TECHNICAL FIELD

The present invention relates to an optical fibre cable f detecting a change in temperature, which fibre cable compris an optical fibre with a core and a cladding having separate r fractive indexes, and a temperature-responsive device whi surrounds the optical fibre along at least a part of its length wherein in a non-influenced state of the fibre cable, a ligh pulse having been sent into the fibre from its one end will be at tenuated substantially uniformly along the length of the fibre and upon microbending of the optical fibre caused by the tempera ture-responsive device, the light pulse will be further atte nuated.

BACKGROUND ART

Microbending will occur in an optical fibre which is subjected t an external force, with subsequent light attenuation. Althoug this is a disadvantage in signal transmission, microbending can b utilized in a manner which will enable the fibre to be used as detector or sensor element. An example of one such force detectin element is founddescribed inEuropean Patent SpecificationNumbe 0188512. An optical fibre having a core and cladding is embrace by a primary cover, and a twisted tape is disposed between th cladding and the primary cover. The twisted tape is responsibl for the microbending when a force is exerted on the fibre.

A fibre cable fordetecting temperature is illustrated in Japanes Patent Application No. 59-50676. The optical-fibre cladding ha wound therearound a wire consisting of memory-metal, whic contracts when exposed to a given detection temperature an therewith exerts pressure on the cladding and causes microbendin of the fibre. Each molecule in the fibre core reflects light b Rayleigh-εcattering and the intensity of the light reflected i directly proportional to the intensity of the passing light pulse The intensity of the reflected light pulse decreases as a resul

of microbending of the fibre. The reflected light pulse can b detected and the position at which microbending occurs along th fibre can be established by so-called OTDR, Optical Time Domai Reflecto etry. The drawback with this device is that in man applications it is difficult to obtain a memory metal which wil produce microbends of desired sizes and at desired temperatures The device is also relatively complicated to manufacture.

DESCRIPTION OF THE INVENTION

According to the invention, an optical fibre has a casing made o polymer material, the coefficient of heat expansion of which ha a desired value in a limited temperature range. The value of t coefficient of heat expansion can be chosen by appropriate choic of the polymer material. Either a positive or a negative coeffi cient can be chosen and the limits of the temperature range can chosenthrough selection of thepolymermaterial. This enables t optical fibre with its casing, the fibre cable, to be given t desired attenuation properties.

The invention has the characteristic features set forth in t following claims.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplifying embodiment of the inventive fibre cable will n be described in more detail with reference to the accompanyi drawings, in which

Figure 1 illustrates schematicallya buildingwiththe fibre cab and a monitoring system;

Figure 2 illustrates the fibre cable in cross-section;

Figure 3 is a schematic illustration of the structure of a polym material;

Figure 4 is a diagram of the coefficient of heat expansion of the polymer material;

Figure 5 is a diagram of the force-effect achieved at differe temperatures;

Figure 6 illustrates the attenuation of a light pulse along th fibre cable;

Figure 7 illustrates the coefficient of heat expansion of a alternative polymer material; Figure 8 is a cross-sectional view of an alternative embodiment o the fibre cable; and

Figure 9 is a diagram showing the coefficient of heat expansion of the materials in the alternative fibre cable o Figure 8.

BEST MODE OF CARRYING OUT THE INVENTION

Figure 1 illustrates schematically a building 1, for instance greenhouse, whose temperature is to be monitored. The greenhous is divided into several rooms 2, 3, 4 and it is of interest to b able to monitor each room individually. It can also be of interes to be able to detect whether or not a small area 5 of one room ha an elevated temperature, such as to expose plants in this part o the room to the risk of injury. The monitoring process is effecte with the aid of temperature-responsive optical fibre cables 6 an 7, which are connected to an OTDR-system 8, 9 having a control uni 10. OTDR is an acronym for Optical Time Domain Reflectometry and, in brief, functions to transmit light pulses of short duratio through the fibre, from one end thereof. When the fibre is sub jected to an external influence within a domain, the attenuatio in the fibres increases as a result of microbending of the fibre. As a result of the aforesaidRayleigh-scattering, the light pulse are reflected back to said one end of the fibre and the reflecte light pulses are attenuated in the influenced domain. Th reflected pulses are detected and the location at which the in fluence has been exerted can be determined by measuring th running time of the light pulses. Amore exhaustive description o OTDR is found, for instance, in US Patent Specifications Number 4,463,254 and 4,713,538.

The optical fibre cables 6 and 7 are inventive, and an exempli fying embodiment of these cables will now be described in mor detail. Figure 2 is a cross-sectional view of the fibre cable

comprising a core 11, a cladding 12 and a primary cover 13 of an optical fibre. In accordance with the invention, a casing 14 of polymer material is disposed externally on the primary cover 13. The casing extends along at least a part of the fibre cable 6, and in the illustrated embodiment, the polymer is an amide resin. Further protective covers can be disposed on the casing 1 . The amide resin is composed chemically of carbon rings 15 containing six carbon atoms, as indicated by the reference C in Figure 3. This material can be treated in manufacture in a manner to extend the carbon rings in a non-influenced state at a temperature TO, such that the rings will have a length dimension of Dl in said direc¬ tion. When this material is subjected to an elevated temperature, the carbon rings contract in their longitudinal direction, to a length extension of D2. This change is reversible, so that the carbon rings return to their larger length Dl at the lower temperature TO. The amide material obtains, in this way, a coefficient of heat expansion α which varies with temperature, referenced T, as illustrated in Figure 4. The coefficient α has a value of small magnitude within a first range extending from the temperature TO to a temperature Tl. In a temperature detection range intended for the fibre cable 6 and lying between the temperature Tl and a temperature T2, the coefficient of heat expansion has a negative value of magnitude αO. In a temperature range between the temperature T2 and a temperature T3, the coeffi- cient of heat expansion has again a low value, which is negli¬ gible in relation to the value αO. It should be noted that the expression "coefficient of heat expansion" is a somewhat improper term, since the expansion can be negative at increasing tempera¬ tures.

The amide casing 14 exerts on the primary cover 13 a force F which varies in the fashion illustrated in the diagram shown in Figure 5. The force F is almost constant in the temperature range up to temperature Tl. In the temperature detection range T1-T2, the force F increases successively to a force F0 and again remains almost constant in the range above temperature T2. The force F exerted on the primary cover 13 causes the aforesaid microbending

by means of which the light in the fibre cable 6 is attenuated and reflected, as will be described with reference to Figure 6.

This Figure is a diagram which shows the light intensity I along the length L of the fibre cable 6 for a light pulse P which is transmitted from an OTDR-unit 8 illustrated in Figure 1. The intensity is given as an attenuation in decibels, dB, and the length L is given in meters. A curve A illustrates attenuation for the fibre cable 6 in a non-influenced state, when the temperature in the rooms 2, 3 and 4 is lower than Tl. A curve B shows the attenuation of the light pulse P when the temperature in the domain 5 exceeds the temperature Tl. The polymer casing 14 of the fibre cable 6 contracts in the domain 5, corresponding to the range Bl and B2 of the fibre cable, and causes microbending of the optical fibre, resulting in higher attenuation of the light pulse P. It is essential that the increased attenuation, measured as dB/m, has a value such that the Rayleigh-reflected pulse can be safely detected and separated from among the interferences which occur, for instance, as a result of the normal variations in at¬ tenuation in the optical fibre. It is also essential that attenua- tion for each meter of fibre will not be too excessive, since it is desired to be able to detect several ranges of elevated temperature along the fibre, such as the ranges Bl and B2 for instance. As illustrated by the curve E, when the attenuation per unit of length is very pronounced, the light wave P will be so powerfully attenuated that only the first range Bl can be detec¬ ted. The strong attenuation according to curve E can be caused by the fact that the polymer casing 14 has a large coefficient of heat expansion of magnitude αl in the temperature range T1-T2, as illustrated in Figure 4. This coefficient of heat expansion is corresponded by a force Fl on the primary cover 13 at the tempra- ture T2, as illustrated in Figure 5. Pronounced attenuation can also be caused by the fact that the coefficient of heat expansion of the polymer casing is αO within a large temperature range, e.g. between the temperature Tl and a temperature T4. Such a casing will exert a force F2 on the primary cover of the fibre at the temperature T4 according to Figure 5 and consequently the light attenuation will essentially follow the curve E in Figure 6. This

attenuation of the light pulse P caused in the fibre does not only depend, however, on the coefficient α of heat expansion and the extent of the temperature range, but also on the intrinsic properties of the core 11, cladding 12 and primary cover 13 of the optical fibre. The final attenuationproperties canbe established by measuring the fibre cable and an attenuation suitable for many applications lies in a range of 0.01-0.1 dB/m within the desired temperature range of T1-T2.

In the aforedescribed example, an OTDR-system has been used to detect the reflected light pulse in the inventive fibre cable 6. This fibre cable can also be used to detect the attenuated light pulse P at the distal end 6a of the fibre cable 6 in Figure 1. In the illustrated case, the distal end of said cable is connected to the unit 8 through an optical fibre 6b, as illustrated by a broken line in said Figure. In the illustrative example, the attenuated light pulse P has the intensity I = -3.8 dB according to the curve B in Figure 6. However, when detecting the attenuated light pulse it is only possible to establish that an increase in temperaturehas occurred somewhere along the fibre, and the actual location of this temperature increase remains unknown. It may be of interest to have knowledge of this location and the location canbeestablishedrelativelyprecisely, particularlywhenthe two fibre cables 6 and 7 are used in the manner illustrated in Figure 1 for detection purposes.

A number of trials have been carried out with inventive fibre cables. In one of these trials there was used an optical standard fibre provided with a casing 14 of amide resin. The fibre core 11 had a diameter 62.5 microns, whereas the cladding 12 had a diameter of 125 microns and the primary cover 13 a diameter of 250 microns. The trial showed that separate amide-resin materials could be chosen which together with the standard fibre gave the desired attenuation values within the aforesaid range of 0.01- 0.1 dB/m. The temperature range T1-T2 in this trial had a span of 8'C. Trials were also carried out in which the casing 14 was made from other polymer materials, such as carbon-fibre material for instance. Similar to the aforesaid amide materials, these carbon

fibre materials comprise extended or stretched carbon rings an have a negative coefficient of heat expansion within a limite temperature range.

In the above embodiment the optical fibre cable 6 has bee described as suitable for use in detecting an increase in tempera ture, it being desired to hold this increase beneath a give value. In accordance with an alternative embodiment of th invention, the polymer casing around the optical fibre, cor responding to the casing 14, has a positive coefficient of heat expansion, as illustrated in Figure 7. The coefficient of hea expansion within a temperature range of T5-T6 is α2. The value o the coefficient of heat expansion in the temperature ranges TO TS and T6-T7 on respective sides of the aforesaid temperatur range is small in comparison with the value α2. When the tem perature falls, the polymer casing contracts within the tem perature range of T5-T6 and causes microbending of the optica fibre in a manner corresponding to that described with referenc to the fibre cable 6. Polyvinyl chloride, PVC-plastic, is a example of material having a positive coefficient of hea expansion. The optical fibre cable according to this alternativ embodiment is utilized to monitor a temperature which shall b maintained above a given value. Such temperature monitorin processes can be employed in foodstuff refrigerating rooms i which freezing of the foodstuffs shall not occur.

Alternative, inventive embodiments of the aforedescribed fibr cables will now be described with reference to Figure 8. Thi Figure illustrates the earlier mentioned standard fibre, compri sing core 11, cladding 12 and primary cover 13. The primary cove is embraced by a casing 16 of polymer material, which in turn i embraced by a secondary protective cover 17. The coefficient o heat expansion of the polymer casing 16 is shown in Figure 9. I the first temperature range of T0-T1 and the second temperatur range of T2-T3, the coefficient of heat expansion has a valu close to zero. In the temperature detecting range T1-T2, th coefficient of longitudinal expansion has a positive value o magnitude α3. The secondary protective cover 17 has a positiv

coefficient of heat expansion ofmagnitude α4, which is relativel small in comparison with α3. When the temperature rises, th polymer casing 16 expands markedly in the temperature detectio range T1-T2, whereas the size of the secondary protective cover 1 remains relatively unchanged. The outer diameter of the polyme casing 16 changes slightly, and at temperatures above th temperature Tl the casing 16 will exert pressure on the primar protective cover 13. This results inmicrobending of the fibre, i a manner corresponding to that described with reference to Figu re 5 and Figure 6. Further alternative embodiments of the inven tive fibre can be mentioned. In the aforedescribed examples, th core 11 of the optical fibre is concentrical with the polyme casing. The fibre, however, can also be positioned non-centrical ly, so that the primary protective cover of the Figure 8 embodi ment lies against the inner surface of the secondary protectiv cover 17. The space thus defined between the primary cover 13 an the secondary cover 17 is filled with a polymeric material whic when subjected to changes in temperaturepresses the primary cove 13 against the inner surface of the secondary cover 17 an therewithproduces microbending of the fibre. The inner surface o the secondary cover may be provided with promontories in order amplify the microbending. Foamed polyurethane is an example polymeric material suitable for use in this embodiment.

With regard to the embodiment illustrated in Figure 2, rubber c be mentioned as a further example of material and materi combinations for the polymer casing of this constructio According to this embodiment, both the primary cover 13 and t polymer casing 14 can be made of acrylate plastics of mutual different hardnesses, the primary cover 13 being softer than t casing 14. This is an advantageous embodiment from the aspect manufacture.