Compared with edge-emitting lasers, vertical-cavity surface-emitting lasers (VCSELs) have superior performance, such as a lower threshold current, single longitudinal-mode output, easy 2D array integration, low power consumption, and low fabrication cost. With the development of large apertures as well as 2D arrays of VCSELs, the output power of VCSELs has been significantly improved, and they are widely used in such fields as optical communication, optical interconnection, and optical information processing. In addition, the applications of VCSEL devices in the consumer field are becoming increasingly extensive, such as LiDAR, distance sensing, autofocusing, 3D sensing, rainbow-mode recognition, air and water quality detection, and virtual reality (VR)/augmented reality (AR)/mixed reality (MR). In recent years, the performances of VCSELs in terms of output power, conversion efficiency, modulation bandwidth, and reliability have improved continuously. However, high-power VCSEL single-tube or array devices are mostly multi-transverse-mode outputs, resulting in poor output beam quality. Therefore, improving device power while obtaining better beam quality in the optical field is a technical challenge that current researchers must solve. In this study, a novel multi-ring cavity structure is used to integrate VCSEL arrays to obtain a better far-field distribution while maintaining a high power output, further expanding the application range of VCSELs in the field of smart devices.
In this study, by analyzing the reasons for the non-uniform carrier distribution of large-aperture VCSELs, a multi-annular cavity structure [Fig. 2(b)] is designed to separate the injected current region into multiple regions to suppress the carrier aggregation effect. The finite-different time-domain (FDTD) method is used for the simulation to optimize the optical field distribution by adjusting the size of the annular cavities and the percentage of the light-out region. On this basis, traditional and new-structure VCSELs with identical external diameters of light exit holes are prepared on the same epitaxial wafer, and their light field uniformity and output characteristics are compared and analyzed.
Near-field photographs of conventional and novel-structure VCSELs tested under the same injection conditions are taken (Fig. 5). The test results show that the light field distribution uniformity of the conventional VCSEL structure [Fig. 5(d)] is extremely poor, and only the annular region near the electrode ring emits light. In contrast, the three ring-cavity structures (A, B, and C) are fully illuminated in all light-emitting regions and have a more uniform light field distribution, which significantly improves the extremely poor light field distribution of the conventional structure owing to the carrier aggregation and space-burning hole effects. By comparison, it can be seen that the structure C not only has better light field uniformity and high utilization of the light-emitting region but, more importantly, has the strongest light field, which is consistent with the theoretical simulation results. In addition, as can be seen from the far-field distribution and spectrogram of the VCSEL with structure C (Fig. 6), the optical field center of the far field has a strong intensity, showing a Gaussian distribution, and the excitation spectrum verifies its excellent single-mode characteristics, with a peak wavelength of 805.03 nm and a spectral full width at half-maximum of 0.82 nm. The device exhibits a very good excitation characteristic. In addition, the conventional VCSEL structure (Fig. 8) has a maximum continuous output power of 90 mW at 0.7 A and a threshold current of 80 mA. The output power and slope efficiency of the new structure are improved compared with those of the traditional-structure device, and the threshold current is reduced. The threshold current of new structure C is 49 mA, and the maximum continuous output power is 140 mW, which is nearly 56% higher than that of the conventional structure.
In this study, by analyzing the reasons for the uneven carrier distribution of large-aperture VCSELs, a multi-annular-cavity-structured VCSEL is designed, and the optical field of the new structure is simulated. The results show that the optical field distribution can be optimized by optimizing the dimensions of the annular cavities and the percentage of the light-output region. Based on this, traditional and new-structured VCSELs with identical external diameters of light exit holes are prepared on the same epitaxial wafer, and the light field uniformity and output characteristics of the new structure are compared and analyzed. The results show that the new structure improves the uneven distribution of the light field caused by the carrier aggregation effect of the traditional structure. The new multi-ring cavity structured VCSEL with a 67% duty cycle has the best near-field distribution, and the threshold current is reduced. On applying an injection current of 0.8 A, the continuous output power at room temperature reaches 140 mW, which is 56% higher than that obtained with the traditional structure, and the far-field shows a Gaussian distribution. In addition, the beam quality is better, which meets the demand for high-power and high-beam-quality semiconductor laser sources for VCSELs in the field of optical communication and further expands the application range of VCSELs in the field of intelligent devices.
