Pulse-shape effects on frequency chirping in single-frequency semiconductor lasers under current modulation.

Directly modulated semiconductor lasers exhibit periodic dynamic frequency shift or chirping due to gain-induced variations of the refractive index. The effect of pulse shape on frequency chirping is analyzed in this paper and the results are compared for the cases of the sinusoidal and square-wave modulations of the applied current. The chirp is generally larger for the square-wave case. However, its magnitude depends on the pulse rise time and decreases for a pulse with softer turn- on and turn-off characteristic. The calculated results are in reasonable agreement with the reported experimental observations.