Fine structures in the broadened line of distributed feedback lasers under high-speed direct modulation

Precise observation of the single longitudinal mode spectrum for distributed feeedback lasers revealed fine structures when the spectrum was broadened by high-speed modulation. A dynamic simulation can explain reasonably the above behavior if the model takes into account the carrier density modulation enhanced by the relaxation oscillation. In this letter, experimental results where both modulation depth and speed were varied are described along with a calculated result.