GB2125572A

GB2125572A - Optical fibre sensors

Info

Publication number: GB2125572A
Application number: GB08222371A
Authority: GB
Inventors: Richard Edward Epworth; Vincent Andrei Handerek
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-08-03
Filing date: 1982-08-03
Publication date: 1984-03-07
Also published as: AU1741183A; JPS5990025A; NZ204955A; ES8500660A1; ES524719A0; AU560013B2; GB2125572B

Abstract

A perturbation (P), such as pressure or vibration, is employed to periodically deform an optical fibre (1) supporting two modes, or two polarisations of one mode, of optical power launched into the fibre by, for example, urging it against an undulating support (2). The period of the undulations is chosen to match the beat length between the two modes, or polarisations, at a predetermined optical frequency. When the fibre (1) is thus deformed coupling between the modes, or polarisations, is achieved; the extent of coupling being a measure of the perturbation causing the deformation. A distributed sensor comprising a number of such sensors is also described; the optical fibre comprising both the sensing elements and the means interconnecting them. Since the beat length varies with optical frequency, the different sensing elements of such a distributed sensor may be resolved by using correspondingly different optical frequencies. <IMAGE>

Description

SPECIFICATION Optical fibre sensors This invention relates to sensor structures and in particularto sensor structures employing optical fibres as sensing elements.

According to one aspect ofthe presentthere is provided a sensor structure including an optical fibre waveguide adapted to support two modes, or two polarisations of one mode, at least along a first length region thereof, and means adapted, in response to a perturbation to be sensed, to mechanically deform the first length region of the fibre periodically.along its length with a period which matches the beat length between the two modes, or polarisations, at a predetermined optical frequency whereby to permit coupling between the modes, or polarisations, respectively, and wherein in use ofthe structure the detection of such coupling in optical power launched into the waveguide corresponds to sensing ofthe perturbation.

According to another aspect ofthe present invention there is provided a distributed optical fibre sensor cable comprising an optical fibre portions of which are adapted to provide sensing elements along its length, and wherein at the sensing elements in use ofthe cable coupling between two modes, ortwo polarisa tionsofone mode, supported bythe optical fibre of optical power launched thereinto is modulated in response to an external perturbation to be sensed acting on the sensing elements.

According to another aspect of the present invention there is provided a sensor arrangement comprising a Iightsource, a light detector and an optical fibre coupling the source and detector,wherein one or more portions of the optical fibre are adapted to provide sensing elements at which in use ofthe arrangement coupling between two modes, ortwo polarisations of one mode, supported by the optical fibre of optical power launched thereinto is modulated in response to an external perturbation, to be sensed, acting on the sensing elements.

Embodiments ofthe present invention will now be described with reference to the accompanying drawings, in which: Figs. 1 a and 1 b illustrate variations in propagation constantwith normalised frequency; Fig. 2 illustrates the beat length for coupling between two modes; Fig. 3 illustrates a basic arrangement for obtaining periodic deformation of an optical fibre by an external perturbation; Figs. 4a and 4b illustrate a view of a section through one specific sensor structure; Figs. 5,6 and 7 show attentative embodiments of sensor structure; Fig. 8 shows a fibre structure which can be used with the sensor structures of Figs. 4to 7, and Fig. 9 illustrates variations in normalised propagation constant with optical frequencyfora tapered fibre.

In a step index multimode optical fibre the propagation constants of modes supported thereby differ as a result of differing propagation velocities of different transverse modes (modes dispersion) or of different polarisations (polarisation dispersion). Fig. 1 a shows howthe normalised propagation constants (ss) of the zero and first ordertransverse modes vary with the normalised frequency (V value) and that the difference in the propagation constants (ass) varies with frequency.Fig. 1 bshows that if a fibre is birefringentthenthe fundamental mode will be split into two orthogonally polarised modes whose difference in propagation constants, Ass, also varies with optical frequency.

In a fibre waveguide with good control of geometry, very little coupling occurs between two modes supported thereby unless the difference, ap, between their propagation constants is very small. However, if a section of a fibre 1 supporting two modes with propagation constants ssl and (32is physicallyde- formed along its length such that it undulates with a longitudinal period which matches the beat length between the two modes (Fig. 2), which deformation may be caused by the effect of an external perturbation indicated by arrows P, for example pressure or vibration, urgingthefibre 1 against a suitably undulating support 2 (Fig. 3),then efficient mode coupling takes place.The beat length is the length of fibre over which two modes slip by one wavelength with respect to each other and is inversely proportional to Ass.

The beat length between modes varies with optical frequency, hence mode coupling only occurs atthe optical frequency at which the beat length and the perturbation length are matched. The perturbation length is the period of the undulations or deformations. At other frequencies coupling does not occur.

The frequency selectively may be maximised by increasing the overall length, but not the periodicity, of the periodically deformed section, and by minimizing the amplitude ofthe periodic deformations.

Asensorfordetecting pressure variations or vibration may thus comprise a length of optical fibre, a section 1 of which is laid against a periodically undulating support 2 such that when the section 1 and support 2 are disposed, for example, in a liquid, pressure variations in the liquid cause corresponding variations in the deformation ofthefibre section 1 relative to the support and corresponding variations in the coupling between modes supported by the optical fibre. The particular optical frequency employed is dependent on the period ofthe undulations ofthe support. The amount of coupling between modes at the particular optical frequency is thus a measure of the size ofthe perturbation.Typically the amount of coupling is measured by means of a mode selective filter and detector atthe output end of the fibre, which device may be directly calibrated in terms of pressure or other perturbation if required.

Fig. 3 shows only a basic arrangement for obtaining the necessary undulating support of the optical fibre in the sensor region thereof. As indicated at 3, however, the optical fibre section 1 is preferably protected from the direct contact with the medium supplying the perturbation by a protective layer 3.

Various embodiments of such undulating support optical fibre sensors will now be described with reference to Figs. 4,5,6 and 7. In Figs. 4a and bthere is shown a sensor comprising an optical fibre 4 laid longitudinally along the surface of a cylindrical supportS whose diameter varies in a periodic manner along its length. A cylindrical sleeve 6 (not shown in Fig. 4a) of an elastic material, such as polypropylene orsilicone rubber, serves to hold thefibre in place and to protect it. Modulation of the pressure acting on the sleeve 6 will thus modulate the magnitude ofthe coupling between modes supported bythefibre.

Alternatively, this configuration may be reversed in that the sleeve may be of a harder material and provided with the undulations and the support may be of a compliant material, such that it yields in response to pressure applied to the sleeve in order to permit the optical fibre to be deformed bythesleeve. Furthermore, arrangements in which both the sleeve and the support are undulated can be envisaged. The support periodic diameter variations illustrated in Fig.

4 may be achieved by suitable machining of a body or alternatively (Fig. 5) may be achieved by helically winding a suitably sectioned elongate element, such as a circularlycross-sectioned wire 7 around a main cylindrical support 8. The fibre 9 is then laid across the turns ofthewire 7 along the length ofthe support 8 and may be held there by a flexible sleeve (not shown).

The support 8 may comprise a nylon former and the wire7 may be of steel. Instead of employing periodic diameter variations of a support and a longitudinal fibre as in Figs. 4 and 5, a fibre 13 may be helically applied to a suitably shaped support, as illustrated in Figs. 6 and 7, in which case an external sleeve (not shown) may or may not be employed. In Fig. 6 a support 10 is provided with longitudinal extending grooves as in a splined shaft. In Fig. 7 a cylindrical support 11 is provided with a stranded layer 12 of wires or other suitably sectioned elongate members.

As mentioned with regard to Fig. 4, when a sleeve is employed these structures may be reversed also. That istheperiodicstructures may be provided on the inside of a sleeve between which and a compliant supportthefibre is arranged.

Whereas only individual sensors have been described so far, the frequency dependence of the mode coupling which they employ means that a so-called distributed sensor may be readily constructed using the same basictechniques. In a distributed sensor a plurality of sensing elements are distributed along a line and connected to suitable apparatus for display ingtheparametersensed bythe elements. In thecase of a known acoustic distributed sensor a number of microphones are distributed along a line and electri cally interconnected to a receiving apparatus such as to provide an average signal from that received by the microphones. Such a distributed sensor may be comprised, alternatively, by mode-coupling optical fibre sensors as described above.One embodiment of optical fibre distributed sensor comprises a length of optical fibre having localised sensor regions each of which is formed by one ofthe sensor configurations described above. For example at discrete intervals along the length ofthe optical fibre, the fibre is positioned between respective support and sleeves which cooperate with one anotherto deform the fibre in dependence on an external perturbation, for example the liquid pressure of a liquid in which the sensor is arranged. Thus the optical fibre as well as acting as the sensor elements also acts as the means connecting the sensor elements.

An optical fibre distributed sensor with localised sensor regions may alternatively be constructed by stepping the diameter of the fibre core 14 (Fig. 8) up and down at intervals so that the sensing regions, such as 15, are capable of supporting two modes, whereas the intermediate regions 16 are single mode regions used onlyfortransmission purposes. The sensing regions 15 are provided with respective support and sleeve arrangements as described above, for example, in order to deform them in dependence on the external perturbation to be sensed. In a single modetwo polarisation fibre, the sensing regions may be comprised by regions of relatively high birefringence in a fibre of otherwise relatively low birefringence.

The distributed optical fibre sensors described so far operate with a single optical frequency and thus provide only an average signal asinthecaseofthe mentioned known acoustic distributed sensor. The frequency dependence ofthe mode coupling may however be used to advantage in orderto distinguish between different sensor regions. Basically, each sensor region of a distributed optical fibre sensor (a sensorcable) isarrangedto beableto produce mode coupling only at a particular and different optical frequencyfrom the other sensor regions and, there- fore, different positions along the length ofthe sensor cable may be resolved simply by the use of different optical frequencies.

Such region resolution may be achieved by employing a fibre with uniform characteristics along its length, for example constant diameter, ellipticity etc., and employing sleeve-substrate structures at each sensing region which provide different perturbation periods (undulation periods) and thus require different beat lengths and corresponding different optical frequencies for mode coupling. Alternatively, region resolution may be achieved by employing a single perturbation period but varying the beat length of the fibre along its length, so that different sensing regions have different beat lengths associated there- with. This may be achieved by using a fibre whose diameter, refractive index profile, ellipticity or stress induced birefringence aredeliberatelyvaried con- tinuously or at intervals along its length.Fig. 9 shows howthe normalised propagation constants of the zero and first order modesvarvwith optical angular frequencyoafortwo different regions land II of a tapered fibre 17 with the same perturbation period, thus enabling thetwo regionsto be resolved by using different optical frequencies wl and (t)2 Using either or both ofthe region resolution configurations described above a distributed sensor cable may be achieved in which a specific region of the cable may be resolved by measuring the coupling at a specific optical frequency. The use of abroad- band laser light source and afrequencydispersive element, for example a grating or prism, together with a linear array of photodetectors, enables simultaneoussensing ata large numberofindependent regions of a sensor cable to be achieved. Alternative ly, fibre optic wavelength multiplex components may be used to demultiplexthe signals from specific sensor regions. Since the sensor is truly distributed, the linear array may be remotelyconfigured.Thatis, since the sensor elements are capable of detecting the phase of the sensed signal it is possible to process the sensor signal to produce phased array detection.

Techniques analogous to those used in radar and radio communication applications may be employed to form steerable lobes of enhanced sensitivity-- a so-called "beamforming" sensor. When such a sensor is used in a medium such as water, the full range of beam steering techniques should be possible.

Whereas the invention has been described with reference to the detection of pressure variations or vibration, it is applicable to the sensing of any perturbing effect which causes a small physical displacement of the sensing fibre and/or its support- ing and perturbing structure. For example, a high expansion coefficient former could be used to detect temperature, or a magnetorestrictive sleeve could be used to detect magnetic fields.

Claims

1. A sensor structure including an optical fibre waveguideadaptedtosupporttwo modes, or two polarisations of one mode, at least along a first length region thereof, and means adapted, in response to a perturbation to be sensed, to mechanically deform the first length region of the fibre periodically along its length with a period which matches the beat length between the two modes, or polarisations, at a predetermined optical frequencywherebyto permit coupling between the modes, or polarisations, respectively, and wherein in use of the structure the detection of such coupling in optical power launched into the waveguide corresponds to sensing of the perturbation.

2. Asensorstructureasclaimed in claim 1, wherein the extent of coupling provides a measure of the perturbation.

3. A sensor structure as claimed in claim 1 or claim 2, wherein the means to mechanically deform thefibre comprisesfirstand second support means between which the first length region ofthe fibre is arranged, and wherein either or both ofthe first and second means are shaped whereby to provide the periodic deformation ofthe fibre when urged together by the perturbation.

4. Asensorstructure as claimed in claim 3, wherein the first means comprises a first cylindrical support whose diameter varies periodically along its length, the period of the diameter variations matching said beat length, wherein thefirst length region of the fibre is laid along the length ofthe first cylindrical support and held in place thereon by a sleeve comprising the second means and upon which in use ofthe structurethe perturbation acts.

5. Asensorstructure as claimed in claim 3 wherein the first means comprises a second cylindrical support on which is helically wound an elongate elementwherebyto present an undulating surface having a period matching said beat length for deformation ofthefirstlength region ofthefibre which is laid along the length ofthe first means and held in place by a sleeve comprising the second means and upon which in useofthestructurethe perturbation acts.

6. A sensor structure as claimed in claim 1 or claim 2, wherein the means to mechanically deform the fibre include an elongate support on which the first length region of thefibre is helically wound.

7. A sensor structure as claimed in claim 6, wherein the elongate support presents an undulating surfaceto the fibre such asto provide the periodic deformation of the fibre when the fibre is urged thereagainst by the perturbation.

8. A sensor structu re as claimed in claim 7, wherein the elongate support comprises a splined shaft.

9. A sensor structure as claimed in claim 7, wherein the elongate support comprises a cylindrical body on which a stranded layer, comprised by a plurality of elongate members, is arranged.

10. A sensor structure as claimed in anyone of claims 7 to 9, including afirstsleeve member arranged over the first length region of the fibre wound on the elongate support.

11. A sensor structure as claimed in claim 6, wherein the elongate support is compliant and a second sleeve member is arranged overthefirst length region of the fibre wound on the elongate support, which second sleeve member presents an undulating surface to the fibre such as to provide the periodic deformation of the fibre when urged thereagainst by the perturbation.

12. A sensor structure as claimed in any one of the preceding claims, wherein the optical fibre includes tail portions at both end ofthe first length region and whereinthetail portions are adapted to support a common single mode or polarisation.

13. Adistributed sensorcomprising a plurality of sensor structures as claimed in any one of the preceding claims, each sensor structure including a respective length region ofthe optical fibre whereby the optical fibre serves both as part of the sensor structures and as means to interconnectthem.

14. A distributed sensor as claimed in claim 13, wherein the sensor structures are substantially identical and the optical fibre's parameters are substantially constant along the length thereof.

15. A distributed sensor as claimed in claim 13, wherein a different predetermined optical frequency for coupling is associated with each sensor structure.

16. A distributed sensor as claimed in claim 15, wherein the parameters of the optical fibre are substantially constant along the length thereof and wherein the mechanised deformation means of the different sensor structures differfrom one another.

17. A distributed sensor as claimed in claim 15, wherein one or more parameters of the optical fibre vary along its length and wherein the mechanical deformation means ofthe different sensor structures are substantially identical.

18. A distributed sensor as claimed in claim 15, wherein one or more parameters ofthe optical fibre vary along its length and wherein the mechanical deformation means ofthe different sensor structures differfrom one another.

19. A distributed opticalfibresensorcablecom- prising an optical fibre portions ofwhich are adapted to provide sensing elements along its length, and wherein atthe sensing elements in use ofthe cable coupling between two modes, or two polarisations of one mode, supported by the optical fibre of optical power launched thereinto is modulated in response to an external perturbation to be sensed acting on the sensing elements.

20. A sensor cable as claimed in claim 19, wherein the optical fibre, at least atthe sensing elements, comprises a common waveguide for both modes, or both polarisation ofthe one mode.

21. A sensor cable as claimed in claim 20, wherein coupling at each ofthesensing elements is achieved at respective optical frequencies whereby the sensing elements can be spatially resolved along the length of the cable.

22. A sensor cable as claimed in claim 21, wherein at each sensing element the external perturbation serves to cause periodic mechanical deformation of the fibre along its length with a period which matches the beat length of the two modes, orthetwo polarisations, at the respective optical frequency.

23. Asensorarrangement comprising a light source, a light detector and an optical fibre coupling the source and detector, wherein one or more portions ofthe optical fibre are adapted to provide sensing elements at which in use ofthe arrangement coupling between two modes, or two polarisations of one mode, supported by the optical fibre of optical power launched thereinto is modulated in response to an external perturbation, to be sensed, acting on the sensing elements.

24. A sensor structure substantially as herein described with reference to and as illustrated in Figs.

1 and 2, Figs. 1,2 and 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7 or Fig. 8 of the accompanying drawings.

25. A distributed sensorsubstantially as herein described with or without reference to Fig. 9 of the accompanying drawings.