Frequency noise and modulation of a four-section DBR laser

A theoretical model is presented to describe the tuning, frequency noise, and modulation characteristics of tunable DBR lasers with two active and two passive sections. Analytical expressions are derived for the intensity and frequency modulation responses, frequency noise spectrum, spectral linewidth, effective linewidth enhancement factor, and spontaneous emission rate. It is shown that a nearly flat red-shifted FM response over several gigahertz can be obtained by nonuniform current injection and modulation of the lower carrier density section in the active region. Increasing the tuning current through the passive sections enhances the FM efficiency. However, a higher FM efficiency is generally accompanied by a larger spectral linewidth. The predictions of our theoretical model are in good agreement with the available experimental data