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Issue #6/2017
D.Hou, J.Wang, X. Li, L.Gao, W.Cai, H.Zhang, C.Ma, Y.Shang, H.Zhang, Y.Fan, X.Liang, X.Liu
Advancements in Packaging of Kilo-watts Level High Power Diode Lasers for DPSSLs Applications. Part 2.
Advancements in Packaging of Kilo-watts Level High Power Diode Lasers for DPSSLs Applications. Part 2.
The second part of the article presents the latest advances in the packaging of high-power diode laser arrays. The key factors in the development of arrays, such as temperature control, failure analysis, and reliability assessment are considered.
Теги: high-power diode laser arrays packaging technology of laser arrays диодные лазерные решетки технология компоновки лазерных стеков
ADVANCES IN KILO-WATTS LEVEL HIGH POWER DIODE LASERS
With the advances in diode laser bar technologies and the development of packaging technologies, the performances of high power diode laser pumping sources have been improved significantly over the past years. The evolution of the output power of diode laser bar is shown in Figure 13 and Figure 14. For reliable operation in mass production from commercial products, the CW output power of an 808nm single bar is up to 200W, and QCW output power of a single bar is up to 500W [24]. The output power reaches 250W in CW mode and 600W in QCW mode in engineering level. In order to further scale the output power, different packaging technologies and structures is developed to achieve KW-watts and even ten KW-watts level high power diode lasers, including horizontal array, liquid cooled diode laser stack and conduction cooled diode laser stack etc.
Single bar package structure
Based on the micro-channel liquid cooler, diode laser array can be constructed with four important parts: MCC, a laser chip, a cathode plate, and an insulator as shown in Figure 15 [13]. The heat is mainly dissipated through the MCC cooler. In order to improve the performance of the MCC laser array, it’s critical to optimize the MCC structure with sophisticated structure design.
Figure 16 shows a typical PVI and spectrum data of a liquid cooled single bar diode laser packaged with improved MCC structure in CW and QCW mode. Figure 16 (a) shows the output power of 200W at 199.8A in CW mode. For QCW mode, the output peak power of 500W at 407A under the condition of 8% duty cycle (200us, 400Hz) was obtained, as shown in Figure 16 (b).
The maximum output power of the MCC diode laser bar is limited either by thermal rollover or catastrophic optical mirror damage (COMD) [25]. Reliable operation of the laser device highly depends on high thermal rollover and COMD value. The maximum QCW power of the MCC diode laser bar is tested with the 940nm wafer under the temperature of 20 °С as shown in Figure 17. The output power reaches 1088W/bar before thermal rollover in the testing condition of 600us, 133Hz, 1000A. The maximum wall plug efficiency is about 75% in the current of 130A. The wall plug efficiency slightly decreases from 75% to 58% when the current increases from 130A to 1000A.
The lifetime data of typical 808nm 500W single bar hard solder bonded diode lasers were also characterized, as shown in Figure 18. It can be seen that there is no power degradation for 500W hard solder MCC single bar after running 1.38Ч109shots. The performance of the hard solder MCC single bar with AuSn bonding technology is stable and reliable.
The non-linearity of the near-field of emitters (or the so called "smile") in a laser diode array poses significant challenges in optical coupling and beam shaping and has become one of the major roadblocks in broader applications of laser arrays. If near field linearity of a laser array is poor, the coupling efficiency of the laser array to a fiber array or micro-optics such as a fast axis collimation lens is very low. Increasing the near-field linearity of a pumping laser diode array enables the laser system manufacturer to improve the laser system compactness, optical coupling efficiency, power, and beam quality while at the same time reducing manufacture cost in the laser system, such as diode pumped solid state lasers and fiber lasers. Therefore, the near-field linearity of a laser bar is one of the key specifications of laser array products and improving the near field performance is especially important in order to increase production yield, reduce cost and gain competitiveness [26].
The enlarged "smile" images of a typical diode-laser array is shown in Figure 19. It was found that when there was a better CTE match between submount and the laser bar, "smile" was decreased significantly. For MCC-packaged high power diode laser arrays, the statistics of smile data of 898 samples were conducted. It is found that the average smile value is 0.52µm. The result shows that high power diode laser arrays using hard soldering MCC-packaged technology have acceptable smile.
Horizontal arrays
For certain applications, such as side pumping of a solid state laser, when higher optical power are required, an array of laser bars are packaged horizontally. The industrial water can be directly used, as the diode laser stacks are electrically isolated from the cooling plate. For the horizontal bar array of 3x10 package structure illustrated in Figure 20, the diode laser is used in QCW mode because of compact design. This array consists of ten 3-bars modules soldered on a macro channel cooler with the expected optical output peak power of 6000W. The CTE-matched submounts are applied for bonding the laser diode bar with hard solder to achieve higher reliability [27].
The numerical simulation of a 30 bars horizontal array of diode laser stacks were conducted using finite element analysis (FEA) as shown in Figure 21. In the simulation, the horizontal array operates in the condition of 6.6% duty cycle (200µ, 330Hz) in QCW mode with the output peak power of 6000W. The temperature of cooling water is 25 °С and the water flow rate is 2L /min. Figure 22 illustrates the temperature distribution of the 3-bars stacks along the cooling plate from the inlet to outlet. It can be seen that the temperature distribution is not uniform.
The scatter wavelengths of different stacks lead to wide spectrum in Figure 23 (a), while the concentrate wavelengths lead to narrow spectrum in Figure 23 (b). In order to achieve narrow spectrum and accurate central wavelength of horizontal array, accurate spectrum control is required for the 3-bars stacks on the ten positions. Typically for GaAs-based 808nm diode laser devices, the wavelength temperature coefficient is 0.28nm/K, which indicates the temperature change of 1K results in 0.28nm change of wavelength. The distribution of selected and output wavelength of the 3-bars stacks are illustrated in Figure 22. The uniform output wavelength is obtained along the cooling plate. The central wavelength is well controlled to 800nm based on the simulation.
Figure 24 shows the example of manufactured horizontal bar arrays. The LIV curves can be obtained by the measurement of output light power or voltage as a function of the driving current. The graphs of power versus input current and corresponding spectrum of the diode laser stack are illustrated in Figure 25. Based on the hard solder packaging technology and the spectrum control method the optical output power reaches as much as 6000W in QCW mode. The spectrum value of full width of half maximum (FWHM) and full width of 90% energy (FW90%E) is 2.36nm and 3.37nm respectively, which is rather small for a 30 bars horizontal array of diode laser stacks. Conventionally the value of FWHM is about 3nm and 90% energy is about 5nm.
Liquid cooled vertical stacks
Vertical stacks offer power scaling by integrating numerous bars vertically. The beam quality of vertical stack lasers along slow axis keeps the same as a single laser bar. With the improvement of the packaging technology, the numbers of laser bars in a vertical stack laser can be up to 70–80 bars, and the maximum output power of a vertical stack laser is from several hundred watts to up to tens of thousands watts [13].
The liquid cooled vertical-stack (V-stack) diode laser can be packaged with MCC diode lasers. Figure 26 (a) show the packaging structures of the V-stack structure with 15 MCC diode lasers. The important constituent parts, i. e. the cathode, the anode, the coolant inlet and outlet, and the packaged bars, are noted in figures. Due to the good heat dissipation capability of the precisely designed MCC, V-Stack laser is able to operate in CW mode and QCW mode with high duty cycle.
Figure 26 (b) shows a typical liquid cooled MCC package vertical stack with collimating lens for QCW application. This commercial product can deliver totally 18KW power with 60 bars packaged in vertical stack as shown in Figure 27 (a). Each diode laser bar obtains the output power of over 300W. Figure 27 (b) illustrates the advanced result of another sophisticated packaged V-stack structure. This product is able to work in the total output power of 13KW with 26 packaged MCC diode laser bars. Each bar can deliver the output power of 506W.
The major challenges in vertical bar stack packaging are the spectrum control and beam control. Although the laser bars in the vertical stack are cooled in micro-channel liquid cooled configuration, there remains temperature non-uniformity among the bars due to thermal crosstalk and/or liquid flow non-uniformity. The screening of the diode laser bars are required to achieve specific wavelength and uniform spectrum.
Conduction cooled stacks
The packaging structure of a conduction cooled stack (G-stack) diode laser is shown in Figure 28. In the figure, three diode laser bars are packaged in one G-stack laser, and the cathode and anode of the laser stack are on the left and right side, respectively [5]. The heat conduction cooler is under the laser bars, and the layer between laser bars and conduction cooler is the insulator layer. There are two insulator layers used for separating cathode and cooler, anode and cooler, respectively. There are four fixing holes in the stack, and the functions of them are to fix the stack and to connect the anode and cathode of the stack laser to the power driver, respectively.
For conduction cooled G-stack structure, e. g. the diode laser bars are sandwiched in the CTE-matched submounts in Figure 29 (a) [14]. An example of the numerical simulation of the thermal behavior characteristics of a 5-bar diode laser stack with different pitches were presented, as shown in Figure 29 (b). The laser stack is simulated with the pulse duration of 30ms, the frequency of 3Hz and optical output power of 100W/bar in QCW mode. The pitch of the laser stack ranges from 0.4mm to 4.0mm under the heatsink temperature of 20 °С. The simulation results indicate that the peak temperature decreases rapidly at the beginning when the pitch of the diode laser stack increases. The pitch can be chosen as the optimized pitch value for both good thermal dissipation and compact device structure.
The output power of the diode laser stack fabricated by multiple laser bars can reach several thousand watts CW power and tens of Watts QCW power. Figure 30 (a) and (b) shows a typical conduction cooled G-Stack module with collimating lens of 30 bars, and a 5-bars module without collimating lens. The diode bars are electrically connected in series and isolated from the heatsink by a thermally conductive and electrically insulating material. For a G-Stack, it can be only used for QCW application due to its poor cooling approach. The upper limit of the duty cycle is related to the pitch of the bars.
As shown in Figure 31, the 30-bars module can deliver 4800W output power in the testing condition of 250us, 40Hz and the temperature of 60 °С.The 5-bars module is developed with the advanced packaging technologies and material [12][28], such as the application of the CuC with high thermal conductivity and CTE-matched property. The 5-bars module is able to work in high duty cycle (200us, 400Hz, 8% duty cycle), and delivers the total output power of 2736W, with each bar delivering the output power of 557W.
The maximum output power of the 5-bars module reaches the value of 3655W with the single bar of 731W in Figure 32. The conventional G-Stack module with the same structure dimension and testing condition, can only reach the power of 400W per bar. The application of the new packaging materials and advanced packaging technologies significantly improve the performance of G-Stack products.
The area array (AA) diode laser is packaged through the G-stack modules, which scales the output power of diode laser bars from KW-watts level to tens of KW-watts level. Sophisticated cooling plated with uniform cooling tunnels and low water pressure drop from the inlet to outlet, is designed to construct the G-stack modules to a area array. Figure 33 (a) shows a 96 bars area of array observed by the near-field illumination. The graphs of power versus input current and corresponding spectrum of the diode laser stack are illustrated in Figure 33 (b). Based on the hard solder packaging technology and the wavelength screening of the diode laser bar, the optical output power reaches as much as 19KW in QCW mode operation under the test condition of 5% duty cycle. The spectrum value of full width of half maximum (FWHM) and full width of 90% energy (FW90%E) is 2.44nm and 4.14nm respectively, which is rather small for a 96 bars area of array of diode laser stacks. Conventionally the value of FWHM and 90% energy is about 4nm and 6nm respectively. The result indicates that the spectrum deviation of this laser device is very small.
SUMMARY AND OUTLOOK
In conclusion, the packaging technologies for high power diode laser bars are reviewed and discussed. The key factors, such as thermal management, thermal stress analysis, processes development, failure analysis and reliability evaluation are presented in developing kW-level diode lasers. The dominant high power diode laser bar bonding technology used in commercial products are still indium bonding and AuSn bonding. To improve reliability and lifetime, especially under harsh conditions and for long pulse on-off power cycling applications, the development trend is that indium solder bonding technology is being replaced by AuSn solder bonding technology. The application of advanced packaging materials (e. g. CuC), and sophisticated design of cooling plate (e. g.MCC), can significantly improve the performance of the diode laser stack. The precise control of wavelength of diode laser bars greatly improves the spectrum with accurate central wavelength and narrow spectrum. A variety of high power diode laser arrays are fabricated and tested, including conductively cooled diode laser stacks, micro-channel water cooled diode laser stacks, and horizontal diode laser arrays. The maximum power of the diode laser stack packaged with the micro-channel cooler reaches the value of over 1000W per bar, while the G-stack diode laser stack reaches the value of 731W per bar. The area of array diode laser scales the output power of de diode laser bars from KW-watts level to tens of KW-watts level, with the delivered value of 19KW. Along with diode laser bare bar improvement, new packaging material, advanced and novel packaging technology need to be developed to enhance high power diode laser bar device performances to achieve higher power, higher brightness and higher reliability.
With the advances in diode laser bar technologies and the development of packaging technologies, the performances of high power diode laser pumping sources have been improved significantly over the past years. The evolution of the output power of diode laser bar is shown in Figure 13 and Figure 14. For reliable operation in mass production from commercial products, the CW output power of an 808nm single bar is up to 200W, and QCW output power of a single bar is up to 500W [24]. The output power reaches 250W in CW mode and 600W in QCW mode in engineering level. In order to further scale the output power, different packaging technologies and structures is developed to achieve KW-watts and even ten KW-watts level high power diode lasers, including horizontal array, liquid cooled diode laser stack and conduction cooled diode laser stack etc.
Single bar package structure
Based on the micro-channel liquid cooler, diode laser array can be constructed with four important parts: MCC, a laser chip, a cathode plate, and an insulator as shown in Figure 15 [13]. The heat is mainly dissipated through the MCC cooler. In order to improve the performance of the MCC laser array, it’s critical to optimize the MCC structure with sophisticated structure design.
Figure 16 shows a typical PVI and spectrum data of a liquid cooled single bar diode laser packaged with improved MCC structure in CW and QCW mode. Figure 16 (a) shows the output power of 200W at 199.8A in CW mode. For QCW mode, the output peak power of 500W at 407A under the condition of 8% duty cycle (200us, 400Hz) was obtained, as shown in Figure 16 (b).
The maximum output power of the MCC diode laser bar is limited either by thermal rollover or catastrophic optical mirror damage (COMD) [25]. Reliable operation of the laser device highly depends on high thermal rollover and COMD value. The maximum QCW power of the MCC diode laser bar is tested with the 940nm wafer under the temperature of 20 °С as shown in Figure 17. The output power reaches 1088W/bar before thermal rollover in the testing condition of 600us, 133Hz, 1000A. The maximum wall plug efficiency is about 75% in the current of 130A. The wall plug efficiency slightly decreases from 75% to 58% when the current increases from 130A to 1000A.
The lifetime data of typical 808nm 500W single bar hard solder bonded diode lasers were also characterized, as shown in Figure 18. It can be seen that there is no power degradation for 500W hard solder MCC single bar after running 1.38Ч109shots. The performance of the hard solder MCC single bar with AuSn bonding technology is stable and reliable.
The non-linearity of the near-field of emitters (or the so called "smile") in a laser diode array poses significant challenges in optical coupling and beam shaping and has become one of the major roadblocks in broader applications of laser arrays. If near field linearity of a laser array is poor, the coupling efficiency of the laser array to a fiber array or micro-optics such as a fast axis collimation lens is very low. Increasing the near-field linearity of a pumping laser diode array enables the laser system manufacturer to improve the laser system compactness, optical coupling efficiency, power, and beam quality while at the same time reducing manufacture cost in the laser system, such as diode pumped solid state lasers and fiber lasers. Therefore, the near-field linearity of a laser bar is one of the key specifications of laser array products and improving the near field performance is especially important in order to increase production yield, reduce cost and gain competitiveness [26].
The enlarged "smile" images of a typical diode-laser array is shown in Figure 19. It was found that when there was a better CTE match between submount and the laser bar, "smile" was decreased significantly. For MCC-packaged high power diode laser arrays, the statistics of smile data of 898 samples were conducted. It is found that the average smile value is 0.52µm. The result shows that high power diode laser arrays using hard soldering MCC-packaged technology have acceptable smile.
Horizontal arrays
For certain applications, such as side pumping of a solid state laser, when higher optical power are required, an array of laser bars are packaged horizontally. The industrial water can be directly used, as the diode laser stacks are electrically isolated from the cooling plate. For the horizontal bar array of 3x10 package structure illustrated in Figure 20, the diode laser is used in QCW mode because of compact design. This array consists of ten 3-bars modules soldered on a macro channel cooler with the expected optical output peak power of 6000W. The CTE-matched submounts are applied for bonding the laser diode bar with hard solder to achieve higher reliability [27].
The numerical simulation of a 30 bars horizontal array of diode laser stacks were conducted using finite element analysis (FEA) as shown in Figure 21. In the simulation, the horizontal array operates in the condition of 6.6% duty cycle (200µ, 330Hz) in QCW mode with the output peak power of 6000W. The temperature of cooling water is 25 °С and the water flow rate is 2L /min. Figure 22 illustrates the temperature distribution of the 3-bars stacks along the cooling plate from the inlet to outlet. It can be seen that the temperature distribution is not uniform.
The scatter wavelengths of different stacks lead to wide spectrum in Figure 23 (a), while the concentrate wavelengths lead to narrow spectrum in Figure 23 (b). In order to achieve narrow spectrum and accurate central wavelength of horizontal array, accurate spectrum control is required for the 3-bars stacks on the ten positions. Typically for GaAs-based 808nm diode laser devices, the wavelength temperature coefficient is 0.28nm/K, which indicates the temperature change of 1K results in 0.28nm change of wavelength. The distribution of selected and output wavelength of the 3-bars stacks are illustrated in Figure 22. The uniform output wavelength is obtained along the cooling plate. The central wavelength is well controlled to 800nm based on the simulation.
Figure 24 shows the example of manufactured horizontal bar arrays. The LIV curves can be obtained by the measurement of output light power or voltage as a function of the driving current. The graphs of power versus input current and corresponding spectrum of the diode laser stack are illustrated in Figure 25. Based on the hard solder packaging technology and the spectrum control method the optical output power reaches as much as 6000W in QCW mode. The spectrum value of full width of half maximum (FWHM) and full width of 90% energy (FW90%E) is 2.36nm and 3.37nm respectively, which is rather small for a 30 bars horizontal array of diode laser stacks. Conventionally the value of FWHM is about 3nm and 90% energy is about 5nm.
Liquid cooled vertical stacks
Vertical stacks offer power scaling by integrating numerous bars vertically. The beam quality of vertical stack lasers along slow axis keeps the same as a single laser bar. With the improvement of the packaging technology, the numbers of laser bars in a vertical stack laser can be up to 70–80 bars, and the maximum output power of a vertical stack laser is from several hundred watts to up to tens of thousands watts [13].
The liquid cooled vertical-stack (V-stack) diode laser can be packaged with MCC diode lasers. Figure 26 (a) show the packaging structures of the V-stack structure with 15 MCC diode lasers. The important constituent parts, i. e. the cathode, the anode, the coolant inlet and outlet, and the packaged bars, are noted in figures. Due to the good heat dissipation capability of the precisely designed MCC, V-Stack laser is able to operate in CW mode and QCW mode with high duty cycle.
Figure 26 (b) shows a typical liquid cooled MCC package vertical stack with collimating lens for QCW application. This commercial product can deliver totally 18KW power with 60 bars packaged in vertical stack as shown in Figure 27 (a). Each diode laser bar obtains the output power of over 300W. Figure 27 (b) illustrates the advanced result of another sophisticated packaged V-stack structure. This product is able to work in the total output power of 13KW with 26 packaged MCC diode laser bars. Each bar can deliver the output power of 506W.
The major challenges in vertical bar stack packaging are the spectrum control and beam control. Although the laser bars in the vertical stack are cooled in micro-channel liquid cooled configuration, there remains temperature non-uniformity among the bars due to thermal crosstalk and/or liquid flow non-uniformity. The screening of the diode laser bars are required to achieve specific wavelength and uniform spectrum.
Conduction cooled stacks
The packaging structure of a conduction cooled stack (G-stack) diode laser is shown in Figure 28. In the figure, three diode laser bars are packaged in one G-stack laser, and the cathode and anode of the laser stack are on the left and right side, respectively [5]. The heat conduction cooler is under the laser bars, and the layer between laser bars and conduction cooler is the insulator layer. There are two insulator layers used for separating cathode and cooler, anode and cooler, respectively. There are four fixing holes in the stack, and the functions of them are to fix the stack and to connect the anode and cathode of the stack laser to the power driver, respectively.
For conduction cooled G-stack structure, e. g. the diode laser bars are sandwiched in the CTE-matched submounts in Figure 29 (a) [14]. An example of the numerical simulation of the thermal behavior characteristics of a 5-bar diode laser stack with different pitches were presented, as shown in Figure 29 (b). The laser stack is simulated with the pulse duration of 30ms, the frequency of 3Hz and optical output power of 100W/bar in QCW mode. The pitch of the laser stack ranges from 0.4mm to 4.0mm under the heatsink temperature of 20 °С. The simulation results indicate that the peak temperature decreases rapidly at the beginning when the pitch of the diode laser stack increases. The pitch can be chosen as the optimized pitch value for both good thermal dissipation and compact device structure.
The output power of the diode laser stack fabricated by multiple laser bars can reach several thousand watts CW power and tens of Watts QCW power. Figure 30 (a) and (b) shows a typical conduction cooled G-Stack module with collimating lens of 30 bars, and a 5-bars module without collimating lens. The diode bars are electrically connected in series and isolated from the heatsink by a thermally conductive and electrically insulating material. For a G-Stack, it can be only used for QCW application due to its poor cooling approach. The upper limit of the duty cycle is related to the pitch of the bars.
As shown in Figure 31, the 30-bars module can deliver 4800W output power in the testing condition of 250us, 40Hz and the temperature of 60 °С.The 5-bars module is developed with the advanced packaging technologies and material [12][28], such as the application of the CuC with high thermal conductivity and CTE-matched property. The 5-bars module is able to work in high duty cycle (200us, 400Hz, 8% duty cycle), and delivers the total output power of 2736W, with each bar delivering the output power of 557W.
The maximum output power of the 5-bars module reaches the value of 3655W with the single bar of 731W in Figure 32. The conventional G-Stack module with the same structure dimension and testing condition, can only reach the power of 400W per bar. The application of the new packaging materials and advanced packaging technologies significantly improve the performance of G-Stack products.
The area array (AA) diode laser is packaged through the G-stack modules, which scales the output power of diode laser bars from KW-watts level to tens of KW-watts level. Sophisticated cooling plated with uniform cooling tunnels and low water pressure drop from the inlet to outlet, is designed to construct the G-stack modules to a area array. Figure 33 (a) shows a 96 bars area of array observed by the near-field illumination. The graphs of power versus input current and corresponding spectrum of the diode laser stack are illustrated in Figure 33 (b). Based on the hard solder packaging technology and the wavelength screening of the diode laser bar, the optical output power reaches as much as 19KW in QCW mode operation under the test condition of 5% duty cycle. The spectrum value of full width of half maximum (FWHM) and full width of 90% energy (FW90%E) is 2.44nm and 4.14nm respectively, which is rather small for a 96 bars area of array of diode laser stacks. Conventionally the value of FWHM and 90% energy is about 4nm and 6nm respectively. The result indicates that the spectrum deviation of this laser device is very small.
SUMMARY AND OUTLOOK
In conclusion, the packaging technologies for high power diode laser bars are reviewed and discussed. The key factors, such as thermal management, thermal stress analysis, processes development, failure analysis and reliability evaluation are presented in developing kW-level diode lasers. The dominant high power diode laser bar bonding technology used in commercial products are still indium bonding and AuSn bonding. To improve reliability and lifetime, especially under harsh conditions and for long pulse on-off power cycling applications, the development trend is that indium solder bonding technology is being replaced by AuSn solder bonding technology. The application of advanced packaging materials (e. g. CuC), and sophisticated design of cooling plate (e. g.MCC), can significantly improve the performance of the diode laser stack. The precise control of wavelength of diode laser bars greatly improves the spectrum with accurate central wavelength and narrow spectrum. A variety of high power diode laser arrays are fabricated and tested, including conductively cooled diode laser stacks, micro-channel water cooled diode laser stacks, and horizontal diode laser arrays. The maximum power of the diode laser stack packaged with the micro-channel cooler reaches the value of over 1000W per bar, while the G-stack diode laser stack reaches the value of 731W per bar. The area of array diode laser scales the output power of de diode laser bars from KW-watts level to tens of KW-watts level, with the delivered value of 19KW. Along with diode laser bare bar improvement, new packaging material, advanced and novel packaging technology need to be developed to enhance high power diode laser bar device performances to achieve higher power, higher brightness and higher reliability.
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