Abstract: A photonic radio frequency, RF, signal generation apparatus has a source to output a coherent optical signal at a carrier frequency; an optical splitter to split the signal into a first and a second optical carrier signal; an RF signal generation apparatus generating an RF signal at an initial frequency; an optical modulation apparatus modulating the first and second signals at the initial frequency to generate optical harmonic signals around each signal; a first optical filter to select an nth order optical harmonic signal of a first sign, +n, of the optical harmonics around the first signal; a second optical filter to select the nth order optical harmonic signal of an opposite sign, ?n, of the optical harmonics around the second signal; and a combiner to combine the selected +n and ?n signals to generate an optical beat signal forming an output photonic RF signal.

Abstract: A photonic radio frequency, RF, signal generation apparatus has a source to output a coherent optical signal at a carrier frequency; an optical splitter to split the signal into a first and a second optical carrier signal; an RF signal generation apparatus generating an RF signal at an initial frequency; an optical modulation apparatus modulating the first and second signals at the initial frequency to generate optical harmonic signals around each signal; a first optical filter to select an nth order optical harmonic signal of a first sign, +n, of the optical harmonics around the first signal; a second optical filter to select the nth order optical harmonic signal of an opposite sign, ?n, of the optical harmonics around the second signal; and a combiner to combine the selected +n and ?n signals to generate an optical beat signal forming an output photonic RF signal.

Abstract: A method (100) of encoding communications traffic bits onto an optical carrier signal in a pulse amplitude modulation, PAM, format. The method comprises: receiving (102) bits to be transmitted; receiving (104) an optical carrier signal comprising optical pulses having an amplitude and respective phases; performing (106) PAM of the optical pulses to encode at least one respective bit in one of a pre-set plurality of amplitudes of a said optical pulse; and performing (108) phase modulation of the optical pulses to encode at least one further respective bit in a phase difference between a said optical pulse and a consecutive optical pulse.

Abstract: A method (100) of encoding communications traffic bits onto an optical carrier signal in a pulse amplitude modulation, PAM, format. The method comprises: receiving (102) bits to be transmitted; receiving (104) an optical carrier signal comprising optical pulses having an amplitude and respective phases; performing (106) PAM of the optical pulses to encode at least one respective bit in one of a pre-set plurality of amplitudes of a said optical pulse; and performing (108) phase modulation of the optical pulses to encode at least one further respective bit in a phase difference between a said optical pulse and a consecutive optical pulse.

Abstract: A method of transmitting communications traffic, the method comprising steps of receiving a sequence of communications traffic bits; and mapping the sequence of communications traffic bits onto a respective one of a plurality of transmission symbols for transmission during a symbol time. Each transmission symbol is identified by a respective first symbol identifier indicative of a respective one or more of a plurality, M, of wavelengths for a transmission signal and a respective second symbol identifier indicative of a respective one or more of a plurality, N, of optical fibers on which to transmit the transmission signal.

Abstract: A digital signal processing, DSP, unit (10) for use in a coherent optical receiver for an optical communications network. The DSP unit comprises an adaptive equalizer (12) and a processing block (22). The equalizer (12) comprises input ports for receiving electrical signals, each corresponding to a different state of polarization of an optical signal received by the coherent optical receiver, and output ports, each connected to a processing branch (14). A processing branch comprises a symbol sequence estimator, SSE, (16) and a carrier phase estimator, CPE, (18) comprising an input for receiving signal taped from an output of the processing branch. An output of the CPE is connected to a phase adjuster (20) interconnecting the respective output port of the equalizer and the SSE. The processing block (22) is connected to an output of the CPE, an output of the processing branch and at least one of the output of the phase adjuster and the outputs of the equalizer.

Abstract: A method of transmitting communications traffic, the method comprising steps of receiving a sequence of communications traffic bits; and mapping the sequence of communications traffic bits onto a respective one of a plurality of transmission symbols for transmission during a symbol time. Each transmission symbol is identified by a respective first symbol identifier indicative of a respective one or more of a plurality, M, of wavelengths for a transmission signal and a respective second symbol identifier indicative of a respective one or more of a plurality, N, of optical fibres on which to transmit the transmission signal.

Abstract: A method (10) of non-linear propagation impairment equalization, the method comprising the steps of: a. receiving (12) communications traffic carried by an optical communications signal transmitted over an optical communications link; b. generating (14) a time dependent filter representation of a nonlinear time-variant impulse response of the inverse of the optical communications link; and c. applying (16) the time dependent filter representation to the received communications traffic to form non-linear propagation impairment equalized communications traffic. An optical communications link nonlinear propagation impairment equalizer and optical communications signal receiver apparatus are also provided.

Abstract: A digital signal processing, DSP, unit (10) for use in a coherent optical receiver for an optical communications network. The DSP unit comprises an adaptive equaliser (12) and a processing block (22). The equaliser (12) comprises input ports for receiving electrical signals, each corresponding to a different state of polarization of an optical signal received by the coherent optical receiver,and output ports,each connected to a processing branch (14). A processing branch comprises a symbol sequence estimator, SSE, (16) and a carrier phase estimator, CPE, (18) comprising an input for receiving signal taped from an output of the processing branch. An output of the CPE is connected to a phase adjuster (20) interconnecting the respective output port of the equaliser and the SSE. The processing block (22) is connected to an output of the CPE,an output of the processing branch and at least one of the output of the phase adjuster and the outputs of the equalizer.

Abstract: A method (10) of non-linear propagation impairment equalisation, the method comprising the steps of: a. receiving (12) communications traffic carried by an optical communications signal transmitted over an optical communications link; b. generating (14) a time dependent filter representation of a nonlinear time-variant impulse response of the inverse of the optical communications link; and c. applying (16) the time dependent filter representation to the received communications traffic to form non-linear propagation impairment equalised communications traffic. An optical communications link nonlinear propagation impairment equaliser and optical communications signal receiver apparatus are also provided.

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 2n-QAM (e.g. 16-QAM) optical modulator comprising cascaded I-Q modulators. The first I-Q modulator applies 2n-2 (e.g. 4) QAM to an optical signal, having a constellation diagram with the 2n-2 (e.g., 4) constellation points located in quadrant I. The second I-Q modulator subsequently applies a quaternary phase-shift keying (QPSK) modulation scheme to the optical signal, thereby rotating the constellation points of the 2n-2-QAM modulation scheme to quadrants II, III and IV, to produce a 2n-QAM modulation constellation diagram. The rotation causes the 2n-QAM modulator to inherently apply four quadrant differential encoding to the optical signal. A method of 2n-QAM optical modulation is also provided and optical signal transmission apparatus comprising the 2n-QAM optical modulator.

Abstract: A method of detecting a signal in an optical receiver is described. The method includes converting a received optical signal to a digital electrical signal comprising a plurality of samples, applying a predetermined phase rotation to said samples to obtain amplitude and phase components of phase range adjusted sample values, and performing a first detection process based on the amplitude and phase components of the phase range adjusted sample values.

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 equalizer (60) processes, in the electrical domain, a signal obtained from a path of an optical transmission system. The equalizer comprises N cascaded stages (where N?1). At least one of the stages comprises a cascade of a linear equalization element (61) and a non-linear equalization element (62). The equalizer (60) is able to compensate for both linear impairments, such as dispersion, and non-linear impairments. The cascaded linear and non-linear elements can simulate the effect of signal propagation through a fiber which has the opposite propagation parameters (e.g. attenuation, dispersion, non-linearity) to those of the propagation path experienced by a signal in the transmission system. The non-linear equalization element (62) can be a non-linear phase rotator which rotates phase of an input signal proportional to the squared modulus of the input signal amplitude.

Abstract: A method of detecting a signal in an optical receiver is described. The method includes converting a received optical signal to a digital electrical signal comprising a plurality of samples, applying a predetermined phase rotation to said samples to obtain amplitude and phase components of phase range adjusted sample values, and performing a first detection process based on the amplitude and phase components of the phase range adjusted sample values.

Abstract: A 2n-QAM (e.g. 16-QAM) optical modulator comprising cascaded I-Q modulators. The first I-Q modulator applies 2n-2 (e.g. 4) QAM to an optical signal, having a constellation diagram with the 2n-2 (e.g., 4) constellation points located in quadrant I. The second I-Q modulator subsequently applies a quaternary phase-shift keying (QPSK) modulation scheme to the optical signal, thereby rotating the constellation points of the 2n-2-QAM modulation scheme to quadrants II, III and IV, to produce a 2n-QAM modulation constellation diagram. The rotation causes the 2n-QAM modulator to inherently apply four quadrant differential encoding to the optical signal. A method of 2n-QAM optical modulation is also provided and optical signal transmission apparatus comprising the 2n-QAM optical modulator.

Abstract: A 2n-QAM (e.g. 16-QAM) optical modulator comprising cascaded I-Q modulators. The first I-Q modulator applies 2n?2 (e.g. 4) QAM to an optical signal, having a constellation diagram with the 2n?2 (e.g., 4) constellation points located in quadrant I. The second I-Q modulator subsequently applies a quaternary phase-shift keying (QPSK) modulation scheme to the optical signal, thereby rotating the constellation points of the 2n?2-QAM modulation scheme to quadrants II, III and IV, to produce a 2n-QAM modulation constellation diagram. The rotation causes the 2n-QAM modulator to inherently apply four quadrant differential encoding to the optical signal. A method of 2n-QAM optical modulation is also provided and optical signal transmission apparatus comprising the 2n-QAM optical modulator.

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 equaliser (60) processes, in the electrical domain, a signal obtained from a path of an optical transmission system. The equaliser comprises N cascaded stages (where N?1). At least one of the stages comprises a cascade of a linear equalisation element (61) and a non-linear equalisation element (62). The equaliser (60) is able to compensate for both linear impairments, such as dispersion, and non-linear impairments. The cascaded linear and non-linear elements can simulate the effect of signal propagation through a fibre which has the opposite propagation parameters (e.g. attenuation, dispersion, non-linearity) to those of the propagation path experienced by a signal in the transmission system. The non-linear equalisation element (62) can be a non-linear phase rotator which rotates phase of an input signal proportional to the squared modulus of the input signal amplitude.