In Transparent Optical Networks (TONs), traffic is carried over end-to-end all optical connections, called lightpaths. Due to the increasingly high data rates and the vulnerabilities related to the transparency of optical networks, security issues in transparent wavelength division multiplexing (WDM) optical networks have become of great significance to network managers. In this paper, we consider the problem of attack-aware routing and wavelength assignment (RWA) in optical networks, for periodic scheduled traffic demands. We present an optimal mathematical formulation to solve the RWA for scheduled lightpath demands in such a way that the overall vulnerability to attacks is minimized for the set of lightpaths as a whole. Our simulations show that the proposed approach can lead to significant reductions in the attack-radius.