Abstract
A fiber-optic sensor array (100) comprises a line array of fiber-optic sensor packages (AX, BX, CX, DX, AY, BY, CY, DY, AZ, BZ, CZ, DZ) each having a package input/output (i/o) fiber and each being arranged to output a finite output pulse series of optical output pulses via the package i/o fiber in response to input thereto of one or more interrogating optical pulses. The array further comprises a fiber-optic bus (104,106, 108, 110) extending along the length of the line array, each package i/o fiber being optically coupled to the fiber-optic bus at a respective positions along the line array. The array allows interrogation at a higher frequency than is the case for a serial array of the same number of fiber-optic sensing packages.
Claims
1. A fibre-optic sensor array comprising a line array of fibre-optic sensor packages each having a package input/output (i/o) fibre and each being arranged to output a finite output pulse series of optical output pulses via its package i/o fibre in response to input thereto of one or more interrogating optical pulses and wherein the array comprises N fibre-optic bus fibres extending in parallel along the length of the line array, each package i/o fibre being optically coupled to one of said N bus fibres at a respective position along the line array such that individual pulses within an output pulse series from corresponding packages coupled to different bus fibres are interleaved in the array i/o fibre.
2. A fibre-optic sensor array according to claim 1, wherein the package i/o fibre of the nth fibre-optic sensor package of the line array is optically coupled to the kth bus fibre (k=1 to N) if n/N=M+k/N, and M is a positive integer or zero.
3. A fibre-optic sensor array according to claim 2 wherein positions along a given bus fibre at which package i/o fibres are coupled thereto are separated by equal lengths of bus fibre, and at least one bus fibre incorporates a respective delay length of optical fibre between the array i/o fibre and the position along the bus fibre at which the package i/o fibre of the first package of that bus fibre is coupled.
4. A fibre-optic sensor array according to claim 3 wherein each of the fibre-optic sensor packages is arranged to output optical pulses of an output pulse series at a rate 1/, each pulse having a duration /N, and wherein the one or more delay lengths of fibre are such that, in use of the array, groups of output pulse series coupled into respective bus fibres form a multiplexed output sequence in the array i/o fibre, the sequence comprising a plurality of said groups in which consecutive groups have a relative delay /N.
5. A fibre-optic sensor array according to claim 4 wherein each fibre-optic sensor package of the array is arranged to output an output pulse series of m optical output pulses in response to one or more interrogating pulses and wherein said equal lengths of bus fibre have a length cpm/2, where p is an integer, p1 and c is the speed of light in fibre.
6. A fibre-optic sensor array according to claim 5 wherein at least one of said equal lengths of fibre incorporates a delay length of optical fibre.
7. A fibre-optic sensor array according to claim 4 wherein each fibre-optic sensor package of the array is arranged to output an output pulse series of m optical output pulses in response to one or more interrogating pulses and wherein said equal lengths of fibre have a length cp(m1)/2, where p is an integer, p1 and c is the speed of light in fibre.
8. A fibre-optic sensor array according to claim 7 wherein at least one of said equal lengths of fibre incorporates a delay length of optical fibre.
9. A fibre-optic sensor array according to claim 7 wherein each fibre-optic sensor package comprises m-2 fibre-optic sensing coils arranged in series, each coil having a length c/2, and wherein each of said coils is provided with a partial reflector on an input side thereof and wherein the package is provided with a terminal reflector.
10. A fibre-optic sensor array according to claim 9 wherein each reflector is a fibre-coupled reflector.
11. A fibre-optic sensor array according to claim 9 wherein each reflector is an in-fibre reflector.
12. A fibre-optic sensor array according to claim 9 wherein one or more fibre-optic sensing coils is comprised in a fibre-optic accelerometer.
13. A fibre-optic sensor array according to claim 9 wherein one or more fibre-optic sensing coils is comprised in a fibre-optic hydrophone.
14. A fibre-optic sensor array according to claim 9 wherein each package has four fibre-optic sensing coils, three coils being comprised in respective fibre-optic accelerometers and one coil being comprised in a fibre-optic hydrophone.
15. A fibre-optic sensor array according to claim 1 wherein the fibre-optic bus is contained within a protective cable extending along the length of the line array.
16. A large-scale fibre-optic sensor array comprising an i/o fibre and plurality of sub-arrays each according to claim 1, the large-scale array further comprising a wavelength multiplexer/demultiplexer arranged to couple radiation of a particular wavelength between the i/o fibre and a corresponding sub-array.
17. A sensor system comprising: a linear array of fibre optic sensor packages, each package including at least one fibre optic sensor each being arranged to output a finite output pulse series of optical output pulses in response to input thereto of one or more interrogating optical pulses, a fibre optic bus extending along the array comprising a plurality of N parallel bus fibres, wherein the system is arranged such that physically adjacent packages of the array are always coupled to different ones of said N bus fibres such that individual pulses within an output pulse series from corresponding packages coupled to different bus fibres are interleaved.
18. A sensor system according to claim 17, wherein each package comprises q individual sensors, and outputs at least q+1 pulses in response to an input pulse.
19. A sensor system according to claim 17, in which each package comprises q individual sensors, and each bus has p packages optically coupled thereto, and in which each package is arranged to output optical pulses of an output pulse series at a rate 1/ wherein the rate of interrogation of the system is substantially 1/(p(q+1)).
Description
DESCRIPTION OF THE FIGURES
(1) Embodiments of the invention are described below by way of example only and with reference to the accompanying drawings in which:
(2) FIG. 1 shows a known type of fibre-optic sensor package;
(3) FIG. 2 shows a pair of pulses suitable for interrogating the FIG. 1 package;
(4) FIG. 3 shows an output pulse series obtainable from the FIG. 1 package in response to input of the pulse pair of FIG. 2;
(5) FIG. 4 schematically illustrates a first example fibre-optic sensor array of the invention;
(6) FIG. 4A illustrates the physical arrangement of the FIG. 4 array;
(7) FIG. 5 shows the timing of groups of output pulse series coupled into bus fibres of the FIG. 4 array in the case where delay coils of the array have zero length;
(8) FIG. 6 shows a multiplexed output sequence obtainable from the FIG. 4 array when the delays coils thereof have suitable lengths;
(9) FIG. 7 schematically illustrates a second example fibre-optic sensor array of the invention;
(10) FIG. 8 shows a group of three output pulse series obtainable from a bus fibre of the FIG. 7 array in response to input of a single interrogating pulse pair;
(11) FIG. 9 shows a group of output pulse series obtainable from a bus fibre of the FIG. 7 array in response to repeated interrogation of that bus fibre;
(12) FIG. 10 shows the timing of groups of output pulse series from bus fibres of the FIG. 7 array in the case where delay coils of the array have zero length;
(13) FIG. 11 shows a multiplexed output sequence obtainable from the FIG. 7 array when delay coils of the array have suitable lengths;
(14) FIG. 12 shows a group of output pulse series obtainable from a bus fibre of another array of the invention in response to repeated interrogation of that bus fibre;
(15) FIG. 13 shows a group of output pulse series obtainable from a bus fibre of a further array of the invention in response to repeated interrogation of that bus fibre;
(16) FIG. 14 schematically illustrates a third example fibre-optic sensor array of the invention;
(17) FIG. 15 shows a pair of input pulses suitable for interrogating the FIG. 14 array;
(18) FIG. 16 shows an output pulse series obtainable from a package of the FIG. 14 array in response to input of the pulse pair of FIG. 15;
(19) FIG. 17 shows a group of output pulse series obtainable from a bus fibre of the FIG. 14 array in response to repeated interrogation of that bus fibre; and
(20) FIG. 18 shows a multiplexed output sequence obtainable from the FIG. 14 array when delay coils of the array have suitable lengths.
DESCRIPTION OF THE INVENTION
(21) In FIG. 1, a known type of fibre-optic sensor package, indicated generally by 10, comprises four individual fibre-optic sensing coils 1, 2, 3, 4 arranged in series. The coils 1, 2, 3, 4 are formed from a single length 13 of optical fibre, a portion 12 of which serves as the package input/output (i/o) fibre. Fibre-coupled mirrors 5, 6, 7, 8, 9 are coupled to the optical fibre 13 at respective locations along it such that each of the coils 1, 2, 3, 4 has a fibre-coupled-mirror coupled at each end of it. Coils 1, 2, and 3 could, for example, form parts of respective corresponding fibre-optic accelerometers with coil 4 forming part of a hydrophone to form a four-component package suitable for seismic surveying applications. (Mechanical parts of the package 10 are not shown in FIG. 1). Each of the coils 1, 2, 3, 4 has a length of 40 m. The total length of the fibre 13 of the sensor 10 is therefore 160 m (ignoring the small length of the i/o fibre 12 and the small connecting lengths of fibre between adjacent coils).
(22) Referring to FIG. 2, a single interrogation of the package 10 of FIG. 1 may carried out by introducing an interrogating pair of optical pulses 20, 22 into the package i/o fibre 12. Pulses 20, 22 have respective frequencies .sub.1, .sub.2 and pulse 22 is delayed by =2l/c with respect to pulse 20, l being the length of coil in the sensor 10 (40 m) and c being the speed of an optical pulse in the fibre 9 (assumed to be 210.sup.8 ms.sup.1 for the purposes of this description), hence =400 ns. Each of the pulses 20, 22 has a duration /4=100 ns. Referring to FIG. 3, the package 10 outputs an output pulse series of six pulses 30, 32, 34, 36, 38, 40 in response to input of the pulses 20, 22. Each of the pulses 30, 32, 34, 36, 38, 40 has a duration /4=100 ns and there is a delay between consecutive pulses. Pulse 30 is generated by a portion of pulse 20 being reflected from the fibre-coupled mirror 5 of the package 10. Pulse 40 is generated by a portion of pulse 22 being reflected from fibre-coupled mirror 9. Each of pulses 32, 34, 36, 38 consists of a portion of pulse 20 coincident with a portion of pulse 22. For example, pulse 34 is made up of a portion of pulse 22 reflected from fibre-coupled mirror 6 and a portion of pulse 20 reflected from fibre-coupled mirror 7. A phase difference between these two pulse portions arises due to a phase-shift experienced by the portion of pulse 20 as it makes a double pass through sensing coil 2. By observing the phase or frequency of a beat signal generated when the second pulse of each of a plurality of output pulse series is incident on a photodetector (in response to multiple interrogating pulse pairs), information on physical conditions near the package 10 may obtained. (Exactly what the phase or frequency of the beat signal corresponds to depends on the type of individual sensor of which the coil 2 is a part). Similar considerations apply to pulses 34, 36 and 38. Output pulses 32, 34, 36, 38 are thus the outputs of individual sensors comprising coils 1, 2, 3, 4 respectively. Output pulses 30, 40 carry no useful information.
(23) FIG. 4 schematically illustrates a first example fibre-optic sensor array 100 of the invention. The array 100 comprises an array input/output (i/o) fibre 102 optically coupled to four bus fibres 104, 106, 108, 110, labelled A, B, C and D respectively. Branches A, B and C incorporate delay loops 113, 114 and 115 respectively. Each of the bus fibres A, B, C, D has three packages of the type shown in FIG. 1 optically coupled to it at intervals of 200 m. The packages coupled to bus fibres B, C and D are offset along the length of the array 100 by distances of x.sub.1=50 m, x.sub.2=100 m, and x.sub.3=150 m respectively from those coupled to bus fibre A. The packages of the array 100 are grouped into three clusters X, Y, Z each containing four packagesone coupled to each of the bus fibres A, B, C, D. The array 100 thus comprises 12 packages AX, BX, . . . CZ, DZ of the type shown in FIG. 1. The positions at which the packages are coupled to the bus fibres A, B, C, D of the array 100 are also indicated by the reference signs AX, BX, . . . CZ, DZ in FIG. 4 (the packages themselves are not shown in FIG. 4).
(24) FIG. 4A shows the physical arrangement of the array 100. The packages AX, BX, . . . CZ, DZ form a line array; bus fibres A, B, C, D extend along the length of the line array and are housed in a protective cable 101. The first, fifth and ninth packages along the line array (AX, AY, AZ respectively) are coupled to bus fibre A. The second, sixth and tenth packages along the line array (BX, BY, BZ respectively) are coupled to the bus fibre B. The third, seventh and eleventh packages along the line array (CX, CY, CZ respectively) are coupled to bus fibre C. The fourth, eighth and twelfth packages (DX, DY, DZ) are coupled to bus fibre D. If one bus fibre before the first package thereof, the set of packages attached to it cannot be interrogated, although the effect of such breakage on the performance of the array 100 is less significant than would be the case if groups of four adjacent packages were to be coupled to each of three bus fibres with a spacing of 800 m between adjacent groups.
(25) In use, the array 100 is interrogated by a series of pulse pairs, each as shown in FIG. 2, which are introduced into the array input/output (i/o) fibre 102. Following input to i/o fibre 102, each pair is divided into four portions; one portion being input to each of the bus fibres A, B, C, D. Output pulse series coupled into a given bus fibre form a group of output pulse series in that bus fibre; groups from respective bus fibre are multiplexed in the array i/o fibre 102 to form a multiplexed output sequence.
(26) FIG. 5 shows the timing of groups 150, 152, 154, 156 of output pulse series from each of the four bus fibres 104, 106, 108, 110 (labelled A, B, C, D respectively in FIG. 4) in response to a single interrogating pulse pair in the case where the delay coils 113, 114, 115 have zero length. The origin of time in FIG. 5 is taken as the time when a portion of the first of a pair of interrogating input pulses returns from package AX. The output sequence from bus fibre A consists of a series of 16 pulses; a particular pulse j=1 . . . 16) of these pulses returns at a time (j1) later than the first pulse. Since packages AX, AY, AZ are spaced along bus fibre A at intervals of 200 m=5c (corresponding to a round-trip delay of 2 s), the sixth pulse of the output pulse series of package AX overlaps with the first pulse of the output pulse series from package AY, and the sixth pulse of the output pulse series output from package AY overlaps with the first pulse of the output pulse series from package AZ. Individual output pulses in the group 150 of output series are labelled to indicate which individual sensing coil they correspond to, e.g. AX3 indicates a pulse carrying information from the third sensing coil of package AX. The groups 152, 154, 156 of output pulse series from bus fibres B, C and D have the same format as those output from bus fibre A, however they are delayed by 500 ns, 1000 ns and 1500 ns respectively with respect to the group 150 from bus fibre A because the packages on bus fibres B, C, and CD are offset with respect to those on bus fibre A by distances of 50 m, 100 m and 150 m respectively along the line array of packages.
(27) If delays coils 113, 114, 115 incorporated in bus fibres A, B and C respectively of the array 100 have lengths 120 m, 80 m and 40 m respectively, groups 150, 152, 154 of output pulse series from bus fibres A, B and C are delayed by 1200 ns (3), 800 ns (2) and 400 ns() respectively, these delays being indicated in FIG. 5. Groups 152, 154, 156 of output pulse series from bus fibres B, C and D therefore become delayed by 100 ns (/4), 200 ns (/2) and 300 ns (3/4) respectively with respect to group 150 from bus fibre A.
(28) Referring to FIG. 6, the delays introduced by delay coils 112, 114, 116 result in a multiplexed output sequence 160 in the array i/o fibre 102 in which groups 150, 152, 154, 156 of output pulse series from bus fibres A, B, C, D are interleaved to form three continuous blocks 170, 172, 174 of useful (i.e. data-carrying) output pulses, each of duration 4=1600 ns, and four periods 162, 164, 166, 168, each of duration =400 ns, which contain pulses from the first and/or last fibre-coupled reflectors of certain packages, and hence no useful information. Blocks 170, 172, 174 are arranged such that output pulses corresponding to sensing coils having the same position in each of the packages within a particular cluster are grouped contiguously in time. Since the period 168 contains no useful information, the array 100 may be interrogated at a rate 1/15167 kHz in order to generate continuous output from the array 100; this is also the rate at which multiplexed output sequences are output from the array i/o fibre 102.
(29) Other embodiments of the invention may be obtained by modifying the array 100 of FIG. 4 so that the distances x.sub.1, x.sub.2, x.sub.3 have values other than 50 m, 100 m, and 150 m. The lengths of the delay coils 112, 114 and 116 then need to be adjusted so that their respective lengths remain equal to x.sub.3-3c/8, x.sub.2-c/4 and x.sub.1-c/8 respectively. A particular embodiment in which the packages on bus fibres B, C, and D are offset by desired distances x.sub.1, x.sub.2, x.sub.3 from those on bus fibre A may therefore be obtained simply by making appropriate choices for the lengths of the delay coils 112, 114, 116. Further embodiments of the invention may also be obtained by modifying the array 100 so that individual sensing coils of each package have a length other than 40 m; if the length of a coil is l then the duration of interrogating pulses must be adjusted so that remains equal to 2l/c.
(30) Referring to FIG. 7, a second example fibre-optic sensor array of the invention is indicated generally by 200. Parts of the array 200 which correspond to parts of the array 100 of FIG. 4 are labelled with reference numerals having a value 100 greater than the reference numerals labelling the corresponding parts of the array 100. Adjacent packages coupled to a given bus fibre have a separation of 800 m, compared to 200 m in the array 100 of FIG. 4. The physical layout of the array 200 is similar to that of the array 100 shown in FIG. 4A, i.e. individual packages form a line array with bus fibres of the array 200 extending along the length of the line array. Each of the packages of the array 200 has the structure shown in FIG. 1.
(31) FIG. 8 shows a group 250 of output pulse series of bus fibre A of the array 200 resulting from input of a single interrogating pulse pair having the form shown in FIG. 2. The origin of time is the time at which the first pulse 20 of the interrogating pair returns from the position AX. Each of the packages AX, AY, AZ generates an output pulse series of six optical output pulses as shown in FIG. 3. The three output pulse series are indicated by AX, AY and AZ in FIG. 8 (the structures of the series are not shown). Each output pulse series AX, AY, AZ has duration 2l/4=2.1 s as indicated in FIG. 3. The delay between consecutive output pulse series is 20=8 s because the packages AX, AY, AZ are spaced apart by 800 m of bus fibre.
(32) FIG. 9 shows a group 249 of output pulse series from bus fibre A when continuously interrogated by pulse pairs of the form shown in FIG. 2 at a rate 1/15. The group 249 is made up of output pulse series from the packages AX, AY, AZ generated by a series of consecutive interrogating pulse pairs n, n+1, n+2, . . . n+9. The origin of time is the time when the first pulse of pair n+2 returns from package AX. Since the delay between consecutive output pulse series resulting from a particular interrogating pulse pair is 20, and since the last /4 of each series contains no useful output, there exists the possibility of interleaving three other output pulse series, generated by other interrogating pulse pairs, between consecutive output pulse series generated by a single interrogating pulse pair. Thus for example, in the group 249 of output pulse series, the following output pulse series appear between the output pulse series of packages AX and AY generated by the (n+2)th interrogating pulse pair: an output pulse series from package AY generated by the (n+1)th interrogating pulse pair an output pulse series from package AZ generated by the nth interrogating pulse pair, and an output pulse series from package AX generated by the (n+3)th interrogating pulse pair.
(33) Interleaving of output pulse series generated by different interrogating pulse pairs allows a higher sampling rate than would be the case if the output pulse series from the last package coupled to the bus fibre generated by a particular interrogating pulse pair were to be received before the output pulse series from the first package generated by the next interrogating pulse pair.
(34) FIG. 10 shows the timing of groups 250, 252, 254, 256 of output pulse series from the four bus fibres 204, 206, 208, 210 (labelled A, B, C and D respectively in FIG. 7) output in response to consecutive interrogating input pulse pairs n, n+1, n+2 in the case where the delay coils 212, 214, 216 have zero length. The origin of time in FIG. 7 is taken as the time when a portion of the first pulse of interrogating pulse pair n+2 returns from package AX. Groups 252, 254, 256 are delayed by 500 ns, 1000 ns and 1500 ns respectively with respect to group 250 because the packages coupled to bus fibres B, C and D are displaced along the array 200 by distances of 50 m, 100 m and 150 m respectively with respect to corresponding packages coupled to bus fibre A.
(35) Delays coils 212, 214, 216 incorporated into bus fibres A, B, and C respectively of the array 200 delay the groups 250, 252, 254 of output pulse series from those branches by 3, 2, and respectively. (Delays coils 212, 214 and 216 therefore have lengths of 120 m, 80 m and 40 m respectively.) The groups 252, 254, 256 of output pulse series from bus fibres B, C and D respectively therefore become delayed by /4, /2 and 3/4 respectively (i.e. by 10 ns, 200 ns and 300 ns respectively) with respect to the group 250 of output pulse series from bus fibre A.
(36) FIG. 11 shows one complete cycle 260 of a multiplexed output sequence which is output from the array i/o fibre 202 of the array 200. The cycle 250 has the same basic structure as the cycle 160 in FIG. 6; there is a difference only in that individual blocks 270, 272, 274 of useful data are generated by consecutive interrogating pulse pairs rather than by a single interrogating pulse pair. The sampling rate of the array 200 is 1/15 (equal to the rate at which interrogating pulse pairs are introduced into the array i/o fibre 202).
(37) The distances by which packages coupled to bus fibre B, C and D are offset along the length of the array 200 with respect to those coupled to bus fibre A may take any desired value, however the lengths of the delays coils must then be adjusted as described above in relation to the array 100.
(38) Referring again to FIG. 9, interleaving of groups of output pulse series output onto bus fibre A and arising from different interrogating pulse pairs is possible because the separation of adjacent packages coupled to a given bus fibre is 800 m. This provides a delay of 20 between consecutive output pulse series generated by a particular interrogating pulse pair. Since the useful information within each output pulse series occurs within the first 5, of each series, the delay of 20 provides a time window of 15 which may be used to accommodate three other output pulse series (35). Other schemes are possible. For example, FIG. 12 shows a group 349 of output pulse series obtainable from a single bus fibre having three packages AX, AY, AZ coupled along it at intervals of 1000 m when interrogated at a repetition frequency 1/15. The group 349 is made up of output pulse series generated by 13 consecutive interrogating pulse pairs n, n+1, n+2, . . . n+12. FIG. 13 shows a group 449 of output pulse series obtainable from a single bus fibre having three packages AX, AY, AZ coupled along it with a spacing of 1400 m between packages. The rate of interrogation is 1/15. In general terms if each bus fibre has q packages coupled to it, and if the duration of useful output in an output pulse series from a single package is T (taking the first pulse in a series to be useful for this purpose, but not the last pulse) then it is possible to interrogate the array using a sampling period qT to obtain contiguous output pulse series from each bus fibre, provided the length x of fibre between adjacent packages coupled to a bus fibre is such that 2x/c=sT, where s=rq+1 and r is an integer. r=0 represents the trivial case where 2x/c=T and output pulse series from a branch generated by a particular interrogating pulse pair are contiguous. Solutions for r=1 and r=2 are illustrated in FIGS. 9 and 13 respectively. There are also some possible combinations of s, r and q that do not meet the condition s=rq+1; one of these is illustrated in FIG. 12.
(39) Although FIGS. 9, 12 and 13 relate to packages separated by bus fibre lengths of 800 m, 1000 m and 1400 m respectively, there is flexibility in the choice of physical package separation because additional delay coils may be employed between packages on a bus fibre in order to give a desired delay between output pulse series of the bus fibre generated by a particular interrogating pulse pair. For example, adjacent packages coupled to a bus fibre of the array 200 of FIG. 7 are connected by an 800 m length of bus fibre, but if a physical separation of only 700 m was required, two 100 m delay coils could be employed to give the timing scheme of FIG. 9: one coil between packages AX and AY and the other between packages AY and AZ. If the desired physical package separation was 1100 m, two 300 m delay coils could be used to allow the timing scheme of FIG. 12 to be employed.
(40) The array 200 of FIG. 7 could also be interrogated by introducing interrogating pulse pairs to the array i/o fibre 202 at a rate which is low enough to allow output pulse series from all the packages of the array, which are generated in response to a particular pulse pair, to be retrieved before the next interrogating pulse pair is introduced. Although delay coils could be used to interleave groups of output pulse series coupled onto the different bus fibres, the maximum interrogation rate would be 1/4555 kHz. Furthermore, no output from the array would be received for 61% of each sample period.
(41) To illustrate how the above ideas may be easily extended to cover arrays having greater numbers of bus fibres and packages per branch, as well as more complex packages, FIG. 14 shows another fibre-optic sensor array 500 of the invention comprising five bus fibres 504, 506, 508, 510, 511 (labelled A, B, C, D, E respectively) each of which has four packages coupled to it. The array 500 therefore has 20 packages AW, AX, . . . EY, EZ. Each package comprises five (sic) sensing coils, each having a length of 40 m. The packages are arranged into four clusters W, X, Y, Z each having five packages (one coupled to each of the five bus fibres A, B, C, D, E). The physical separation of adjacent packages coupled to any given bus fibre is 1000 m. Packages coupled to bus fibres B, C, D and E are displaced along the array 500 by distances of 50 m, 75 m, 150 m and 175 m respectively with respect to corresponding packages coupled to bus fibre A. Delay coils 512, 514, 516, 518 have lengths of 143 m, 101 m, 84 m and 17 m respectively. Delays coils 513A, B & C, 515A, B & C, 517A, B & C, 519A, B & C, 521A, B & C each have a length of 200 m. The physical layout of the array 500 is similar to that shown in FIG. 4A; individual packages of the array are arranged in a line array with bus fibres extending along the length of the line array. The bus fibres are preferably house in a protective cable.
(42) Interrogation of the array 500 is performed by introducing interrogating pulse pairs of the form shown in FIG. 15 into the array i/o fibre 502 of the array 500. The delay between individual pulses of a pair is =240/c=400 ns, and since there are five bus fibre in the array 500, the width of the individual interrogating pulses is /5. FIG. 6 shows an output pulse series of seven pulses output by a single package in response to a single interrogating pulse pair. The duration of the output pulse series is 31/5.
(43) FIG. 17 shows a timing scheme for interleaving output series coupled onto bus fibre A, the output pulse series being generated by ten consecutive interrogating pulse pairs n, n+1, n+2, . . . n+9. The same scheme is used for the other bus fibres. The 200 m delay coils 513, 515, 517, 519, 521, together with the physical cluster separation of 1000 m results in a delay of 30 between consecutive output pulse series from a particular bus fibre and generated by a single interrogating pulse pair. Contiguous output pulse series are output from a bus fibre in response to interrogation at a rate 1/24=104 kHz.
(44) FIG. 18 shows a complete cycle 560 of a multiplexed output sequence of the array 500, the cycle 560 being generated by four consecutive interrogating pulse pairs n, n+1, n+2, n+3. The duration of the cycle is 24%, hence the output rate of the array 500 is 104 kHz.
(45) It will be understood that the present invention has been described above purely by way of example, and modification of detail can be made within the scope of the invention.
(46) Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.
