The optical link suffers from degradation due to attenuation, increase of bit rate and due to increase in the propagation distance, it also depends on the characteristics of the optical laser source. The direct modulated laser (DML) source radiation parameters such as photon lifetime, optical confinement factor, and gain coefficient are some of the important parameters which affect the performance of the optical communication link. The system bit error rate (BER) has been monitored to determine the effect of different radiation parameters of the DML laser on the performance of the Free Space Optical (FSO) link.