Currently, the growing bandwidth demands of cloud services are accelerating the hyperscale expansion of data centers. 850 nm vertical-cavity surface-emitting lasers (VCSELs) are widely used in data centers owing to their high-cost performance and high power. However, current commercial VCSELs operate in the multitransverse mode, and their applications are limited to the short connections of a few hundred meters. Owing to reduced mode partition noise and modal dispersion, single transverse-mode VCSELs can transmit over long distances in multimode fiber. Furthermore, in multitransverse-mode VCSELs, high-order modes consume a large fraction of the injection current, resulting in low relaxation oscillation frequencies. Therefore, 850 nm single-mode surface-emitting lasers are an urgent requirement in emerging hyperscale data centers to provide cost-effective, high-bandwidth, and long-distance optical communication systems. This paper presents an 850 nm single-mode surface-emission distributed feedback (SEDFB) laser with a rectangular oxide aperture and a shallow etched surface grating. A threshold current of 1.8 mA and a side-mode rejection ratio (SMSR) of 47 dB are achieved.
A first-order grating is used to provide adequate optical feedback for the laser. A second-order grating is used to achieve upward diffracted light. A λ/4 phase-shift structure is used to achieve stable single longitudinal mode lasing. Moreover, the simulation calculation is used to optimize the material and thickness of each layer of the waveguide structure and the coupling coefficient of the grating while ensuring the light confinement factor in the active region. Large-area rectangular oxide apertures are designed to confine current injection. Furthermore, the laser chip is epitaxially grown using metal-organic chemical vapor deposition (MOCVD). Wet etching with a citric acid-hydrogen peroxide solution is used to etch surface GaAs. The reactive coupled plasma (ICP) is used to etch the waveguides and gratings. Using a high-temperature wet oxidation method, a rectangular oxidation aperture is formed.
A low threshold current of 1.8 mA is obtained for the SEDFB laser with an active region length of 50 μm (Fig. 11). Its differential resistance is 46 Ω, which is smaller than that of VCSELs with the same oxidized aperture area. The laser exhibits good single-mode characteristics, resulting in an SMSR of approximately 47 dB (Fig. 11). The laser temperature increases from 20 ℃ to 60 ℃, and the SMSR remains above 40 dB (Fig. 12). The laser’s calculated thermal resistance is approximately 0.74 ℃/mW, which is smaller than the thermal resistance of a VCSEL with the same oxidized aperture area. The full width at half maximum of far-field pattern divergence angle is approximately 21°×26° (Fig. 13), and the quasicircular spot output is achieved. Simultaneously, the laser exhibits a relaxation oscillation frequency of 17 GHz at five times the threshold current (Fig. 14). An SEDFB laser with an active region length of 150 μm exhibits a 3 dB modulation bandwidth of 15 GHz.
In summary, this paper presents an 850 nm single-mode surface-emitting distributed feedback laser. It exhibits a threshold current of 1.8 mA, and the low threshold current shows our device’s application potential in energy-saving systems. To considerably reduce series resistance, we use a large-area rectangular oxide aperture. The SEDFB laser has a single transverse mode that is confined by the ridge waveguide structure and a single longitudinal mode that is selected by the surface grating. Therefore, the SEDFB laser exhibits a high SMSR of 47 dB. The relaxation oscillation frequency of the SEDFB laser is approximately 17 GHz.
