High-brightness fiber coupled pumps


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High-brightness fiber coupled pumps

  1. 1. High-brightness fiber coupled pumps V. Gapontsev, N. Moshegov, P. Trubenko, A. Komissarov, I. Berishev, O. Raisky, N. Strougov, V. Chuyanov, G. Kuang, O. Maksimov, and A. Ovtchinnikov* IPG Photonics Corp., 50 Old Webster Rd, Oxford, Ma, USA, 01540 ABSTRACTAdvanced high volume applications require pumps with high power, high brightness, and high power efficiency. Newgeneration devices meet all of these challenging requirements, while still maintaining the advantages of distributedpumping architecture including high reliability inherent to single emitter sources. Based on new-generation long-cavitydiode chips, new pumps are capable of more than 60W CW power ex-fiber output (100 μm core diameter) into NA ~0.12. Peak power efficiency stays over 60%. All of the above is provided at room heatsink temperature, maintained bybasic air- or water-cooling.Keywords: high-power, high-brightness, high-efficiency, single emitter, pump, diode 1. INTRODUCTIONRecent industry development efforts have been primarily targeted on pump diodes’ power and efficiency. As a result ofthese efforts, clear dominance of single emitter over traditional bars has developed in the last few years. For practicalapplications, such as industrial, telecom, medical and similar, the single emitter approach means cheaper and brighterdiodes, as well as more reliable overall kilowatt-class pump engines not based on overcomplicated and high-maintenance micro-channel cooled bar stacks. Single emitters have prevailed not only due to obvious reason of superiorperformance, reliability and cost of ownership, but also due to the not so obvious upfront cost advantage traditionallymeasured in $/W.This paper focuses on IPG Photonics most recent developments for high-power high-brightness single emittermultimode pumps operating in the wavelength range of ~ 8xx nm to ~ 9xx nm. These diodes are best suitable for fiberlaser pumping and other applications, including direct diode systems, graphic arts, medical, dental, and numerous others.. 2. NEW GENERATION SEMICONDUCTOR CHIP DEVELOPMENT2.1 Diodes operating in the 9xx nm rangeIPG Photonics Chips-On-Submount (COS) and fiber coupled pumps of the previous generation demonstrate state-of-the-art performance and reliability1. Due to optimized chip design diodes with a cavity length (L) of 3 mm we were able toachieve industry best performance2,3,4,5 in power, brightness and power efficiency without compromising pumpreliability1. These parameters were achieved for fiber coupled pumps rated to operate up to ~ 20W-25W ex-fiber. Furtherincrease of rated power in the L=3 mm chip design was impossible without sacrificing reliability of the chip. Scaling ofcavity length is a natural approach to increase the level of reliable output from a single emitter device2,3. Our choice wasa 50% scaling factor resulting in L=4.5 mm. In order to maintain a decent slope efficiency and to keep the thresholdcurrent low, we have performed a layered structure, as well as ridge stripe geometry optimization targeting acompromise between “high slope and low threshold current”, and the sacrifice in “the far-field divergence and seriesresistance”; the latter has a direct impact on power efficiency that is the key parameter for the multi-kilowatt systems.The far-field divergence greatly determines the coupling efficiency, which in turn affects the rated power level andpower efficiency ex-fiber.* aovtchinnikov@ipgphotonics.com; phone 1 508 373 1100; fax 1 508 373 1203; www.ipgphotonics.com High-Power Diode Laser Technology and Applications VII, edited by Mark S. Zediker, Proc. of SPIE Vol. 7198, 71980O · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.809456 Proc. of SPIE Vol. 7198 71980O-1
  2. 2. The results presented in Figure 1 summarize the layered structure and the ridge waveguide development effort. As onecan see from the plot, the best COS (W=90 μm wide aperture, L=4.5 mm, 25°C heatsink temperature) launches ~18WCW free-space at a 25A current not reaching the thermal roll-over. This performance is archived at a very low operatingvoltage: CW power efficiency peaks at ~ 72% (WPE > 60% recorded up to 14W CW power) and stays above 50% up tothe ex-facet output of > 17W. To the best of our knowledge, this is the most power-efficient high-power diode reportedso far to operate above cryogenic temperature. In order to reach such efficient operation in manufacturing environment,we had to take full advantage of AlGaInAs material grown in the most precise and controllable manner by Solid StateMolecular Beam Epitaxy growth method. Power Efficiency, % 16 1.6 60 12 1.2 L = 4.5 rrrr W = 90 irn 25CC > 40 -r....-- 0.8 or 9) 0 WPE > 710 0 WPE 600o up to - 13.5W 4 WPE > 50°o: up to 17.5W 0.4 20 0 30 0 = 8-9 976 rrr 0 0 5 10 15 20 0 5 10 15 20 Current CW, AFigure 1 Power and Power Efficiency dependencies on current of the 4.5 mm-long cavity, W=90 μm COS optimized for efficient performance.In order to demonstrate ultimate CW power output, we chose a slightly different layered structure and processed it in aridge waveguide structure of W, which is slightly wider than 100 μm. The results of CW THS=25°C test are presented inFigure 2. An output greater than 20W CW was achieved; this power is very close to earlier reported data for diodesgrown by MOCVD2. peek effioiency 2 650/0 18 60 - 25°C heatsink 2 600/0 WPE op to 15 2 20W CW 13A - 12W CW, 250C 45 2 500/o WPE up to -- 12 22A - 19W CW, 250C U U >flWcW__ - 30 0 85CC heatsink 6 15 3 A = 974 nm 0 0 I I 0 5 10 15 20 25 0 5 10 15 20 25 Current CW, A Current CW, A Figure 2 Power and Power Efficiency dependencies on current of the 4.5 mm-long cavity, W~100 μm COS optimized for high-power performance. Proc. of SPIE Vol. 7198 71980O-2
  3. 3. Results of the cavity length analysis and the tests of temperature sensitivity of lasing are given in the next two graphs.While temperature coefficients yielded numbers of T0=167K and T1=358K, the internal quantum efficiency and internalloss were ηi=99% and αi=0.35 cm-1 respectively. At the time of preparing this paper, the authors were not aware ofpreviously reported data detailing such low internal loss for semiconductor lasers. 50 - 45 >, 0 = @9 C) 0 0 0 w C) 30 0 15 4 8 12 16 20 Current CW A Figure 3 Set of Power and Power Efficiency dependencies on current of the 4.5 mm-long cavity, W=90 μm COS recorder at different heatsink temperatures. 700 E = a) 500 0 0 -c Q) U) a) 1 -c F- 500 30 60 90 30 60 90 Heatsink temperature, °CFigure 4 Results of cavity length and power-current characteristics analysis performed on laser diodes of 4.5 mm-long cavity design, W=90 μm COS at different heatsink temperature. Proc. of SPIE Vol. 7198 71980O-3
  4. 4. 2.2 Diodes operating in the 8xx nm rangeThe modeling, and layered structure and ridge geometry design rules developed in the course of our investigations,appear to be rather universal and applicable to devices operating at different wavelengths. The very first attempts toproduce COS with L=4.5 mm operating in the wavelength range of 8xx nm yielded a COS with typical performancepresented in Figure 5; the L=4.5 mm COS performance is plotted against L=3.0 mm COS previously reported in 1. Asone can clearly see from the graph a 50% increase in cavity length (for this particular wavelength) gives the benefit of a~50% increase in roll-over power and a ~ 50% increase in roll-over current. Efficiency, 0/0 THIS.k = 25°C 12 50 10 40 8 30 6 fi 20 4 10 ---W=9Ogm,L=4.5mm - W = 90 m, L = 3.0mm 0 10 15 0 5 10 15 Current CW, A Figure 5 Room temperature Power-Current CW characteristics comparison performed on COS operating in 808 nm wavelength range of different cavity length (L=4.5 mm and L=3.0 mm); both have similar lateral stripe geometry W=90 μm. 3. FIBER COUPLED PUMPS BASED ON L=4.5 MM COS3.1 Low power pumps operating in the 9xx nm rangeThe pumps reported in this section are based on the same package design, packaging and fiber coupling technology andpreviously reported techniques6. The fiber coupled pumps of the early design were based on COS with 2-3 mm cavitylength and the details of their performance were reported elsewhere6,7. Those devices were qualified and proven innumerous applications, including telecommunications and space operation.The performance of the L=4.5 mm COS-based pumps is presented in Figure 6. Ex-fiber power (105 μm core diameter)versus current characteristic is linear, with over 13W CW power launched at a 15A driving current. In the entire drivingcurrent range, the power is confined within a numerical aperture of 0.12 or less. Despite of the long-cavity chips, thesefiber coupled pumps arrear to be very power efficient. As one may see from the efficiency-current dependence, the peakpower efficiency is well above 60%, the efficiency stays above 50% up to the highest driving current of 15A (> 13WCW ex-fiber output).Another attractive feature of these devices is low thermal resistance; we recorded thermal resistance as low as 2.5-2.9°C/W, which corresponds to less than 30°C junction temperature overheat at a 10W CW output. Low chip overheatfavorably contributes to overall pump reliability. Proc. of SPIE Vol. 7198 71980O-4
  5. 5. 12 -Io E 0 Wlenqth 974uiiii 10 rIiuiI 2.4S - .0 °C/W U 0 0 q 12 0 3 0 12 Curr.nt CW AFigure 6 Room temperature Power and Power Efficiency characteristics for “low-power” fiber-coupled pump based on L=4.5 mm and W=90 μm (λ ~ 975nm) COS3.2 Mid-power pumps operating in the 9xx nm rangeMid-power pumps are based on the package design and technology previously reported elsewhere1. Replacing L=3mmCOSs with the new-generation chips allowed us to achieve higher power ex-fiber, while keeping the same package. 60 40 55 so C) L 30 I 0 20 . 20 S 10 9 15 5 10 15 20 25 30 35 40 Current CW A OW Power ex-fiber, W Figure 7 a. Room temperature Power and Power Efficiency characteristics for “mid-power” fiber-coupled pump based on L=4.5 mm and W=90 μm (λ ~ 975nm) COS. b. Junction overheat as a function of ex-fiber CW output.Figure 7 presents the power and efficiency characteristics recorded for one of these packages. Power of slightly less than40W is reached at a driving current of ~15A. Even at the highest driving currents, the power is still confined within NA≤ 0.12. Such high efficient operation, stable fiber coupling efficiency and high brightness of radiation were madepossible due to optimized stripe geometry design providing negligible slow-axis far-field blooming with current. Powerefficiency peaks at > 60% and stays over 50% with ex-fiber output up to 35W - 40W CW. Even though this high-brightness pump has a exceptionally small footprint it demonstrates very stable temperature performance and very littlejunction temperature overheat. As one may see from the right-hand plot in Figure 7, the junction temperature overheatstays below 30°C even at high ex-fiber output, such as 30W CW. Such an efficient and temperature-insensitive Proc. of SPIE Vol. 7198 71980O-5
  6. 6. performance makes these pumps ideal candidates for multi-kWatt system pumping by providing the ideal balancebetween performance, reliability, and manufacturing cost.3.3 High power pumps operating in the 9xx nm rangeAlthough the mid-power pumps reported in the previous section provide power and brightness sufficient to satisfy therequirements of vast majority of most modern pumping applications by providing almost an ideal balance ofperformance, reliability and cost ($/W), some specialty applications or experimental units require even higher pumpingpower and brightness. The pump sources reported in this section are designed to address these needs. 60 E 45 60 9) > 40 0 C) r C 0 9) - C) uJ r U 15 C n Wavelength 974 nm NA 6.12 9) I I I 0 3- J 6 9 12 15 18 0 20 40 60 Curr6nt CW, A Power CW ex-fiber, W Figure 8 a. Room temperature Power and Power Efficiency characteristics for “high” power fiber-coupled pump based on L=4.5 mm and W=90 μm (λ ~ 975nm) COS. b. Junction overheat as a function of ex-fiber CW output. The insert photograph shows the mid- and the high-power pumps placed side-by-side.The graph on the left in Figure 8 depicts the power and power efficiency dependencies of the high-power pumps of thenew design. In the right-hand graph, one will find the dependence of junction temperature overheat with CW ex-fiberoutput; the insert photo shows mid- and high-power pumps placed side-by-side to provide the footprint comparison ofthe pumps of both designs. As one can see from the graphs, there is almost virtually no penalty in fiber couplingefficiency, numerical aperture, and power efficiency with power scaling to > 60-70W CW. Stability of these parameterswith power scaling is possible due to proprietary fiber coupling techniques (combined with narrow and stable withcurrent far-field6). One can see that the junction-temperature-overheat with ex-fiber output is maintained low for thispump design: the ~30°C-junction-overheat is recorded at a ~ 60W CW output.3.4 Pumps for specialty applicationFew specialty applications require “low-power”, low-cost, ultra-high-brightness pumping. 9xx nm pumps with 50 mmfiber core diameter may be the perfect solution for these applications. As one can see in Figure 9, the thermal roll-overpower of ~ 10W CW ex-fiber (50 μm fiber core diameter) may be achieved at about 16A at a room temperature heatsink.These pumps offer brightness of pumping comparable to that of the more expensive higher power devices reported in theprevious sections (100 μm fiber core diameter pumps rated for 20W-30W CW output) due to low NA ~ 0.11 - 0.12 andhigh ex-fiber power output. Although these pumps do not provide the lowest $/W pumping solution, they offer thecheapest solution providing the brightest and the most efficient pumping in the power range up to ~5-7W CW. Proc. of SPIE Vol. 7198 71980O-6
  7. 7. 10 ORO "5 w o*r .*-fIb.: S0pr ear., nA nçntr.$. to 0W 100 on icr. OQutvJIOrlt o 30W LOC tarn or. 1Dor 0 GS 0 2 4 6 0 10 12 14 16 4 ( 1) 11 Current CW, A Figure 9 Room temperature Power and Power Efficiency characteristics for 50 μm core diameter fiber-coupled pump based on L=4.5 mm and W=90 μm (λ ~ 976 nm) COSThe spectral range of 8xx-nm is another wavelength that requires a significant amount of pumping power. Recentlysingle emitter-based pumps started to provide not only the low-power (2-4W CW), but also higher power pump systemsbased on power scaling solutions described elsewhere1. Figure 10 provides an example of the pump based on long-cavityL=4.5 mm COS. As one may see from the plot ex-fiber (100 μm core fiber diameter), an output of ~ 10W CW with peakpower efficiency of ~ 50% can be achieved from this device. The performance of this device qualifies it as one thebrightest and the most efficient of presently available pumps8 for this wavelength range. 10 > C C 4 8 10 1.! I Ctwre t CW, AFigure 10 Room temperature Power and Power Efficiency characteristics for “low” power fiber-coupled pump based on L=4.5 mm and W=90 μm (λ ~ 808nm) COS Proc. of SPIE Vol. 7198 71980O-7
  8. 8. 3.5 Reliability of 9xx nm pumps’ rated for ~20W output for industrial applicationsAt the time of this publication, we have not accumulated sufficient data to assess reliability of the pumps based on L=4.5mm chips. The reliability data are available for the previous generation devices: mid-power range pumps rated for ~20Woutput that were reported earlier1. These devices are based on a 3 mm-long cavity COS; these pumps are primarily usedin material processing, medical, dentistry, and specialty applications. Typical power current characteristic of thesepumps is presented alongside with power efficiency dependence in the right graph of Figure 11b.To assess the reliability of these devices we have randomly selected 153 pumps manufactured in different IPG facilitiesworldwide. They were put on a multi-cell test that provided stress conditions by operating current overdrive and elevatedheatsink temperature. In the course of this test, over 2,750,000 COS-real-time-hours-on-test (not-accelerated) wereaccumulated. Random COS failures were proven to not have any noticeable effect on the reliability of the other singleemitter diodes operating within the same pump enclosure, i.e. COS failures within the same pump can be consideredindependent events. The results of COS failure in time distribution analysis yielded the results summarized in the leftgraph of Figure 11. As one may can in the graph, MTBF value greater than 100,000 hours at ~20W ex-fiber pump outputwas determined for packaged COS of 3 mm cavity length. This MTBF value for packaged chips L=3.0 mm cavity lengthensures less than a 10% drop in output power of a pumping ensemble over a period of time greater than two years ofcontinuous operation. As of today, numerous devices of this design are deployed in the field serving many diverseapplications. These pumps provide additional proof of the validity of this reliability assessment data, as the actual in-field reliability exceeds the estimate obtained in this accelerated test. 250220 so 200000 45 l5o00 30 100,000 50,000 C 14 20 21 22 20 24 25 6 12 Power ex-fiber, W Current CW, A Figure 11 a. Dependence of COS Mean-Time-Between-Failures as a function of ex-fiber output for mid-power pumps rated for ~20W output1. b. Room temperature Power and Power Efficiency characteristics for the fiber-coupled pump rated for ~20W output and based on L=3.0 mm and W=90 μm (λ ~ 975nm) COS 4. CONCLUSIONSSolid Source MBE-grown AlGaInAs lasers performing in the 8xx-9xx nm range demonstrate the highest powerefficiency and lowest internal loss previously reported. These are the key pump laser parameters for high power, highvolume applications. The recent design and manufacturing packaging solutions result in high brightness pumps operatingin the output power range of >60W CW ex 100 μm fiber core diameter. The combination of these parameters makesthese devices the most efficient presently available pumping solutions. Proc. of SPIE Vol. 7198 71980O-8
  9. 9. 5. ACKNOWLEDEGEMENTSThe authors would like to thank their co-workers at IPG Photonics; without their support, contribution, andencouragement, this work would not be possible. REFERENCES1 V. Gapontsev, I. Berishev, V. Chuyanov, G. Ellis, I. Hernandez, A. Komissarov, N. Moshegov, O. Raisky, V. Rastokine, N. Strougov, P. Trubenko, L. Wright, and A. Ovtchinnikov, “8xx – 10xx nm highly efficient single emitter pumps”, SPIE Proceedings, V. 6876, paper 6876-172 P. Ylamanchili, V. Rossin, J.Skidmore, K. Tai, X. Qui, R. Duesterberg, V. Wong, S. Bajwa, K. Duncan, D. Venables, R. Verbera, Y. Z. Dai, J. F. Feve, E. Zucker, “High-power, high-efficient fiber-coupled multimode laser diode pump module (XX nm) with high-reliability”, SPIE Proceedings, V. 6876, paper 6876-373 N. Lichtenstein, M. Krejci, Y. Manz, J. Boucart, B. Valk, J. Muller, C. Buttons, S. Weib, S. Pawlik, B. Sverdlov, “Recent development for BAR and BASE: setting the trends”, SPIE Proceedings, V. 6876, paper 6876-0C4 R. Martinsen, K. Price, and S. Karlsen, “Next-generation fiber laser pumps”, Applications of High Power Semiconductor Lasers, San Diego, 06 October 20085 E. Wolak, K. Kuppuswamy, B. Fidric, S.-K. Park, D. Liu, S. Cutilas, H. Li, I. Chyr, F. Reinhardt, R. Miller, X. Jin, T. Nguyen, T. Towe, P. Cross, T. Truchan, R. Bullock, J. Mott, J. Harrison, “Reliability of ensembles multi-stripe laser diodes”, SPIE Proceedings, V. 6876, paper 6876 0N.6 V. Gapontsev, I. Berishev, G. Ellis, A. Komissarov, N. Moshegov, A. Ovtchinnikov, O. Raisky, P. Trubenko, V. Ackermann, E. Shcherbakov, “9xx nm single emitter pumps for multi-kW systems”, SPIE Proceedings V. 6104, paper 61040K, 20067 V. Gapontsev, I. Berishev, G. Ellis, A. Komissarov, N. Moshegov, O. Raisky, P. Trubenko, V. Ackermann, E. Shcherbakov, J. Steinecke, and A. Ovtchinnikov, “High efficiency 970 nm multimode pumps”, Photonics West, San Jose, CA, January 24th 2005, SPIE Proceedings 5711-68 Schleuning, D.A., Haapamaa, J., Luong, C., Morales, J., Pathak, R., Watson, L., Winhold, H., and Hasenberg, T., “Lateral Modes and Slow Axis Divergence in Broad Area Semiconductor Lasers”, Applications of High Power Semiconductor Lasers, San Diego, 06 October 2008 Proc. of SPIE Vol. 7198 71980O-9