20.2 mW! Single-mode VCSEL power efficiency sets a new record
12/11
2024
Photonics Research Articles from Issue 9, 2024:
Research Background
With the rapid development of artificial intelligence technology, high-speed data centers have become key infrastructure supporting cutting-edge technologies such as AI, big data analytics, cloud computing, and 5G networks. Currently, data centers extensively use multimode Vertical-Cavity Surface-Emitting Lasers (VCSELs) as the core technology for short-distance high-speed optical communication, due to their low cost and low power consumption, suitable for high-density optical interconnects. However, as demand for higher data rates (exceeding 100 Gbps) increases, multimode VCSELs face challenges such as mode dispersion, bandwidth limitations, and signal noise, making them insufficient. Existing methods face challenges in expanding the single-mode gain volume and controlling lateral modes via surface microstructures, limiting breakthroughs in single-mode VCSEL power and efficiency. The development history of single-mode VCSELs is shown in Table 1. From the table, it can be seen that since 2006, the power of single-mode VCSELs has developed slowly, remaining around 10 mW, with relatively low electro-optical conversion efficiency.
Table 1 Development History of Single-Mode VCSELs
In recent years, in the field of high-speed communication, with the adoption of PAM4+ modulation schemes, the importance of power has become increasingly prominent; the rapid development of AI technology has led to a significant increase in data throughput, making device power consumption a key concern. Therefore, researching single-mode VCSELs with characteristics of low cost, high power, high efficiency, and low divergence angle is crucial for advancing high-speed optical communication.
Research Highlights
Firstly, under low-threshold operating conditions, this work compared the control ability of surface phase layer thickness variation on reflectivity between multi-junction VCSELs with low-reflectivity output mirrors and single-junction VCSELs with high-reflectivity output mirrors. As shown in Figure 1, simulation results indicate that the reflectivity variation of a 9-pair p-DBR multi-junction VCSEL design reaches up to 20%, with surface structure modulation capability far exceeding that of traditional single-junction VCSEL designs.
Figure 1 (a) Schematic of 6-junction VCSEL; (b) Schematic of p-DBR for single-junction VCSEL; (c) Schematic of p-DBR for multi-junction VCSEL; (d) Relationship between output mirror reflectivity and surface phase layer Si 3 N 4 thickness under different DBR pair numbers
Next, based on the previously constructed ultra-high efficiency breakthrough multi-junction VCSEL model (Light Sci Appl 13, 60, 2024), the threshold gain variations of single-junction and multi-junction VCSELs based on surface phase layer thickness were analyzed, as well as the efficiency extension characteristics of multi-junction VCSELs based on the current level of single-junction single-mode VCSELs. As shown in Figure 2, results indicate that with changes in surface Si 3 N 4 thickness, different amplitudes of reflectivity variation cause significant differences in the maximum threshold gain between the two types of VCSELs, with the maximum threshold gain of multi-junction VCSELs about twice that of single-junction VCSELs. The core idea to achieve single-mode VCSELs is to increase the threshold gain difference between higher-order modes and the fundamental mode, thereby maintaining single-mode operation within a certain operating range. However, most current methods rely on various surface microstructures, causing higher-order modes with mode field distributions concentrated outside the emission aperture to experience greater loss. Therefore, this work first revealed that multi-junction VCSELs can enhance the modulation ability of surface microstructures on higher-order modes while maintaining low-threshold operation, and the proposed method can significantly increase the threshold gain difference between higher-order modes and the fundamental mode, thus achieving higher-order mode suppression over a larger range. Simulation experiments on the extension characteristics of multi-junction VCSELs indicate that the efficiency of high-power single-mode VCSELs is expected to exceed 60%.
Figure 2 19-pair p-DBR single-junction and 9-pair p-DBR 6-junction VCSEL, (a) Relationship between surface Si 3 N 4 optical thickness and threshold gain; (b) Efficiency extension characteristics of multi-junction VCSELs
Subsequently, the team fabricated 6-junction VCSEL samples with surface relief structures of different sizes and characterized their optoelectronic properties. The results are shown in Figure 3. The 6-junction VCSEL achieved 20.2 mW laser output power under continuous current drive, with a side-mode suppression ratio greater than 35 dB, corresponding electro-optical conversion efficiency of 42%, and divergence angle of 9.8°. Near-field spots indicate operation in the single fundamental mode laser mode. According to the research team members known, under room temperature continuous operation conditions This is the highest single-mode power of a single VCSEL to date, and the power value is nearly twice the known record. In addition, this method only requires simple micron-scale silicon nitride etching on the surface of the VCSEL, without special photolithography processes and complex process flows, ensuring the high reliability and low cost advantages of single-mode VCSELs.
Figure 3 (a) L-I-V curve; (b) Spectra at different currents when SR=1 μm; (c) Near-field image at 20.5 mW; (d) Far-field spot image at 20.5 mW; (e) Far-field divergence angle at 20.5 mW.
Summary and Outlook
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