Aculight has demonstrated spectral beam combining of four diode laser bars in a single optical cavity; each 1 cm wide diode bar included 200 individual single mode laser emitters. The beam combining was accomplished in the plane of the diode bar -- slow direction. In earlier work, Aculight has reported near diffraction limited performance from single diode laser bars where we have spectrally beam combined 200 laser emitters while maintaining a beam quality near the diffraction limit. Without spectral beam combination these diode laser bars will have a beam quality, in the plane of the bar, corresponding to an M² of 1000. In current work, Aculight is extending this technology to demonstrate a spectrally beam combined, diode laser system of 50 Watts, with near diffraction limited beam quality. To accommodate multiple diode laser bars, optical modeling was used to design and complete sensitivity analysis of a unique optical cavity based on the Schmidt telescope principal to remove off-axis aberrations. Error trees have been developed for beam quality and efficiency that illustrates just how the efficiency and beam quality have been maintained within this system.

We present experimental results on the locking of a 19-broad-area semiconductor laser array using a novel design of external cavity containing a lens array, projection optics, and a diffractive grating. All lasers are locked to single longitudinal mode. Significant improvement of the spatial profile of the entire laser array output beam has been observed. The center lobe of the far-field pattern of the laser array shows a single wavelength which can be tuned over a range more than 10 nm. The proposed technology can be applied to larger arrays including the stacked arrays.

Since the introduction of laser-cooling techniques for neutral atoms, the enhancement of high-power lasers with excellent spectral and spatial quality has been an important research subject. We report a new principle of using high-power laserdiodes directly in an external cavity. The very compact design offers an output power of up to 1 W and an excellent beam quality (M² < 1.2). The coupling efficiency for a single mode fiber exceeds 60%. The center wavelength can be tuned between 775 nm and 785 nm. This laser operates single mode with a mode-hop free tuning range of up to 15 GHz without current modulation and a side-mode suppression better than 55 dB. Demonstrating the suitability for neutral atom cooling we used this laser as light source in the production of a BEC of over a million ⁸⁷Rb atoms.

Pulsed lasers with pulse durations of nanosecond to millisecond are very important tools for free-space optical communication, LADAR, laser material processing, and optical sensing. Although Q-switched solid-state lasers or gas lasers are currently the most popular light sources for these purposes, pulsed semiconductor lasers have the potential for the above applications because of their compactness, accessibility of direct modulation, and inherently large electrical to optical conversion efficiency. The drawbacks with high-power semiconductor lasers are their poor beam quality and low coherence factors. This work addresses the above issues through experimental demonstration of frequency locking, wavelength tuning, and synchronization of nanosecond pulsed broad-area semiconductor lasers. Nanosecond optical pulses with the peak power of 25 W and the repetition rates of 4 KHz to 240 KHz are generated from a broad-area laser. An external cavity with a diffractive grating is used to reduce the linewidth of the laser from over 5 nm to less than 0.1 nm. The wavelength of the pulsed laser is tunable over more than 10 nm. We have conducted injection locking of a nanosecond pulsed broad-area laser with optical injection from a frequency-locked master laser. Successful injection locking strongly support the feasibility of synchronization and beam combination of pulsed broad-area lasers.

In this paper, the experimental results of self-injection phase locking in a 10 W, single element wide broad-area diode laser are presented. The width of the emitting area of this diode is 1000 μm, to our knowledge it is the broadest single element diode laser that until now has been used in an external feedback cavity. The beam quality of the diode laser is improved by the asymmetric self-injection phase locking technique. The far-field and near-field profiles are measured with and without the self-injection phase locking at different operating currents. 3.65 W output power is obtained with the operating current of 13.0 A, with a beam quality factor M² of 16.6, which is improved with a factor of 14 by the self-injection phase locking technique.

In this paper, we demonstrate lateral mode selection and amplification in a broad area laser (BAL) diode in an external cavity. The cavity is based on self-injection locking of an 807 nm, 3W broad area diode using a mirror stripe as the feedback unit. At the optimum mirror stripe position, the lateral far-field intensity profile is narrowed 8.5 times compared with the profile from the freely running laser when running at a drive current of twice the threshold current. We have determined the lateral angular range, in which, different array modes can be exited and, only, within a narrow range around 2.3° from the beam center a high, spatial beam coherence can be obtained.

High power fiber lasers have strong potential for use in both commercial and military applications. Improved wall plug efficiency over Nd:YAG and CO₂ lasers combined with up to a 10-fold improvement in beam quality, make fiber lasers extremely attractive for industrial applications such as welding and cutting. In military applications, fiber lasers offer a simplified logistic train, a deep magazine limited only by electric power, and a compact footprint, allowing theater defense and self-protection of combat platforms with speed of light engagement and flexible response. Commercial viability of these systems, however, is limited by the availability of compact, cost effective, and reliable diode laser pump sources in the multi-kilowatt regime. The relatively low brightness of diode laser sources has complicated the task of building high power pumps at a reasonable cost. In response to this need, Nuvonyx, Inc. in conjunction with the University of Illinois at Urbana-Champaign, has been developing a new technology for producing high power, single lateral mode devices which do not suffer form the instabilities mentioned above. The waveguide consists of a narrow section, approximately 2 μm wide, which flares to approximately 12 μm wide at the output facet. The flaring of the waveguide increases the gain volume and reduces the optical power density at the facet allowing for higher output power capability. The index guide is defined using an epitaxial process which allows the confinement of the mode to be reduced as the width of the guide expands. Thus, the mode is confined in a single mode waveguide throughout the cavity maintaining stability of the mode to the emitting facet. In November 2002, Nuvonyx, Inc. was awarded a contract with the Air Force Research Lab, Kirtland AFB, Albuquerque, NM, to transition these devices to production quality for use in high-power fiber laser pumps. Partnered with Alfalight, Inc. and the University of Illinois, we have begun initial device fabrication and testing of these devices with the goal of achieving production quality, long lifetime, 50W bars exhibiting stable single lateral mode operation. The goal of this program is to ultimately deliver multi-kilowatt fiber laser pumps and direct diode laser systems for both military and industrial applications. Currently, we are in the process of developing the necessary device growth, processing, and packaging technologies. Several devices have been made and tested yielding promising results. In this paper, we present some of these results along with an examination of the system implications and capabilities of these devices.

Reliable, high-power, single-mode, GaAlAs/GaAs, laser-diodes in the spectral region of 780 - 900 nm have been designed with procedures developed for telecom-grade, 980 nm, InGaAs/GaAlAs/GaAs pump diodes. Fifteen 808 nm, single-mode laser-diodes, mounted epitaxial-side up onto AlN submounts with eutectic Au₈₀Sn₂₀ solder, have been operated reliably for 3500 hours at 150 mW.

For further development in beam quality of high power diode laser systems a new type of diode laser bars has been realized recently. Compared to commercially available broad area diode laser bars the lateral design of tapered diode laser bars consists of a ridge waveguide and a tapered section, where the tapered structure determines high output power and the high brightness is provided by the ridge waveguide structure. Unfortunately the different lateral structure leads to astigmatism effects which are also thermally affected in continuous wave operation. Hence in terms of diode laser systems with essentially fast- and slow-axis collimation micro-optics the full characterization of tapered diode laser bars is necessary. Where the wavelengths and divergences of tapered diode bars are relatively easy to measure, the beam profile with its apparently inside the tapered structure located lateral source is not. With a telecentric optics the caustic of the individual emitters of the bars are detected by the use of a fiber sensor with a resolution of 10 μm in fast- and slow-axis direction. Subsequently with beam analysis software the astigmatism can be calculated. As a result of several measurements of tapered bars of different wavelengths we detected good homogeneity over the bar concerning the amount of astigmatism but a slight current dependency. So particularly with regard to beam shaping micro-optics the application to high power diode laser systems seems to be sophisticated so far.

A major problem with coupling high power diode lasers into optical fibers is the counter effects of requiring a synthetic silica core for its energy carrying capacity but needing also high numerical aperture and/or core size to capture the highly diverging fast axis light. Coupling the output of diode arrays efficiently and for maximum brightness retention involves additional problems, some of which are discussed. A possible solution to these probelms is having the availability of higher numerical aperture [NA] fibers based primarily on a silica core and silica cladding. New deposition techniques have permitted the formation of preforms leading to fibers with pure silica cores which have an NA of 0.30 versus the standard 0.22 value. Further new structures and approaches to the problem have lead to optical fibers with a doped silica core having an NA above 0.50. Properties of these fibers are presented along with advantages they have for improving coupling high power diode lasers and also some newer concepts for improving retention of high brightness output from such laser systems.

High power diode laser was the first semiconductor laser used for materials processing. It has found some areas of applications in production. The most promising applications of HPDL in mechanical engineering are in thin steel sheet welding and hardening. The HDPL welding process is at this moment usually performed as conduction limited welding process. In this study the effect of joint configuration and beam properties on the efficiency of welding with HPDL were examined. Joint types tested were bead on plate and butt joint with different joint manufacture. The laser parameters tested were beam intensity, welding speed, incidence angle of laser beam and focal spot size. The tested parameters had an effect on the weld quality and the welding speed. The higher the beam intensity is the higher the welding speed can be achieved. The reliable welding parameters can be established for the welding of various industrial products.

Great expectations were expressed about the future of high power diode lasers for direct materials processing applications, when they have been introduced in the mid-nineties. Nevertheless, based on their technology, there are physical limitations, especially if beam quality is considered: Kilowatt power must be generated from individual elements, which do not provide more than a few watts for the time being, if they shall last for a reasonable time; such elements are incoherently combined. In this article the technology will be analyzed about their potential and a comparison with "conventional" high power lasers will be performed. Fortunately not all laser materials processing techniques require extremely high power density, i.e. high beam quality, but rather need a larger spot or a spot with a special shape. Such applications are the niches for the highly efficient and reliable diode lasers. In this context, high power diode lasers have proven to be a perfect supplement to the existing CO₂ and Nd:YAG materials processing lasers rather than a competitor for them.

Direct diode laser will become much more prevalent in the solar system of manufacturing due to their high efficiency, small portable size, unique beam profiles, and low ownership costs. There has been many novel applications described for high power direct diode laser [HPDDL] systems but few have been implemented in extreme production environments due to diode and diode system reliability. We discuss several novel applications in which the HPDDL have been implemented and proven reliable and cost-effective in production environments. These applications are laser hardening/surface modification, laser wire feed welding and laser paint stripping. Each of these applications uniquely tests the direct diode laser systems capabilities and confirms their reliability in production environments. A comparison of the advantages direct diode laser versus traditional industrial lasers such as CO₂ and Nd:YAG and non-laser technologies such a RF induction, and MIG welders for each of these production applications is presented.

Multi-mode pumps based on single emitter diodes deployed in distributed pump architectures offer significant advantages in thermal management and reliability for pumping high-power fiber lasers and amplifiers. In a distributed architecture, while individual diode failures do not directly generate failures of other diodes in the distributed ensemble, failures do cause the rest of the sources to drive to higher power levels to compensate for the loss of power. A model of the ensemble lifetime based on module failure rates and power-scaling factors demonstrates that the distributed pump architecture requires random failure rates corresponding to better than 200,000 h mean time between failure (MTBF) to meet typical application requirements. A high power multi-mode pump module suitable for commercial aplications is shown. Critical elements are based on telecom architectures, including the optical train and the fiber alignment. The module has a low thermal resistance of 4 C/W from the laser diode junction to the external heat sink, couplng efficiency of over 80% into 0.2 NA, and demonstrated reliable output power of over 5W CW with peak wavelengths near 915 nm. Telecom qualified modules have random failure rates corresponding to better than 1,000,000 h MTBF. Stability of the critical fiber alignment joint for single mode packages has been demonstrated at elevated temperatures (e.g. 85 C) for thousands of hours. The reliability of the commercial multi-mode package can be estimated by similarity to the telecom package, and is verified by testing of conditions considered to be at risk based on the differences between the known telecom, and the new commercial package, designs. Test results are shown for temperature cycling, CW operation, and damp heat. The relationships between anticipated MTBF requirements, test duration and test population are shown.

Transient thermal properties of actively cooled InAlGaAs-based high-power diode laser arrays, so-called 'cm-bars', are investigated for 'low-frequency' pulsed operation with a repetition rate of 0.1-10 Hz. Under these operation conditions the devices experience almost complete thermal cycles between the temperature of the heat sink and the operation temperature typical for continuous wave operation. We analyze the thermal tuning behavior that is governed by the thermal bandgap shift (85%) but substantially modified by pure pressure tuning (15%). During operation further modification arises from inhomogeneous temperature distribution along the 'cm-bar.' Our results offer the possibility specifying optimum operation condition of 'low-frequency' pulsed operation resulting in improved reliability.

Operation of 808-nm laser diode pumps at elevated temperature is crucial to many applications. Reliable operation at high power is limited by high thermal load and low catastrophic optical mirror damage (COMD) threshold at elevated temperature range. We demonstrate high efficiency and high power operation at elevated temperatures with high COMD power. These results were achieved through device design optimization such as growth conditions, doping profile, and materials composition of the quantum-well and other layers. Electrical-to-optical efficiency as high as 62 percent was obtained through lowered threshold current and lowered series resistance and increased slope efficiency. The performance of single broad-area laser diodes scales to that of high power single bars on water-cooled copper micro-channel heatsinks or conductively-cooled CS heatsinks. No reduction in bar performance or significant spectral broadening is seen when these micro-channel coolers are assembled into 6-bar and 18-bar cw stacks for the highest power levels.

Computer simulation was done to determine the effect of the position of a laser diode bar on a laser diode cooler compared to the effect of changing the thickness of the top layer of copper on the laser diode cooler. The range of copper layer thicknesses tested was from 0.1mm to 1.4mm. The thicker the top layer, the better the thermal performance of the cooler up to 1.4mm. Above 1.4mm there was not measurably increased performance. The range of power input was 40W and 80W per bar. The position of the laser diode bar was only distinguishable at the 80W power level and the impractical top layer thicknesses of 0.1mm and 0.2mm, meaning that top layer thickness is much more important for improved thermal performance than the positioning of the bar on the cooler.

Traditional materials have serious deficiencies in meeting requirements for thermal management and minimization of thermal stresses in high-power laser diode packaging. Copper, the standard material for applications requiring high thermal conductivity, has a coefficient of thermal expansion (CTE) that is much larger than those of ceramics and laser diodes, giving rise to thermal stresses when packages are subjected to thermal excursions. Traditional materials with low CTEs have thermal conductivities that are little or no better than that of aluminum. There are an increasing number of new packaging materials with low, tailorable CTEs and thermal conductivities up to four times those of copper that overcome these limitations. The ability to tailor material CTE has been used to solve critical warping problems in manufacturing, increasing yield from 5% to over 99%. Advanced materials fall into six categories: monolithic carbonaceous materials, metal matrix composites, carbon/carbon composites, ceramic matrix composites, polymer matrix composites, and advanced metallic alloys. This paper provides an overview of the state of the art of advanced packaging materials, including their key properties, state of maturity, using composites to fix manufacturing problems, cost and applications.

During the last years high power diode lasers have become increasingly established for direct material processing. The advantages are the high efficiency (more than 50%) and long lifetime of more than 10.000h. An important factor believed to be responsible for the aging of diode lasers is the thermo-mechanical stress. High stress levels arise from the packaging process. The mismatch between the thermal expansion coefficient of the heat sink (typically copper 16.5x10-6 K-1) and the laserbar (GaAs 6.7x10-6 K-1) cause high mechanical stress. The change in length during the cooling process of a 10mm wide laserbar is more than 10μm. If a hard solder is used, the stress is much higher, because hard solder typically has a higher melting point and stress can not be reduced by relaxation. Typically material with lower thermal expansion coefficient have a lower thermal conductivity than copper. This increases the thermal load of the laserbar, which decreases the life-time in this sense. The expansion-matching and the lower thermal conductivity of this material are working against each other. In order to find a good compromise, different active cooled expansion matched heat sinks are simulated. Very promising heat sinks have been fabricated and characterized. Also the solder selection has influence on the long term stability. A very soft solder is more critical in terms of long term stability. A higher diffusion takes place, so that the properties of the solder change during the lifetime of the diode-laser. Hard solder, especially AuSn, are well tested solders with a very high long term stability. (No changes of the intermetallic structure even at higher temperature.) The disadvantage of the hard solder is the incapability to reduce the mechanical stress through relaxation. Different solders are being used and investigated.

We have demonstrated record high cw and quasi-cw operation of InP-based 1.5 μm laser arrays (bars) and water-cooled stacks. High-efficiency and high-power operation were achieved through device design optimization including the multi-quantum well design, crystal growth process, doping profile, and material composition. Internal quantum efficiency, mode loss, gain parameters, and temperature sensitivity parameters are reported. Single-stripe devices produced 3 watts of cw output power and 35 percent electrical-to-optical efficiency. We demonstrated 40 watts of cw power from single bars on water-cooled copper-microchannel heatsinks. A stack of 20 bars that were collimated using fast axis microlenses achieved greater than 350 watts of cw power.

A new generation of very efficient high power laser diodes has been developed. The design was optimized for efficient operation of a long cavity device necessary to reduce electrical and thermal resistance. CW operation of a 100 μm wide laser at 25C yielded slope efficiency as high as 1.14W/A and 64% electrical-to-optical conversion efficiency. Optical power as high as 13.5 W for thermally limited CW operation and 17.3 W for pulsed operation were also recorded.

2-micron solid-state lasers operating at moderate to high pulse energies require high power quasi-CW laser diode arrays (LDAs) operating at a nominal wavelength of 792 nm with pulse durations of at least one millisecond. This long pulse duration is one of the main causes of limited lifetimes for these arrays. Such relatively long pulse durations cause the laser diode active region to experience high peak temperatures and drastic thermal cycling. This extreme localized heating and thermal cycling of the active regions are considered the primary contributing factors for both gradual and catastrophic degradation of LDAs. This paper describes the thermal characteristics of various LDA packages, providing valuable insight for improving their heat dissipation and increasing their lifetime. The experiment includes both direct measurement of thermal radiation of the LDA facet using a high resolution IR camera and indirect measurement of LDA active region temperature by monitoring the wavelength shift of the near-IR light. The result of thermal measurements on different quasi-CW LDA packages and architectures is reported.

The reliability, long-term performance and lifetime of high power diode arrays are important issues for pumping of solid state and fiber laser systems. Operation of high power arrays in these systems has resulted in greater degradation rates than the reported lifetime data. We report on lifetime testing of a commercial high power array using an automated diode array reliability experiment. This computer controlled setup operates the laser array 24 hours a day in a cyclical format of 10 minutes on and one minute off. The array currently being tested was operated for more than 2500 hours at which time it experienced a sudden drop in power. Analysis of the array and the data suggest that the micro channel coolers corroded and that a sudden plugging of one or more channels caused the failure.

The 40-year-old laser diode technology underpins applications such as data storage, industrial lasers and telecommunications but still suffers from reliability and longevity issues in high power applications, most notably in pumping of Nd:YVO₄ and Nd:YAG lasers. Despite thermal advantages allowing expansion matched Au/Sn hard soldering, the main problem for InAlGaAs lasers is facet oxidation, which leads to increased absorption and COMD device failure. This article presents a novel process, which atomically seals the surface and eliminates oxidation by forming stable nitrides on the facet. Pulsed testing of 805 nm of Al_>0.20InGaAs single mode devices with a protective nitride layer demonstrates stable median 1.3W COMD (30MW/cm²), after one hour of CW screening at 12.5mW/μm (50W bar power). A 200h burn-in at 12.5mW/μm (50W bar power) resulted in an initial power drop of 1-2% and a linear degradation rate of 0.1%/1000h, compared to an initial power drop of 5-18% and a degradation rate of 46%/1000h for lasers with only AR/HR-coatings. A subsequent 1000h life-test at 22.5mW/um (90W bar power) demonstrated a degradation rate of only 3%/1000h under stress test conditions due to p-side up mounting, 10°C higher ambient temperature and 57% higher operating current over typical high power bar operating power levels. The QW temperature was 53°C. No sudden device failures occurred.

For the usage of diode lasers in industrial applications, customers ask today for expected lifetimes of more then 30.000 hours. To match the request for low costs per Watt as well, the output power has to be as high as possible. To achieve a maximum power out of a diode laser bar, heat removal must be extremely efficient. Today, micro channel heatsinks (MCHS) are the only way to achieve the high power level of 50W. But due to erosion/corrosion effects the lifetime of MCHS is limited at 15000...20.000 hours today. Finally we have to determine that for selected semiconductor materials not the semiconductor but the heatsink is limiting the expected lifetime of high power diode lasers today. Passive heat sinks based on solid copper are not limiting lifetime expectations in any way. But as cooling efficiency is lower, the power has to be reduced to a level of 30...40W. The first time ever, the JENOPTIK Laserdiode can present today a cooling technique that combines the passive cooling of a diode laser bar with the optical output a power of a bar, mounted on a MCHS. Using a special heat exchanger called DCB (patent pending) we were able to increase the power to 50W per bar while looking forward to extend the expected lifetime to more than 30.000 hours for selected materials. Restrictions on the quality of the water by means of deionization grade or PH- level are no longer necessary. The device is operating with regular water. The flow rate is as low as on MCHS, the pressure drop over the DCB is comparable. Additionally, the measurements will show an even lower thermal resistance compared to MCHS. The second generationof engineering samples is built up for pumping rows. A vertical stack design will be available for evaluating purposes soon. All these efforts are part of the JENOPTIK Laserdiode's LongLifeTechnology.

Diode-pumped solid state laser (DPSSL) and fiber laser (FL) are believed to become the dominant systems of very high power lasers in the industrial environment. Today, ranging from 100 W to 5 - 10 kW in light output power, their field of applications spread from biomedical and sensoring to material processing. Key driver for the wide spread of such systems is a competitive ratio of cost, performance and reliability. Enabling high power, highly reliable broad-area laser diodes and laser diode bars with excellent performance at the relevant wavelengths can further optimize this ratio. In this communication we present, that this can be achieved by leveraging the tremendous improvements in reliability and performance together with the high volume, low cost manufacturing areas established during the "telecom-bubble." From today's generations of 980-nm narrow-stripe laser diodes 1.8 W of maximum CW output power can be obtained fulfilling the stringent telecom reliability at operating conditions. Single-emitter broad-area lasers deliver in excess of 11 W CW while from similar 940-nm laser bars more than 160 W output power (CW) can be obtained at 200 A. In addition, introducing telecom-grade AuSn-solder mounting technology on expansion matched subassemblies enables excellent reliability performance. Degradation rates of less than 1% over 1000 h at 60 A are observed for both 808-nm and 940-nm laser bars even under harsh intermittent operation conditions.