With the emerging of advanced technologies in the global market, now-a-days optical networks are the most ideal medium for high bandwidth communication. Optical wireless networks predicated on free space optics (FSO) technology must be designed to combat changes in the atmosphere, which can affect FSO system performance capacity. FSO is a security module due to Line of sight operation and does not require any system security upgradations. However, FSO requires secured communication for longer distance. FSO has been used in different ways to provide fast communication connectivity in inaccessible areas. Various methodologies are introduced to enhance FSO performance in terms of security that leads to the development of the secured FSO system. Optical code division multiple access (OCDMA) is becoming more popular in the field of optical communication, due to asynchronous transmission, flexibility, security and scalability. The major advantage of OCDMA is its efficient use of spectrum as well as the multiuser communication capabilities. OCDMA requires spectral amplitude coding (SAC) to minimize the impact of multiuser interference and also suppress the effect of phase induced intensity noise (PIIN) with fixed in phase cross-correlation. In this work, SAC-OCDMA over FSO communication design is proposed. In this paper, SAC-OCDMA FSO communication system is evaluated at 10 Gbps using Modified random diagonal (MRD) code that possesses better auto correlation, minimum cross correlation and high cardinality. The proposed SAC-OCDMA MRD is compared with the existing Multi-diagonal (MD) and Random diagonal (RD) codes under different weather conditions. The simulation results signify that optical system based on MRD code is better than MD and RD codes in terms of bit error rate (BER). The simulation results are validated using Optisystem version 14.