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

Directly modulated semiconductor lasers exhibit dynamic frequency shifts (chirping) due to gain-induced variations of the refractive index. Using the small-signal analysis of the single-mode rate equations, the effect of current-pulse shape on frequency chirping is analyzed, and the results are compared for the cases of sinusoidal and square-wave modulations. The chirp is generally larger for the square-wave case. However, its magnitude depends on the pulse rise and fall times, decreasing for a pulse with slower turn-on and turn-off characteristics. Chirp analysis presented here includes the effect of power-dependent gain changes arising from the processes such as spectral hole-burning.