Design of Low Threshold and Temperature Insensitive 1.3 μm Surface Emitting Lasers toward Optical Interconnection by Wavelength Domain Addressing

Vertical cavity surface emitting laser (VCSEL) is attractive light source in a future massively parallel computing system with many processor elements (PEs). We proposed optical interconnection by wavelength domain addressing for realizing switch-less and reconfigurable interconnection [1]. For such a system, 1.3 µm VCSEL has advantages over short wavelength VCSEL such as 0.98 µm VCSEL because of its low driving voltage, which enables direct drive by CMOS circuits. However, the performances of 1.3 µm VCSEL are still insufficient for practical applications [2]. Small optical gain of active layer and low reflectivity mirrors severely limit the performances of conventional 1.3 µm VCSELs on InP substrate. Here, we propose high performance 1.3 µm VCSEL grown on InGaAs ternary substrate. We present the results of theoretical investigation and primary experiment. Figure 1 shows direct energy gap versus lattice constant relation. On InP substrate, materials of barrier layer we can design are limited, while on InGaAs ternary substrate whose lattice constant is in-between those of GaAs and InP, wider band gap materials are available for a barrier layer. According to theoretical calculation [3], deep potential well on the ternary substrate enables larger optical gain, leading to low threshold current and temperature insensitive operation. Use of the ternary substrate should also provide high-reflectivity mirrors at 1.3 µm. Refractive index difference between InGaAs and InAlAs on InGaAs substrate can be much larger than that between InP and InGaAsP on InP substrate. This results in higher reflectivity and wider stop bandwidth mirrors at 1.3 µm. We theoretically investigated the performances of 1.3 µm VCSEL on InGaAs substrate. We assumed a l-cavity structure with InGaAs strained QWs shown in fig. 2. We calculated the optical gain based on the band calculation, and estimated the threshold current and the temperature characteristics. Figure 3 shows an example of calculation, where the threshold current of 1.3 µm VCSEL on the InGaAs substrate is shown as a function of compositional wavelength of barrier layer. It is found that extremely low threshold current density below 200 A/cm 2 could be obtained on the InGaAs substrate, indicating possible improvement in the performances of 1.3 µm VCSEL. This improvement is mainly due to the enlarged optical gain and the enhanced mirror reflectivity. We experimentally demonstrated the increase in mirror-reflectivity at 1.3 µm [4]. We fabricated In 0.29 Ga 0.71 As/In 0.28 Al 0.72 As DBR stacks by MOVPE on GaAs substrate with InGaAs graded buffer …