The characteristics of the light source in fiber optic communications were analyzed. According to the coherence of light, the principle, structure , performance and coupling efficiency of the light-emitting diode(LED) and laser diode(LD) were compared, so that the characteristics of laser diode were obtained, and that laser diode was found to adapt to the fiber optic communications with large capability and long-distance transmission. In the new-generation optical soliton communications system, whether as a light source of optical soliton or a pumping source of EDFA, laser diode exerts very important function. The Laser diode exerts more important effect in future.