Conductively cooled high-power, high-brightness bars and fiber-
`coupled arrays
`Hailong Zhou, Mark Mondry, Michael Fouksman, Eli Weiss
`Serguei Anikitchev, Keith Kennedy, Jun Li, Erik Zucker and Paul Rudy
`Coherent Inc., Santa Clara, CA, 95054, USA
`
`Jukka Kongas, Jouko Haapamaa, and Sami Lehkonen
`Coherent Finland, Kauhakorvenkatu 52, Tampere, 33720, Finland
`
`Abstract
`
`Solid-state-laser and fiber laser pumping, reprographics, medical and materials processing applications require high
`power, high-brightness bars and fiber-coupled arrays. Conductively cooled laser diode bars allow customers to simplify
`system design and reduce operational size, weight, and costs. We present results on next generation high brightness,
`high reliability bars and fiber-coupled arrays at 790-830 nm, 940 nm and 980 nm wavelengths.
`
`By using novel epitaxial structures, we have demonstrated highly reliable 808 nm, 30% fill-factor conductively cooled
`bars operating at 60W CW mode, corresponding to a linear power density (LPD) of 20 mW/μm. At 25°C, the bars have
`shown greater than 50% wall-plug-efficiency (WPE) when operating at 60W. Our novel approach has also reduced the
`fast-axis divergence FWHM from 31° to less than 24°. These bars have a 50% brightness improvement compared to our
`standard products with this geometry.
`
`At 980nm, we have demonstrated greater than 100W CW from 20% fill-factor conductively cooled bars, corresponding
`to a LPD of 50 mW/μm. At 25°C, the WPE for 976nm bars consistently peaks above 65% and remains greater than
`60% at 100W. We coupled the beam output from those high-brightness bars into fiber-array-packages (“FAPs”), and
`we also achieved high-brightness and high-efficiency FAPs. We demonstrated 60W from a 600μm core-diameter fiber-
`bundle with a high WPE of 55%, and a low numerical aperture of 0.115. The brightness of such FAPs is four times
`higher than our standard high-power 40W FAP products at Coherent. Ongoing life test data suggests an extrapolated
`lifetime greater than 10,000 hours at 80W CW operating-condition based on 30%FF conductively cooled bar geometry.
`
`Keywords: diode-arrays, fill-factor, conductively cooled, fiber-coupling, wall-plug-efficiency
`
`Introduction
`
`High power diode laser integrators continue to place more demanding requirements on component manufacturers, with
`the goals of improving performance, reliability, and manufacturability. Coherent is well known for its family of high
`reliability conductively cooled products between 0.8-1.0 μm, with a wide variety of free space and fiber-coupled bar
`devices, , and we have leveraged our expertise in high power diode lasers to develop a new generation of products to
`address applications requiring increased power, efficiency and brightness. By combining our field proven packaging
`technology with our expertise in epitaxial growth and semiconductor laser design, we are developing the next
`generation of commercially available 808-830 nm and 940-980 nm bars for a variety of pumping and direct diode
`applications.
`
`Our efforts toward next generation bars have included two development programs. The first involved improving overall
`conversion efficiency at 940-980 nm by a combination of epitaxy and design optimization. By increasing the efficiency
`of our 940-980 nm bars, the overall heat dissipation and junction temperatures are reduced, translating not only into
`higher operating powers, but improved lifetime as well. The second component of our development efforts focused on
`improving the amount of useful power, i.e. brightness of our laser diodes. This is particularly important for fiber-
`coupled devices to get as much power into the delivery conduit as possible.
`
`High-Power Diode Laser Technology and Applications III, edited by Mark S. Zediker, Proc. of SPIE
`Vol. 5711 (SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.589860
`
`37
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`ASML 1127
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`790-830nm high-power high-brightness bars at Coherent
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`At Coherent, a new generation epitaxial structure design coupled with improved semiconductor material quality and
`new bar design allowed us to achieve 60W CW operations for 30% fill-factor (FF) conductively cooled bars. This type
`of bar consists of 19 emitters, each emitter having 150 μm stripe width, with a 500μm distance between two adjacent
`emitters. As shown in Figure 1, these 60W bars demonstrated 53% wall plug efficiency (WPE) at operating point. The
`full-width at half-maxima (FWHM) of spectrum at operating power for these bars is approximately 2.5 nm.
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`Figure 1. Performance of 60W conductively cooled 30% fill-factor bars at 25°C heat sink temperature
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`One advantage of the new structure is the reduction of fast-axis and slow-axis divergence, which enhances beam
`brightness and facilitates micro-lensing. As show in Figure 2, these improved structure designs have reduced the
`FWHM fast axis divergence of 808nm bars from 31° to 21°. The associated decrease in facet power density, coupled
`with improved semiconductor material quality, enabled an improvement in the CW post-aging catastrophic optical
`mirror damage (COMD) level from 3.6 W to 4.8 W, measured on 60um stripe width single emitters. This improvement
`in COMD is very important to achieve reliable bar operation at higher brightness.
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`Figure 3. 808nm post-aging COMD improvement
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`FWHM(new design)=21°
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`To assess reliability of our new generation bars, life-testing is being conducted on several groups of conductively cooled
`bars at 65A (~60W CW), 25 °C. Figure 4 shows the reliability data to date. The earliest batch of 3 devices has reached
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`38 Proc. of SPIE Vol. 5711
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`5000 hours and only shows an average power drop of 4%. Based upon this life test data collected on these 17 bars, we
`expect mean-time-to- failure (MTTF) to be greater than 15,000 hours with 90% confidence level.
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`808nm, 30% FF, 60W CW Lifetest
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`Figure 4. Life test of 808nm 30%FF bars at 60W CW condition
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`980nm high-power high-brightness bars at Coherent
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`In the past few years, there has been a growing need for high-power high-brightness 976 nm bars and fiber-coupled
`arrays for pumping fiber-lasers, as well as high-power, high-efficiency and low-cost 940 nm bars/stacks to pump disk-
`based diode pumped solid state lasers (DPSSL). Here we summarize our recent progress with our 980nm and 940nm
`bars.
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`Optimisation of epitaxy designs and growth processes has resulted in a dramatic improvement in efficiency with 980nm
`bars. Figure 5 shows an example 46W 30% FF bar with 69% WPE, operated CW with a 25°C heat sink temperature.
`The lasing wavelength is centered at 976.5 nm at operating conditions with a spectral FWHM value of 2.8 nm, and the
`peak efficiency occurs near the operating point. Far-field profiles of θ⊥ (FWHM) = 33.5° and θ// (FWHM) = 6.5° are
`typical.
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`Figure 5. 980 nm 69% WPE bar operating at 46W CW , 25 °C
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`Further redesign and optimisation work has resulted in lower fill-factor and longer cavity length bars with enhanced
`output beam brightness. Figure 6 shows the test result for a 20% fill-factor 980nm bar operating at 100W CW,
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`Proc. of SPIE Vol. 5711 39
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`corresponding to a linear power density of 50 mW/μm. This bar design operates at 63.8% wall-plug efficiency, with a
`peak WPE of 66.7%. An increase of the spectral FWHM to 3.8 nm has been observed and indicates an increase in the
`junction temperature and current density.
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`coupling efficiency = 87%coupling efficiency = 87%
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`Figure 6. WPE as high as 63.8% is maintained at
`100W CW with conductively cooled 20%FF bars,
`heat sink temperature = 25 °C
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` Figure 7. 60 W was measured from a 600 μm
` fiber bundle. Coupling efficiency from bar to
` fiber bundle is 87%.
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`These high efficiency bars have been integrated into Fiber-Array-Packages (FAP), and example performance data from
`the output of a 600μm-diameter fiber bundle with 0.115 N.A. (numerical aperture) is shown above in Figure 7. With 60
`W coming out of fiber bundle, wall plug efficiency of 55% was observed, with typical coupling efficiencies of 85%-
`90% from the bar into the fiber. A typical currently available high-power FAP product operates at 40 Watts from a
`800μm diameter fiber-bundle with N.A. of 0.15, and the improvements described above have increased the brightness
`by more than 4 times.
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`To assess the reliability, we have life-tested a several groups of conductively cooled bars. Figure 8 shows a batch of six
`20%FF bars operating at 40A (~40W) for 3000 hours with no degradation, and then we increased the current to 50A
`(~50W) and continued the test for another 3500 hours. The test is still ongoing, and the average power degradation is
`only 1.3% to date. With the data we calculated mean-time-to-failure (MTTF) > 15,000 hours with 90% confidence level
`at this time. With this very slow degradation rate, we should be able to get much longer MTTF expectation as we keep
`running those bars.
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`980nm 20%FF 40W/50W Lifetest
`(LPD = 25mW/um)
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`980nm 30%FF 80W CW Lifetest
`(LPD = 27mW/um)
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`40 Proc. of SPIE Vol. 5711
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`To meet the increasing demand for power, we also designed conductively cooled bars targeting 80 watts reliable
`operation. The bar geometry in this case is 30% FF. Figure 9 shows an on-going life test with 5 bars running at 90A
`(~80W). We have so far collected 2500 hours of lifetime on each bar, and the bars showed an average power
`degradation of 1.8%. The data set does not yet allow accurate assessment MTTF due to wear-out, but we estimate it be
`greater than 10,000 hours.
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`Demonstrating the robustness of our epi-design and growth capability, 940 nm 30% FF bars have been fabricated with a
`WPE of 68% at 50W, again operating CW with a heat sink temperature of 25 °C. Pilot production bars exhibit a typical
`WPE of 65% at operating conditions.
`
`Summary
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`Product improvements for commercially available 808-830nm, 940nm, and 976nm bars has been described. More than
`60 watts CW at 808nm from 30% fill-factor conductively cooled bars has been presented. At 25 °C, 53% wall-plug-
`efficiency (WPE) and far field FWHM divergence of 21° has been realized at a 60W operating point. Reliability tests so
`far indicate that a MTTF > 15,000 hours is achievable with high confidence. At 976nm, more than 100W CW from a
`20% fill-factor conductively cooled 65% WPE bars has been demonstrated, with a corresponding LPD of 50 mW/μm.
`At 25 °C, the WPE for these 976nm bars consistently peaks above 65% and remains greater than 60% at 100W CW.
`Our ongoing life test data suggests an extrapolated lifetime greater than 10,000 hours at 80W CW operating-condition
`based on 30%FF conductively cooled bar geometry.
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`The authors would like to acknowledge the support of DARPA FHOENICS program, Penn State Electro-Optics Center
`and Missile Defense Agency / Advanced Systems.
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`Acknowledgements
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`Proc. of SPIE Vol. 5711 41