A method for detecting and locating faults in an optical transmission path, and to an optical transmission system having such a functionality, is provided. By measuring the temporal change in the state of polarization SOP, external influences on a glass fiber cable of a long-haul transmission system, e.g., a WDM transmission system, may be detected. Such fault(s) may be localized with the aid of the relative arrival time of the polarization signal at both sides of a bidirectional transmission system.