Abstract: A WDM system comprising a first WDM module having a first multiplexer unit for multiplexing a WDM optical signal, a second WDM module having a first demultiplexer unit for demultiplexing the WDM optical signal, and wherein the system is arranged, in use, such that optical losses experienced by individual channels of the WDM optical signal in the first multiplexing unit and the first demultiplexing unit and optical losses experienced by the channels during un-amplified transmission between the first and second WDM modules are substantially balanced.

Abstract: A method of effecting failure protection switching in an optical network, the optical network comprising a ring structure carrying a bi-directional optical data signal, and a plurality of network hubs arranged in-line within the ring structure, each network hub being arranged, in use, to transmit and receive signals bi-directionally along the ring structure, the method comprising the steps of a) detecting a “no signal” at a primary pre-amplifier located at one of the network hubs and arranged to pre-amplify a primary optical signal received from a first direction along the ring structure b) shutting down the primary pre-amplifier and powering up a secondary pre-amplifier located at the one network hub and arranged to pre-amplify a redundant optical signal corresponding to the optical signal and received from the opposing direction along the ring structure.

Abstract: A hub structure for use in an optical network, the optical network comprising a ring structure carrying a bidirectional optical data signal and a plurality of hub structures arranged in-line within the ring structure, the optical data signal comprising primary and secondary path transmission signals having opposing transmission directions on the ring structure, the hub structure comprising a drop unit arranged, in a normal state, to drop and through-connect a primary receive path signal for further processing at the hub structure, and, in a protection state, to drop and through-connect a secondary receive path signal for further processing at the hub structure.

Abstract: An optical network hub structure comprising a WDM unit arranged in line with a fiber trunk carrying a bi-directional optical network signal to drop/add blocks of wavelengths destined to/originating from the network hub structure and to through connect other blocks of wavelengths, and at least one amplifier structure disposed in line on the fiber trunk at each side of the WDM unit, each amplifier structure comprising at least two propagation dependent optical junction elements, at least two optical paths optically connected in parallel between the two junction elements, and a first amplifier in only one of the optical paths, whereby a bi-directional, multiplexed optical signal comprising different blocks of wavelengths, each block of wavelengths having a specified propagation direction with respect to the amplifier structure, is, in use, uni-directionally amplified, whereby at each side of the WDM unit the optical network signal is, in use, uni-directionally amplified.

Abstract: An optical ring network structure comprising two or more network elements, and a single optical fiber connection between each pair of neighboring network elements for carrying an optical signal, wherein the ring network structure is arranged in a manner such that, in use, band allocation utilizing multiplexing on each single fiber connection is chosen in a manner such that groups of wavelengths for bi-directional data transfer and for bi-directional redundant data transfer for protection respectively are provided on each single fiber connection.

Abstract: A bi-directional optical amplifier arrangement for use in an optical path along which a bi-directional optical signal is travelling, the arrangement comprising at least two amplifier units disposed to amplify, in use, in different propagation directions, at least two propagation direction dependent optical junction elements, at least two optical paths disposed in parallel between the junction elements, wherein each optical paths comprises one of the amplifier units; and wherein gain settings of the amplifier units are chosen in a manner such that, in use, a total gain experienced by an optical signal re-circulating along both optical paths is less than unity.