Free Space Optical communication system is a technology that solves the problem of network connectivity. In this system, it contains a coding and technique to control the data that transferred using a laser beam from transmitter station through the atmosphere to receiver station. This technique is to optimize the error rate and improve the signal received. In this paper, the comparison between conventional technique and DDM technique has been recorded. By using a varies beam divergence, DDM technique can support 6.7mrad beam divergence higher than OOK technique and can receive weaker power signal - 90dBm with a minimum level of BER 10-9 compare to OOK technique. This improvement shows DDM technique can perform better performance compared to OOK technique.
