Effects of bandgap, lifetime, and other nonuniformities on diode laser thresholds and slope efficiencies

We examine the effect of various types of spatial nonuniformities on the performance of semiconductor diode lasers. Light versus current (L-I) characteristics are modeled by solving the coupled rate equations for the carrier density, photon density, carrier temperature, and lattice temperature. In cases of macroscopic nonuniformities, that is, when the fluctuation correlation length (Λ_F) is longer than the carrier diffusion length (L_D), the cavity is divided into a large number of segments, and separate rate equations are solved for each segment. Type-II antimonide quantum wells are used as the model diode laser system. Macroscopic nonuniformities of the nonradiative lifetime are shown to have little effect on the slope efficiency at high injection levels, but produce a signature softening of the threshold abruptness if the nonuniformity is large enough. This results from the nonuniformity of the carrier concentration and consequent mixture of gain and loss regions. For the case of microscopic bandgap fluctuations (L_D≫Λ_F), inhomogeneous broadening of the gain spectrum leads to an increase of the threshold current density, which is accompanied by only a small reduction of the efficiency. However, in the complementary limit of macroscopic bandgap fluctuations (L_D<Λ_F), the regions with distinct bandgaps become isolated, and photons emitted from the regions with larger bandgaps tend to be absorbed by the regions with smaller bandgaps. For an example with a Gaussian bandgap distribution of width σ=30 meV, the nonuniformities have the effect of increasing the threshold current by a factor of 2.6, while reducing the slope efficiency by a factor of 2. We also consider lateral nonuniformities due to the temporal instability of the lasing mode, which tend to have a rather modest effect on the threshold current, slope efficiency, and even on the maximum output power despite the severe degradation of the beam quality