Patent ID: 7650075

Claim:
An optical add-drop multiplexer having a function of demultiplexing an inputted wavelength-division multiplexed signal on a wavelength length basis, and adding or dropping part or all of the demultiplexed optical signals on a wavelength basis, and then multiplexing these optical signals again before outputting the multiplexed signal, said optical add-drop multiplexer comprising: an optical switch having a function of adding at least one optical signal having a desired wavelength to a WDM signal line through which the wavelength-division multiplexed signal is transmitted, or a function of dropping at least one optical signal having a desired wavelength from the WDM signal line; said optical switch including an add switch capable of switching the optical signal from an add state to a non-add state and vice versa, and a drop switch capable of switching the optical signal from a drop state to a non-drop state and vice versa, said add switch and said drop switch being provided on the output side, and on the input side, of the optical add-drop multiplexer respectively; a failure detection part for detecting a failure of an optical signal that may occur on the WDM signal line, or a failure information receiving part for receiving failure information from the outside; an optical switch control circuit for switching the states of the add switch and the states of the drop switch; and means for inputting, into the optical switch control circuit, failure occurrence information coming from the failure detection part or the failure information receiving part, and for, when the failure occurrence information is detected or received, switching either the add switch or the drop switch, or both the add switch and the drop switch, on the basis of the information, wherein: in the case where the optical add-drop multiplexers which are connected to active and backup WDM signal lines respectively are located on the farthest downstream side of the WDM signal lines, the optical add-drop multiplexer can be configured so that if the optical add-drop multiplexer is connected to the active WDM signal line, at the time of normal operation, the drop switch is switched to a drop state, whereas if a failure occurs on the WDM signal line, the drop switch is switched to a non-drop state, and so that if the optical add-drop multiplexer is connected to the backup WDM signal line, at the time of normal operation, the drop switch is switched to a non-drop state, whereas if a failure occurs on the WDM signal line, the drop switch is switched to a drop state.