SENSOR ARRANGEMENT

Abstract: A substance sensor comprises an optical fibre having a core (1) and a cladding (2) having a lower refractive index. A layer (4) of cladding is left between a sensor material (3) and the core (1). The thickness (t) of the layer (4) is sufficiently small that the sensor material (3) lies within the evanescent wave which occurs within the cladding (2) when optical radiation is conducted through the core (1). The refractive index of the sensor material (3) varies in the presence of a substance to be sensed over a range which extends on either side of the refractive index of the cladding (2). When its refractive index is lower than that of the cladding the sensor material operates, as far as the radiation is concerned, similary to the cladding (2). Conversely, when its refractive index is higher than that of the cladding (2) the sensor material (3) couples out some of the radiation, reducing the transmissivity of the fibre. The sensor material may be polyethylene oxide, in which case the arrangement operates as a humidity sensor.

Description: SENSOR ARRANGEMENT This invention relates to a sensor arrangement comprising an optical waveguide having a core of material which is transmissive for optical radiation having a given wavelength and a cladding which surrounds the core, the cladding being of material which has a refractive index at the given wavelength which is less than the refractive index of the adjacent surface of the core, the arrangement including a sensor material which is capable of taking up a given substance or substances within itself and has a refractive index which is dependent upon the amount of such substance(s) taken up, the sensor material being positioned sufficiently close to the core as to be capable, at least when the refractive index of the sensor material has a first value corresponding to a first said amount of substance(s) taken up, of coupling radiation having the given wavelength out from the core and thereby reducing the transmissivity of the waveguide for radiation having the given wavelength. An arrangement of this general kind is known, for example, from US patent specification 4634856. The known arrangement is for sensing moisture and comprises an optical fibre core surrounded by cladding material The cladding material has a refractive index which is a function of its moisture content and may be constituted by sintered or porous plastic. In this known arrangement the sensor material and the cladding material are therefore one and the same It appears that the refractive index of the dry plastic is less than that of water and that the effective refractive index increases towards that of the core with increasing moisture content Light signals are transmitted through the fibre from a light source at one end to a detector unit at the other The increase in the refractive index of the cladding towards that of the core occurring with increasing moisture content results in a modification of the modal power distribution in the core and hence in attenuation of the light signals received by the detector unit It appears that, for this to occur, the fibre operates as a multimode waveguide The specification mentions that information on optical signal loss throughout the length of the fibre can be obtained by means of an optical time-domain reflectometer (OTDR) Thus moisture content can be measured at any point along the entire length of an elongated optical fibre arranged about or along a substantial area being monitored. Moisture measurements can be made at many points along a single optical fibre cable containing numerous individual optical fibres tor monitoring at the desired locations. Single-mode optical fibres are being increasingly employed to carry signals in telecommunications networks. Multi-fibre cables are conventionally used, couplings being made to individual fibres at convenient "splitter" nodes In particular, individual fibres are often "split", that is coupled to two or more other optical fibres so that signals carried by one fibre are fed to two or more other fibres. The splitting is often accomplished with a device known as an optical coupler, often in the form of a "fused fibre" coupler As the name suggests, fused fibre couplers are made by fusing (under the action of heat and, typically, tension) two or more fibre lengths. Such couplers are sensitive to moisture So called passive optical networks, PONs, make much use of such couplers. Although not yet widely deployed these PONs will make extensive use of optical couplers and these couplers will often need to be distributed throughout the network - typically being located at distribution points which are no more than street cabinets or access points In such locations moisture ingress is very difficult to avoid. Because the operation of these nodes tends to be degraded by moisture, steps have to be taken to minimise this risk, for example by hermetically sealing the nodes and packing them with silica gel. Even so there is still a finite risk of the relative humidity within the nodes rising to an unacceptable level. If this should occur it is highly desirable that the situation be detected promptly, so that remedial action can be taken before unacceptable degradation occurs. It is of course possible to provide one of the many known humidity sensors in each node and interrogate it, and power it also if necessary, from a central monitoring point by means of electrical wiring However this is cumbersome and costly and there is a need for a humidity sensor which can be included in, and monitored via, a length of single-mode optical fibre. Preferably too it should be possible to construct such a sensor in such a way that it does not result in an unacceptably large attenuation of signals passing through the fibre, whatever the ambient humidity is, so that several such sensors can be included at successive locations along the same fibre and be satisfactorily monitored via that fibre. It is an object of the invention to enable the above needs to be met. According to the invention a sensor as defined in the first paragraph is characterised in that the sensor material extends within the cladding material towards the core but is separated from the core by cladding material, and in that the cladding material has a refractive index at the given wavelength which is intermediate said first value of the refractive index of the sensor material and a second value of the refractive index of the sensor material which is lower than the first value and corresponds to a second said amount of substance(s)taken up As the sensor material is separated from the core by cladding material the insertion loss of the arrangement for radiation transmitted through the core can be small when the refractive index of the sensor material has the second value Moreover, the insertion loss when the refractive index of the sensor material has the first value can be tailored to particular circumstances by suitably choosing the separation between the sensor material and the core. Preferably the optical waveguide is a single-mode optical waveguide at the given wavelength. If the arrangement is required to operate as a humidity sensor arrangement the sensor material may suitably be a hydrophilic polymer, for example polyethylene oxide. In order to facilitate construction the optical waveguide may be an optical fibre which is embedded in a block of material having a flat surface, the surface of a thinned portion of the cladding forming part of the flat surface and the sensor material being present as a layer on the flat surface. Embodiments of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which: Figure 1 shows a longitudinal section through a first embodiment; Figure 2 shows a longitudinal section through a second embodiment; Figure 3 is a plan view of part of the embodiment of Figure 2; and Figure 4 shows a plurality of sensor arrangements included in succession within a single optical transmission path. Figure 1 is a section, taken in the length direction, through an optical waveguide in the form of an optical fibre having a core 1 surrounded by a cladding 2. The core 1 may, for example, carry optical signals from an Optical Time Domain Reflectometer (not shown). In usual fashion the refractive index N : of the core 1 is higher than the refractive index N 2 of the cladding 2 at the wavelength(s) of the optical signals, so that the signals are guided by the core 1 The core 1 can be circular in cross-section and its diameter d is preferably sufficiently small that the waveguide is a single-mode waveguide at the wavelength(s) of the optical signals. The section of waveguide shown operates as a humidity sensor To this end a sensor material 3 extends within the conventional cladding 2 towards the core 1 The sensor material 3 is capable of taking up water vapour within itself and its refractive index N 3 is, at least at the wavelength(s) of the signals carried by the core 1 , dependent upon the amount of water vapour taken up. More particularly, as the concentration of water in the material 3 increases, the refractive index N 3 moves through a range of values which includes the refractive index N 2 of the cladding mateπal so that, at a particular concentration, N 3 = N 2 . The material 3 extends from the exterior towards the core 1 but is spaced from the material of the core by a thin layer 4 of the cladding material 2. The thickness t of this layer, i.e. the spacing between the material 3 and the core 1 , is sufficiently small that the portion of the material 3 adjacent the core lies within the evanescent waves created in the cladding 2 by the optical signals carried by the core 1 . At concentrations of water in the material 3 at which its refractive index N 3 is equal to or less than the refractive index N 2 of the cladding material 2 the material 3 behaves in a similar way to the cladding material 2 as far as the signals carried by the core 1 are concerned. However, at concentrations of water at which N 3 is greater than N 2 the material 3 couples out some of the signal energy from the core 1 and thereby reduces the transmissivity of the waveguide This reduction in transmissivity can be sensed, for example, by means of an optical radiation detector (not shown) positioned at one end of the waveguide to receive optical radiation transmitted by the waveguide from a source (also not shown) positioned at the other end, or by means of an Optical Time Domain Reflectometer positioned at one end of the waveguide to transmit optical radiation pulses into the waveguide and detect the backscattered radiation caused by the coupling out of energy by the material 3. The amount of energy couoled out by the material 3 at relative humidities at which its refractive index is higher than that of the cladding material 2 is highly dependent on the thickness t of the layer 4 Thus the reduction in the transmissivity of the waveguide 1 ,2 which occurs at these concentrations can be chosen to suit particular circumstances by suitably choosing the thickness t For example, if only a single sensor as shown in Figure 1 is provided along a given optical fibre it may be appropriate to choose the reduction in transmissivity to be large, i.e to choose a very small value of t, so as to achieve maximum sensitivity On the other hand, if several such sensors are provided at successive locations along the same fibre a large reduction in transmissivity caused by one sensor is likely to hinder the monitoring, via the fibre, of the others. In such a situation it may be appropriate to choose a relatively small reduction in transmissivity, i e to choose a relatively large value of t. It will be noted from Figure 1 that the cladding 2 is, at the region where it adjoins the core 1 , continuous along the length of the waveguide or fibre This enables the loss caused by the presence of the sensor material 3 to be kept very low in situations where the refractive index of the sensor material 3 is equal to or lower than that of the cladding material 2. The configuration shown in Figure 1 is somewhat impractical to construct, and a preferred configuration will now be described with reference to Figures 2 and 3 of the drawings In Figures 2 and 3 items which have counterparts in Figure 1 have been given the same reference numerals. The sensor arrangement of Figures 2 and 3 is, similarly to Figure 1 , shown in longitudinal section in Figure 2 This sensor arrangement again comprises an optical fibre having a core 1 and a cladding 2. However the fibre 1 ,2 is now held by means of an appropriate adhesive 5, for example heat-cured epoxy resin, in a groove cut in the top surface of a quartz block 6. see also the plan view of the arrangement, minus the sensor material 3, shown in Figure 3 The groove deepens on going away from the centre of the block 6 towards its edges, so that the fibre 1 ,2 is curved rather than straight in this embodiment. (The curvature is greatly exaggerated in Figure 2) The top surface of the assembly of the block 6, the adhesive 5 and the embedded fibre 1 ,2 has been polished so that a portion 7 of the cladding 2 of the fibre 1 ,2 in the centre of the block 6 has been removed to leave a much thinner layer 4 of cladding at this area. The polishing has been continued until the layer 4 has the required thickness; cf . the discussion above with reference to Figure 1 of the dependence of the coupled out energy on the thickness t of the layer 4 (It is noted at this point that so called D fibre, that is to say fibre manufactured with a non circularly symmetric 'D' shaped profile having its core located in close proximity to the planer surface, may be used in place of such 'polished' fibre) . Once the desired thickness of the layer 4 has been achieved a layer of the sensor material 3 has been provided over the whole of the top surface of the assembly of the block 6, the fibre 1 ,2 and the adhesive 5. The material 3 thus extends within the space the cladding material 2 would have occupied without 'polishing' towards the core 1 but is again separated from the core 1 by cladding material 2. In one example of a sensor arrangement constructed and configured as described with reference to Figures 2 and 3 the material of the core 1 was fused silica doped with germania so that its refractive index was approximately 0.3% greater than that of the cladding 2 at a wavelength of 435nm. Its diameter was 8μm. The material of the cladding 2 was fused pure silica having a refractive index of 1 .433 at a wavelength of 435nm. Its radial thickness was 58.5μm. The thickness of the sensor material 3 was at least enough to restore the original cladding thickness at the region 7; a thickness of 1 00μm was found to be satisfactory. Such a film thickness, of the order of hundreds of microns, will mean that there will be an almost continuous set of modes in the sensor material layer to which the fundamental fibre mode can couple to. The radius of curvature of the fibre 1 ,2 was 25cm. The dimensions of the quartz block 6 were 10 x 10 x 30nm. The sensor material 3 used was polyethylene oxide (PEO), which is highly hydrophilic and also reversible in its response to water vapour. It was brushed on to the top surface of the block 6/fibre 1 ,2/adhesιve 5 assembly in dissolved form and allowed to dry. This material expands as it takes up water vapour and contracts as it gives up water vapour, causing its refractive index to fall and rise respectively. A typical range of refractive index through which the polyethylene oxide material will pass as the relative humidity changes from typically 40% to 90% is from n = 1 .5 to n = 1 .4. It will be clear to the skilled reader, therefore, that with increasing relative humidity the device will pass from a regime in which the refractive index of the sensor material is higher than that of the cladding material, such that light will be coupled out from the fibre into the sensor material, to a regime in which the refractive index of the sensor material is lower than that of the cladding material, such that light will not be coupled out from the fibre. The relative humidity at which its refractive index becomes equal to that of the cladding mateπal 2 is approximately 80% at room temperature. (This value can be changed if desired by doping the PEO with a suitable dopant Such suitable dopants might include ionic salts such as cobalt chloride, although this has been found to lead to problems with film degredation) . The thickness of the layer 4 was chosen to be such that approximately 50% of single-mode optical radiation of a given wavelength carried by the fibre 1 , 2 was coupled out by the sensor material 3 in situations (relative humidities) in which the refractive index of the sensor material 3 was greater than that of the cladding material 2. (This figure of 50% occurred for a wavelength of 1 .3μm and increased with increasing wavelength due to the corresponding increase in the mode field diameter of the light in the fibre) . This thickness of the layer 4 was achieved by carrying out the polishing of the top surface of the block 6/fιbre 1 ,2/adhesιve 5 assembly in stages, and testing the properties of the assembly after each stage The test method employed was the so-called "liquid drop" method for details of which reference may be made to an article "Measurement of the Core Proximity in Polished Fibre Substrates and Couplers" by Digonnet et al in Optics Letters, J_Q ( 1 985) at pages 463-5. This method basically consists in measuring the loss of transmission in the fibre 1 ,2 when liquid of an appropriate refractive index (higher than that of the cladding) is applied to the exposed fibre. Several sensor arrangements, each as described with reference to Figures 2 and 3, may be included serially in a single optical fibre transmission path, as shown in Figure 4 In Figure 4 the sensor arrangements 8 are included serially in a single transmission path formed by a single-mode optical fibre 9 The fibre 9 is cut at each location where a sensor arrangement 8 is required and the two fibre ends thus created at each location are spliced to respective ends of the fibre 1 ,2 of that sensor arrangement The arrangements 8 may be interrogated by means of an Optical Time Domain Reflectometer 10 positioned at one end of the fibre 9 for launching optical test signals into the fibre 9 and analysing signals received from the fibre 9 in response. It will be noted that this arrangement and in particular the use of such a sensor material as polyethylene oxide has the advantage that the system operates at the worst case, with respect to the power budget, when all sensors are in their 'safe' or rather 'dry' state. Any change from this, such as when at least one sensor detects the appropriate threshold relative humidity level causes the overall light loss from the fibre to be lowered. This eliminates the risk of 'obscuring' sensors toward the end of a serial array. It will be evident that many modifications may be made to the embodiments described, while remaining within the scope of the invention as defined by the claims. For example the sensor material 3 may be something other than polyethylene oxide, for example gelatine or polyvinyl alcohol. Indeed, sensor arrangements in accordance with the invention may be used to sense substances other than water vapour, if the sensor material 3 is chosen appropriately. In this last connection reference may be made, for example, to a paper by Ronot et al "Detection of chemical vapours with a specifically coated optical-fibre sensor" in Sensors and Actuators B, 11 ( 1 993) pages 375-381 for a discussion of sensor materials whose refractive index varies in the presence of vapour of a given chemical or chemicals. As a modification to the embodiments described a thin layer of a further material may be provided between the thin layer 4 of cladding material and the sensor material 3. In such a case the variation of the refractive index of the material 3 upon take up of the substance or substances to be sensed should be capable of encompassing a range which includes the 'effective' refractive index of the new 'effective' cladding material formed from the combination of the further material and the 'old' cladding material rather than just the refractive index of the 'old' cladding material.

Claims: CLAIMS 1 . A sensor arrangement comprising an optical waveguide having a core of material which is transmissive for optical radiation having a given wavelength and a cladding which surrounds the core, the cladding being of material which has a refractive index at the given wavelength which is less than the refractive index of the core, the arrangement including a sensor material which is capable of taking up a given substance or substances within itself and has a refractive index which is dependent upon the amount of such substance(s) taken up, the sensor material being positioned sufficiently close to the core, yet still separated from the core by cladding material, as to be capable, at least when the refractive index of the sensor material has a first value corresponding to a first said amount of substance(s) taken up, of coupling radiation having the given wavelength out from the optical waveguide and thereby reducing the transmissivity of the waveguide for radiation having the given wavelength, characterised in that the cladding material has a refractive index at the given wavelength which is intermediate said first value of the refractive index of the sensor material and a second value of the refractive index of the sensor material which is lower than the first value and corresponds to a second said amount of substance(s) taken up. 2. An arrangement as claimed in Claim 1 , wherein the optical waveguide is a single-mode optical waveguide at the given wavelength. 3. An arrangement as claimed in Claim 1 or Claim 2, wherein the sensor material is a hydrophilic polymer and the given substance is water so that the arrangement constitutes a humidity sensor arrangement. 4. An arrangement as claimed in Claim 3, wherein the hydrophilic polymer is polyethylene oxide. 5. An arrangement as claimed in any preceding claim, wherein the optical waveguide is an optical fibre which is embedded in a block of material having a flat surface, the surface of a thinned portion of the cladding forming part of the flat surface and the sensor material being present as a layer on the flat surface 6. An arrangement as claimed in Claim 5, wherein the layer of sensor material has a thickness of at least 100 microns. 7. An arrangement as claimed in any preceding claim, provided with an optical time domain reflectometer coupled to the waveguide for launching optical radiation of the given wavelength into the waveguide and receiving backscattered such radiation from the waveguide. 8. A multiple sensor arrangement comprising a plurality of sensor arrangements, each as claimed in any of claims 1 to 6, having their optical waveguides included in succession in a single transmission path for optical radiation of the given wavelength. 9. A multiple sensor arrangement as claimed in Claim 8, provided with an optical time domain reflectometer coupled to the transmission path for launching optical radiation of the given wavelength into the transmission path and receiving backscattered such radiation from the transmission path.

Inventors: BARTON JAMES STEPHEN, BOWNASS DAVID CASSON, JONES JULIAN DAVID CLAYTON, BARTON, JAMES, STEPHEN, BOWNASS, DAVID, CASSON, JONES, JULIAN, DAVID, CLAYTON
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1996
Main classification: G01N21
IPC codes: G01N21, G02B6