Terry Towe

Near 1 kW of Continuous-Wave Power From a Single High-Efficiency Diode-Laser Bar

Near 1 kW of continuous-wave output power has been obtained from a single 1-cm-wide diode-laser bar. Mounted on a water-cooled microchannel heat sink, the operating wavelength was near 940 nm (at ~400 W) and the measured thermal resistance was as low as 0.08degC/W. Maximum power conversion efficiencies (PCEs) of 70% and 67% were obtained from high-fill-factor bars with cavity lengths of 4 and 5 mm, respectively. A maximum PCE of 72.2% was achieved with 3-mm broad-area single emitters. On-going lifetime data may signal the stable operation at unprecedented powers.

High-efficiency, high-power diode laser chips, bars, and stacks

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

Ongoing Development of High-Efficiency and High-Reliability Laser Diodes at Spectra-Physics

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.

Next-generation high-power, high-efficiency diode lasers at Spectra-Physics

This paper gives an overview of recent product development and advanced engineering of diode laser technology at Spectra-Physics. Focused development of device design, heat-sinking and beam-conditioning has yielded significant improvement in both power conversion efficiency (PCE) and reliable power, leading to a family of new products. CW PCEs of 60% to 70% have been delivered for the 880 to 980 nm wavelength range. For 780 to 810 nm, PCE are typically between 50% and 56%. Comprehensive life-testing indicates that the reliable powers of devices based on the new developments exceed those of established, highly reliable, production designs. For the progress of ultra-high power bars, CW output power in excess of 1000 W and 640 W have been demonstrated from single laser bars with doubled-side and single-side cooling, respectively. Spatial power density of greater than 2.8 kW/cm2 and FWHM spectral widths of 3.5 nm have been obtained from laser stacks.

Next-generation active and passive heatsink design for diode lasers

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). CTEs 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.

Reliability of ensembles multi-stripe laser diodes

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

High-power, fiber-coupled stack arrays for pump applications

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

Advances in high-power laser diode packaging

High-power, packaged diode-laser sources continue to evolve through co-engineering of epitaxial design, beam conditioning and thermal management. Here we review examples of improvements made to key attributes including reliable power, brightness, power per unit volume and value.

Continuous improvement of high-efficiency high-power 800-980nm diode lasers at Spectra-Physics

New-generation multi-mode 9xx mini-bars used in fiber pump modules have been developed. The epitaxial designs have been improved for lower fast-axis and slow-axis divergence, higher slope efficiency and PCE by optimizing layer structures as well as minimizing internal loss. For 915nm mini-bars with 5-mm cavity length, maximum PCE is as high as ~61% for 35W operation and remains above 59% at 45W. For 808nm, a PCE of 56% at 135W CW operation has been demonstrated with 36%-fill-factor, 3-mm-cavity-length, water-cooled bars at 50°C coolant temperature. On passive-cooled standard CS heatsinks, PCE of >51% is measured for 100W operation at 50°C heatsink temperature. Leveraging these improvements has enabled low-cost bars for high-power, high-temperature applications.

Advances in Fiber Combined Pump Modules for Fiber Lasers

Fiber combining multiple pump sources for fiber lasers has enabled the thermal and reliability advantages of distributed architectures. Recently, mini-bar based modules have been demonstrated which combine the advantages of independent emitter failures previously shown in single-stripe pumps with improved brightness retention yielding over 2 MW/cm2Sr in compact economic modules. In this work multiple fiber-coupled mini-bars are fiber combined to yield an output of over 400 W with a brightness exceeding 1 MW/cm2Sr in an economic, low loss architecture.