Low-Threshold, High-Temperature Operation of Surface Emitting Laser toward Optical Interconnection by Wavelength Domain Addressing

In a future massively parallel computing system with many processor elements (PEs), interconnection among the PEs would be a bottle-neck for improving system performance especially in the power consumption. For a highly parallel system, conventional electrical interconnection requires many high-speed switches that consume much electrical power, restricting the system performance. Concerning this problem, we proposed optical interconnection by wavelength domain addressing. In this scheme, a pair of a wavelengthtunable laser and a wavelength-selective detector is assigned to each PE module and is connected to a common optical bus, which enables switch-less interconnection as shown in Fig. 1. By assigning a different wavelength to each PE module, simultaneous and re-configurable interconnection among arbitrary PE modules can be achieved with low power consumption. Vertical cavity surface emitting laser (VCSEL) is the most promising light source for this application because of its potential for ultra-low power operation and 2D-integration. Here we report 0.98 m VCSELs with low-threshold current and excellent temperature characteristics [1]. Although our target is to realize wavelength-tunable 1.3 m VCSELs, we could successfully obtain excellent VCSELs as the first step. The VCSEL structure was grown by molecular beam epitaxy (MBE) on n-GaAs substrate. It consists of a pair of distributed Bragg reflector (DBR) surrounding 2 8 nm strained In0:2Ga0:8As quantum wells separated by 10 nm GaAs barriers. The top DBR is 25 pairs of p-doped GaAs/AlAs, while the bottom DBR is 22.5 pairs of n-doped GaAs/AlAs. Note that Al0:5Ga0:5As spacers are inserted to reduce series resistance between GaAs layers and AlAs layers in the top DBR [2]. The lasers were chemically etched through the active region to form a mesa structure, stopping at the middle of the bottom DBR, which provides both carrier and optical confinement. The wet etching is a simple process and quite effective to avoid the nonradiative recombination at the sidewall of the active region which degrades low threshold operation [1,3]. Figure 3 shows light output and voltage versus driving current characteristics of a 17 m-square VCSEL at 20 C. The threshold current was 1.3 mA, corresponding to the threshold current density of 450 A/cm2, which was to date the lowest value. The voltage at the threshold was 2.8 V, which thermally limited the maximum output power to 4.5 mW. In fact, under pulsed operation, the maximum output power of 14 mW was obtained in the same device. Further reduction of series resistance would lead to higher output power. The temperature dependence of threshold current is shown in Fig. 4. Two types of VCSEL in which detunings of the cavity wavelength to the gain peak wavelength ( = cavity gain) were -2 nm (type A) and 16 nm (type B), respectively. In both VCSELs, we observed stable CW operation up to 160 C, which is the highest operation temperature of VCSELs ever reported [4]. In addition to this result, the threshold current of the type B device takes a minimum value at around 50 C, exhibiting a flat response over a broad temperature range of 20 80 C. The threshold current of the type B device at 160 C was only as low as 5.2 mA. In conclusion, we have fabricated VCSELs emitting at 0.98 m as the first step toward 1.3 m VCSELs used in the optical interconnection by wavelength domain addressing. Low threshold current and temperature insensitive operation were achieved, and thus fundamental design theories and process techniques were established.