Trevor Crum

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.

High-efficiency high-power 808-nm laser array and stacked arrays optimized for elevated temperature operation

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.

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.

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.

High power laser diodes at 14xx nm wavelength range for industrial and medical applications

We report on the development of the latest generation of high power laser diodes at 14xx nm wavelength range suitable for industrial applications such as plastics welding and medical applications including acne treatment, skin rejuvenation and surgery. The paper presents the newest chip generation developed at II-VI Laser Enterprise, increasing the output power and the power conversion efficiency while retaining the reliability of the initial design. At an emission wavelength around 1440 nm we applied the improved design to a variety of assemblies exhibiting maximum power values as high as 7 W for broad-area single emitters. For 1 cm wide bars on conductive coolers and for bars on active micro channel coolers we have obtained 50 W and 72 W in continuous wave (cw) operation respectively. The maximum power measured for a 1 cm bar operated with 50 μs pulse width and 0.01% duty cycle was 184 W, demonstrating the potential of the chip design for optimized cooling. Power conversion efficiency values as high as 50% for a single emitter device and over 40% for mounted bars have been demonstrated, reducing the required power budget to operate the devices. Both active and conductive bar assembly configurations show polarization purity greater than 98%. Life testing has been conducted at 95 A, 50% duty cycle and 0.5 Hz hard pulsed operation for bars which were soldered to conductive copper CS mounts using our hard solder technology. The results after 5500 h, or 10 million “on-off” cycles show stable operation.

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.