Raman Srinivasan

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.

1.2-kW single-mode fiber laser based on 100-W high-brightness pump diodes

We have demonstrated a monolithic (fully fused), 1.2-kW, Yb-doped fiber laser with near-single-mode beam quality. This laser employs a new generation of high-brightness, fiber-coupled pump sources based on spatially multiplexed single emitters, with each pump providing 100 W at 915 nm within 0.15 NA from a standard 105/125 μm fiber. The fiber laser is end pumped through the high-reflector FBG using a 19:1 fused-fiber pump combiner, eliminating the need for pump/signal combiners. The output wavelength is 1080 nm, with a linewidth of < 0.5 nm FWHM. A peak power of 1.5 kW was reached in modulated operation (1-ms pulse duration) with M2 < 1.2.

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.

4-kW fiber laser for metal cutting and welding

We have developed a commercial 4-kW fiber laser consisting of seven, 600-W modules whose outputs are combined with a fused-fiber combiner. The system architecture has several practical advantages, including pumping with reliable single-emitter diodes, monolithic fused-fiber construction (no free-space beams), and end pumping using a 91:1 pump combiner (eliminating the need for complex pump/signal combiners). Typical results at 4-kW output power are a beamparameter product of 2.6 mm-mrad, 8-hr power stability of < 0.5% rms, central wavelength of 1080 nm, and linewidth of 1.2 nm FWHM. These lasers have been incorporated into Amada machines used for cutting metal sheet and plate and have been used to cut aluminum, mild steel, stainless steel, brass, titanium, and copper with a thickness up to 19 mm. A world-record cutting speed of 62 m/min has been demonstrated for 1-mm aluminum sheet metal.

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.

Advances in high-power 9XXnm laser diodes for pumping fiber lasers

A multi-mode 9XXnm-wavelength laser diode was developed to optimize the divergence angle and reliable ex-facet power. Lasers diodes were assembled into a multi-emitter pump package that is fiber coupled via spatial and polarization multiplexing. The pump package has a 135μm diameter output fiber that leverages the same optical train and mechanical design qualified previously. Up to ~ 270W CW power at 22A is achieved at a case temperature ~ 30ºC. Power conversion efficiency is 60% (peak) that drops to 53% at 22A with little thermal roll over. Greater than 90% of the light is collected at < 0.12NA at 16A drive current that produces 3.0W/(mm-mr)2 radiance from the output fiber.