The frequency response of a group of 1.3 μm InGaAsP vapor-phase-regrown buried heterostructure lasers of various cavity lengths is analyzed by fitting the measured response curves. The dependence of resonant frequency f0and damping rate on bias power is determined. The differential gain coefficient for InGaAsP is determined as cm2. The damping rate is found to be proportional to the square of the resonant frequency with a proportionality factor which is independent of device geometry and facet reflectivity. The existence of such a universal relationship between and f0and the observed magnitude of the damping rate is explained by the interband relaxation model of nonlinear gain.