Effects of transport limited nonuniform pumping for multiple quantum well semiconductor lasers

Multiple quantum well (MQW) lasers have exhibited improved performance over conventional double heterostructure (DH) lasers for low threshold, high frequency, and high power applications. Ideally, the modulation bandwidth should increase with the number of wells due to the enhancement of differential gain. However, in reality, the maximum number of wells that can work effectively is governed by carrier transport. Because of the lower mobility of holes, those wells far away from the hole injection side (p-electrode) receive less carriers and have a smaller gain; and those wells close to the p-electrode receive more carriers and have a higher gain. The excess carriers in the wells near the p-electrode tend to reduce the differential gain, thus degrade the high frequency characteristics. A carrier diffusion model is used to theoretically analyze the effect of nonuniform pumping in a 1.55 μm InGaAs/InGaAsP MQW (20 wells) laser. A substantial nonuniformity in gain distribution among the wells limits the ultimate modulation bandwidth. Possible schemes of overcoming the transport-limited nonuniform pumping such as using p-doping are discussed