous-wave (q-cw) operation of monolithically stacked laser diode bars. Monolithically stacked laser diode bars consist of more than one laser diode grown on top of each other. In between every two laser diodes a tunnel junction is included to ensure proper current injection to all lasers. In comparison to a standard laser operated at the same optical power level, the monolithic laser stack has a significantly reduced optical mirror load. Furthermore the required current is reduced drastically, which has positive consequences on both laser lifetime and diode driver costs. If one otherwise compares a monolithic integrated laser bar stack with a setup of three separate standard laser bars, the monolithic laser bar stack is characterized by very low costs per watt as well as high brilliance. By using monolithically stacked laser diode bars we were able to exceed an optical power of 500 W in q-cw mode and are moving to even higher output power levels. Typical wavelengths are in the range between 800 and 1000 nm.

808 nm, QCW laser bars delivering peak power higher than 150 Watts were developed. The optimization of the tensile strain in the QW structure, the design configuration of the laser cavity together with an improved packaging technology lead to more than 55% wall plug efficiency when assembled as stacks. Due to the high characteristic temperature (T₀, T₁) values and high efficiency, the output power of these devices is almost insensitive to elevated heat sink temperatures. In addition, a collimation technique which significantly improves the beam quality of the laser stacks was developed. The active collimation method is flexible and control over the level of collimation is achievable. The use of this collimation technique alongside with high quality micro lenses allows for a reduction of the fast axis divergence to values as low as 3 mrad with minimal power losses. An automatic process control was developed allowing for the efficient attachment of the collimating micro lenses in a highly reproducible fashion. The combination of the collimation technique with a reliable mounting and stacking technology supports the serial manufacturing of devices delivering 1 kW peak power in QCW operation. These QCW collimated diode laser stacks demonstrate stable operation and high reliability in the course of more than 6*10⁸shots at 2% duty cycle. Another important advantage of the collimated stacks is their capability to withstand severe environmental conditions, maintaining high beam quality and performance.

Successful thermal and stress management of edge-emitting GaAs-based diode lasers is key to their performance and reliability in high-power operation. Complementary to advanced epitaxial structures and die-fabrication processes, next-generation heatsink designs are required to meet the requirements of emerging applications. In this paper, we detail the development of both active and passive heatsinks designed to match the coefficient of thermal expansion (CTE) of the laser die. These CTE-matched heatsinks also offer low thermal resistance, compatibility with AuSn bonding and improved manufacturability. Early data representing the performance of high-power devices on the new heatsinks are included in the presentation. Among the designs are a water-cooled, mini-channel heatsink with a CTE of 6.8 ppm/°C (near to the nominal 6.5 ppm/°C CTE of GaAs) and a thermal resistance of 0.43 °C/W (assuming a 27%-fill-factor diode-laser bar with a cavity length of 2 mm). The water flow in the heatsink is isolated from the electrical potential, eliminating the possibility of electrolytic corrosion. An additional feature of the integrated design is the reduction in required assembly steps. Our next-generation, passive, CTE-matched heatsink employs a novel design to achieve a reduction of 16% in thermal resistance (compared to the predecessor commercial product). CTE's can be engineered to fall in the range of 6.2-7.2 ppm/°C on the bar mounting surface. Comparisons between simulated performance and experimental data (both in CW and long-pulse operation) will be presented for several new heat-sink designs.

New technology regarding manufacturing of micro cooling systems and selective laser melting was presented at Photonics West 2005; at Photonics West 2006 the first non-corrosive active micro coolers manufactured by this procedure were introduced to the auditorium. This presentation shows the latest results regarding active micro cooling systems for high power diode laser with a matched CTE. After the first non corrosive micro coolers passed the tests regarding cooling performance, there were still two tasks which had to be accomplished: First one was the homogeneity of cooling. In order to achieve the homogeneity, the design of the supporting structure in front of the micro cooling structures was modified. Measurements of cooling performance and the range of the wavelength along the laser bar showed that with the modified structure a homogeneous inflow was achieved as well as homogeneous cooling along the laser bar. The presentation highlights the measurements and results. Second point of development was the quest for a material combination with an adequate thermal conductivity for keeping the cooling performance, but with a coefficient of thermal expansion that matches with the CTE of diode laser bar material. After testing several materials fitting to these requirements, only one material was left, which was capable to machine workable micro coolers with the SLM procedure. The presentation contains the first results of the micro coolers with a matched CTE regarding thermal resistance, CTE and packaging with gold - tin solder.

The purpose of this paper is to report life-testing results of a copper micro-channel cooler [MCC] for high power diode laser [HPDL] bars. The testing is being done on MCC's with 3 different internal designs at three different flow rates, ranging from 0.2 l/min. to 0.5 l/min. Water conductivity is kept constant at 5 micro-Siemens/cm, and the water temperature is maintained at 25C. The MCC's do not have a voltage applied to them. The test is ongoing, but results up to 15,000 hours will be reported. Preliminary results indicate that lifetimes over 20k hours can be achieved with proper control of conductivity, and that flow rate and internal structure are secondary considerations compared to water quality.

In the paper, the dynamics in the microsecond time-range of thermally induced wavelength shifts in emission spectra of pulse operated semiconductor lasers is studied. The experimental technique of time-resolved laser spectra mapping, developed to assess thermo-optical properties of pulse operating lasers is described. The presented technique enables measurements of the laser mode spectral characteristics at an arbitrarily selected moment within the duration of driving current pulse. The measured shift of the dominant mode wavelength in the spectrum envelope enables to find out temperature change of the laser active region. The values of the temperature change have been compared with the ones numerically calculated employing the finite element method.