TELECOMMUNICATION SYSTEMS

Abstract: A simple two-way communication link by means of which data signals from a number of data inputs 17 at spaced positions along a highway 11 can be fed in individu­ ally in any desired sequence comprises a highway 11 consti­ tuted by an optical fibre; a gas laser 13 or other source of electromagnetic radiation of optical frequency at the end of the highway; a number of outstations 17 at spaced positions along the highway, each of which outstations comprises a modulator 18 for modulating a data signal onto a subcarrier which periodically varies the optical path lenght of a section of the highway with the modulated data signal, and has an associated receiver 20 for signals for controlling operation of the modulator; and, connected to the other end of the highway, a demodulator 16 and transmitter 21 for transmit­ ting control signals to the receivers 20. Preferably the trans­ mitter 21 and the receivers 20 are electrically intercon­ nected by a pair of electric conductors constituting a leaky electric cable 12.

Description: TELECOMMU1 I CAT ION SYSTEMS This invention relates to telecommunication systems and is particularly concerned with telecommunication systems of the kind employing modulated optical signals. One known form of telecommunication system employing modulated optical signals comprises a highway formed from an optical fibre, a source of coherent electromagnetic radiation of optical frequency, as hereinafter defined, at one end of the highway, a plurality of outstations spaced along the highway, each outstation having associated means for applying a data signal to the highway modulated onto a respective subcarrier and comprising means for periodically varying the optical path length of the highway, and demodulating means connected to the other end of the highway. The term "electromagnetic radiation of optical frequency" as used herein means visible light and other electromagnetic radiation of adjacent frequency bands which can be handled by the same optical techniques as are used for visible light. This known form of telecommunication system operates on the principle that an ultrasonic pressure wave when launched across an optical fibre causes a consequent phase modulation of the light within the fibre. Provided that the light is adequately coherent, this phase modulation can be detected at one end of the fibre by using suitable demodulating means. If the ultrasonic wave is a modulated data signal, then a filter at the output end of the fibre centred on the frequency of the modulated data signal will form the basis of a data communications link. By using a range of ultrasonic frequencies, one frequency being unique to each data input, and demodulating means incorporating a plurality of receiver filters, one for each frequency, a data highway accommodating a plurality of data inputs can be assembled. This data highway concept is a data collection system offering two notable advantages. Firstly, since the ultrasonic modulation technique can be successfully applied to an optical fibre when it constitutes a component element of an optical cable, there is no necessity to break into the cable or fibre to input the data. This simplifies the apparatus at the data input position, makes the optical insertion loss at the position of each data input substantially zero, and also provides flexibility for selection of the data input positions. The second major advantage stems from the fact that the ultrasonic modulation technique is inherently very efficient, and for low bandwidth data transmission, as would be appropriate for many military and industrial process monitoring applications, transmission of the data can be successfully implemented using data input electronics consuming less than lmW of total electric power. This enables a battery-powered modulator to be employed, which is an attractive concept for remote or intrinsically safe environments. It is an object of the present invention to provide an improved telecommunication system which, in addition to the two important advantages outlined above, provides a simple two-way communications link by means of which data signals from a plurality of data inputs at spaced positions along the highway can be fed in individually in any desired sequence. According to the present invention the improved telecommunication system comprises a highway formed from an optical fibre; a source of coherent electromagnetic radiation of optical frequency, as hereinbefore defined, at one end of the highway; a plurality of outstations at spaced positions along the length of the highway, each of which outstations comprises means for modulating a data signal on to a subcarrier and means for periodically varying the optical path length of a section of the highway with the modulated data signal, and has an associated receiver for signals for controlling operation of the modulating means or ancillary equipment; and, at the other end of the highway, demodulating means and means for transmitting control signals to the receivers associated with the outstations. Preferably, the source of radiation is a gas laser but a solid state or semi-conductor light source of adequate coherence may be employed. The control signal transmitting means and the receivers associated with the outstations are preferably electrically interconnected by a pair of electric conductors constituting a leaky electric cable and, in this case, the optical fibre constituting the highway is preferably, but not necessarily, a component element of the cable. For the composite highway and leaky electric cable, it is preferred to employ an optical cable as described and claimed in the Complete Specification of our Patent No 1479426 and comprising an extruded elongate body of plastics insulating material having throughout its length a bore in which an optical fibre is loosely housed, and, embedded in the elongate body on opposite sides of and parallel to the bore, two strain wires of electrically conductive metal or metal alloy. In one alternative form of composite highway and leaky electric cable, the optical fibre is loosely housed in a bore in an extruded elongate body of plastics insulating material, and embedded in the elongate body on opposite sides of and parallel to the bore are a non-metallic elongate strain member and a pair of coaxial conductors, the outer conductor of which pair of coaxial conductors being of such a form that the pair of conductors constitutes a radiating or leaky cable. It will be appreciated, however, that any convenient form of composite highway and leaky electric cable can be employed. Alternatively, the control signal transmitting means and the receivers associated with the outstations may constitute components of a broadcasting system, but, in this case, problems of frequency allocation and electromagnetic interference may arise. Each modulating means preferably takes the form of means for subjecting a length of the optical fibre forming the highway to periodic variations of stress in response to a modulating signal. For this purpose, a piece of piezo electric material may be secured to the optical fibre1 or to the cable of which the optical fibre constitutes a component element. Some suitable modulating means for this purpose are described and illustrated in the Specifications of Patent No 1488253 and Patent Application No 2019561A. Where the optical fibre is loosely housed in a bore in the cable and the modulating means of each outstation includes means for subjecting a length of the optical fibre to periodic variations of stress in response to a modulating signal, at the positions of each outstation, preferably the cable is so bent or flexed that the optical fibre is in direct contact with the wall of the bore. Alternatively, at the position of each outstation and preferably throughout the length of the cable, the space in the bore not occupied by the optical fibre is filled with petroleum jelly or other material of a jelly- or grease-like nature. Where the telecommunication system is to be employed in conditions where it is likely to be subjected to poisonous or noxious fluid, filling the bore with a material of a jelly-like nature throughout its length, or at the positions of the outstations, substantially reduces the risk of any poisonous or noxious fluid flowing along the bore of the cable. The telecommunication system of the present invention has the major advantage that it provides for simple two-way communication between outstations feeding data signals to modulating means and the demodulating means at the control station. Since the control signal transmitting means can be employed automatically or manually to effect operation of the plurality of outstations individually in any desired sequence, each data signal can be modulated by its associated modulator on to a common sub-carrier, thereby enabling a single receiver filter to be incorporated in the demodulating means alternatively, some or all of the data inputs may be modulated on to two or more than two sub-carriers differing from one another, provided that the appropriate number of receiver filters are incorporated in the demodulating means. Additionally, the telecommunication system of the present invention can be used to provide a means of control of ancillary equipment or voice communication. The leaky electric cable can be used to monitor for total cable integrity; the light output of the optical fibre provides for monitoring of fibre integrity. It is possible to employ the telecommunication system of the present invention as a hybrid system, with signals of some of the data input being fed to the control station via the leaky electric cable and the signals of the other data inputs being fed into the optical fibre. The telecommunication system of the present invention represents a significant development in the concept of optical fibre cable telemetry. A data collection highway based on the use of low power modulators for the data inputs to the optical fibre has many applications in industrial telemetry and process control, and in military systems. The additional flexibility allowed by the use of two transmission concepts in one cable is even more attractive. Furthermore, the telecommunication system of the present invention retains the advantages of the known system for intrinsic safety, and the security of data and electromagnetic immunity aspects of optical transmission. The leaky electric cable is, of course, susceptible to electromagnetic interference, and will obviously radiate. However, the data band width can be arranged to be very narrow and the carrier power and frequency can be chosen with electromagnetic interference aspects of a particular application in mind. Electromagnetic interference can be kept to an acceptable low level, and the distance along the leaky electric cable that signals can be transmitted can be increased, by employing such an operating frequency and a leaky electric cable of such a construction that the radiation extends only a few centimetres from the cable. The amount of radio frequency power required to be transmitted through the cable is quite low - in the order of a few milliwatts - so that the originally established intrinsic safety aspects of the optical system are not jeopardised. The invention is further illustrated by a description, by way of example, of a preferred two-way telecommuncation system with reference to the accompanying drawings, in which: Figure 1 is a schematic transverse cross-sectional view of a preferred leaky electric cable for use in the telecommunication system, and Figure 2 is a schematic representation of the telecommunication system, for clarity only one data input station being shown. The leaky electric cable shown in Figure 1 comprises an extruded elongate body 4 of polyethylene having a bore 2 and, embedded in the extruded body on opposite sides of the bore, two strain wires 3 so arranged that their axes and the axis of the bore are substantially parallel and lie in a substantially common plane. Each strain wire 3 comprises a core 5 of steel and a cladding 6 of copper. A separate optical fibre 1 is housed loosely in the bore 2. The two-way telecommunication system shown in Figure 2 comprises a highway 11 constituted by the optical fibre 1 of the leaky electric cable shown in Figure 1, the electrically conductive strain wires 3 of the cable constituting a leaky feeder path 12. At one end of the highway 11 is a gas laser 13 which constitutes a source of coherent electromagnetic radiation of optical frequency and, at the other end of the highway, a photodetector 14 feeds a demodu lator and demultiplexer 16 via a single filter 15. Associated with the demodulator and demultiplexer 16 is a transmitter 21 which is connected to one end of the leaky feeder path 12, the-other end of the leaky feeder path being connected to a matched termination 22. At spaced positions along the length of the highway 11 are a plurality of outstations 17 of which, for clarity, only one is shown. Each outstation 17 comprises a modulator 18 for modulating a data signal onto a sub-carrier and an ultrasonic modulator 19 which is secured to the leaky electric cable and which is employed to vary periodically the optical path length of a section of the highway with tha modulated data signal. At the position of each outstation 17, the leaky electric cable is so bent that the optical fibre 1 is in direct contact with the wall of the bore 2 as shown in Figure 1. Each outstation 17 also includes a receiver 20 for signals for controlling operation of the modulator 18 and other ancillary equipment transmitted by the transmitter 21 along the leaky feeder path 12. Syncronous data to the receivers 20 and outputs for control decisions can be transmitted back and forth between the demodulator and multiplexer 16 and the transmitter 21.

Claims: CLAIMS 1. A telecommunications system comprising a highway formed from an optical fibre; a source of coherent electromagnetic radiation of optical frequency, as hereinbefore defined, at one end of the highway; a plurality of outstations at spaced positions along the length of the highway, each of which outstations comprises means for modulating a data signal onto a sub-carrier and means for periodically varying the optical path length of a section of the highway with the modulated data signal, and has an assocated receiver for signals for controlling operation of the modulating means or ancillary equipment; and, at the other end of the highway, demodulating means and means for transmitting control signals to the receivers associated with the outstations. 2. A telecommunication system comprising a highway formed from an optical fibre; a source of coherent electromagnetic radiation of optical frequency, as hereinbefore defined, at one end of the highway; a plurality of outstations at spaced positions along the length of the highway, each of which outstations comprises means for modulating a data signal onto a sub-carrier and means for periodically varying the optical path length of a section of the highway with the modulated data signal, and has an associated receiver for signals for controlling operation of the modulating means or ancillary equipment; demodulating means and means for transmitting control signals to the receivers associated with the outstations at the other end of the highway; and, electrically interconnecting the control signal transmitting means and the receivers associated with the outstations, a pair of electric conductors constituting a leaky electric cable. 3. A telecommunication system as claimed in Claim 2, wherein the optical fibre constituting the highway is a component element of the leaky electric cable. 4. A telecommunication system as claimed in Claim 3, wherein the leaky electric cable comprises an extruded elongate body of plastics insulating material having throughout its length a bore in which an optical fibre is loosely housed, and, embedded in the elongate body on opposite sides of and parallel to the bore, two strain wires of electrically conductive metal or metal alloy. 5. A telecommunication system as claimed in Claim 3, wherein the leaky electric cable comprises an extruded elongate body of plastics insulating material having throughout its length a bore in which an optical fibre is loosely housed, and, embedded in the elongate body on opposite sides of and parallel to the bore, a non-metallic elongate strain member and a pair of coaxial conductors, the outer conductor of which pair of coaxial conductors being of such a form that the pair of conductors constitutes a radiating or leaky cable. 6. A telecommunication system as claimed in Claim 4 or 5, wherein each modulating means takes the form of means for subjecting a length of the optical fibre forming the highway to periodic variations of stress in response to a modulating signal and wherein, at the positions of each outstation, the cable is so bent or flexed that the optical fibre is in direct contact with the wall of the bore. 7. A telecommunication system as claimed in Claim 4 or 5, wherein each modulating means takes the form of means for subjecting a length of the optical fibre forming the highway to periodic variations of stress in response to a modulating signal and-wherein, throughout the length of the cable, the space in the bore not occupied by the optical fibre is filled with petroleum jelly or other material of a jelly- or grease- like nature. 8. A telecommunication system as claimed in any one of Claim 1 to 3, wherein each modulating means takes the form of means for subjecting a length of the optical fibre forming the highway to periodic variations of stress in response to a modulating signal. 9. A telecommunication system as claimed in any one of Claims 6 to 8, wherein each modulating means comprises a piece of piezo electric material secured to the optical fibre, or to the cable of which the optical fibre constitutes a component element. 10. A telecommunication system as claimed in any one of the preceding Claims,wherein the source of radiation is a gas laser.

Inventors: BALL, PETER RAYMOND, CULSHAW, BRIAN, POND, JOHN CHARLES, SADLER, ALAN ARTHUR
Applicants: BICC PUBLIC LIMITED COMPANY
Application year: 1981
Main classification: H04B9
IPC codes: H04B10