Abstract: A method and apparatus for determining propagation delay of a first path and or of a second path which connect a first transceiver unit associated with a first clock to a second transceiver unit associated with a second clock in a communications network. The apparatus comprises a control unit configured to cause the first transceiver unit to transmit a first signal to the second transceiver unit over the first path and to receive a reply to the first signal from the second transceiver unit over the second path, and to transmit a second signal to the second transceiver unit over the second path and to receive a reply to the second signal from the second transceiver unit over the first path.

Abstract: A transport network (10) is configured to connect a plurality of remote radio units (3) with a plurality of digital units (5) in a radio access network. The transport network comprises an electronic cross-connect (31) common to the plurality of remote radio units and digital units, and a control unit (32) configured to control the electronic cross-connect. The transport network further comprises an optical link (40) between the electronic cross-connect and remote radio units. The electronic cross-connect is a multi-layer switch. The electronic cross-connect is configured to switch data flows between one or more of said plurality of digital units (5) and one or more of said plurality of remote radio units (3).

Abstract: The present invention provides an optical source comprising a laser, a first optical power splitter, phase changing equipment, polarisation state setting equipment, a polarisation beam coupler and an output. The laser is configured to generate an optical signal. The first optical power splitter is configured to split the optical signal into a first optical signal and a second optical signal. The phase changing equipment is configured to change the phase of at least one of the first optical signal and the second optical signal such that the first optical signal has a first phase and the second optical signal has a second phase different from the first phase by a preselected phase difference.

Abstract: Validating a path in an optical layer of a communications network, for client traffic having an associated service level, involves setting an optical quality margin according to the service level associated with that client traffic. The optical quality margin indicates how close an estimated optical quality of the path can approach a level which produces a threshold error rate. This margin is used to check whether the estimated optical quality is within the optical quality margin set according to the client traffic service level. Making the optical quality margin dependent on client traffic service level, can enable increased optical reach. This can give more flexibility in path selection and enable better matching to service levels of client traffic.

Abstract: An optical network comprising an optical network element comprising a first optical transmitter, a first controller, an optical receiver, a second optical transmitter, a second controller and optical receiver apparatus. Said first controller is arranged to control said first optical transmitter to generate and transmit a first optical signal in response no second optical signal being detected. Said first controller is arranged to iteratively generate and transmit said first optical signal at different wavelengths of a plurality of wavelengths until said second optical signal is detected, and is further arranged to subsequently maintain generation and transmission of said first optical signal at said wavelength at which said second optical signal is detected. Said second controller is arranged to control said second optical transmitter to generate and transmit said second optical signal following detection of said first optical signal by said optical receiver apparatus.

Abstract: A transceiver (4) comprising a receive part (70) configured to receive and detect a first signal carried on an optical carrier, wherein the signal is in a first part of a RF spectrum. The transceiver (4) further comprises a modulator (68) configured to modulate the same optical carrier with a second signal in a second part of the RF spectrum. The transceiver comprises a transmit part (60) configured to transmit the optical carrier modulated with the second signal. The first part of the RF spectrum is separate to the second part of the RF spectrum. The first signal and/or second signal are spectrally compressed signals.

Abstract: A bidirectional WDM optical communications link has WDM signals sent in opposite directions along a shared optical path and using at least one common wavelength. An optical amplifier (20, 21, 22, 70, A1D, A2U, A2D) optically amplifies (144) a first WDM signal separately from a second WDM signal in the other direction. This separated optical amplification is controlled (134) according to indications of transmission quality at the common wavelength, to alter the relative optical powers of the first and second WDM signals to enable crosstalk at the common wavelength to be limited. Cross talk at the common wavelength can be improved by rebalancing relative amounts of cross talk in the different directions, to enable the capacity benefits of using a common wavelength for both directions to be obtained while using greater optical signal power. This is particularly useful where the optical power is asymmetric, such as in WDM PON systems.

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 switch configured to connect a plurality of remote radio units with a plurality of main units in a radio access network. The switch comprises a first stage comprising one or more first units configured to receive a signal originating from a remote radio unit, and configured to output one or more spectral component having a wavelength. The switch further comprises a central stage configured to receive the one or more optical wavelength, and transfer the spectral component to an output port determined by the wavelength. The switch further comprises a second stage comprising one or more second units configured to receive the one or more spectral component from the central stage and output a signal towards a main unit. The first unit is configured to control the wavelength of the output spectral component such that the spectral component is transferred by the central stage to a selected one of said second units.

Abstract: A radio-over-fibre transmitter comprising: an optical splitter arranged to receive an optical carrier signal having a carrier optical frequency, and split it into a plurality of portions; electro-optic modulation apparatus each arranged to receive a respective optical carrier signal portion and a respective modulated radio frequency subcarrier signal, and arranged to modulate the respective optical carrier signal portion with the respective modulated radio frequency subcarrier signal and arranged to suppress onward transmission of the respective optical carrier signal portion, to form a respective carrier suppressed optical subcarrier signal; an optical combiner arranged to receive the carrier suppressed optical subcarrier signals and one of the optical carrier signal portions and arranged to combine them to form a subcarrier multiplexed optical signal; and polarisation apparatus arranged to ensure that the carrier suppressed optical subcarrier signals and said optical carrier signal portion each have the sam

Abstract: A supervisory channel is provided on an optical path (31) between nodes of an optical communication network. The nodes are arranged to use a set of wavelengths allocated for carrying traffic channels. An optical signal (16) which carries a supervisory channel is generated at a supervisory channel transmitter (15) and added (12) to the optical path (31) downstream of an optical amplifier (11). The optical signal (16) has a wavelength which is one of the set of wavelengths allocated for carrying traffic. The method is performed at a time when the wavelength is not being used to carry traffic. An impairment parameter of the received optical signal is measured at a supervisory channel receiver (15). The receiver is a coherent receiver and the impairment parameter is chromatic dispersion or polarization mode dispersion.

Abstract: The invention provides a free space optical (FSO) communications terminal for a first telecommunications card or a backplane. The FSO terminal comprises a plurality of transmission interfaces. The FSO terminal further comprises a light signal generating unit adapted to generate a plurality of light signals. Each of the plurality of light signals carries the same information as the other one or more of the plurality of light signals and is arranged for transmission through a respective one of the plurality of transmission interfaces. Each of the plurality of light signals is at a different orthogonal mode from the other one or more of the plurality of light signals. The invention further provides a free space optical (FSO) communications terminal for a second telecommunications card or a backplane. The FSO terminal comprises a plurality of receive interfaces. Each of the plurality of receive interfaces adapted to receive a light signal carrying information.

Abstract: An optical modulator is described, comprising—an optical splitter to receive an optical carrier signal and split it into sub-signals; modulation assemblies, each comprising a binary optical modulator to apply a binary phase shift keyed (BPSK) modulation to a respective optical sub-signal to form a BPSK modulated optical sub-signal. Modulation assemblies comprise a phase-shifter, each to apply a phase shift to the respective BPSK modulated optical sub-signal in dependence on a pre-selected modulation format; an encoding apparatus to receive communications traffic to map into symbols, and to generate and transmit a respective drive signal to the optical modulator of a modulation assembly, each drive signal to cause the optical modulator to apply said BPSK modulation, to encode the symbols onto the respective optical carrier sub-signal; and a combiner to receive each BPSK modulated optical sub-signal to form an output optical signal having said 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 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: An optical switch has four optical ports; a first optical waveguide coupled between a first of said ports and a second of the ports; a first switch element provided between the first waveguide and a second optical waveguide that is coupled to a third of the ports; a second switch element provided between the first waveguide and a third optical waveguide that is coupled to a fourth of the ports. Each switch element has a micro-ring resonator having an active state in which it is coupled to the first waveguide and to a respective one of the second and third waveguides for optical signals at a preselected wavelength, and an inactive state in which no coupling occurs. Each switch element has a control element arranged to receive a respective control signal configured to cause it to switch the micro-ring resonator between said states.

Abstract: A method of automatically tuning subcarriers of a superchannel transmission. The method comprises determining (71) a received parameter of transmission quality of one or more of the plurality of subcarriers. The method further comprises tuning (72) a frequency of the one or more subcarrier based on the determined parameter of the transmission quality.

Abstract: A node (20, 25) for a single fiber bidirectional WDM optical ring network has a first optical protection switch (100) having first and second ports for coupling to the single fiber bidirectional ring, while providing a pass through optical path for wavelengths on the bidirectional ring. A further port is coupled to an external optical path. In operation the switch couples optically bidirectional selected wavelengths between the external optical path and either of the first and second ports selectively, according to an indication of a fault on the ring, so as to use different portions of the bidirectional ring respectively as working path and protection path. This combines coupling wavelengths with the ring, with the selection of protection or working path, which simplifies the optical equipment, and upstream and downstream optical delays can be symmetrical.

Abstract: A receiver (1) is configured to receive an optical transmission, wherein the receiver is configured to determine a plurality of values over time of the quality of the optical transmission. The receiver comprises one or more low pass filter (26) configured to low pass filter the values of quality of the transmission. Based on the low pass filtered values, the receiver (1) is configured to determine a FEC code and/or modulation type for a further optical transmission.

Abstract: An optical multi carrier signal has a modulation format and has many individual carrier signals. Parameters of the signal are controlled by receiving an indication of individual carrier transmission performance of the individual carrier signals, and selecting parameter values for the individual carrier signals, the parameter values comprising both a carrier FEC overhead and a carrier bandwidth for the modulation format. Selection is made according to the indicated individual carrier transmission performance and according to an overall spectral efficiency of the multi carrier signal. The selected parameter values are output for control of the optical multi carrier signal. By selecting values for both parameters rather than either one, better optimisation can be obtained since they are interdependent. The control can have better granularity than changing modulation format, and can make better use of bandwidth or improve the overall capacity.