Objective Semiconductor lasers have the advantages of high efficiency, easy integration, high reliability, and easy tuning; hence, they are used in various applications such as fiber coupling, seed laser sources, medical equipment, and communication systems. To increase the output power of semiconductor lasers, a wide-ridge waveguide structure is usually employed; however, in such lasers, the lateral mode is poor and the energy distribution is uneven. These observations are mainly reflected in the “multilobe” phenomenon of the near-field spot, thereby limiting the wide applications of semiconductor lasers. To improve the lateral mode performance of semiconductor lasers, researchers have proposed many solutions including an external cavity laser structure, tapered waveguide structure, and curved waveguide structure. These approaches can improve the lateral mode and quality of the lateral beam. However, they have the disadvantages of low output power, complicated process, and high threshold. In this study, a wide-ridge waveguide semiconductor laser with a lateral microstructure (LMWR-LD) was proposed. Results show that the output power, slope efficiency, and electro-optic conversion efficiency of LMWR-LD were improved, while the mode competition inside the cavity was reduced. Moreover, the lateral microstructure and wide-ridge waveguide structure were formed in the same step of photolithography, which is beneficial to simplify the process and reduce the cost.
Methods Based on the threshold gain theory, the influence of the mode loss on the mode threshold gain was analyzed. As the mode loss increased, the threshold gain of the mode increased. The lateral mode characteristics of wide-ridge waveguide semiconductor laser (WR-LD) were simulated using PIC3D, and the optical field distribution of each order lateral mode was shown (Fig. 2). The fundamental mode optical field was a single spot with a Gaussian distribution, and the energy was mainly concentrated at the center of the optical field. A broken line was observed in the middle of the first-order lateral mode optical field, which was divided into two spots. Thus, the energy distribution of the laser was uneven. When the order index of the lateral mode increased, the number of broken lines in the corresponding lateral mode optical field increased and the energy distribution became more uneven. Based on the characteristics of the optical field distribution of each order lateral mode in WR-LD, the suppression mechanism of the lateral microstructure on lateral modes (Fig. 3) was observed using FDTD simulation software. The influence of the microstructure width on the mode loss of each order lateral mode was investigated [Fig. 4(a)]. When the microstructure width was fixed, each order lateral mode exhibited different losses. By analyzing the loss difference between the fundamental and high-order lateral modes [Fig. 4(b)], the optimal lateral microstructure width can be obtained. In this case, the loss of the fundamental mode was small and that of the high-order lateral mode was large. Thus, the lateral mode characteristics of LMWR-LD were improved owing to the introduction of microstructure.
Results and Discussions Experimental results show that owing to the fierce competition among each order lateral mode, the near-field spot showed a “mutilobe” appearance in WR-LD [Fig. 5(a)]. After introducing the lateral microstructure, the near-field spot “mutilobe” phenomenon of the LMWR-LD was clearly eliminated [Fig. 5(b)]. Comparing the near-field optical distribution of these two devices, the intensity at the edge of near-field optical distribution for LMWR-LD was observed to decrease significantly [Fig. 5(c)]. This proves that the lateral microstructure had a good inhibition effect on the high-order lateral mode. Because the “multilobe” phenomenon of the near-field optical distribution of LMWR-LD was eliminated, the far-field optical distribution presented a “single-lobe” phenomenon and the full width at half-maximum divergence angle of LMWR-LD decreased from 2.07° to 2.05° (Fig. 6). At 0.5 A input current, the LMWR-LD achieved an output power of 130 mW, showing an improvement of 58.5% compared with WR-LD [Fig. 7(a)]. Moreover, the slope efficiency and electro-optic conversion efficiency of LMWR-LD were 0.27 W/A and 14.5%, respectively, which are 80% and 55.9% higher than those of WR-LD [Fig. 7(b)]. The improvement in slope efficiency and electro-optic conversion efficiency was mainly attributed to the introduction of lateral microstructure. Because the high-order lateral mode was suppressed in the LMWR-LD cavity, the mode competition in the cavity decreased to a certain extent and the optical field distribution of the output laser became more uniform. Therefore, the matching degree of the optical field and injection current was improved.
Conclusions A LMWR-LD structure was proposed in this study. The diffraction loss of the high-order lateral mode was increased by introducing microstructures on both sides of the wide ridge, leading to the suppression of high-order lateral modes. Moreover, the “multilobe” phenomenon in the output laser of LMWR-LD was eliminated. After the fabrication and packaging of LMWR-LD, the test and analysis were conducted. Experimental results show that compared with WR-LD at 0.5 A, the output power of LMWR-LD increased from 82 mW to 130 mW, slope efficiency increased from 0.15 W/A to 0.27 W/A, and the electro-optic conversion efficiency increased from 9.3% to 14.5%. Additionally, the lateral microstructure of LMWR-LD was simultaneously formed with the wide-ridge waveguide structure in ultraviolet lithography. Further, LMWR-LD offers the advantages of simple process and low cost. Based on this research, by optimizing the structure of the device, high-power semiconductor laser devices with good lateral mode characteristics can be achieved.
