Nortel Networks Corporation

Northern Telecom Limited

Stc, Inc

Ciena Corporation

Alan Robinson

Richard E Epworth

Jonathan Paul King

Maurice M O'Sullivan

Kim Byron Roberts

Simon P Parry

Robin Paul Rickard

Terry Victor Clapp

Vincent Handerek

Kim Bryon Roberts

Nigel Lawrence Bragg

Kevan Peter Jones

Terry Bricheno

Robert William Keys

Rongqing Hui

Adrian Paul Sparks

Mark R Hinds

Barrie Flintham

Nigel Edward Jolley

Julian Fells

Kevin Stuart Farley

Ross Alexander Saunders

Christopher R Fludger

Daniel A Watley

Bram Peeters

Stephen Wilson

Mark E Bray

James Regan

Gary S Sapsford

Alan A Solheim

Huai Kee

Joseph Mun

Dietmar Garthe

Ian F Scanlan

Benoit Charbonnier

Hual Kee

Marco Cavallari

Henry John Harvey

Richard Goodwin

Gregory John Milner

Martin P Poettcker

Robert A Baker

Roger M Gibb

Toby J Reid

A method of encrypting an optical communications signal involves determining an encryption function, filtering an electrical input signal using the encryption function to generate an encrypted electrical signal, and modulating an optical source using the encrypted electrical signal to generate a corresponding encrypted optical signal. This is then transmitted through an optical communications system. The encryption is selected such as to substantially remove symbol definition from the optical signal. This method provides digital signal processing of an electrical input signal in order to derive a signal for controlling an optical modulator in such a way that the optical signal transmitted over the link is a continuous analogue signal rather than a series of discrete symbols which alternate between well-defined signal values. This makes it difficult for a third party to derive the binary bit sequence encoded by the optical signal.

Data security in optical communications systems

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

An optical communications system has a plurality of spans between a transmitter and a receiver. The receiver has optical to electrical conversion circuitry for converting the received optical signal to an electric signal, analogue to digital conversion circuitry and digital signal processing means for analysing the electrical digital signal. The digital signal processing means derives information concerning characteristics of individual spans from the electrical digital signal. This enables parameters such as per-span variations in provisioned power, local dispersion and span loss to be measured. In-service measurements of system characteristics can be used to enable optimisation of the network operation.

Analysis and control of optical communications systems

An optical receiver for receiving an optical differential phase shift keyed signal has an optical component sensitive to the optical phase of the signal, such as an interferometer, a device arranged to generate a control signal by non linear limiting of an output of the optical component, such as an RF amplifier arranged to operate in a region near saturation point, and a phase controller for tuning a phase response of the optical component to the received signal according to the control signal.

DQPSK receiver phase control

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

The invention provides a method of balancing the power level of an optical signal within an aggregate of optical signals in an amplified dense wavelength division multiplexing DWDM optical network, as well as power balancing apparatus therefor. The method involves measuring the power level of the optical signal received at an optical receiver through the optical link and referencing said measured receive power level to a nominal power level in the multiplex immediately preceding the receiver; measuring the power level of the optical signal at an optical transmitter and referencing said measured transmit power level to a nominal power level in the multiplex immediately following the transmitter; calculating a mean power level of the optical signal from said optical measurements, the mean power level of the optical signal being the mean of the power levels of the optical signal at the transmitter and at the receiver referenced to the nominal power levels in the associated multiplexes; and adjusting the mean power level of the optical signal independently of the power levels of other optical signals carried on the same optical link to the receiver so that a desired OSNR at the receiver is achievable.

Power balancing in DWDM optical networks

An optical amplifier comprises a doped fiber core and a cladding layer surrounding the core. The mode field diameter of the fiber is greater than 8 &#x3bc;m and the refractive index difference between the core and the cladding layer is selected such that the cut-off wavelength at which the fiber becomes single mode lies in the range 1000-1550 nm. This amplifier uses a large made field diameter fiber, which reduces the intensity for a specified output power. This results in reduced filtering of the low frequency components of the signal. The refractive index difference between the core and cladding is selected such that the fiber is multi-mode at 980 nm, which enables bend performance to be improved.

Optical fiber amplifier

An optical dispersion compensation device includes a first optical compensation unit that applies non-linear dispersion compensation across a signal band, the first optical compensation unit being coupled to a second optical compensation unit that applies a degree of linear dispersion compensation across the signal band. The approach taken is to provide broadband dispersion compensation by applying dispersion slope compensation across the signal band to equalise residual dispersion slope and by applying a degree of linear compensation separately to affect the required linear dispersion compensation. Using these two degrees of freedom it is possible to set the desired dispersion slope and linear dispersion (whether positive or negative) to affect broadband dispersion compensation without needing to demultiplex the optical signal.

Chromatic dispersion compensation

Dispersion compensation devices are described which comprise waveguides including sampled Bragg gratings which exhibit comb-like reflectance characteristics. The profile of effective refractive index along the length of the grating is controlled to adjust the position of the teeth and/or to control the dispersion exhibited by the device (i.e. to control the chirp of the grating). The devices can thus be used to provide dispersion compensation to any one of a number of wavelength channels in a WDM system. In preferred arrangements, the effective refractive index distribution is set by a applying a temperature distribution along the length of the grating, or by setting an applied electric field.

Dispersion compensation apparatus and method utilising sampled Bragg gratings

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

The fibre fuse effect, also termed self propelled self focusing (SPSF) is a catastrophic damage mechanism resulting from a runaway thermal affect which can melt optical fibres. A method and apparatus for limiting the damage caused by a fibre fuse is provided by utilising a detector external to the fibre for monitoring the condition of the fibre.

Fibre fuse protection

A method of measuring the polarization mode dispersion (PMD) of an optical transmission path carrying clocked digital data traffic involves modulating the state of polarization (SOP) of the light launched into the transmission path, and detecting consequential frequency components in an electrical signal detected at the far end of the transmission path. 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 measurement

An optical waveguide format tuneable optical filter is formed using Bragg grating reflectors to define one or more optical cavities of a Fabry P&eacute;rot etalon, the or each cavity of the etalon including one or more slots containing a controllable refractive index medium, for instance a silicone gel whose index is regulated by Joule heating, or a polymer dispersed liquid crystal whose index is electric field regulated. Strong coupling coefficient (&tilde; a few tens mm ) gratings may be made using lithography to form linear arrays of buried high index inclusions extending along the waveguide core axis.

Tuneable optical filters

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Harlow, United Kingdom

Alan Robinson