Study of far-field reduction in high power 940 nm vertical-cavity surface-emitting lasers cascaded by tunnel junctions

This paper characterizes the performance of 940 nm single-junction (1 J) and triple-junction (3 J) vertical-cavity surface-emitting laser (VCSEL) arrays, tested at room temperature under 1.8 ns pulsed current injection. By suppressing thermal effects, the slope efficiency (SE) of the 1 J VCSEL array reaches 1.05 W/A, while the 3 J VCSEL array achieves 3.2 W/A, with a peak output power exceeding 120 W, demonstrating a significant performance enhancement. Furthermore, we observe that in the 3 J VCSEL array, the far-field (FF) divergence angle gradually decreases with increasing injection current, reducing from approximately 17° to about 5°. The far-field beam profile exhibits a Gaussian distribution, and spectral measurements indicate that the fundamental mode is dominant. We further analyze the characteristics of the VCSEL through both simulations and measurements. Current path analysis reveals that in the 3 J structure, the presence of a highly doped tunnel junction (TJ) and multiple oxide layers alleviates current crowding compared to the 1 J structure, resulting in a different gain distribution. Calculations show that the overlap between the gain region and the fundamental mode is greater than that of higher-order modes, which may explain the dominance of the fundamental mode. The results from single-device testing align with the observations in the VCSEL array, consistently demonstrating fundamental mode dominance. This phenomenon contributes to a reduced divergence angle, presenting a significant advantage for future optoelectronic applications.