Nortel Networks Corporation

Northern Telecom Limited

Stc, Inc

International Standard Electric Corporation

Northern Telecom Europe Limited

Richard E Epworth

Alan Robinson

Kim Byron Roberts

Terry Bricheno

Jonathan Paul King

Robin Paul Rickard

Seb J Savory

Daniel A Watley

Maurice M O'Sullivan

Stephen Wright

Nigel Edward Jolley

Julian Fells

Kevin Stuart Farley

Christopher R Fludger

Norman Derek Leggett

Roger J Brambley

Fiona Davis

George H Thompson

Kevin C Byron

Vincent Handerek

Paul Anthony Kirkby

James E Whiteaway

Martin Chown

Andrew Niall Robinson

Duncan John Forbes

Peter Jeremy Anslow

John S Leach

Peter Richard Selway

Robert A Baker

Harish Ram Sunak

David F Smith

Huai Kee

Nigel Taylor

Peter J Anslow

Benoit Charbonnier

Moira J Stewart

Marco Cavallari

Henry John Harvey

A transmitter for an optical transmission system transmits an optical sub carrier multiplexed signal comprising number of sub-carriers, onto an optical transmission path, and provides spectral shaping by different magnitudes of the sub-carriers, or different modulation formats for different sub carriers. This spectral shaping can reduce performance degradation by Kerr effect optical non linearities. This can mean higher input powers may be launched. The magnitudes can provide a signal spectrum which is lower near a center of a band of sub carriers than near an edge of the band. Such spectral shaping can be provided in the receiver either to undo the pre emphasis in the transmitter, or to reduce non linearities from components at the receiving side.

Spectral shaping for optical OFDM transmission

A coherent receiver receives a set of signals which are spaced in phase, or phase and polarization, and a reference signal. The receiver processes the set of signals to determine which of the set of signals has a predetermined association (e.g. closest in phase) with the reference signal and selects that signal as an optimum output. The receiver has the effect of de-rotating the phase slip between the input signal and reference signal in discrete steps. The selecting occurs on a repeated basis to select an optimum output. The set of signals can be processed in the analogue or digital domains, by such techniques as: comparing the amplitude of each of the set of signals with a threshold, comparing signals with each other or cross-correlation. The processing can be implemented, if desired, without the need for complicated components, which would either be expensive or become unreliable at the extreme high operating frequencies used for optical communication.

Coherent receiver

Use in an optical communications system of multiple detectors to separately detect respective multiple spectral modes of a received optical signal. The invention also provides for corresponding multi-channel, dispersion-tolerant optical receivers. Embodiments are presented both for direct detection and for coherent detection of optical signals.

Multi-detector detection of optical signals

A polarization diversity receiver has an optical section for converting the received optical signal into four or five polarization diverse component optical signals that substantially represent amplitude and polarization state information of the received optical signal, by respective polarization transformations to respective points on a Poincar&#xe9; sphere, the points being equally spaced apart to maximize polarization diversity, even in the worst case input polarization state. Detectors produce component electrical signals from each of the component optical signals, for electronic processing to compensate for PMD. By reducing the number of component optical signals significant cost and size reductions are enabled. The need for precise polarization tracking in the receiver can be reduced or eliminated completely. Balanced detectors can be used to reduce the number of electrical signals. The electrical processing can use sequence detection.

Polarization diversity receiver for optical transmission system

An optical fiber for communications systems, the fiber being designed to ensure a compensation of Kerr effects. The fiber has a profile which ensures that changes in power produce changes in distribution of power between core and cladding, such that the phase change associated with the changed spatial distribution of the power, is equal and opposite to the phase change due to Kerr Effect.

Optical waveguide

A coherent optical intradyne receiver is described which comprises an optical image reject mixer capable of dividing, mixing and recombining signals from three inputs so as to produce an inphase and a quadrature channel. On each of the channels a same set of image products is cancelled and from which channels, phase modulated signals can be recovered.

Coherent optical receiver

A variable gain optical amplifier, in which homogeneous gain broadening is dominant, has first and second fixed gain rare-earth doped optical waveguide amplifiers () optically in series together with an intervening variable attenuation optical attenuator (). This arrangement circumvents the problem of gain tilt encountered when operating such amplifiers under variable gain conditions. An alternative form of the module has variable gain waveguide amplifiers, but these are co-regulated so that the aggregate of their gain at a wavelength within the gain spectrum is maintained constant. A further alternative form of module is employed in a concatenation of such modules. In such a concatenation, the gain of individual modules is allowed to vary, but the aggregate of the gain, at a wavelength within the gain spectrum, of all the waveguide amplifiers of all the modules of the concatenation is maintained constant.

Optical amplifier modules

An optical couplercomprising at least three fibresand, portions of the fibres being fused to each other to form a coupling regionoperable to couple light therebetween. At least two of the fibres have inputsand/orand/orand at least two of the fibres have outputsand/orand/or. In use, on illumination of at least two of the inputs light intensity is emitted from a plurality of the outputs having a substantially asymmetric phase relationship, such as, for example, a quadrature phase relationship between two of the outputs. The invention also provides for methods of making such a device.

3 Fibre I and Q coupler

An optical packet is provided with an amplitude modulated (ASK) payload and a differential phase shift keyed (DPSK) header so that a routing address in the header can be read and modified at a network node without having to inject light at this node to do this modification. With ASK, though it is easy to convert a high-level bit to a low-level bit by simple attenuation, the conversion of low-level bit to high-level bit requires light injection. With DPSK, bit conversion can be effected without light injection, merely requiring passage through a phase modulator.

Optical packet switching

A method of compensating the polarisation mode dispersion (PMD) of an optical transmission path carrying clocked digital data traffic involves modulating the state of polarisation (SOP) of the light launched into the transmission path, and employing consequential frequency components in an electrical signal detected at the far end of the transmission path to regulate the birefringence of a polarisation state controller portion of a birefringent element inserted between the transmission path and the receiver's photodetector. The SOP modulation, when represented on a Poincar&eacute; sphere, has an oscillatory rotational component at a frequency f about a first axis of the sphere and an oscillatory rotational component at a frequency f about a second axis of the sphere that is orthogonal to said first axis, and where f &ne;f , f &ne;2f , and f &ne;2f .

Polarization mode dispersion compensation

A hybrid optical fiber comprises a first fiber section comprising a fiber or a concatenation of fibers having first dispersion characteristics at a predetermined operating wavelength and a second fiber section coupled to the first fiber section to form the hybrid fiber. The first fiber dispersion characteristics are selected to maintain the signal dispersion within desired limits, whereas the second fiber is optimised for low loss. Dispersion compensation is provided in the first section of the fiber span where the signal intensity is highest, and therefore the region of the fiber span where these non-linearities have greatest effect. In the second section of the fiber span, the non-linear effects can be ignored, so it is optimised for low loss rather than for dispersion compensation.

Dispersion managed fiber optic cable and system

In an optical communication system, a train of optical pulses is transmitted such that each pulse carries a set of multiplexed channels. In a receiver, the error rate for each channel is monitored and a subset of channels having the most favorable rate is selected to carry a first data stream. The remaining channels carry a second data stream. Error rate performance is determined on the basis of error detecting codes carried by the channels in the first and second data streams.

Multiplexed transmission of optical signals

Conventional quadratically chirped fiber Bragg gratings are typically apodized at both their high and low chirp ends. The present specification describes an improved Bragg grating reflector in which a second quadratically chirped region is arranged in front on the high chirp end of a substantially conventional quadratically chirped portion. The high chirp end of the first portion is not apodized; instead this apodization takes place in the second portion, and so enables the first portion to exhibit appreciable reflectivity to signals having wavelengths extending to the Bragg wavelength corresponding to the high chirp end of the first portion. The present invention thus enables the useable bandwidth of a quadratically chirped grating to be increased, and so enables an increased tuning range to be achieved in adjustable dispersion (and adjustable dispersion compensation) apparatus incorporating such gratings.

Chirped Bragg grating reflectors and adjustable dispersion apparatus incorporating such gratings

An apparatus and a method for compensating for polarisation mode dispersion in an optical signal. The apparatus includes a chirped Bragg reflector extending longitudinally along an optical waveguide, the waveguide being susceptible to stress birefringence; and at least one tuning element located at a position along the length of the waveguide. The tuning element is operable to, in use, apply a stress to the waveguide so as to alter the magnitude and the orientation of the birefringence of the waveguide for compensation of the polarisation mode dispersion of the optical signal. Higher order PMD compensation can be achieved by providing two such tuning elements along the waveguide.

A variable gain optical amplifier, in which homogeneous gain broadening is dominant, has first and second fixed gain rare-earth doped optical waveguide amplifiers ( ) optically in series together with an intervening variable attenuation optical attenuator ( ). This arrangement circumvents the problem of gain tilt encountered when operating such amplifiers under variable gain conditions. An alternative form of the module has variable gain waveguide amplifiers, but these are co-regulated so that the aggregate of their gain at a wavelength within the gain spectrum is maintained constant. A further alternative form of module is employed in a concatenation of such modules. In such a concatenation, the gain of individual modules is allowed to vary, but the aggregate of the gain, at wavelength within the gain spectrum, of all the waveguide amplifiers of all the modules of the concatenation is maintained constant.

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Sawbridgeworth, United Kingdom

Richard E Epworth