Fig. 1. (a) Schematic diagram of six-junction VCSEL based on surface microstructure, including n-type and p-type DBR and the active area. Each active region includes multiple quantum well structures, an oxidation layer, and a reverse-biased tunnel junction. (b) p-DBR schematic diagram of single-junction single-mode VCSEL. (c) High discrimination capability p-DBR designed in this article. (d) Relationship between the reflectivity of the output mirror and the surface Si3N4 thickness under different numbers of top DBR layers. The reflectivity change of nine pairs of p-DBR designs is greater than 20%, and its surface structure modulation ability is much greater than that of traditional DBR designs.

Fig. 2. 19-pair p-DBR single-junction and nine-pair p-DBR six-junction VCSELs. (a) Relationship between surface Si3N4 optical thickness and threshold gain. The maximum threshold gain of the six-junction VCSEL is approximately twice that of the single-junction VCSEL. (b) Power conversion efficiency; electro-optic conversion efficiency of the multi-junction VCSEL has been significantly improved. The maximum efficiency will reach about 60%.

Fig. 3. SEM image of the six-junction VCSEL with SR=3  μm: (a) top view, (b) cross section.

Fig. 4. Measured L-I-V results of six-junction VCSEL devices. SR=1  μm, the L-I-V curve did not show a kink, the max power is 20.2 mW. In other VCSELs, a kink is observed. The figure displays the spectrum at a current of 5.9 mA. It is evident that after the kink manifests, they transition to multi-mode operations.

Fig. 5. Spectral characteristics under different driving currents. (a) SR=1  μm; even when the device undergoes thermal rollover, it still maintains single-mode operation. (b) SR=2  μm; when the current is 5.6 mA, two modes appear in the spectrum, corresponding to the kink phenomenon on the L-I-V curve.

Fig. 6. Schematic diagram of (a) near-field and (b) far-field testing setup. Beam characteristics of the six-junction VCSEL with SR=1  μm at 20.5 mW: (c) near-field beam, exhibiting a quasi-Gaussian beam intensity distribution; (d) far-field pattern; (e) far-field divergence angle of 9.8° (at 1/e2).