High Brightness Laser Diode Bars Norbert Lichtenstein *, Yvonne Manz, Jürgen Müller, Jörg Troger, Susanne Pawlik, Achim Thies, Stefan Weiß, Rainer Baettig, Christoph Harder Bookham (Switzerland) AG, Binzstrasse 17, CH-845 Zürich/Switzerland ABSTRACT Based on the most recent generation of Bookham s laser diode bars in the 9xx nm wavelength range which are able to deliver in excess of 25 W of output power from 5% filling factor 2.4 mm cavity length design, we have developed low 2% fill-factor bar devices for high brightness applications. Close to 2 W of output power has been achieved in CW mode from actively cooled micro-channel cooler devices without signs of damage. Mounted on conductively cooled copper blocks, still more than 13 W (CW) has been obtained, indicating the high conversion efficiency of >6% reducing the thermal load on the mounting assembly. Based on extensive reliability testing in excess of 5 h and at power densities ranging up to 36mW / um and beyond, highly reliable operation of 2% fill-factor bars is expected. To facilitate fiber coupling into wide-core multi-mode fibers a further reduction of the emitter aperture has been realized. From a single 3.6 mm cavity length by 8 um wide emitter design ( MaxiChip ) about 5 W output power has been obtained in CW mode from devices mounted on standard conductively cooled 1x1 inch copper blocks. While CW operation has been thermally limited, extremely high peak power operation can be expected in qcw operation. Due to the narrow aperture of this MaxiChip efficient and easy coupling into wide aperture multimode fibers can be achieved. Keywords: Semiconductor, laser diode bar, stack, array, AlGaAs, high-power, reliability, far field, brightness, AuSn, material processing, fiber coupling 1. INTRODUCTION Future high power laser systems are using fiber laser (FL) or direct laser diode (DLD) architectures as light sources. While today s high power laser diode bars are able to deliver in excess of 3 W under CW operation from 5% [1] or 8% [2] high fill-factor designs, most important for FL and DLD applications is excellent reliability at enhanced brightness levels at competitive cost. Bookham s advanced (Al)Ga(In)As laser technology has been continuously improved over the last two decades with respect to power, reliability and cost maintaining the classical ridge-waveguide design and the effective elimination of catastrophic optical mirror damage (COMD) using E2 facet passivation. For superior long-term stability of the packaged devices AuSn-based mounting technologies are used similar as for our telecom products. Based on the most recent generation of Bookham s laser diode bars in the 9xx nm wavelength range which are able to deliver in excess of 25 W of output power from 5% filling factor 2.4 mm cavity length design, we have developed low 2% fill-factor bar devices for high brightness applications. The bar design contains standard 19 emitters at 5 um pitch while the width of the individual emitters amounts to 1 um to enable high brightness fiber coupling schemes. Close to 2 W of output power has been achieved in CW mode from actively cooled micro-channel cooler devices without signs of damage. Mounted on conductively cooled copper blocks, still more than 13 W (CW) have been * mailto: norbert.lichtenstein@bookham.com; phone ++41-1-455 8582; fax ++41-1-455 8586; www.bookham.com High-Power Diode Laser Technology and Applications IV, edited by Mark S. Zediker, Proc. of SPIE Vol. 614, 614E, (26) 277-786X/6/$15 doi: 1.1117/12.648721 Proc. of SPIE Vol. 614 614E-1
obtained, indicating the high conversion efficiency of >6% reducing the thermal load on the mounting assembly. The high power density at the front facet of more than 1mW / um depicts the robust device design and enables high operating power levels. Based on extensive reliability testing in excess of 5 h and at power densities ranging up to 36mW / um, highly reliable operation of these 2% fill-factor bars is expected. To facilitate fiber coupling into wide-core multi-mode fibers a further reduction of the emitter aperture has been realized. From a single 3.6 mm cavity length by 8 um wide emitter design, our so called MaxiChip, about 5 W output power has been obtained in CW mode from devices mounted on standard conductively cooled 1x1 inch copper blocks. While CW operation has been thermally limited, extremely high peak power operation can be expected in qcw operation. Due to the narrow aperture of this MaxiChip efficient and easy coupling into wide aperture multimode fibers can be achieved. From extensive multi wafer stress tests of multi-mode single emitters reliable operation in excess of 1 kh is anticipated. While first samples have been realized at 9xx nm emission wavelength, scaling at wavelengths ranging from.8 to 1.1 um is expected in the future. 2. HIGH BRIGHTNESS LASER BAR TECHNOLOGY 2.1. Process and technology Bookham s high brightness laser diode technology in this presentation is based on well known building blocks featuring the (Al)Ga(In)As / GaAs material system for optimum optical, electrical and thermal performance. The high quality material grown by molecular beam epitaxy (MBE) allows volume manufacturing with excellent material quality covering the whole wavelength range from 78 nm to 16 nm. For the device technology a simple but robust ridge-waveguide design [3] has been chosen. For high power applications proven long term stability against catastrophic optical mirror damage (COMD) is crucial. Bookham s well known E2 facet passivation technology provides maximum protection from chip fails due to catastrophic optical mirror damage (COMD) [4] while standard mirror technologies only guarantee the absence of COMD until the level of COMD has dropped below the operation power [5]. To accommodate for the heat load associated with different output power levels, laser bars with cavity lengths of 2.4 mm or 3.6 mm have been realized. For optimum performance and reliability the laser diode bars are mounted episide down onto expansion matched sub-assemblies using AuSn hard-solder [6] effectively eliminating potential degradation modes like thermal fatigue as well as whisker growth and electromigration which are otherwise often seen in the context of hard pulse operation. Cooling is achieved using standard micro-channel coolers or conductively cooled copper blocks. 2.2. High brightness laser diode bars Addressing high brightness operation, various aspects of laser diode performance require optimization. Straightforward power scaling linearly increases the brightness, however it needs to demonstrate its reliability. In addition, a narrow far field intensity pattern reduces the solid angle of the emission. Both aspects are demonstrated in this paragraph closing with a rigorous reliability assessment at accelerated conditions. 2.2.1. High power operation Bookham s latest generation of laser diode bars realized from this process is the BAC12C-9xx product family. This device contains 48 emitters resulting in approximately 5% filling factor. The typical Light-Current-Voltage (LIV) characteristic of such a device is plotted in Figure 1. More than 25 W of CW output power can be obtained at approximately 3 A drive current demonstrating the robustness of the technology against extreme current and power densities. Proc. of SPIE Vol. 614 614E-2
4 Power (W) / Wallplug Efficiency (%) 25 2 15 1 5 Popt [W] Ef f [%] Vact [V] 3.5 3 2.5 2 1.5 1.5 Voltage (V) CW 25 C 1 2 3 Figure 1: Typical light-current-voltage characteristics of Bookham s BAC12C-9xx product family. 2.2.2. Lateral far field engineering Laser diode bars with significantly improved brightness over conventional laser diode bars have been realized, based comprehensive modeling and testing. In Figure 2 the lateral far field pattern is shown for both a conventional laser diode bar and the Bookham high-brightness bar. To assess the power contained in a defined angle the integral power emitted in the lateral angular space is plotted in the same figure. Since the value of the full angular width at half intensity maximum (FWHM) for most applications is not a good figure of merit, the angle for 95% of the power contained has been analyzed. From this a typical value of around 7 for 95% power content has been obtained at 12 W output power in CW mode. Compared to values close to 12 for conventional laser diode bars at the same power level this results in a 7% increase in the brightness available for demanding applications. In Figure 3 values for the angle for 95% power content are plotted for devices rated at 8 W (@8 A drive current) and 12 W (@14 A drive current) optical output power. Although the far field pattern is influenced from changes in current density, thermal load and optical power density, only a slight dependency in the far field angles on these parameters has been measured. Together with the similar far field values for both device generations obtained at the operating point this demonstrates the stability of the design. Proc. of SPIE Vol. 614 614E-3
1% 9% 8% 7% intensity [%] 6% 5% 4% 3% 2% 1% % -12-1 -8-6 -4-2 2 4 6 8 1 12 divergence [ ] Figure 2: Lateral far field pattern for conventional and Bookham s high brightness laser diode bars at 12 W output power level (solid line). To assess the power contained in a certain angular distribution the integral power distribution is overlaid (dotted line). Slow Axis Width 95% PowerContent ( ) 12 11 1 9 8 7 6 5 4 Conventional LD Bar Conventional LD Bar Bookham BAC8C-9xx Bookham BAC12C-9xx 2 4 6 8 1 12 14 16 Figure 3: Angular width for 95% power content as a function of the drive current for Bookham s latest laser bar generations BAC8C-9xx and BAC12C-9xx (circles), rated for 8W (open symbols) and 12W output power (closed symbols) respectively. For comparison the values obtained from conventional laser diode (LD) bars are given (squares). 2.3. Technology robustness Rigorous reliability assessments have been performed to demonstrate the robustness of the technology. To realize real operating conditions in our long term aging tests the laser diode bars are operated in intermittent operation mode (hard pulse: 1.3 Hz, to 1% modulation, 5% duty cycle). From the analysis of the degradation mode physics [7] it has been established that the predominant failure mode can be characterized by linear wear-out behavior, defined by a reduction of the output power. In this case, linear extrapolation towards the time for 2% output power reduction is appropriate. From the statistical evaluation of the data applying a log-normal distribution the mean-time-to-failure (MTTF) amounts to more than 3 kh. Since such low degradation rates are sensitive to measurement errors for extrapolation beyond 5x the measured time, the average power from the ensemble might be a good indicator for the reliability of the system. In this case the average power is expected to amount to more than 97% of the initial value after 1 kh of operation. Proc. of SPIE Vol. 614 614E-4
1.1 1 Rel. power (a.u.).9.8.7.6.5 1 2 3 4 5 6 Time (h) Figure 4: Aging test at nominal output power of 12 W in intermittent operation mode (hard pulse) For the prediction of a reliable long-term operation it is important not only to assess begin-of-life (BOL) condition but also to perform stress tests, for example applying end-of-life (EOL) operating current. This is demonstrated in Figure 5, where the current (hard pulse) has been chosen to obtain nominally 12 W, 15 W and 18 W output power at time h, 5 h and 25 h respectively. From such severe overstress tests the robustness of the laser diode bar itself can be demonstrated even at power densities of 36 mw / um emission area. 1.8 1.6 1.4 Rel. power (a.u.) 1.2 1.8.6.4.2 5 1 15 2 25 3 35 Time (h) Figure 5: Step-stress test in intermittent operation mode (hard pulse) at nominal output power of 12W, 15W and 18W. Proc. of SPIE Vol. 614 614E-5
3. APPLICATION: PUMPING AND DIRECT MATERIAL PROCESSING Applying laser diode bars to pumping and direct material processing various techniques for collimation and beam shaping [8] are used. Depending on the beam shaping technique either integrated arrays and stacks or discrete elements like individual laser diode bars are favorable. 3.1. Stacks and arrays Based on the results obtained from individual laser diode bars on micro channel coolers, lateral bar arrays (LBA) and stack as vertical bar arrays (VBA) have been realized. Figure 6 demonstrates the electro-optical performance of a LBA consisting of 6 bars. From the array in excess of 1 W can be obtained at less than 2 A drive current. The slope efficiency of 6.5 W/A measured in the range up to 2 W output power corresponds well with the values measured on the individual bars. 1 9 8 Output Power, W 7 6 5 4 3 2 1 2 4 6 8 1 12 14 16 18 2 Drive Current, A Figure 6: Electro-optical performance of a 6-bar LBA at 94 nm wavelength demonstrating more than 1 W optical output power. The arrangement of the bars in a vertical bar array (VBA) is demonstrated in Figure 7. Due to the high brightness of the laser diode bars a stack,consisting of only 2 laser bars is sufficient to obtain 2.4 kw output power after fast-axis collimation at a drive current of 14 A. Applying appropriate beam-shaping optics to such VBAs this technology is enabling multi-kw fiber-coupled output power required for direct material processing without need for inefficient conversion of pump light in solid-state laser systems. 25 2 Power (W) 15 1 5 2 4 6 8 1 12 14 16 Drive Figure 7: 2-bar VBA at 92nm wavelength delivering 2.4 kw output power Proc. of SPIE Vol. 614 614E-6
3.2. Reduced filling factor devices While our vertical and lateral bar arrays providing multi-kw power levels are extremely compact, some beam-shaping optics are based on distributed pump sources. For such systems individual laser diode bars with high brightness light emitting areas are of great importance. Leveraging the technology described for our 5% filling factor laser diode bars, derivatives with a reduced filling factor of 2% have been realized. Such bars containing 19 emitters at 1 um aperture have been mounted using the concept of hard soldering on standard 1x1 inch conductively cooled copper blocks. Typical data for the electro-optical performance can be found in Figure 8. More than 13 W output power has been obtained just limited by thermal rollover. 16 22 2.5 Optical Power (W) 14 12 1 8 6 4 2 Optical Power (W) / Wallplug Efficiency (%) 2 18 16 14 12 1 8 6 4 2 2 1.5 1.5 Voltage (V) 2 4 6 8 1 12 14 16 18 5 1 15 2 25 Figure 8: a) Electro-optical performance of conductively cooled 2% filling factor 9xx laser diode bar. b) Electro-optical performance (CW, 18 C) of actively cooled 2% filling factor 9xx laser diode bar on micro-channel cooler. To demonstrate the capabilities of the devices actively cooled micro channel coolers have been used. A maximum of 2 W output power is obtained routinely without signs of degradation even the power is thermally limited as demonstrated in Figure 8b). Such power level of 11 W from 1 um wide emitters at an electro-optical conversion efficiency of 65% corresponds well with the results obtained from broad area single emitter devices of same width [9] enabling highly reliable 8 W operating power and beyond. Based on the reliability assessments demonstrated on laser bars at power levels of up to 3 mw / um emission area and beyond [1], highly reliable operation of these 2% filling factor devices is expected. 3.3. Outlook Complementary to solutions introducing 1-cm wide single-mode emitter array laser (SEAL) bars based on narrow stripe emitters [11],[12], we have chosen a reduction of the width of the laser diode bar as a next step of brightness improvement. From a single 3.6 mm cavity length by 8 um wide emitter design ( MaxiChip ) about 5 W output power has been obtained in CW mode from devices mounted on standard conductively cooled 1x1 inch copper blocks (Figure 9). While CW operation has been thermally limited at this power level, extremely high peak power operation can be expected in qcw operation. From extensive multi wafer stress tests of multi-mode single emitters [13] reliable operation in excess of 1 kh is anticipated. While first samples have been realized at 9xx nm emission wavelength, scaling at wavelengths ranging from.8 to 1.1 um is expected in the future. Proc. of SPIE Vol. 614 614E-7
6 2.5 Optical Power (W) 5 4 3 2 1 2. 1.5 1..5 Voltage (V) / Wallplug Efficiency. 1 2 3 4 5 6 Figure 9: Electro-optical characteristics of 8 um aperture MaxiChip under CW operation. Due to the narrow aperture of this MaxiChip, efficient and easy to achieve coupling into wide core-diameter multimode fibers can be achieved. Applying cleaved and uncoated 91 um diameter multi-mode fibers a 75% coupling efficiency has been obtained yielding 32 W coupled power at 5 A. Eliminating the need for optics or fiber lenses cost-reduced solutions for low-power applications like plastic-welding or soldering are enabled. Leveraging the full benefit of the high brightness of the MaxiChip by state-of-the-art collimation and beam-shaping, similar multi-1w power level are expected in fiber diameters of 1 um and below. 5 1% 4 8% Optical Power (W) 3 2 6% 4% Coupling Efficiency (%) 1 2% Ex -Facette Fiber Coupled Coupling Eff. % 1 2 3 4 5 6 Figure 1: MaxiChip delivering 32 W fiber-coupled power into 91 um core as-cleaved fiber without additional optics or fiber lenses. Proc. of SPIE Vol. 614 614E-8
4. CONCLUSION We have presented our latest results on 9xx nm wavelength laser diode bars based on telecom proven device technology and highly reliable AuSn hard soldering. Excellent brightness is obtained from the capability to deliver up to 25 W output power and the narrow lateral far field pattern of 7 at 95% power content. Excellent robustness at accelerated conditions up to 18 W in hard pulsed operating mode has been demonstrated. Based on this technology lateral arrays yielding 1 kw from 6-bar assemblies and vertical arrays yielding 2.4 kw from 2-bar assemblies have been realized. Leveraging this technology in 2% low filling factor 1-cm wide bars, very high output power and electro-optical conversion efficiency has been shown from conductively cooled heatsinks. Extremely high 2 W output power reached on actively cooled micro-channel cooler assemblies demonstrate the capabilities of this device. Further improvement of the brightness of laser diodes is expected from the introduction of 8 um wide aperture MaxiChips. Ex-facet output power of 5 W and 32 W output power from as-cleaved wide core-diameter fibers has been achieved. We believe that these results are enabling the direct use of laser diode bars in stacks or discrete assemblies for fiber laser pumping and direct material processing. REFERENCES 1. N. Lichtenstein, Y. Manz, P. Mauron, A. Fily, B. Schmidt, J. Müller, S. Arlt, S. Weiß, A. Thies, C. Harder 325 Watt from a single 1-cm 9xx Laser Diode Bar on standard Micro-Channel Cooler, Proc. of the 17th Annual Meeting of the IEEE Lasers and Electro-Optics Society, Vol. 2, pp. 955-956 (24). 2. P. Crump, J. Wang, T. Crum, S. Das, M. DeVito, W. Dong, J. Farmer, Y. Feng, M. Grimshaw, D. Wise, S. Zhang > 36W and > 7% Efficient GaAs-Based Diode Lasers, Proc. of SPIE 5711-3, San Jose, California, 25 3. B. Schmidt, N. Lichtenstein, B. Sverdlov, N. Matuschek, S. Mohrdiek, T. Pliska, J. Müller, S. Pawlik, S. Arlt, H.-U. Pfeiffer, A. Fily, C. Harder "Further development of high-power pump laser diodes," in Proceedings of SPIE Vol. 5248, Semiconductor Optoelectronic Devices for Lightwave Communication, edited by Joachim Piprek (SPIE, Bellingham, WA, 23) 42-54. 4. A. Oosenbrug, Reliability Aspects of 98-nm Pump Lasers in EDFA Applications, Proc. of SPIE, San Jose, California, 1998, pp. 2-27. 5. A. Moser, E. Latta, Arrhenius parameters for the rate process leading to catastropic damage of AlGaAs-GaAs laser facets, J. Appl. Phys. 71(1), 1992, pp. 4848. 6. N. Lichtenstein, B. Schmidt, A. Fily, S. Weiß, S. Arlt, S. Pawlik, B. Sverdlov, J. Müller and C. Harder, DPSSL and FL Pumps Based on 98nm-Telecom Pump Laser Technology: Changing the Industry, Proc. of SPIE 5336-31, San Jose, California, 24 7. N. Lichtenstein Application: Determination of laser diode bar and array lifetime based on degradation mode physics in Quantifying high power diode laser lifetime - a seminar and discussion, Photonics West 26, San Jose, California, 26 8. S. Heinemann, L. Leininger, Fiber coupled diode lasers and beam-shaped high-power stacks, Proc. of SPIE 3267, San Jose, California, 1998, pp. 116-124 9. S. Pawlik, B. Sverdlov, R. Bättig, B. Schmidt, N. Lichtenstein, H. Pfeiffer, J. Müller, B. Valk, C. Harder 9xx high power pump modules, Proc. of SPIE 614-18, San Jose, California, 26 1. N. Lichtenstein, Y. Manz, P. Mauron, A. Fily, B. Schmidt, J. Müller, S. Arlt, S. Weiß, A. Thies, J. Troger, C. Harder 325 Watt from 1-cm wide 9xx Laser Bars for DPSSL- and FL-applications, Proc. of SPIE 5711-1, San Jose, California, 25 11. N. Lichtenstein, Y. Manz, P. Mauron, A. Fily, B. Schmidt, J. Müller, S. Pawlik, B. Sverdlov, S. Weiß, A. Thies, C. Harder High-Brightness 9xx and 14xx Single-Mode Emitter Array Laser Bars, Proc. of SPIE 5711-13, San Jose, California, 25 12. N. Lichtenstein, Y. Manz, P. Mauron, A. Fily, S. Arlt, A. Thies, B. Schmidt, J. Müller, S. Pawlik, B. Sverdlov, C. Harder Singlemode Emitter Array Laser Bars for High-Brightness Applications, Proceedings of the 24 IEEE 19th International Semiconductor Laser Conference, pp. 45-46 (24) 13. B. Schmidt, B. Sverdlov, S. Pawlik, N. Lichtenstein, J. Mueller, B. Valk, R. Baettig, B. Mayer, C. Harder 9xx high-power broad-area laser diodes, Proc. of SPIE 5711-25, San Jose, California, 25 Proc. of SPIE Vol. 614 614E-9