Muxponder and method of converting a plurality of tributary optical communications signals having a first bit rate into an optical line signal having a second, higher bit rate

A muxponder comprising: modulation format conversion apparatus comprising: first and second inputs each arranged to receive an amplitude modulated tributary optical signal carrying a communications traffic bit stream; first and second optical to electrical signal conversion apparatus each arranged to receive a respective tributary optical signal and to convert it into a corresponding tributary electrical signal carrying the communications traffic bit stream; a delay element arranged to synchronise the communications traffic bit streams; and an optical IQ modulator arranged to receive an optical carrier signal and the tributary electrical signals. The optical IQ modulator having an in-phase arm and a quadrature arm, each arm being arranged to receive one of the tributary electrical signals such that each tributary electrical signal drives the respective arm of the optical IQ modulator to encode the communications traffic bit streams onto the optical carrier signal in a multilevel modulation format.