The frequency modulation characteristics of 1.3 ?m InGaAsP DFB BH lasers are measured when the bias current is slightly modulated by a sinusoidal waveform. Two deviations from the prediction by the rate equations are observed. One is that the phase difference between the frequency deviation and the modulation current approaches 180? below the relaxation resonance, and the other is that the ratio of the frequency modulation index to the amplitude modulation index has a strong frequency dependence. Both of the results are well explained by the two-section model, in which the inhomogeneous distribution of the spectral width enhancement factor ? is assumed.