Threshold-current analysis of InGaAs-InGaAsP multiquantum well separate-confinement lasers

An analysis of the threshold behavior of long-wavelength ( lambda =1.55 mu m) multiquantum well separate-confinement lasers with InGaAs wells and quaternary ( lambda g=1.3 mu m) barriers is presented. Using the effective mass approximation and Fermi statistics for carriers, an approximately logarithmic dependence of optical gain on carrier density for quantum well lasers with one confined electron state is predicted theoretically. This prediction is verified by measured threshold currents of broad-area lasers of various cavity lengths and different numbers of quantum wells. Moreover, the characteristic parameters, such as transparency current density, gain constant, and absorption outside the active region, are determined.