Abstract: A method for generating an optical single sideband signal comprising the steps of splitting an optical field into two parts and introducing a relative phase delay of +/??/4 radians in each direction of transmission to one of the parts, intensity reflection-modulating each part with electrical signals having a relative phase delay of +/??/2 radians and then recombining the reflection-modulated signals.

Abstract: This concerns a protected long-reach PON having a plurality of terminals connected to a distribution network that is fed by both a main and back up feed, each feed including a head end and a repeater. The back up head end had access to a ranging table with data previously obtained by the main head end, thereby speeding up the switch over in the event of a fault with the main feed. In one embodiment, the repeater has a virtual ONU therein, allowing the back up repeater to be ranged by the back up head end, thereby yet further speeding up the ranging procedure. The main and back up repeaters are sufficiently equidistant from the distribution network to allow the back up head to perform normal scheduling without performing a ranging operation on each of the terminals, even if the different terminals transmit at slightly different wavelengths. This is achieved using the ranging information obtained with regard to the back up repeater.

Abstract: The present invention relates to communication, in particular to communication involving analogue-like signals that are multiplexed together. There is provided a method of communicating with a plurality of devices, each of which of which provides a signal whose amplitude varies in an analogue fashion, the method including the steps of: temporally interleaving the signals from the devices so as to form a multiplexed signal having a plurality of sequential pulses, which pulses can, in an analogue fashion, each have an amplitude within a continuous range, each pulse originating from a respective device, and each pulse having a plurality of temporal portions; and, forming a synchronisation signal within the multiplexed signal for demultiplexing the multiplexed signal, the synchronisation signal being formed by reducing the amplitude within a temporal portion of at least some of the pulses.

Abstract: The present invention relates to a method of and apparatus for evaluating the position of a time-varying disturbance on an optical waveguide. The steps include: transmitting sensing signals onto the optical waveguide, which sensing signals have imposed thereon a modulation which is dependent, at least in part, on their time of transmission; receiving returned sensing signals, which signals have been exposed to the disturbance; and, from the previously imposed modulation on the returned sensing signals, evaluating the position of the disturbance. Because the sensing signals have a modulation that is dependent at least in part on their transmission time, the round-trip time for the signals to travel to and from a disturbance can be inferred. From this round trip time, or a value related thereto, it is possible to directly or indirectly evaluate the position of the disturbance.

Abstract: An optical fiber network comprises a laser source (1a) configured to generate laser light of a plurality of wavelengths. A first optical fiber (4a), transmits multi-wavelength light from the laser source to a location remote from the laser source. A wavelength division multiplexer (2) at the remote location (203) is connected to a plurality of second optical fibers (8). A plurality of optical modulators (9) are each connected optically to the wavelength division multiplexer (2) via a respective second optical fiber (8). The wavelength division multiplexer (2) is arranged to de-multiplex the multi-wavelength light received from the first optical fiber (4a) into a plurality of wavelengths and to supply a respective wavelength to each of the second optical fibers (8). The optical modulators (9) are reflective optical modulators each arranged to modulate light received from the associated second optical fiber (8) with a data signal and to reflect the modulated light back along the second optical fiber (8).

Abstract: The present invention relates to testing a passive optical network having a head end and a plurality of terminals, which terminals are connected to the network at a respective plug and socket arrangement located at customer premises. A test device is provided having a standard plug for connection in place of a terminal in the socket of the relevant customer premises. An identification device is provided in the socket. The test device is arranged to (i) read the identity of the identification device and (ii) test the line and transmit a result to the head end together with the identity so that the line to the customer premises can be tested remotely. When the line has been tested, the test device can be removed. Because the test device uses an existing connection, the optical (insertion) loss is reduced.

Abstract: 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.

Abstract: A method for generating an optical single sideband signal comprising the steps of splitting an optical field into two parts and introducing a relative phase delay of +/??/4 radians in each direction of transmission to one of the parts, intensity reflection-modulating each part with electrical signals having a relative phase delay of +/??/2 radians and then recombining the reflection-modulated signals.

Abstract: 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.

Abstract: The present invention relates to a system for sensing of a disturbance on an optical link. Data traffic from an optical source with a short coherence length is transmitted along the link to a receiver station on one or more of a plurality of time-division-multiplexed channels. One of the channels is used to transmit encoded phase information relating to the phase characteristics of the optical source output. At the receiver station, the actual phase characteristics of the arriving light from the optical source is compared with the encoded phase information. Since a physical disturbance of the link is likely to alter the actual phase characteristics of the arriving light but not the encoded phase information, it is possible to determine if a physical disturbance has occurred. The system can conveniently be used to monitor an optical link carrying communications traffic.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: The present invention relates to a system for sensing a disturbance in a sensing region along an optical path. The sensing system includes: a source of optical radiation; an upstream junction for channelling light along a first path and along a second path, the first and second paths each extending through the sensing region in an acoustically coupled relationship, such that, in use, a disturbance experienced at one of the first and second paths is experienced at the other of the first and second paths; and, a downstream junction for combining light from the first path with light from the second path so as to provide an interferometric signal from which the presence of a disturbance can be inferred.

Abstract: This concerns a protected long-reach PON having a plurality of terminals connected to a distribution network that is fed by both a main and back up feed, each feed including a head end and a repeater. The back up head end had access to a ranging table with data previously obtained by the main head end, thereby speeding up the switch over in the event of a fault with the main feed. In one embodiment, the repeater has a virtual ONU therein, allowing the back up repeater to be ranged by the back up head end, thereby yet further speeding up the ranging procedure. The main and back up repeaters are sufficiently equidistant from the distribution network to allow the back up head to perform normal scheduling without performing a ranging operation on each of the terminals, even if the different terminals transmit at slightly different wavelengths. This is achieved using the ranging information obtained with regard to the back up repeater.

Abstract: 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.

Abstract: An optical fibre network comprises a laser source (1a) configured to generate laser light of a plurality of wavelengths. A first optical fibre (4a), transmits multi-wavelength light from the laser source to a location remote from the laser source. A wavelength division multiplexer (2) at the remote location (203) is connected to a plurality of second optical fibres (8). A plurality of optical modulators (9) are each connected optically to the wavelength division multiplexer (2) via a respective second optical fibre (8). The wavelength division multiplexer (2) is arranged to de-multiplex the multi-wavelength light received from the first optical fibre (4a) into a plurality of wavelengths and to supply a respective wavelength to each of the second optical fibres (8). The optical modulators (9) are reflective optical modulators each arranged to modulate light received from the associated second optical fibre (8) with a data signal and to reflect the modulated light back along the second optical fibre (8).

Abstract: The present invention relates to a method of sensing a disturbance along an optical link including the steps of: using a first interferometer arrangement connected to an optical source so as to generate pairs of signal copies, which signal copies of a given pair are temporally delayed relative to one another; transmitting the time-displaced signal copies from the first interferometer arrangement onto the optical link; receiving the time-displaced signal copies transmitted over the optical link; and, using a second interferometer arrangement to temporally recombine the received signal copies from a given pair so as to generate an interferometer signal, which interferometer signal is representative of environmental effects on the optical link.

Abstract: 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.