It is well known that directly modulated semiconductor lasers have significant amounts of frequency chirping during modulation and this limits the performance of long-distance high speed fiber-optic communication systems [l]. One of the methods that can overcome this limitation is using injection-locked semiconductor lasers. Injection-locked semiconductor lasers can offer reduced chirping  and larger modulation bandwidth [3-41 with the proper adjustment of injection parameters such as injection ratio and frequency detuning. In this paper, the large signal characteristics of injection-locked semiconductor lasers and their performances in optical transmission systems are analyzed in detail. The rate equations for a single-mode semiconductor laser under optical injection  are numerically solved for the NRZ modulation format at the bit rate of 2.5 Gbps. The configuration used for the simulation is illustrated in Fig. 1. The laser parameters are obtained from [ 11, where the linewidth enhancement factor of 5 is used. The emitting power of the slave laser (SL) is maintained at about 3 mW for on state and 1 mW for off state, regardless of the injected optical power.