John L. Hostetler

Thermal and strain characteristics of high-power 940 nm laser arrays mounted with AuSn and In solders

As diode pumped solid state lasers gain more market share, the performance, stability and lifetime of the diode pump source faces unprecedented scrutiny. Lifetimes of diode pumps in excess of 35,000 hrs are sought with no intervention or maintenance from the end user. One lifetime and power limiting phenomena for arrays is that of solder creep typical with traditional mounting using soft solders such as Indium. Harder solders such as Gold/Tin on Copper-Tungsten submounts provide a more robust and stable mounting system for long term high power pump sources. Furthermore, beam multiplexing of laser bars require tight wavelength and polarization purity which are affected by mounting induced strain. In this investigation, high power 940 nm laser bars, operating in the 100 to 200 W power range, were mounted using AuSn/CuW and In soldering schemes. The differences in thermal and strain characteristics are investigated through the examination of the emitter wavelength, nearfield measurements, polarization and smile. The measurements are correlated with finite element modeling to predict the 3-dimensional thermal distributions within the laser bars.

High-Power, High-Brightness Direct-Diode Lasers

Fifty years of laser history have led us to the next stage of the laser’s evolution: This is the century of the diode laser. These authors describe how direct-diode lasers are entering the high-power laser market, enabling materials-processing applications such as welding, soldering and cutting.

Comparison of facet temperature and degradation of unpumped and passivated facets of Al-free 940-nm lasers using photoluminescence

Influences of facet degradation of Al-free InGaAsP-GaAs 940-nm laser diodes were studied at power densities well below catastrophic optical mirror damage level using photoluminescence (PL) during normal operation and after a rigorous burn-in procedure. The shift in the PL peak of the cladding layer of the device is used to calculate the temperature of the facet. Devices with different facet treatments: untreated electron beam evaporation, untreated ion beam deposition, unpumped and passivated facets were compared. The results indicate that the degradation of facet is more severe for untreated and unpumped facets as compared to passivated facets. The results were also compared with power measurements, which show that the drop in the power during the first 50 h of operation is nonexistent for passivated facet devices leading to the conclusion that photo-induced oxidation is the major cause of the degradation of the facet and thus oxide removal and surface passivation are crucial to make stable laser diodes.

Dense Spatial Multiplexing Enables High Brightness Multi-kW Diode Laser Systems

The materials processing industry has recently mandated the need for more efficient laser systems with higher beam quality and longer life. Current multiplexing techniques, state-of-the-art laser diodes and novel cooling designs are now emerging as possibilities to meet the ever demanding industry needs. This paper describes the design and initial results of a direct diode system that is aimed at delivering 1.5 kW of output power and a beam divergence of 40 mm mrad on a long life macro-channel cooler. The design entails multiplexing 2 wavelength combined beams and 2 polarization combined beams. Each of the four branches of the direct diode system utilizes a novel stacking and cooling design. The results from one of these branches, 1 wavelength and 1 polarization, are presented where the light is coupled into a fiber with a 400 μm core diameter and a NA of 0.22. Each branch consists of 60 diode laser mini-arrays, where each mini-array consists of four 100 μm wide emitters and a lateral fill factor of 50%. An output power of 500W at 10°C water temperature and 420 W at 25°C are demonstrated through the 400 μm fiber.

Coherently combined diode laser arrays and stacks

We have coherently combined up to 7.2 W CW using an individually addressable 10-element-array of 960-nm Slab-Coupled Optical Waveguide Lasers (SCOWLs). We are currently scaling the phase-locked output power to 100 W using SCOWL stacks.

Single-Shot, High-Speed, Thermal-Interface Characterization of Semiconductor Laser Arrays

Through a detailed characterization of thermally induced output power degradation it is possible to use junction heating as a tool to resolve thermal interfaces on mus timescales using a single-shot characterization technique. In this work, the deleterious effect junction heating has on the optical output power of a laser array is characterized and then used to infer the time-dependent junction temperature in response to current pulses of varying widths. The extracted parameters are also used numerically to model the laser as a temperature-dependent heat source for thermal simulations. This treatment allows realistic packaging and emitter-placement studies to be parametrically performed by incorporating the relationship between temperature and output power/efficiency for each emitter. In this respect, once the temperature behavior of a single emitter is quantified, the operating temperature and output power performance can be accurately predicted for any realistic physical arrangement of laser array and packaging. The experimental method presented in this work is also compared to other techniques and numerical simulations using the nonlinear heat source; this demonstrates the utility of this approach and the convenience of using easily measured parameters in thermal simulations.

Diode laser pumping of thin disk lasers

Disk lasers provide the highest brightness conversion factor of commercially available diode pumped solid state lasers.The geometry of disk lasers is particularly well suited for low cost diode excitation, very high brightness and cw power extraction as well as high power ultra short pulse lasers.The input beam parameter product (BPP) requirement of commercially deployed disk media lies around 400 mm mrad, while an output BPP of 2 - 8 mm mrad can be guaranteed in the kW range. With such values the brightness conversion factor (ratio of output vs. input power/BPP²) reaches 2e4. A high value for the brightness conversion factor is important as it allows high cw output powers with highly integrated diode pump sources at nearly 100% optical pump launch efficiencies - and therefore at very low cost per Watt.The large value for the brightness conversion also effectively suppresses non-linear effects and allows achieving ps-pulses with average power levels in excess of 50 Watts.Disk lasers provide the highest brightness conversion factor of commercially available diode pumped solid state lasers.The geometry of disk lasers is particularly well suited for low cost diode excitation, very high brightness and cw power extraction as well as high power ultra short pulse lasers.The input beam parameter product (BPP) requirement of commercially deployed disk media lies around 400 mm mrad, while an output BPP of 2 - 8 mm mrad can be guaranteed in the kW range. With such values the brightness conversion factor (ratio of output vs. input power/BPP²) reaches 2e4. A high value for the brightness conversion factor is important as it allows high cw output powers with highly integrated diode pump sources at nearly 100% optical pump launch efficiencies - and therefore at very low cost per Watt.The large value for the brightness conversion also effectively suppresses non-linear effects and allows achieving ps-pulses with average power levels in excess of 50 Watts.

TruDiode: Highest efficiency and brilliance for keyhole welding

Emerging high brightness laser diode emitter make the realization of industrial high power Direct diode laser systems possible where the power from the diode is used directly for materials processing instead of pumping an intermediate media.A significant increase in system wall-plug efficiency with outstanding beam quality distinguishes these systems from conventional laser systems. Deployable power levels are reaching > 10 W per broad area emitter resulting in brightness levels of > 60 MW/cm2str on the emitter level. Advanced fiber coupling techniques at TRUMPF have allowed 100 watts of power out of an optical fiber with a diameter of 100 µm, a divergence less than 120 mrad and a beam parameter product, BPP, of less than 6 mm*mrad. Furthermore, as reliability is always considered to be a primary design focus, the high brightness diodes are mounted with hard-solders and are passively cooled. The passively cooled modules form the basic building block for the series of direct diode lasers, the TruDiode, which allow achievements in the kilowatt arena with a high beam quality. Results of the fiber coupled diodes are discussed as well as system architecture for power scaling.Emerging high brightness laser diode emitter make the realization of industrial high power Direct diode laser systems possible where the power from the diode is used directly for materials processing instead of pumping an intermediate media.A significant increase in system wall-plug efficiency with outstanding beam quality distinguishes these systems from conventional laser systems. Deployable power levels are reaching > 10 W per broad area emitter resulting in brightness levels of > 60 MW/cm2str on the emitter level. Advanced fiber coupling techniques at TRUMPF have allowed 100 watts of power out of an optical fiber with a diameter of 100 µm, a divergence less than 120 mrad and a beam parameter product, BPP, of less than 6 mm*mrad. Furthermore, as reliability is always considered to be a primary design focus, the high brightness diodes are mounted with hard-solders and are passively cooled. The passively cooled modules form the basic building block for the series of direct diode lasers, the TruDiode, whi...

Passive cooling effects of low and high fill-factor 937 nm 1 cm arrays

Diode-pumped solid-state lasers are gaining acceptance as the desired laser source for materials processing as well as a host of new applications that are expanding rapidly. Because of this, the performance, stability and lifetime of the diode-pump source face unprecedented scrutiny. Increasing the lifetime of the diode, while increasing power, remains a primary focus of the industry. One lifetime limiting issue is that of a voltage potential in the water cooling channels which can cause cooler degradation and lower efficiency over time. Studies have been carried out that explore different cooling approaches based on passive schemes where insulation layers are present to shield the voltage from the water channels. However, with the introduction of insulation layers, a reduction of the deployable power from that of microchannel coolers is seen. This report explores the effects of passive cooling approaches on the power and divergence of 1 cm AuSn/CuW mounted bars with fill factors ranging from 10% to 50%. It is shown that a 150 W array can be realized on a passive cooler and multiplexed to give a 1600 W stack. Thermal modeling is presented along with life-test data for passively cooled devices.

Diode laser pump sources for advanced solid-state lasers

Advanced solid-state lasers rely on diode laser pump sources. In particular, for industrial customers, cost and reliability of the laser systems are of upmost importance. Improved diode laser pumps significantly contribute to this. To fulfill all customer requirements, not only output power, but the complete diode laser assembly must be optimized. Diode laser bars are the most cost-effective solution, because of their integration on the chip level as compared to single-emitter diode lasers. Packaging and cooling technologies must be integrated with chip design for optimum overall performance.We demonstrate diode laser operation at 200 W and 60 °C junction temperature when mounted to a micro-channel heat sink. As we use stable AuSn hard-solder long-term operation even in fast power modulation mode is ensured. In cases when established micro-channel coolers are not accepted, these diode laser can operate at a derated output of 150 W on passive heat sinks.Advanced solid-state lasers rely on diode laser pump sources. In particular, for industrial customers, cost and reliability of the laser systems are of upmost importance. Improved diode laser pumps significantly contribute to this. To fulfill all customer requirements, not only output power, but the complete diode laser assembly must be optimized. Diode laser bars are the most cost-effective solution, because of their integration on the chip level as compared to single-emitter diode lasers. Packaging and cooling technologies must be integrated with chip design for optimum overall performance.We demonstrate diode laser operation at 200 W and 60 °C junction temperature when mounted to a micro-channel heat sink. As we use stable AuSn hard-solder long-term operation even in fast power modulation mode is ensured. In cases when established micro-channel coolers are not accepted, these diode laser can operate at a derated output of 150 W on passive heat sinks.