British Telecommunications Public Limited Company

British Telecommunications Corporation

Edmund S R Sikora

Peter Healey

Mark A Shackleton

David J T Heatley

The present invention relates to the communication of information where the information is introduced at a plurality of spaced apart locations onto an optical waveguide. The following steps are performed: (i) transmitting test signals onto the waveguide, components of the test signals being returned in a distributed manner along the waveguide; (ii) receiving first component signals returned from beyond a first location at which information is introduced; (iii) receiving second component signals returned from beyond a second location at which information is introduced, the second component signals being returned through the first location; and, (iv) processing the second component signals using the first component signals to distinguish information introduced at the second location from information introduced at the first location.

Communicating information

A network is assessed, in particular, a network having a main line and a plurality of branch lines. The method includes the steps of: (i) introducing test signals into the main line, the main line being coupled to the branch lines; (ii) imposing a modulation on test signals which propagate along the branch line, and (iii) monitoring test signals returned along the main line. The imposed phase modulation is detected interferometrically such that the test signals from the designated branch line can be distinguished with respect to the signals returned from other branch lines.

Assessing a network

The present invention relates to a method of and apparatus for evaluating the position of a disturbance, in particular using a waveguide having a plurality of overlap regions. A position sensor is provided including: an optical waveguide; a transmission stage for launching a sensing signal into the waveguide; a receiving stage arranged to receive a returned sensing signals which returned sensing signal is a time distributed signal derived from backscattered components of the sensing signal, the waveguide being arranged along a path having a plurality of overlap regions such that a disturbance in an overlap region causes a first disturbance feature and a second disturbance feature in the returned sensing signal; and, monitoring means for monitoring the returned signal, such that a respective time of return can be associated with the first and second disturbance feature. Both return features can then be used to evaluate the position of the disturbance so as to enhance the resolution of the position sensor.

Evaluating the position of a disturbance

The present invention relates to the communication or reproduction of sound, in particular audible sound. There is provided a method of communicating a sound, including the steps of: transmitting, onto an optical link, pairs of signal copies, the signal copies of a given pair having a time offset relative to one another; applying the audible sound to the optical link, such that the audible sound causes a phase modulation to the transmitted signal copies; receiving modulated signal copies previously transmitted onto the link; and, for received pairs of signal copies, causing one signal copy of a pair to mix with the other signal copy of that pair so as to produce a signal representative of the sound. The sound is an audible sound. It has been found that a delay of at least 75 micro seconds allows for a better reproduction of audio sounds, since these have a relatively low frequency content.

Communicating or reproducing an audible sound

The present invention relates to a method for detecting or inferring a physical disturbances on a communications link, in particular by using distributed backscattering. The method includes the steps of: transmitting test signals onto a link; receiving test signals returned from a remote portion of the link; performing a function on the returned test signals; and in dependence on at least one characteristic of the combination signal, inferring the presence of a disturbance. The test signal are returned by a process Rayleigh backscattering along the fibre, so existing fibre installations can be used without requiring a mirror to be specifically introduced.

Distributed backscattering

The position of a disturbance on an optical link is evaluated, in particular where the disturbance is a time-varying disturbance. An optical time domain reflectometry technique is used in which a series of low coherence test pulses is launched by means of an optical pulse source into an the optical link, and the backscattered return signal is monitored. The test pulses pass through an unbalanced Mach Zhender interferometer with the result that for each test pulse, a pair of time-displaced pulse copies is launched onto the link. The backscattered return signal is passed through the same interferometer, which causes the pulse copies of each pair to become realigned and to interfere with one another. A time-varying disturbance is likely to affect each pulse copy of a pair differently. As a result, an abnormality such as a step is likely to occur in the backscattered signal. From the time position of an abnormality, the distance of the disturbance responsible is evaluated.

The location or identification of a waveguide is determined, in particular where a disturbance is applied to the optical waveguide. This is achieved by: (a) monitoring the waveguide at a first location to sense a disturbance along the waveguide; (b) causing a disturbance to the waveguide at a second location; (c) transmitting a sensor signal to the second location; and (d) inferring, from the sensor signal, the presence or identity of the waveguide at the second location.

Identifying or locating waveguides

A secure optical communication scheme uses differential delay D in an unbalanced Mach-Zehender interferometer to provide two copies of the optical source signal at a remote phase modulator separated in time by D. As D is much bigger than the coherence time source, the two copies of the signal are effectively uncorrelated. Both signals are phase-modulated by the remote sender's data and returned to the unbalanced interferometer. The phase modulator will be converted into amplitude modulation by the action of the interferometer.

Secure optical communication

An optical sensing method transmits radiation from a source onto an optical link, the radiation being passed through an unbalanced interferometer connected between the source and the link. The radiation which is reflected back and has travelled through a portion of the link which is subject to an external disturbance is received and passed through the same unbalanced interferometer. A detector is used to detect the radiation that has returned through the unbalanced interferometer. To improve sensitivity, the unbalanced interferometer includes a coupler for coupling radiation to the detector, the coupler being an N &#xd7;N coupler where N is an integer greater or equal to 3.

Optical sensor with interferometer for sensing external physical disturbance of optical communications link

The present invention relates to an disturbance sensing system, in particular an optical system in which a disturbance can be inferred. The system includes: a first waveguide portion and a second waveguide portion disposed in a side to side arrangement relative to one another; launch means for launching a first signal and a second signal onto the first waveguide portion and the second waveguide portion respectively, the first and second waveguide portions being optically coupled such that each of the transmitted first and second signals travels along each of the first and second waveguides; and, combining means for combining the transmitted first and second signals so as to produce a combination signal, the first and second signals being related to one another such that a disturbance in either one of the first and the second waveguide portions can be inferred from the combination signal. The noise or other signal resulting from a disturbance applied to both waveguide portions will be at least partially suppressed.

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Ipswich, United Kingdom

Edmund S R Sikora