Dependence of self-pulsation characteristics of multisection index-coupled distributed feedback lasers on section lengths

We investigate the effect of section lengths on the self-pulsation (SP) characteristics of a multisection index-coupled distributed feedback (DFB) laser that is composed of two spectrally detuned DFB sections and a phase-tuning section between them. Compound-cavity modes in multisection DFB lasers can be obtained by the concept of Fabry-Perot cavity modes. The simple equation for a CS number, which represents the number of compound-cavity modes within the stop-band width of a DFB section, is derived. By using the CS number, we were able to predict the SP characteristics such as abrupt changes of SP frequency due to mode hopping. Stable SP characteristics without abrupt changes of SP frequency for the variation of the spectral detuning and the phase shift in a phase-tuning section were obtained in cases with small CS numbers, which can be obtained by adjustment of section lengths.