Abstract: A method for dual polarisation optical transmission is disclosed. The method comprises splitting a continuous wave light source into first and second sub-channels, optically modulating each sub-channel with a data signal, and superimposing a first pilot tone onto the first optically modulated sub-channel and a second pilot tone, different from the first pilot tone, onto the second optically modulated sub-channel. The method further comprises polarisation multiplexing the first and second sub-channels to form a polarisation multiplexed signal, in which the first and second sub-channels have orthogonal states of polarisation, and transmitting the polarisation multiplexed signal.

Abstract: An optical transmission method (10) comprising steps of receiving (12) a communication signal comprising symbols for transmission. The method comprises performing (16) linear amplitude modulation of an optical carrier with the communication signal to generate an amplitude modulated optical carrier. The method comprises performing low-pass filtering (14) to reduce a bandwidth of the symbols to less than a Nyquist bandwidth of the symbols. The method comprises transmitting (18) the amplitude modulated optical carrier. The method further comprises receiving (20) the amplitude modulated optical carrier following transmission and performing direct detection of the received amplitude modulated optical carrier to generate a received electrical signal; and determining (22) received symbols from the received electrical signal using an indication of a nonlinear impulse response of the direct detection.

Abstract: A method for dual polarisation optical transmission is disclosed. The method comprises splitting a continuous wave light source into first and second sub-channels, optically modulating each sub-channel with a data signal, and superimposing a first pilot tone onto the first optically modulated sub-channel and a second pilot tone, different from the first pilot tone, onto the second optically modulated sub-channel. The method further comprises polarisation multiplexing the first and second sub-channels to form a polarisation multiplexed signal, in which the first and second sub-channels have orthogonal states of polarisation, and transmitting the polarisation multiplexed signal.

Abstract: An optical transmission method (10) comprising steps of receiving (12) a communication signal comprising symbols for transmission. The method comprises performing (16) linear amplitude modulation of an optical carrier with the communication signal to generate an amplitude modulated optical carrier. The method comprises performing low-pass filtering (14) to reduce a bandwidth of the symbols to less than a Nyquist bandwidth of the symbols. The method comprises transmitting (18) the amplitude modulated optical carrier. The method further comprises receiving (20) the amplitude modulated optical carrier following transmission and performing direct detection of the received amplitude modulated optical carrier to generate a received electrical signal; and determining (22) received symbols from the received electrical signal using an indication of a nonlinear impulse response of the direct detection.

Abstract: A method of controlling a parameter in the generation of a coherent optical signal, the method comprising the steps of: receiving a set of signal samples relating to detection of a coherent optical signal; transforming the set of signal samples into a set of spectrum samples in the frequency domain, the set of spectrum samples being an estimation of the spectrum of the coherent optical signal; calculating at least one feedback variable based on the spectrum samples; and adjusting the parameter based on the at least one feedback variable.

Abstract: A method of controlling a parameter in the generation of a coherent optical signal, the method comprising the steps of: receiving a set of signal samples relating to detection of a coherent optical signal; transforming the set of signal samples into a set of spectrum samples in the frequency domain, the set of spectrum samples being an estimation of the spectrum of the coherent optical signal; calculating at least one feedback variable based on the spectrum samples; and adjusting the parameter based on the at least one feedback variable.

Abstract: An optical communication system comprising an optical link comprising an optical fiber, a first network element coupled to a first end of the optical link and a second network element coupled to a second end of the optical link. The first network element is configured to generate a first optical signal for transmission to the second network element, the first optical signal having a first propagation mode corresponding to a first guided mode of the optical fiber. The second network element is configured to generate a second optical signal for transmission to the first network element, the second optical signal having a second propagation mode, different to the first propagation mode, corresponding to a second guided mode of the optical fiber.

Abstract: 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 synchronize 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.

Abstract: 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.

Abstract: An optical communication system comprising an optical link comprising an optical fibre, a first network element coupled to a first end of the optical link and a second network element coupled to a second end of the optical link. The first network element is configured to generate a first optical signal for transmission to the second network element, the first optical signal having a first propagation mode corresponding to a first guided mode of the optical fibre. The second network element is configured to generate a second optical signal for transmission to the first network element, the second optical signal having a second propagation mode, different to the first propagation mode, corresponding to a second guided mode of the optical fibre.

Abstract: An optical device comprises an optical device stage, which comprises an optical input, an optical AND gate, an optical flip-flop and an optical output. The optical input is arranged to receive an optical input pulse at an input wavelength. The optical AND gate comprises a first input arranged to receive a part of said optical input pulse, a second input arranged to receive at least a part of a flip-flop optical output signal, and an output. The optical AND gate is arranged to generate an AND gate optical output pulse dependent on said flip-flop optical output signal. The optical flip-flop comprises a first input arranged to receive a further part of said optical input pulse, a second input arranged to receive a said AND gate optical output pulse, and an output. The optical flip-flop is arranged to generate said flip-flop output signal at a flip-flop output wavelength. At least a part of the flip-flop output signal is provided to said output.

Abstract: A method (10) of compensating phase noise in a coherent optical communications network. The method comprises: receiving a traffic sample (12); receiving an optical carrier and determining a phase noise estimate for the optical carrier (14); and removing the phase noise estimate from the traffic sample to form a phase noise compensated traffic sample (16).

Abstract: An optical signal processor may include an optical waveguide loop, and first and second phase modulator loops. Each of the first and second phase modulator loops may be in optical communication with the optical waveguide loop. The first and second phase modulator loops may include respective control signal input ports to control phase modulation applied by the first and second phase modulation loops. The optical waveguide loop may include two input ports to direct input signals in opposite directions in the optical waveguide loop and may further include an output port to output resulting signals.

Abstract: An optical device comprises an optical device stage, which comprises an optical input, an optical AND gate, an optical flip-flop and an optical output. The optical input is arranged to receive an optical input pulse at an input wavelength. The optical AND gate comprises a first input arranged to receive a part of said optical input pulse, a second input arranged to receive at least a part of a flip-flop optical output signal, and an output. The optical AND gate is arranged to generate an AND gate optical output pulse dependent on said flip-flop optical output signal. The optical flip-flop comprises a first input arranged to receive a further part of said optical input pulse, a second input arranged to receive a said AND gate optical output pulse, and an output. The optical flip-flop is arranged to generate said flip-flop output signal at a flip-flop output wavelength. At least a part of the flip-flop output signal is provided to said output.

Abstract: A method (10) of compensating phase noise in a coherent optical communications network. The method comprises: receiving a traffic sample (12); receiving an optical carrier and determining a phase noise estimate for the optical carrier (14); and removing the phase noise estimate from the traffic sample to form a phase noise compensated traffic sample (16).

Abstract: Optical signal processor (1) comprising an optical waveguide loop (3), and first and second phase modulator loops (6, 7), each of the first and second phase modulator loops is in optical communication with the optical waveguide loop, and the first and second phase modulator loops each comprises a respective control signal input port (8, 9) to control phase modulation applied by the phase modulation loops, and the optical waveguide loop comprising two input ports (20a, 20b) to direct input signals (10, 11) in opposite senses in the optical waveguide loop and further comprising an output port (20c) to output resulting signals. The first and second phase modulator loops may comprise nonlinear optical loop mirrors. The processor may be an optical logic gate device.