James Harrison

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.

Composite-copper, low-thermal-resistance heat sinks for laser-diode bars, mini-bars and single-emitter devices

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

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.

Compact high-brightness and high-power diode laser source for materials processing

A compact, reliable semiconductor laser source for materials processing, medical and pumping applications is described. This industrial laser source relies on a combination of technologies that have matured in recent years. In particular, effective means of stacking and imaging monolithic semiconductor laser arrays (a.k.a., bars), together with advances in the design and manufacture of the bars, have enabled the production of robust sources at market-competitive costs. Semiconductor lasers are presently the only lasers known that combine an efficiency of about 50% with compact size and high reliability. Currently the maximum demonstrated output power of a 10-mm-wide semiconductor laser bar exceeds the 260 W level when assembled on an actively cooled heat sink. (The rated power is in the range of 50 to 100 W.) Power levels in the kW range can be reached by stacking such devices. The requirements on the stacking technique and the optic assembly to achieve high brightness are discussed. Optics for beam collimation in fast and slow axis are compared. An example for an optical setup to use in materials processing will be shown. Spot sizes as low as 0.4 mm X 1.2 mm at a numerical aperture of 0.3 and output power of 1 kW are demonstrated. This results in a power density of more than 200 kW/cm2. A setup for further increase in brightness by wavelength and polarization coupling will be outlined. For incoherent coupling of multiple beams into a single core optical fiber, a sophisticated beam-shaping device is needed to homogenize the beam quality of stacked semiconductor lasers.

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.

Stackable air-cooled heatsinks for diode lasers

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 MonsoonTM 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.