In an optical network having a terrestrial terminal and an open cable interface (OCI) connecting a
submarine cable to a terrestrial cable, the OCI may include a filter positioned on an
optical path between the terrestrial cable and the
submarine cable and configured to pass first communication signals of a first
frequency band, and filter out secondary signals of a second
frequency band that does not overlap with the first
frequency band. The secondary signals may be looped back to the terrestrial terminal. The terrestrial terminal may detect the looped back secondary signals, and in response, determine the presence of the OCI and that the supervisory signals were rerouted by the OCI.