Users of an optical communications system send signals to one another by modulating an optical carrier wave in a prescribed manner. In addition to the normal communications signals sent and received by users of an optical communication system, it may be advantageous for there to be other signals sent from one location within the optical communications system to another location in the system without interfering with the signals of the users. For example, it may be advantageous for the operator of the optical communications system to further modulate an optical carrier wave at a prescribed point within the communication system and monitor that modulated carrier wave at another prescribed point after it has passed through one or more components of the communications system to identify and isolate faults in the communications system. See, for example, U.S. Pat. Nos. 5,296,956 and 5,317,439.
To ensure proper and reliable operation of modem all-optical networks, the performance of its various nodes and links has to be continuously monitored, similarly to today's electrical networks, to detect and locate failures immediately after they occur to allow for quick repairs. Ideally such monitoring should be performed continuously during normal operation of the optical network without interrupting or interfering with the optical information signals carried over the network.
Moreover, modem optical networks simultaneously carry multiple optical signals at different wavelengths over each link. These wavelength-division-multiplexed optical information carriers will be separated and re-arranged at certain nodes in the network and may even be translated to different carrier wavelengths, if necessary, in order to be routed to their different designated destinations (locations). Such complicated networks require sophisticated schemes to track the proper routing of the various individual information carriers through the network and at the same time to monitor the performance of the nodes and links.
A method for monitoring the performance of an optical network that carries only one optical information signal over each line connecting the various nodes of the network is described in the U.S. Patents previously referenced. In this scheme, a unique, unambiguous optical maintenance signal either at the same wavelength as the optical carrier or at a different wavelength is injected at various points in the network and detected at one or more central locations in the network, allowing unambiguous detection and location of faults in the network.
Wavelength-multiplexed optical networks, however, may not allow injection of additional optical maintenance signals at wavelengths other than the predetermined optical carrier frequencies. More importantly, such additional maintenance signals do not permit monitoring of the proper routing of the various carriers since they cannot be selectively routed along the different paths of the re-arranged optical carriers. Common simultaneous modulation of the amplitudes of all multiplexed carriers entering or exiting a node provides for unique performance monitoring but will have to be repeated many times at each node and the modulation index will thus grow rapidly, eventually severely impairing the signal transmission. Moreover, such common modulation of the multiplexed optical carrier also does not permit end-to-end monitoring and tracking of their routing through the network.