CONTROL OF DIFFERENTIAL GAIN, NONLINEAR GAIN, AND DAMPING FACTOR FOR HIGH-SPEED APPLICATION OF GAAS-BASED MQW LASERS

Utilizing small-signal direct modulation and relative intensity noise measurements, we investigate changes in the modulation response, the d ifferential gain partial derivative g/partial derivative n, the nonlin ear gain coefficient epsilon, and the damping factor K, which result f rom the following three structural modifications to GaAs-based multipl e quantum well lasers: 1) the addition of strain in the quantum wells; 2) an increase in the number of quantum wells; and 3) the addition of p-doping in the quantum wells. These modifications are assessed in te rms of their potential for reducing the drive current required to achi eve a given modulation bandwidth, for increasing the maximum intrinsic modulation bandwidth of the laser, and for improving the prospects fo r monolithic laser/transistor integration. The differential gain is in creased both by replacing unstrained GaAs-Al0.25Ga0.75As QW's with str ained In0.35Ga0.65As-GaAs QW's and by increasing the number of straine d QW's, ultimately leading to substantial improvements in modulation b andwidth at a given drive current. However, in both cases, the increas ed differential gain is offset by corresponding increases in the nonli near gain coefficient, leading to relatively constant values of K and hence little variation in the maximum intrinsic modulation bandwidth. By further adding p-doping to the In0.35Ga0.65As-GaAs MQW active regio n, we have been able to simultaneously increase partial derivative g/p artial derivative n and decrease K, yielding very efficient high-speed modulation (20 GHz at a dc bias current of 50 mA) and the first semic onductor lasers to achieve a direct modulation bandwidth of 30 GHz und er dc bias (heat-sink temperature = 25-degrees-C). Since our laser str uctures show no significant carrier transport limitations, the measure d K factor for the p-doped devices implies a maximum intrinsic 3 dB mo dulation bandwidth of 63 GHz.