中国激光, 2024, 51 (7): 0701017, 网络出版: 2024-03-29  

高效率高功率976 nm半导体激光芯片设计与制备

Design and Fabrication of High‑Efficiency and High‑Power 976 nm Semiconductor Laser Chips
作者单位
1 广东先导院科技有限公司,广东 广州 510535
2 度亘核芯光电技术(苏州)有限公司,江苏 苏州 215124
摘要
976 nm高功率半导体激光芯片是光纤激光器的核心部件,具有极为重要的产业价值。报道了课题组在高效率高功率半导体激光芯片的设计、制作与测试方面的研究成果。为了最大限度地提高器件的功率转换效率,同时满足苛刻的寿命要求,在设计上采用双非对称大光腔波导结构,同时对量子阱结构、波导结构、掺杂以及器件结构进行了优化;在外延生长方面,系统地优化了生长工艺参数,确保了外延材料具有极高的内量子效率及低内损耗。大量测试表明:所制作的器件(腔长为5 mm、发光条宽为200 μm的芯片)在室温、连续波(CW)测试条件下,阈值电流约为1 A,斜率效率为1.14 W/A;当电流为9 A时,最高功率转换效率高达72.4%;当电流为30 A时,输出功率达到29.4 W,功率转换效率为61.3%;对应于95%光场能量的水平远场发散角低至8.7°。上述参数性能已经达到了国际同类产品的先进水平。
Abstract
Objective

High-power semiconductor laser diodes emitting at approximately 976 nm are in high demand in Yb-doped fiber lasers (YDFL) because YDFLs exhibit strong absorption peaks at approximately 976 nm. Specifically, the absorption cross section is as strong as three times that at a wavelength of approximately 915 nm. Thus, by using lasers with an emission wavelength of approximately 976 nm for optical pumping, the length of the active fiber can be significantly shortened, leading to cost savings and reduced nonlinear effects. With the ongoing advancements in various industrial applications, the emitting power of high-power lasers increases from 12 W (about 10 years ago) to approximately 30 W. Beyond the high-power requirement, one of the most sought-after features of high-power lasers is their power conversion efficiency (PCE). A superior PCE results in higher optical power emission, enhanced reliability, and reduced system costs.

In this study, we demonstrate the design and fabrication of high-efficiency, high-power 976-nm lasers. The tests on our fabricated devices show that, at room temperature and under the continuous-wave (CW) operation condition, the power conversion efficiency (PCE) reaches as high as 72.4% when the injection current is 10 A. However, the efficiency decreases to 61.3% when the current rises to 30 A, at which the operating power is 29.4 W.

Methods

The epitaxy material is grown using the metal-organic chemical vapor deposition (MOCVD) method, and the structure contains an 8-nm-thick In0.175GaAs single quantum well (SQW) sandwiched between two separate confinement heterostructure (SCH) Al0.17GaAs layers. A wide optical cavity waveguide design is employed to reduce power density and cavity loss. The total thickness of the SCH layers is 1.5 μm. N-and P-doping are optimized to ensure the lasers produce the highest PCE when the emitting power is 25 W. Figure 1 shows the refractive index profile of our material structure, and Table 1 lists the detailed material structure.

After designing the material structure, we proceed with the optimization process to determine the optimum cavity length and anti-reflection (AR) reflectivity. In our optimization, we assume the aperture width is 200 μm, and the laser chip is mounted onto 350-μm-thick AlN ceramics in a P-side down manner, which, in turn, is mounted onto a copper block. Figure 3 displays the dependence of the laser parameters on the cavity length and the AR reflectivity. Based on this, it is clear that when the cavity length is in the range of 4?5 mm, a longer cavity yields better efficiency because the long cavity has a smaller series resistance and thermal impedance. Based on the theoretical simulation, we choose the cavity to be 5-mm-long, and the AR reflectivity is approximately 1%.

We conduct several material growth iteration processes to investigate the effect of growth conditions on device performance. Figure 4 shows the test results before growth optimization. In the figure, it can be observed that when tested at ambient temperature of 25 ℃ under the CW condition, the chip on sub-mount (COS) threshold current is approximately 1.2 A, slope efficiency is approximately 1.13 W/A, maximum PCE is 70.8% (corresponding optical power is 8.2 W). To further improve the device performance, we conduct growth optimization by optimizing the Al mole fractions for different layers in the structure, doping, and layer thickness.

Results and Discussions

The test results for the devices before and after growth optimization are shown in Fig. 6. Figure 5 shows the light-current curves and power conversion efficiencies under different test temperatures for chips with growth optimization. The highest PCE is 72.4% at a current of approximately 10 A. The efficiency reduces to 61.3% when the current reaches 30 A (the corresponding optical power is 29.4 W). Our devices appear to perform even better than those published in the literature. Figures 8 and 9 show the energy ratios under different slow-axis far-field divergence angles and lasing spectrum, respectively. In Fig. 8, it can be observed that more than 95% of the optical power is within horizontal far-field divergence angle of 9°, indicating good beam quality. The reliability, which is of critical importance for real-world applications, is also carefully evaluated by placing a number of devices in an accelerated lifetime test (Fig.10). Based on the lifetime test, there is no observable power degradation after 2000 h accelerated testing at an elevated temperature of 45 ℃.

Conclusions

In summary, we demonstrate the design and fabrication of high-efficiency, high-power 976-nm lasers. These devices are reliable and efficient. The PCE is as high as 72.4% at a current of 10 A; however, the efficiency decreases to 61.3% when the current reaches 30 A.

付鹏, 张艳春, 赵涛, 赵勇明, 唐松, 李颖, 韩沈丹. 高效率高功率976 nm半导体激光芯片设计与制备[J]. 中国激光, 2024, 51(7): 0701017. Peng Fu, Yanchun Zhang, Tao Zhao, Yongming Zhao, Song Tang, Ying Li, Shendan Han. Design and Fabrication of High‑Efficiency and High‑Power 976 nm Semiconductor Laser Chips[J]. Chinese Journal of Lasers, 2024, 51(7): 0701017.

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