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The differential gain of 1.3 μm InGaAsP lasers is found to be a strong function of the active layer doping level. Using devices with doping enhanced differential gain and short cavity lengths, a modulation bandwidth of 15 GHz is achieved. The parasitic shunt capacitance which often severely limits the modulation bandwidth of diode lasers is greatly reduced by using a 0.4 μm thick oxide isolation layer and a device structure which minimizes parasitic capacitances. It is found that nonlinear gain saturation predominantly determines the damping factor of these devices, and thus greatly influences their modulation characteristics. The contribution of nonlinear gain to the FM modulation index and relaxation oscillation damping factor is calculated and measured. The nonlinear gain contribution to the damping factor obtained from the measured FM modulation index is used to predict the optical intensity modulation response. Excellent agreement between prediction and observation is obtained. These data directly show that the damping factor of multilongitudinal-mode lasers is smaller than that of single-mode lasers, suggesting that wider modulation bandwidths can be achieved with multimode devices than with single-mode devices.