Optical fiber networks are considered an attractive solution for increasing data transmission rates in communication systems. Laser diodes (LDs) are common light sources used to transform electrical signals into optical ones in optical fiber systems. The direct modulation of LDs is preferred as a data transmission technique to decrease power consumption and reduce the overall system cost. However, direct intensity modulation of LDs causes a time variation in the lasing frequency (i.e., frequency modulation). The interdependence between intensity and frequency modulation is referred to as "frequency chirp". We present a review on the effect of laser parameters such as linewidth enhancement factor and nonlinear optical gain suppression on the chirping characteristics of directly modulated LDs under high transmission speeds.