A method is provided for detecting intrusion into an optical cable of a single mode optical fiber network comprising where monitoring light signals are transmitted along a telecommunications optical fiber to be monitored either along a fiber additional to a data fiber or by multiplexing onto a common fiber. The received monitoring light signals after transmission along the telecommunications optical fiber are analyzed for changes indicative of movement of the optical fiber for detecting an intrusion event. The monitoring light signals at the receive end of the fiber signals are monitored by feeding the signals from the single mode fiber into a multi-mode fiber in a manner which causes changes in modal power distribution which can be detected by taking a portion only of the modes.