An optical signal amplification device comprising: four ports comprising a first pair of ports for connecting to a first pair of optical fibres and a further pair of ports for connecting to a further pair of optical fibres; four multiplexing devices, each multiplexing device being associated with a corresponding port, each of said multiplexing devices being operable to transform between a wider band signal and two narrower band signals; optical paths for transporting said two narrower band signals between said four multiplexing devices, said optical paths being configured to provide paths between said multiplexing devices associated with said first pair of ports and said multiplexing devices associated with said further pair of ports; two of said optical paths comprising amplifiers for amplifying signals travelling in a first direction from said first pair of ports to said further pair of ports and two of said optical paths comprising amplifiers for amplifying signals travelling in a reverse direction from said further pair of ports to said first pair of ports; wherein said optical signal amplification device comprises a path converting configuration in which said optical paths are configured such that paths are provided between each of said two multiplexing devices associated with one pair of ports and each of said two multiplexing devices associated with the other pair of ports, such that co-propagating signals transported by one pair of optical fibres are directed along said paths between said multiplexing devices such that they are transported as counter propagating signals by the other pair of optical fibres. four ports comprising a first pair of ports for connecting to a first pair of optical fibres and a further pair of ports for connecting to a further pair of optical fibres; four multiplexing devices, each multiplexing device being associated with a corresponding port, each of said multiplexing devices being operable to transform between a wider band signal and two narrower band signals; optical paths for transporting said two narrower band signals between said four multiplexing devices, said optical paths being configured to provide paths between said multiplexing devices associated with said first pair of ports and said multiplexing devices associated with said further pair of ports; two of said optical paths comprising amplifiers for amplifying signals travelling in a first direction from said first pair of ports to said further pair of ports and two of said optical paths comprising amplifiers for amplifying signals travelling in a reverse direction from said further pair of ports to said first pair of ports; wherein said optical signal amplification device comprises a path converting configuration in which said optical paths are configured such that paths are provided between each of said two multiplexing devices associated with one pair of ports and each of said two multiplexing devices associated with the other pair of ports, such that co-propagating signals transported by one pair of optical fibres are directed along said paths between said multiplexing devices such that they are transported as counter propagating signals by the other pair of optical fibres.