Although All-Optical Network (AON) are a viable technology for future telecommunication and data networks, it creates many security vulnerabilities that do not exist in traditional networks. Transparency and non regeneration features make attack detection and localization difficult, because the effects of an attack connection can propagate quickly to different parts of a transparent All-Optical Network. Among all attack types in AON, crosstalk attack's propagation capability is second to none. Quick detection and localization of such attack source can avoid losing large amounts of data in an All-Optical Network. However, to detect attack sources, it is not necessary to put monitors on all nodes. Since those connections affected by the attack connection would provide valuable information for diagnosis, we show that placing a relatively small number of monitors on a selected set of nodes in a network is sufficient to achieve the required level of performance. This paper introduces crosstalk attack model and monitor-segment concept. After showing a necessary and sufficient condition for one-OAF diagnosable AON, a new sparse monitor placement method is proposed.