A New Continuous Wave 2500W Semiconductor Laser Vertical Stack

Transcription

1 A New Continuous Wave 2500W Semiconductor Laser Vertical Stack Xiaoning Li 1,2, Chenhui Peng 1, Yanxin Zhang 1, Jingwei Wang 1, Lingling Xiong 1, Pu Zhang 1, Xingsheng Liu 1,3 1 State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics Chinese Academy of Sciences No. 17 Xinxi Road, New Industrial Park, Xi'an Hi-Tech Industrial Development Zone, Xi'an, Shaanxi, , P.R. China, tel , fax , smto@opt.ac.cn 2 Key Laboratory for Physical Electronics and Devices of the Ministry of Education & Shaanxi Key Lab of Information Photonic Technique, Xi an Jiaotong University, No.28, Xianning West Road, Xi'an, Shaanxi, , P.R. China 3 Xi an Focuslight Technologies Co., LTD No. 60 Xibu Road, New Industrial Park, Xi'an Hi-Tech Industrial Development Zone, Xi'an, Shaanxi, , P.R. China Abstract With the increasing applications of high power semiconductor lasers in industrial, advanced manufacturing, military, aerospace, medical systems, display, entertainment. etc., semiconductor lasers with high power and high performances are required. The performance of semiconductor lasers is greatly affected by packaging structure, packaging process and beam shaping. In this work, a high power semiconductor laser was successfully fabricated. A series of techniques such as spectrum control and beam control were used to achieve marrow spectrum and high beam quality. The performances of the semiconductor laser vertical stack were characterized. A high power of 2500 W, a narrow spectrum of 3.11 nm and an excellent rectangular beam shape were obtained. The lifetime of the laser was tested as well. Introduction High power semiconductor lasers have been found increasing applications in pumping of solid state laser systems and fiber amplifiers, frequency doubling, medical systems and material processing, such as welding, cutting and surface treatment [1]. Although the output power of a single emitter has been improved significantly in recent years, the power is still generally limited to below 20 watt for a 9xxnm laser, which is unable to satisfy new applications with higher power demands obviously. Laser bar can provide a magnitude of output power of single emitter by integrating the multiple individual emitters. To further increase the output power, several packaging technologies have been developed, including multiple single-emitter modules, horizontal bar arrays, and area bar arrays [2]. Naturally, integration of single bar unit as a is another choice to increase the output power. The multiple bars are electrically connected in series and can be operated at continuous wave (CW) or quasicontinuous wave (QCW) mode. In this work, the traditional packaging structure has been optimized and improved. Based on the new packaging structure, a new 976 nm semiconductor laser with 2500 W in CW mode was fabricated and its performances were characterized. A series of techniques such as spectrum control and beam control was used to improve the performance of the. Structure design of single bar The performance and reliability of a semiconductor laser are greatly affected by its packaging structure and thermal properties. Since the is assembled by single bars, the structure of the single bar has a significant impact on the performance of the. Therefore, not only the structure of the but also that of the single bar should be considered. Fig. 1 The traditional (a) and double-sided (b) cooling package structure Fig. 1(a) shows a traditional packaging structure. The heat generated from the laser bar is one-sided conducted to heat sink with a low cooling efficiency. Fig. 1 (b) shows the double-sided cooling packaging structure we designed. The heat can be conducted through both the anode and cathode; therefore, the thermal dissipation efficiency is improved significantly. The transient thermal behavior of double-sided cooling packaging was studied using finite element analysis (FEA), as shown in Fig. 2. The simulation result shows that the heat is not only conducted to the heat sink from bottom directly, but also conducted by the cathode, which improves the cooling efficiency. Moreover, according the double-sided cooling packaging design, the heat dissipation efficiency from the cathode can be up to 20% th International Conference on Electronic Packaging Technology & High Density Packaging /10/$ IEEE 1350

2 Intensity/a.u Fig.2 Thermal flux vector graph of CW micro-channel water cooled semiconductor laser Performance of the single bar Fig. 3 shows the LIV curve of the single bar under the condition of CW current measure at room temperature. The output power of 150 W at a drive current of 144 A is obtained. Although the output power can be up to 150W, the single bar we used was operated at 100W in order to keep reliable performance of the device Time/h Fig. 5 Lifetime curve of a single bar 976 nm CW microchannel water cooled semiconductor Near-field beam can directly reflect the performance of each individual emitter. Therefore, checking the necrosis and the uniformity of emitters is useful for studying overall performance of the semiconductor lasers. Accurately testing equipments were used to test the near-field beam of the single bar. Fig. 6 shows the near-field beam spot of 976nm CW micro-channel water cooled single semiconductor laser bar. Obviously, the output light intensity of each emitter in the laser bar is uniform, which is contributed from the homogeneous distribution of drive current on individual emitter in the laser bar. Fig. 3 LIV curve of single bar 976nm CW Micro-channel water cooled semiconductor laser The Full Width at Half Maximum (FWHM) of the single bar we designed is only 2.79nm and 90% energy width is 4.07 nm after using the spectral control technology of the single bar, as shown in Fig. 4. Fig. 4 Spectrum character of single bar 976 nm CW Micro-channel water cooled semiconductor laser The constant power lifetime test (25 ) of single bars under output power of 100W at CW mode is shown in Fig. 5. The chart exhibits that the power degradation of most devices is less than 5% after working 2700 hours, which indicates a good reliability. The lifetime test is still ongoing. Fig. 6 Laser spot of 976nm CW micro-channel water cooled single semiconductor laser bar at near field The Near-field Non-linearity ( smile ) is another key parameter of laser array products and improving the Nearfield performance is especially important. The Near-field Nonlinearity of laser diode array is caused by the coefficient of thermal expansion (CTE) mismatch among the different layers of a bare bar, the packaging process and CTE mismatch between the laser bar and the bonding heat sink [3]. Using advanced packaging technology, a good Near-field linearity of the laser bar was obtained, as shown in Fig. 7. Fig. 7 The smile image of a typical good semiconductor laser bar th International Conference on Electronic Packaging Technology & High Density Packaging 1351

3 Temperature Rise/ Nearly 900 single bars were tested. It was found that about 99% Near-filed nonlinear were less than 1μm, as illustrated in Fig. 8. This result shows that the linearity of near-filed optical cavity of the laser bars and its uniformity are both good. Process Capability of SMILE 值 Process Data LSL * Target * USL 1 Sample Mean Sample N 898 StDev(Within) StDev(Overall) USL Within Overall Potential (Within) Capability Cp * CPL * CPU 1.05 Cpk 1.05 Overall Capability Pp * PPL * PPU 0.81 Ppk 0.81 Cpm * Observed Performance PPM > USL PPM Total Exp. Within Performance PPM > USL PPM Total Exp. Overall Performance PPM > USL PPM Total Fig. 8 Smile statistics of the single bar 976 nm CW micro-channel water cooled semiconductor laser Design of the laser In the design process of the laser, one of the main problems is the thermal crosstalk, which seriously affects the cooling efficiency. In order to avoid thermal crosstalk, a parallel format of liquid cooling is designed to overcome the heat unevenness between the bars, which can effectively improve the thermal dissipation. Fig. 9 shows the design of parallel format of liquid cooling Bar No. Fig. 10 The thermal simulation results of the 20 bar semiconductor laser Based on the thermal simulation, the structure is optimized from cooling water flow, micro-channel cooler selection, water distribution and other aspects. The heat can be taken away as quickly as possible by the cooling water, which can ensure that no thermal accumulation exists between the bars. Based on the double-sided cooling packaging technology and the parallel format of liquid cooled structure, a 2500W semiconductor laser with 25 bars was fabricated, as shown in Fig. 11. Fig. 9 the design of parallel format of liquid cooling A laser with 20 bars was simulated for the thermal design and the structure optimization, which can also be used in 25 bars laser design. The simulation results are shown in Fig. 10. Most of the heat is dissipated via the cooling flow liquid. There is no large accumulation of heat and the temperature gradient of each bar is uniform. The maximum temperature is 36.92, which has only difference with the water temperature in the entrance. Fig. 11 Semiconductor laser Fig. 12 shows the LIV curve of the 25 bar semiconductor laser at 976 nm under the condition of CW current measure at room temperature. The output power of 2500 W at a drive current of 99 A was obtained. The photoelectric conversion efficiency is more than 61% th International Conference on Electronic Packaging Technology & High Density Packaging 1352

4 Fig. 12 LIV curve of the 25 bar semiconductor laser The FWHM of the 25 bar semiconductor laser vertical stack is only 3.11nm, and 90% energy width is 4.15 nm, as shown in Fig. 13. used, such as: using a higher CTE matched materials as packaging materials, design of a suitable heat sink structure, application of new technologies such as the "void free" patch technology and vacuum solder reflow system [6-7]. Based on the spectral control technology, the FWHM of the single bar is only 2.79nm, as shown in Fig.4. Spectral width is also one of the key parameters of semiconductor laser products and it is very important to improve the spectral performance to improve production yield, reduce cost and gain competitiveness. Although the laser bars in the are cooled in parallel in both conduction cooled and micro-channel liquid cooled configurations, there remains temperature nonuniformity among the bars due to thermal crosstalk and/or liquid flow non-uniformity. This would alter the wavelength of the bars and broaden the spectrum of the stack, as shown in Fig.15. Fig.13. spectral result of 25 bar semiconductor laser The Near-field beam spot is shown in Fig. 14, which shows a good rectangular beam shape. Fig. 14 Laser beam of Discussion The major challenges in vertical bar stack packaging are the spectrum control and beam control [2]. 1. spectral control technology With the increasing demand on the high power laser performance, commercial semiconductor laser bar products are required to have narrow spectral width for applications. The challenge of spectral control of single bar is how to achieve the temperature uniformity and stress uniformity across the laser bar to eliminate the thermal effect and stress effect [4]. Local temperature rise is mainly caused by solder voids in bar bonding interface. So we used the advanced process which can minimize solder voids or even achieve void free bonding to reach temperature uniformity across the bar [5]. For the CTE mismatch problem, many methods were Fig.15 The broadened wavelength of semiconductor laser In this work, advanced packaging process was used to maintain temperature distribution uniformity. First, total temperature distribution was simulated and calculated, as shown in Fig. 10. Second, the wavelength of each chip was selected to match the temperature distribution based on the simulation results. The third step was using optimized packaging technology to achieve the same output wavelength. Using this method the spectrum broadening of can be controlled effectively, as shown in Fig Beam control technology For semiconductor laser, beam control includes beam size, intensity uniformity and pointing direction control [5]. Because the we designed is assembled by 25 single bars, the initial beam size is large. Therefore, beam shaping optical systems need to be designed and assembled to control beam size. First, fast-axis collimation components are added to keep each bar pointing direction consistent. And then using advanced real-time monitoring equipment, the position of each bar is fine-tuned from vertical and horizontal orientation to ensure accurate positioning th International Conference on Electronic Packaging Technology & High Density Packaging 1353

5 Semiconductor Laser Arrays, Chinese Journal of Lasers, No. 37 (2010), pp92. Fig. 16 The process of pointing direction control Near-field beam spot is shown in Fig. 14. It was found that the output beam spot of each bar was very uniform and the directivity was excellent. Conclusions In a summary, based on the double-sided cooling packaging technology and parallel format of liquid cooled structure, a 2500 W semiconductor laser with 25 bars was fabricated. A series of techniques such as spectrum control and beam control was developed. The FWHM is only 3.11nm and a rectangular beam shape was obtained. The lifetime testing result shows that the power degradation of most devices is less than 5% even after working 2700 hours. Acknowledgments This work was supported by the project of the Hundred Talents Research Fund of Chinese Academy of Sciences, the Instrument Developing Project of the Chinese Academy of Sciences, and the National High Technology Research and Development Program of China, Grant No. 2009AA References 1. Xingsheng Liu, Ronald W. et al, A study on the reliability of indium solder die bonding of high power semiconductor lasers, Journal of applied physics, No. 100 (2006), pp Xingsheng Liu, Martin H. Hu. et al, Thermal Management Strategies for High Power Semiconductor Pump Lasers, IEEE transactions on components and packaging technologies, No. 29 (2006). 3. Jingwei Wang, Zhenbang Yuan. et al, Study of the mechanism of "smile" in high power diode laser arrays and strategies in improving near-field linearity, IEEE Proceedings of 59th Electronic Components and Technology Conference (ECTC), 2009, pp Xingsheng Liu and Wei Zhao, Technology Trend and Challenges in High Power Semiconductor Laser Packaging, Electronic Components and Technology Conference, 2009, pp Xingsheng Liu, Kechang Song. et al, A Metallization Scheme for Junction-Down Bonding of High Power Semiconductor Lasers, IEEE Transactions on Advanced Packaging, Vol. 29, No. 3 (2006), pp Xingsheng Liu, Jingwei Wang. et al, Study of the Mechanisms of Spectral Broadening in High Power Semiconductor Laser Arrays, IEEE 58th Electronic Components and Technology Conference, 2008, pp Jingwei Wang, Zhenbang Yuan. et al, Study of the Mechanisms of Spectral Broadening in High Power th International Conference on Electronic Packaging Technology & High Density Packaging 1354