We report a novel approach for the assembly of monolithic diode laser stacks with up to 56 laser diode bars at 0.48mm pitch. These stacks are based on 940nm laser bars AuSn soldered on CuW-submounts. By joining the 56 bar-on-submounts (BoS) using an electrically conductive material the complete continuous stack is formed. A robot is used to load these BoS into fixtures for the bonding process. We show the attachment of this monolithic stack to an insulated micro-channel cooler using electrically insulating bonding material to complete the laser module. Use of the novel bonding materials at lower joining temperatures, compared to conventional soldering processes, results in a stack assembly with lower induced stress which allows monolithic stacks with higher number of elements. We demonstrate peak power above 500W per bar at 0.3% duty cycle and 500A peak current with an average junction temperature of 45°C, with over 29kW total peak power. Analysis of the dynamic temperature behavior within the pulses is presented using fast spectral measurements and simulation along with initial reliability test results.
Fiber lasers are becoming an increasingly important option for LIDAR light sources in autonomous driving technology due to their operation in the eye safe 1550 nm spectral region and their intrinsic high beam quality, power and pulse characteristics. As an essential component for the pumping of fiber lasers, semiconductor laser diodes with high temperature stability, power and reliability are necessary. In this report we present the results of a continuous-wave (CW) single edge emitting laser diode designed to operate at 94x nm at 25 °C heat sink temperature and 97x nm at 100 °C. Various epitaxial and laser geometry designs have been implemented to optimize the laser performance over this wide environmental temperature range. The laser epitaxy is based on the AlGaAs/GaAs material system, with an InGaAs strained quantum well (QW). With various designs of laser geometry including emitting area and cavity length, devices are designed, grown, fabricated, and tested with the optimized design improving the temperature stability, power, and efficiency of the laser chip. A peak efficiency of over 54% at heat sink temperature of 105 °C and over 12 W before thermal roll-over occurs has been achieved. In addition to the thermal performance we also report the slow axis beam parameter product of the chip of <5 mm.mrad with polarization purity >98% at operating current and show the preliminary reliability data at the high temperature operation.
Fiber lasers are becoming dominant in the industrial metal cutting market up to 6kW levels due to their increasing performance delivered at aggressive price levels. A key force behind this is the availability of low-cost diode pump modules, consisting of combined single emitter laser chips. To further develop low pump cost an increase the reliable power available from each chip is required, and in principle this can be achieved by reducing facet power density, current density and lowering operational temperature by widening the emitter and increasing cavity length. Here we present the latest development of high-power laser diode chips at 976 nm wavelength designed for operation in 300W+ per pump module applications. For high power/high brightness applications we show a 150 μm contact opening, 5 mm cavity length chip delivering 20W at 20A CW at BPP of <5.5 mm.mrad. This laser has been deployed in several pump module product configurations from 155 W to 320 W with multi-cell module life test demonstrating over 20,000 hours at 90% survivability. For further power scaling we demonstrate design and performance of single emitter lasers with emitting facets of 220 μm to 300 μm. With 300 μm contact opening and 500 μm single emitter chip width 30W operation power is achieved at 32A with far field divergence less than 12°. Life testing is running at 46A with optical power of 41W for 2000 hours.
KEYWORDS: Semiconductor lasers, High power lasers, Temperature metrology, Continuous wave operation, Near field, Optical testing, Near field optics, Thermography
Results of an extended series of experimental studies into the beam parameter product (BPP) of high-power diode lasers are summarized, covering efforts to clearly diagnose the limiting factors and develop novel device technology to address these limits. We review diagnostic studies, separating BPP empirically into bias-dependent (thermal) and bias-independent (non-thermal) terms for convenience of analysis. First, we use monolithically grating-stabilized lasers to confirm the presence of a well-defined series of guided modes, rather than filaments. Second, we present results from a series of custom devices and tests (guided by targeted simulations). Third, we show that effects driven by thermal lensing and current spreading dominate the variation in BPP with bias. The residual bias-independent BPP background remains around 30- 50% of the total, and is most likely partly limited by gain-guiding effects. Fourth, the presence of longitudinal temperature variation due to non-uniform optical intensity along the resonator further degrades the bias-independent background level. Lateral current blocking technology is shown to reduce current spreading, and improve the bias-dependent BPP. Thermal engineering also improves bias-dependent BPP, achieved by varying epitaxial layer structure and by targeted changes in bar layout, clarified using measurements in thermography cross-referenced to simulation. In summary, we contend that experimental studies have allowed the effects that limit lateral BPP to be largely clarified, so that research efforts can now focus on developing device technology suitable for reducing BPP without other penalties. The background level to BPP remains an open topic, and further study is needed to better understand and address this.
The industrial laser market has rapidly expanded over the past decade with the emergence of advanced high brightness solid state laser technology. Thin disk laser systems are important examples of these powerful tools enabling a range of high-end CW materials processing applications such as 2D sheet metal cutting and remote welding applications, and the rising demand for a range of demanding high-energy pulsed applications of high average power. Commercial applications with power in the range of 8 kW- 20 kW can be cost competitive using disk lasers in moderate volumes compared to more commoditized solid-state laser sources such as fiber lasers.
Reduction in the cost structure of disk laser pump sources requires an increase in brightness, efficiency and power of diode lasers bars within. Here we show the development of thin disk laser pump modules from an original common cooler platform with ~180 W per laser bar to recently developed individually cooled laser bars each operating continuously over 300 W. We demonstrate pump modules utilizing these bars with total power of up to 2.4 kW at 940 nm. Cooling in such laser modules is provided by mounting laser bars on isolated laser coolers (ILASCO). The ILASCO cooler comprises a multi-layer structure of aluminum nitride and copper sheets that are designed to decouple the direct current path from the water cooling eliminate electro-corrosion and to maximize heat dissipation and match the thermal expansion of the diode laser bar.
We demonstrate advances in the single quantum well InGaAs/AlGaAs laser epitaxy design and chip layout that enables high power operation at operating temperatures up to 80°C. We show increase in peak electro-optic efficiencies from 55% to over 60% at this temperature. With the application of advanced facet passivation technology, we demonstrate >35 khr reliable operation in the application through accelerated aging tests.
The advance of high power semiconductor diode laser technology is driven by the rapidly growing industrial laser market, with such high power solid state laser systems requiring ever more reliable diode sources with higher brightness and efficiency at lower cost. In this paper we report simulation and experimental data demonstrating most recent progress in high brightness semiconductor laser bars for industrial applications. The advancements are in three principle areas: vertical laser chip epitaxy design, lateral laser chip current injection control, and chip cooling technology. With such improvements, we demonstrate disk laser pump laser bars with output power over 250W with 60% efficiency at the operating current. Ion implantation was investigated for improved current confinement. Initial lifetime tests show excellent reliability. For direct diode applications <1 um smile and >96% polarization are additional requirements. Double sided cooling deploying hard solder and optimized laser design enable single emitter performance also for high fill factor bars and allow further power scaling to more than 350W with 65% peak efficiency with less than 8 degrees slow axis divergence and high polarization.
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