Frequency dependence of the chirp factor in 1.55 mu m distributed feedback semiconductor lasers

The authors report on measurements of the chirp factor versus modulation frequency and bias for 1.55 mu m distributed feedback lasers. A scanning Michelson interferometer is used to measure the phase response of the optical field under small-signal modulation of the injection current. The magnitude of the phase response is then quantified in terms of an effective chirp factor corresponding to the well known alpha -parameter. The authors find this factor to increase monotonically and nearly double as the modulation frequency increases from below to above the resonance frequency of the relaxation oscillation. The results contradict the commonly held view of alpha as a constant parameter for a given laser structure. >