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The development trend and challenges of high-power semiconductor laser packaging technology

this paper summarizes the existing packaging technology of high-power semiconductor lasers (including single emitting cavity, bar, horizontal array and vertical stacked array), and discusses its development trend; The problems and challenges of semiconductor laser packaging technology are analyzed, and the methods and strategies to solve the problems and meet the challenges are given

high power semiconductor lasers and solid-state lasers pumped by them have the advantages of small volume, light weight, high photoelectric conversion efficiency, stable performance, high reliability, long service life and so on. They have become the most promising products in the photoelectric industry and are widely used in industry, military, medical treatment, direct material processing and other fields. The basic unit of high-power semiconductor lasers is a single emitting cavity or a single array (a single array is formed by linear arrangement of multiple single emitting cavities). Figure 1 and Figure 2 are the light emission diagrams of single emitting cavity semiconductor lasers and single array semiconductor lasers respectively

Figure 1: light emission diagram of single emitting cavity semiconductor laser

Figure 2: light emission diagram of high-power single array semiconductor laser

for semiconductor lasers, output power, conversion efficiency and reliability are the three main parameters to describe the performance of devices. With the maturity of chip preparation technology, the reduction of cost and the improvement of performance, semiconductor lasers have new development trends, mainly including high output power, high brightness, indium free packaging, narrow spectrum and low "smile" effect. Next, we will introduce the packaging technology and development trend of existing high-power semiconductor lasers, as well as its existing problems, challenges, corresponding solutions and countermeasures

high output power

many new applications require semiconductor lasers to have higher output power. There are two ways to increase the output power: 1. Improve the chip growth technology and increase the output power of single emitting cavity semiconductor lasers. 2. Increase the number of semiconductor laser light-emitting units in the array, so as to improve the output power. In order to further improve the optical output power, a variety of packaging technologies can be used, including multiple single tube modules, horizontal stacked arrays, vertical stacked arrays, and area arrays

single emitting cavity: the maximum optical output power of single emitting cavity semiconductor laser is limited by catastrophic optical cavity surface damage (comd) or thermal rollover phenomenon, and the relationship between its output power and these two parameters is shown in Figure 3. The main reason of comd is that the cavity surface is overheated due to the combination of light absorption and non radiation, which destroys the cavity. At present, some new technologies have been developed to overcome comd and improve the output power, such as cavity surface passivation, non absorption mirror and non pump window. Thermal rollover phenomenon is that the heat generated is higher than the heat that the refrigeration device can cool. Usually, a large amount of heat will be accumulated in the cavity at this time, which will significantly increase the temperature in the cavity. In order to avoid thermal rollover phenomenon, the thermal resistance of devices should be reduced as much as possible. Increasing the cavity length and the width of the emitting region can significantly reduce the thermal resistance, so the longer the cavity length of a single emitting cavity semiconductor laser, the higher the output power. With the improvement of comd and thermal rollover phenomena, the output power is 5 ~ 8W, the wavelength is 808nm, the output power is 8 ~ 12W, the wavelength is 9xxnm, and the width of the light-emitting region is 200 respectively μ M and 100 μ M single emitting cavity semiconductor lasers have been widely used

Figure 3: schematic diagram of the relationship between the optical output power and the driving current of a single emitting cavity semiconductor laser

single array: in order to increase the output power of the chip, the single emitting cavity is arranged into a one-dimensional linear array to form an array. This structure is usually called a bar, and its structure is shown in Figure 2. The most common bar packaging structures include conductive cooling CS packaging and microchannel liquid cooling packaging. Their structures are shown in Figure 4 (a) and (b) respectively. According to the filling factor and cavity length, the output power of the array semiconductor laser can be up to hundreds of Watts under continuous wave conditions. In order to ensure the reliability of commercial products, the filling factor of semiconductor lasers usually used in the market is 20% or 30%, the wavelength is 808nm, the output power is 60W, and the conduction cooling method is adopted; For single array semiconductor lasers with higher fill factor, the output power can be as high as 80 ~ 100W. For bars with an output power of 100W, liquid refrigeration is usually used. Figure 5 shows the power voltage current and spectral characteristic curves of commercial single array semiconductor lasers using conduction cooling and microchannel liquid cooling, respectively

Figure 4: physical diagram of array laser. (a) : single array conductive cooling package (b): single array microchannel liquid cooling package with alignment (right) and without alignment (left)

Figure 5: typical power voltage current versus spectral curves for commercial applications. (a) : conduction cooling type (b): liquid cooling type

the main problem of improving the output power of array semiconductor lasers is thermal management and thermal stress management [1]. Thermal management includes the design of heat dissipation system and "hole free" patch technology: for single array semiconductor lasers, due to the mutual interference of heat generated by each light-emitting unit of the array semiconductor lasers, as well as the uneven overall heat dissipation, the stability of device performance is reduced and the power rise is limited; If there are holes in the chip layer, it will significantly affect the performance of the array semiconductor laser, including output power and reliability. Although many heat dissipation methods have been proposed for thermal management, such as diamond conduction heat dissipation and microchannel heat dissipation technology, how to improve the heat dissipation efficiency is still the main factor that hinders the high power output of array semiconductor lasers. There are two methods to reduce the cavity in the patch layer: one is to use the patch technology under the reasonable control of ambient temperature and pressure; Another method is vacuum reflux technology. Thermal stress is usually caused by the mismatch of the coefficient of thermal expansion (CTE) between the array laser and the substrate. Thermal stress not only limits the choice of substrate material/heat sink for packaging, but also affects the reliability, spectral width and "smile" effect of semiconductor laser bars. In order to reduce thermal stress, substrate/heat sink materials with high thermal conductivity and thermal expansion coefficient are being developed

multi single tube module: Although the output power of single emitting cavity laser has increased in recent years, its output power is still low on the whole. The combination of multiple single tubes is another way to increase the output power. Figure 6 shows the schematic diagram of the multi spool module. In the figure, each independent light-emitting unit is connected in series, and the output beam of each single light-emitting unit of the module is converged through the optical system and coupled into the optical fiber for output. Using multiple single tube modules, there is no thermal interference between each light-emitting unit, and the output power of each light-emitting unit does not affect each other. However, when optical system is used for beam convergence and fiber coupling, there is optical energy loss. The output power of multiple single tube modules on the market has reached tens of watts or even hundreds of watts

Figure 6: schematic diagram of multiple single tube combination mode

due to the limited output power of a single emitting cavity and the complex beam convergence system, the main problem faced by the multi single tube module is how to increase the output power to hundreds of watts. Because there are more and more single emitting cavities in the module, which are equipped with different fixtures, the design of optical system is more complex, and the cost of micro optical system is higher, which leads to the lack of practical significance of this method. Another challenge of multi single tube module is how to match the output wavelength of each emitting cavity. Each light-emitting unit in the module must undergo wavelength matching screening to ensure that the module has a narrow spectrum

horizontal array: for specific applications, such as side pumped solid-state lasers, higher optical output power is required, but beam convergence is not required. Therefore, packaging multiple bars into a horizontal array can meet the requirements. Figure 7 is the physical diagram of two types of horizontal arrays. Figure 7 (a) is a horizontal array packaged by three bars connected horizontally in series. The three bars are independent of each other and packaged with heat conduction refrigeration and electrical insulation materials; In Figure 7 (b), the bars are also connected in series, but each bar is cooled by microchannel liquid. The output power of the horizontal array ranges from tens of watts to hundreds of watts or even up to kilowatts, depending on the number of bars encapsulated and the power range of a single bar. Figure 8 shows the output power current and spectral characteristic curves of the quasi continuous output horizontal array

Figure 7: physical diagram of horizontal array. (a) : 1x3 (b): 1x4

figure 8: optical power current and spectral curve of the horizontal array under quasi continuous conditions

as shown in Figure 7 (a), the bar of the horizontal array is insulated from the cooler, and industrial water can be used as the cooling medium; In addition, because each bar is insulated from the cooler, the heat generated between the bars affects each other, and the heat generated by the front-end bar will be transferred to the rear-end bar, causing the junction temperature of the rear-end bar to rise, causing the temperature of each bar to be inconsistent, resulting in reduced reliability of the laser, wavelength drift and spectral broadening. Therefore, the horizontal array packaging structure shown in Figure 7 (a) is limited by the number of packaging bars, and the total output power is also limited

vertical stack array: to obtain high output power, vertical stack array becomes the preferred structure. Figure 9 (a) shows a typical quasi continuous output conduction cooled g-stack semiconductor laser, and Figure 9 (b) shows a microchannel liquid cooled vertical stacked semiconductor laser. Both arrays are connected in series. As shown in Figure 9 (a), each bar adopts conduction cooling. Figure 9 (b) shows the microchannel liquid refrigeration with each bar independent of each other. Taking g-stack as an example, due to the limitation of heat dissipation capacity, this structure can only be applied to quasi continuous output, and the choice of duty cycle is related to the thickness of heat sink between bars. At present, the maximum output power of the commercialized single bar can reach 250W, and a g-stack product can package 20 bars at the same time. The output power of each bar of the vertical stacked array shown in Figure 9 (b) can be as high as 300W, and the stacked array can realize the packaging of 30 bars. Among them, the output power of 20 bar vertical stacked arrays is 2kW under continuous conditions and 5kW under quasi continuous conditions

Figure 9: physical diagram of vertical stacked array. (a) : conduction cooled g-stack (b) for quasi continuous applications: microchannel liquid refrigeration stack with and without collimation system (right figure) and (left figure)

the main technical challenge of vertical stacked array packaging is the control of beam and spectrum. The thermal interference between the bars of the vertical stacked semiconductor laser, and the uneven water flow will lead to the uneven distribution of the cooling temperature of the bars, which will lead to the wavelength shift of the bars and the spectral broadening of the stacked array. Beam control includes output spot size control, light intensity density uniformity control and beam transmission direction control, so it is necessary to design and install beam shaping system to realize beam control. Figure 10 shows the square light spot and light intensity distribution of the shaped vertical stacked array cooled by microchannel liquid

Figure 10: square light spot and light intensity distribution of the shaped vertical array of microchannel liquid refrigeration

high brightness

for most applications, whether it is multi single tube module, bar, stacked array or area array structure, the spot size of the output beam is required to be small. Beam brightness is a parameter characterizing beam quality, which is defined as the laser source in unit area or single

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