Providing data security in the optical layer enables low latency data encryption and enlarges the capacity for secure data transmission. As the demand for both secure and high speed transmission grows dramatically, secure data transmission is pushing the requirements for processing speed and network capacity. As fiber-optic systems form the backbone of communication networks, optical approaches for protecting the network security increases the available capacity of the network. Moreover, compared to electric circuits, fiber-optic systems are immune to electromagnetic interference and have high processing speeds. In this chapter, we discuss using fiber-optic-based techniques to defend against threats in the network, including optical encryption, optical code-division multiple access (CDMA), optical key distribution, optical steganography, anti-jamming, and optical chaos-based communication. These approaches are categorized by their different applications for protecting the properties of network security, including confidentiality, privacy, and availability.