Vertical cavity surface emitting lasers with 14GHz bandwidth

Abstract

The possibility of high speed operation is one of the most attractive features of vertical cavity surface emitting lasers (VCSELs). This arises from the small size and high photon density of the cavity. However, while relaxation oscillations at frequencies as high as 70 GHz [l] have been reported, the highest measured modulation bandwidth is in the 5-8 GHz range [2]. The low bandwidth is attributed to large parasitics of the VCSEL and specifically to the high resistance of the p-type mirror. We have investigated small diameter VCSELs and have observed a maximum bandwidth of 14 GHz. In this study, gain-guided VCSELs from two wafers were used, nominally lasing at either 780 nm or 960 nm. The epitaxial structure consisted of a bottom Si-doped mirror, a cavity region, and a top C-doped mirror. The mirrors consisted of quarterwavelength layers of AIAs/A10.3Ga0.7As or AIAs/GaAs for the 780 nm and 960 nm lasers, respectively. The active region of the 780 nm VCSEL was bulk A10,15Ga0.85As while the active region of the 960 nm wafer had three GaAs/ln0,2Ga0.gAs quantum wells. The structures were grown by MBE on 3” diameter n-type GaAs substrates and fabricated by completely planar technology. Threshold currents were 4-6 mA for laser diameters of 6-10 pm and the cw peak powers exceeded 0.4 mW. Modulation experiments showed a very high frequency-square root of power coefficient D=10 GHz/mW1/2. The maximum bandwidth of 13.7 GHz was reached at 20OC (and 14.7 GHz at 7OC) at a current of 8 mA for the 780 nm laser with a cavity diameter of 6 pm. The cavity region of these lasers received both a broad area proton implant as well as a second deep implant centered at the active region to better define the cavity diameter. The 960 nm lasers received only the first broad area implant and their measured response of 10 GHz indicates the importance of minimizing lateral carrier diffusion. Fitting of the response curves yields a damping rate r of more than 10 GHz, indicating an intrinsic response fmax greater than 50 GHz. The S-parameter data can be fitted to high accuracy with a realistic equivalent circuit.