British Telecommunications Public Limited Company

British Telecommunications Corporation

Corning Incorporated

Andrew David Ellis

David M Spirit

David Arthur Davies

William A Stallard

Dmitri Vladislavovich Kuksenkov

Shenping Li

Kevin Smith

David Graham Moodie

Julian Kazimierz Lucek

Robert John Manning

David M Patrick

Danny Robert Pitcher

David Arthur Owen Davies

Derek Malyon

Terrence Widdowson

Robert Michael Percival

Derek John Malyon

William Andrew Stallard

Apparatus for recovering clock pulses of wavelength division multiplexed optical signals passed therethrough. The apparatus comprises an optically-pumped laser cavity defining a cavity length and comprising a nonlinear medium pumped at a wavelength selected to give efficient parametric amplification within said medium. The cavity length corresponds to an integer multiple of bit periods of at least one of the multiplexed optical signals. The optical signals copropagate through the medium with the pump radiation. The apparatus further comprises an optical path for recirculating a proportion of the output from the laser cavity back through the laser cavity. In this way, idler waves are generated symmetrically about the pump wavelength by four wave mixing with the at least one of the multiplexed optical signals and recirculated through the laser cavity to be amplified by parametric amplification in order to recover wavelength division multiplexed clock pulses.

Recovery of clock pulses of wavelength division multiplexed optical signals

An optical network includes a number of nodes coupled to an optical transmission medium, such as an optical fiber bus. Each of the nodes includes a dark pulse generator. Different nodes output dark pulses in different time slots onto the transmission medium, forming a dark pulse OTDM (optical time division multiplexed) signal. The network may have a re-entrant bus topology.

Dark pulse TDMA optical network

A number of optically cascaded electroabsorption modulators (EAM) are electrically biased to provide an optical time division multiplexed (OTDM) signal. The optical input into the first of the cascaded EAMs is a substantially cw light beam In operation, each EAM is normally biased to allow transmission of light, and data is represented by blocking transmission of light for a short period, thereby imposing dark pulses onto the light. Thus, light between dark pulses remains unperturbed and is available for further modulation by subsequent EAMs. In this way, each EAM can be arranged to rpovide a single data channel of the overall OTDM signal.

Dark pulse generation and transmission

Optical signals used for telecommunications in time division multiplexed format are processed to remove channels for local demodulation and to replace the removed channels with locally modulated data. The received signals are subjected to a wavelength modulation so that signals in selected channels are given distinctive wavelengths. After wavelength modulation, the time division multiplexed signals are separated using wavelength sensitive splitters and the various channels are sent to their correct destinations. In one embodiment, one channel is selected and given a distinctive wavelength and all the non-selected channels retain their original wavelength. The channels with the original wavelength are provided to an output terminal for onward transmission whereas the selected channel is provided for local demodulation.

Optical switch

Chromatic dispersion compensating node

Node in optical TDM network

All-optical processing in communications systems

Optical switching device

A fault signalling system for an optical fiber-based communications link having erbium doped fiber amplifier repeaters comprises a broadband optical source for generating a fault signal in the event of a fault occurring. Each repeater includes such a broadband source, which will output a characteristic fault signal onto the link for transmission to a receiving station, in the event of a fault being detected at that repeater. Advantageously, a control circuit which enables the source to emit its fault signal when appropriate is also used as part of an automatic gain control for the output of the amplifier repeater in normal operation.

Optical communication link fault signalling

An AGC for a laser amplifier uses a control tone modulated onto the optical signals. During use the modulation on the optical signals is superimposed on the drive current for the laser. The AGC derives a control signal by detecting the modulation in the drive current, multiplying it by the drive current. The drive current for the laser is adjusted to keep the control signal constant. In a preferred embodiment a low level sensor detects when the control tone falls below a threshold level and switches the circuit to a default configuration. The default configuration provides a distress signal to indicate a fault and to identify the affected amplifier. This preferred embodiment is particularly suitable for submarine telecommunications capable which include a plurality of inaccessible amplifiers.

Amplication of optical signals

Growing community of inventors

Ipswich, United Kingdom

Andrew David Ellis