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The frequency modulation characteristics of 1.3 Â¿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 180Â° 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 Â¿ is assumed.