GB2347209A

GB2347209A - Fibre optic sensing

Info

Publication number: GB2347209A
Application number: GB9904042A
Authority: GB
Inventors: Gerard Fernando; Mandeep Singh; Tongyu Liu
Original assignee: Cranfield University
Current assignee: Cranfield University
Priority date: 1999-02-22
Filing date: 1999-02-22
Publication date: 2000-08-30
Anticipated expiration: 2019-02-22
Also published as: GB9904042D0; GB2347209B

Abstract

Fibre optic sensing apparatus, for sensing at least one measurand of interest, comprises individual fibre optic interferometric physical and/or chemical sensors (2<SB>1</SB>, 2<SB>2</SB>, 2<SB>3</SB>) multiplexed in series and/or in parallel at desired positions along one or more optical fibres (1) in a region of interest. Each such sensor (2<SB>1</SB>, 2<SB>2</SB>, 2<SB>3</SB>) may be Fabry-Perot type etalon structure with an optical pathlength difference which may be different from that of the other sensors (2<SB>1</SB>, 2<SB>2</SB>, 2<SB>3</SB>) of the apparatus. A sensor interrogation system, comprising a Fourier transform spectrometer (3), reads the output of any physical sensor (2<SB>1</SB>, 2<SB>2</SB>, 2<SB>3</SB>) using time domain analysis, and reads the output of any chemical sensor using spatial domain analysis. Fibre optic interferometric sensors (2<SB>1</SB>, 2<SB>2</SB>, 2<SB>3</SB>) may also be multiplexed with one or more spectroscopy-based chemical sensors (7), which may be interrogated in the frequency domain of the same spectrum.

Description

FIBRE OPTIC SENSING The present invention relates to fibre optic sensing, in particular using optical sensors based on the Fabry-Perot etalon.

Over the last few years it has been demonstrated that optical sensors based on the Fabry-Perot etalon can be used to measure a number of different parameters, notably strain, temperature and pressure, for example as discussed in"Interferometric Fibre Optic Temperature Sensor Using a Low-Coherence Light Source", C. E. Lee and H. F. Taylor, Proc. SPIE-Int. Soc.

Opt. Eng. 1370,356 (1991);"Demodulation of a Fibre Fabry-Perot Strain Sensor using White Light Interferometry" : G. Zuliani, D. Hogg, K. Liu, R. M.

Measures, Proc. SPIE-Int. Soc. Opt. Eng. 1588,308 (1992);"White-Light Interferometric Multimode Fiber Optic Strain Sensor", Belleville, C. and Duplain, G., Optics Letters 18,78 (1993) ;"Fiber-Optic Displacement Sensor with 0.02ym Resolution by White-Light Interferometry", Koch, A. and Ulrich, R., Sensors and Actuators A-Physical, 25,201 (1991) ;"Displacement Sensor and Electronically-Scanned White-Light Interferometer", Koch, A. and Ulrich, R., Proc. SPIE Int. Soc. Opt. Eng. 1267,126 (1990); and"Signal Processing Techniques for Interferometric Fiber-Optic Strain Sensors", Liu, K. and Measures, R. M., J.

Intell. Mater. Syst. and Struct., 3,432 (1992). In particular, it has been established that, by using the output of such a sensor to generate an interferogram, the position of the side burst signal relative to the dominant"centre-burst"signal at zero path difference is directly related to the Fabry-Perot cavity length d and the refractive index n of the medium in the cavity.

Thus, measurements of perturbations of the Fabry-Perot cavity length d or the refractive index n of the medium in the cavity caused by changes in a parameter of interest can be used to derive values for those parameter changes.

It would be desirable to multiplex several Fabry Perot sensors, so as to be able to employ a number of sensors whilst speeding up the measurement process and/or avoiding duplication of analysis equipment, but hitherto such multiplexing has not been thought feasible, owing to the complex responses obtained in the frequency and spatial domains and low total light throughput owing to losses at each sensor.

According to a first aspect of the present invention there is provided a method of interrogating individual fibre optic interferometric physical and/or chemical sensors multiplexed in series and/or in parallel along one or more optical fibres, each sensor having a Fabry-Perot type etalon structure with an optical pathlength difference which is different from that of the other fibre optic interferometric sensors, in which method the absolute value of the optical pathlength difference of each sensor is determined using a Fourier transform spectrometer, the output of any such physical sensor being read in the time domain and the output of any such chemical sensor being read in the spatial domain, and values for at least one measurand of interest are derived from the absolute values of the optical pathlength difference so obtained.

Thus, a time-domain or spectral-domain analysis is employed to retrieve the characteristic"signature"of each sensor, which is distinct and is clearly resolvable as a result of all sensors having different optical pathlength differences.

From the applicant's preliminary measurements, it seems that a method embodying the present invention will be particularly advantageous for high resolution quasi-static strain measurements. New applications can also now be envisaged that previously would have been considered difficult. The proposed fibre Fabry-Perot sensor interrogation technique, having the significant advantages of sensor multiplexing and internal selfreferencing, is therefore an advance on anything reported to date in the literature. Furthermore, the use of FFP sensors surpass the performance of other sensor systems where measurements at high temperatures ( > 400 C) are required, and where strain-temperature measurand crosstalk is undesirable.

According to a second aspect of the present invention there is provided a method of simultaneously interrogating one or more fibre optic interferometric physical and/or chemical sensors and one or more spectroscopy-based chemical sensors using a Fourier transform spectrometer, in which method the absolute value of the optical pathlength difference of the or each interferometric sensor is determined in the time domain, in the case of a physical sensor, or in the spatial domain, in the case of a chemical sensor, whilst the or each spectroscopy-based chemical sensor is interrogated in the frequency domain of the same spectrum.

Preferably in a method embodying the first or second aspect of the present invention an interferogram is produced by the Fourier transform spectrometer and the positions of the peaks of each first-order side burst relative to the position of the peak of the centre-burst are determined, from which positional information the said absolute values of the optical pathlength difference of the said fibre optic interferometric sensors are derived.

The interferogram may be generated by standard intensity-splitting interferometers. In the case of the Michelson interferometer of a Fourier transform spectrometer which is internally laser-referenced, each of the sensor"signatures"can be referenced to the dominant"centre-burst"at zero path difference.

The sensors may be interrogated simultaneously for more than one measurand of interest.

The measurand, or one of the measurands, may be one of the following quasi-static strain; dynamic strain; quasi-static temperature; dynamic temperature; periodic vibrations; random vibrations; acoustic emission; static pressure; dynamic pressure; presence and/or concentration of specified organic and inorganic substance (s).

According to a third aspect of the present invention there is provided fibre optic sensing apparatus, for sensing at least one measurand of interest, characterised by individual fibre optic interferometric physical and/or chemical sensors which are multiplexed in series and/or in parallel at desired positions along one or more optical fibres to be deployed in a region of interest, each such sensor having a Fabry-Perot type etalon structure with an optical pathlength difference which is different from that of the other fibre optic interferometric sensors of the apparatus, and by a sensor interrogation system, comprising a Fourier transform spectrometer, which is operable to read the output of any such physical sensor using time domain analysis and to read the output of any such chemical sensor using spatial domain analysis, from which output values for the said at least one measurand are derived.

According to a fourth aspect of the present invention there is provided fibre optic sensing apparatus characterised by at least one fibre optic interferometric sensor, having a Fabry-Perot type etalon structure, at least one spectroscopy-based chemical sensor, and a sensor interrogation system, comprising a Fourier transform spectrometer, which is operable to interrogate said sensors simultaneously such that the absolute value of the optical pathlength difference of the said at least one interferometric sensor is determined in the time domain, in the case of a physical sensor, or in the spatial domain, in the case of a chemical sensor, and the said at least one spectroscopy-based chemical sensor is interrogated in the frequency domain of the same spectrum.

The spectroscopy-based chemical sensor may be one of the following sensor types: transmission-based chemical sensor; reflection-based chemical sensor; total internal reflection-based chemical sensor; fibre optic transmission type chemical sensor; fibre optic reflection type chemical sensor; fibre optic evanescent type chemical sensor.

The spectroscopy-based chemical sensor is desirably suitable for use in carrying out at least one of the following: determining the concentrations of chemical species; measuring pH in solution; measuring temperature; monitoring the kinetics of chemical reactions.

At least one of the said fibre optic interferometric sensors is preferably one of the following types: intrinsic fibre Fabry-Perot interferometer; extrinsic Fabry-Perot interferometer; in-line fibre Fabry-Perot etalon; Michelson interferometer; Mach-Zehnder.

The fibre optic interferometric sensors may consist of one of the following elements: (i) perpendicularly cleaved fibre ends; (ii) polished fibre ends; (iii) fibre ends coated with thin metallic or dielectric films acting as partial reflectors; (iv) reflectors comprising a poor fusion splicing joint between two sections of fibre; (v) narrow optical band pass reflecting surfaces including reflectors made of fibre Bragg gratings; (iv) fabricated, micro-machined and/or etched cavities; (vii) reflectors comprising any combination of elements (i) to (vi).

The Fourier transform spectrometer is desirably one of the following types: moving mirror; spatial domain imaging; CCD imaging; and preferably employs an interferometer of one of the following types: Michelson; Mach-Zehnder; Sagnac.

The said Fourier transform spectrometer preferably comprises one of the following types of scanning unit: mechanically moving mirror or mirrors; Charge Coupled Devices (CCD) as a spatial imager; one or more photodiode detector arrays as a spatial imager; integrated optical phase modulators.

Reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a schematic diagram of fibre optic sensing apparatus embodying the second aspect of the present invention; Figure 2a is a schematic diagram of a fibre Fabry Perot sensor in an optical fibre and Figure 2b is a schematic diagram of several fibre Fabry-Perot sensors multiplexed in a single optical fibre; Figure 3 shows an interferogram of a fibre Fabry Perot etalon; Figure 4 is a diagram illustrating the relative position of the side bursts of three fibre Fabry-Perot sensors in series; Figure 5 is a graph illustrating the strain response of a fibre Fabry-Perot sensor; Figure 6 shows a first fibre optic sensing apparatus embodying the second aspect of the present invention; Figure 7 shows a second fibre optic sensing apparatus embodying the second aspect of the present invention; Figure 8 shows a third fibre optic sensing apparatus embodying the second aspect of the present invention ; Figure 9 shows a fourth fibre optic sensing. apparatus embodying the second aspect of the present invention; and Figure 10 shows a fifth fibre optic sensing apparatus embodying the second aspect of the present invention.

Figure 1 is a schematic diagram of fibre optic sensing apparatus embodying the present invention comprising an optical fibre 1 in which a series of three fibre Fabry-Perot (FFP) sensors 21, 22, 23 have been multiplexed. The FFP sensors 21, 22, 23 have respective cavity lengths dl, d2, d3. Modulated light is launched into the optical fibre 1 by a Fourier transform (FT) spectrometer 3 which inclues an interferometer 4. This embodiment of the present invention will be described with reference to use of a Michelson interferometer of a standard off-the-shelf FT spectrometer.

The characteristic optical response from each sensor is recovered using standard Fourier transform spectrometric analysis. The use of Fourier transform spectrometry (FTS) for the measurement of multi-layered thin epitaxial semiconductor and glassy films is a well-established metrology tool in the semiconductor industry, as described in :"Characterization of Semiconductor Silicon Using Fourier Transform Infrared Spectrometry" : K. Krishnan, P. J. Stout and M. Watanabe, Practical Fourier Transform Infrared Spectroscopy, Academic Press (1990), p. 285; and American Society for Testing Materials, Annu. Book ASTM Stand., F95-76 (1983). The principle of this technique is the exploitation of the physical phenomenon of interference due to multiple internal reflections within a thin film epitaxial layer system. It is used in a method embodying the present invention to observe interferometrically the multiple internal reflections within the FFP etalons multiplexed along the optical fibre 1.

Rather than using the transmittance spectrum computed from the recorded interferograms, an embodiment of the present invention uses the information in the interferogram itself. Figures 2a and 2b represent schematically the FFP etalon cavity measurement using the interferometric technique. If there were only one reflecting surface, ie. a cleaved fibre end, the resulting interferogram would exhibit a maximum signal (known as the centre burst) at the point of zero retardation of the moving mirror of the Michelson interferometer 4. At this point the light beams in the two arms of the interferometer 4 have travelled equal distances. The confinement of the centre burst in the time domain is a direct result of the extended response of the light sources of the FT spectrometer 3 in the frequency (spectral) domain.

However, the FFP etalon 21,... necessarily consists of two parallel optical flats. This will result in multiple intra-cavity reflections, the most prominent of which is the reflection that induces an optical path difference (OPD) of 2nd where d is the FFP cavity length and n is the refractive index of the medium between the optical flats-in this case air. This in turn results in a feature in the interferogram commonly known as a side burst at a separation from the centre burst corresponding directly to the OPD of 2nd, as can be seen in Figure 3 in respect of an FFP sensor having a cavity length d of gram. Since the position of the moving mirror of the interferometer 4 may be internally referenced by counting the fringes of the internal HeNe laser-an integral part of all modern FT spectrometers -the position of the centre burst can be accurately known. The separation of the side burst from the centre burst is therefore an absolute measurement of the cavity length d and/or refractive index n. It therefore follows that a series of FFP etalons 21, 22, 23,... with varying cavity lengths dj, d2, d3,... (or varying refractive indices n1, n2, n3) will exhibit sharp side burst"signatures"in the interferogram at varying separations from the internally-referenced centre burst. Thus, as has been shown to be the case experimentally in both multi-and mono-mode optical fibres, all FFP sensor-related side burst signatures are multiplexed in one interferogram. Furthermore, as shown in Figure 4, a small change in the cavity length d due to an external perturbation, e. g. axial strain, or in the refractive index n, results in a shift in the side burst signature relative to the centre burst.

Importantly the shift in the side burst must necessarily exhibit a linear relationship to the cavity length d assuming a constant refractive index of the cavity medium n (and vice versa), as shown by the experimental results illustrated in Figure 5, which illustrates the strain response of an FFP sensor of gauge length (GL) 2.2 cm and cavity length d. Since the cavity length d is the function of applied axial strain, a linear response to strain is inherent in this system. The strain resolution of this system is a data point spacing in the interferogram as determined by the fast Fourier transform algorithm which, for an 8 cm-1 scan, was determined to be 10y strain for a 1 cm gauge length sensor. The data point spacing and the gauge length are the parameters that determine the strain resolution. For higher spectral resolution and longer gauge lengths, a strain resolution of lu strain is considered possible for this system.

The use of white light (low coherence) interferometry (WLI) necessitates, unlike laser interferometry, the use of a receiving (interrogating) interferometer. This makes possible absolute-and internally-referenced measurements, in contrast to high coherence laser interferometry with which only incremental or relative measurements can be obtained.

Furthermore, the self referencing properties of the WLI method obviate the need for complex referencing techniques. Thus, a system shut-down would not require recalibration or referencing.

The interrogation system described above will operate in a quasi-static measurement mode in which the strain response at each sensor can be measured every 0.2 to 1.0 s. Although quasi-static measurement is adequate for many applications, a dynamic strain sensing system will be required ultimately in, for example, aerospace applications. In these circumstances spatial domain imaging of the interferogram may be employed. For this, the use of a linear focal plane array CCD detector for dynamic time domain interrogation of multiplexed FFP sensors is proposed. In this case the dynamic response will be limited only by the frequency response of the CCD detector-typically several kHz. A possible instrument would be a tilted-mirror fixed path difference (Fizeau) interferometer-based"static"FT spectrometer, as mentioned in"Fourier Transform Spectrometer with a Self-Scanning Photodiode Array", Okamoto, T., Kawata, S., and Minami, S., Appl. Opt. 23, 269 (1984); and"Photodiode Array Fourier Transform Spectrometer with improved Dynamic Range", Barnes, T. H., Appl. Opt. 24,3702 (1985). In such a spectrometer the path difference is created by a tilt in the mirror in one of the interferometer arms. The advantage of this system is that no moving parts exist and it is therefore able to operate at the frequency limit of the CCD detector.

Thus, as explained above, the proposed method allows (in principle) an indefinite number of individual fibre optic sensors to be multiplexed along a single optical fibre, the sensor output signature of each sensor being read exclusively in the time domain in which it is distinct and clearly resolvable.

Advantageously, the interferometer employed can be based on conventional designs such as the Michelson, Mach-Zehnder, Sagnac and other similar designs.

The internal laser referencing of all commercially available FT spectrometers (moving mirror, spatial domain CCD imaging, etc.) allows the sensor output signature of each sensor to be referenced with respect to the dominant centre-burst signature of the interferogram, but where internal referencing is not required an optical fibre analogue of the abovementioned interferometer designs can be employed.

Any perturbation of the Fabry-Perot cavity of each sensor can be detected where the perturbation is caused by a change in the cavity length d, a change in the medium in the cavity or a change in the refractive index n of the medium in the cavity induced by any means. Accordingly, as mentioned above, in principle a large number of different measurands can be monitored via this technique. Indeed, as a consequence of the multiplexibility of this sensor system, one or more of different measurands can be measured simultaneously.

As mentioned above in more detail, this system may employ any Fabry-Perot type etalon structure.

Figure 6 shows apparatus embodying the present invention in more detail. As can be seen in Figure 6 the Fourier transformer spectrometer 3 includes a light source 31 and a Michelson interferometer 4. The Michelson interferometer 4 comprises a beam splitter 41, a moving mirror 42 and detector 43. Light is launched from source 31 into fibre optic interferometric sensors 2i, 22,... 2N, multiplexed in series along optical fibre 1 via a first fibre connector 5 and fibre couplers 6. The return signal is directed, via fibre couplers 6 and a second fibre connector 5, onto the beam splitter 41 of the interferometer 4.

Figure 7 shows a second embodiment of the present invention similar to that of Figure 6 except in that the fibre optic interferometric sensors 21, 22,.... 2N are multiplexed in parallel, rather than in series, along respective optical fibres 1.

A further variation is shown in Figure 8 in which fibre optic interferometric sensors 211, 212,..., 21n are multiplexed in series along one optical fibre 1 and also in parallel with further fibre optic interferometric sensors 221,.-.., 2N1 along further optical fibres 1.

Figure 9 is a modification of the Figure 8 embodiment. In addition to the fibre optic interferometric sensors 211 to 21n and 221 to 2N1, spectroscopic chemical sensors 7, multiplexed in parallel with the fibre optic interferometric sensors, are provided. Whereas the interferometric sensors are interrogated in the time domain, in the case of a physical sensor, or in the spatial domain, in the case of a chemical sensor, the spectroscopic sensors are interrogated within the frequency domain of the same spectrum. Any of the following types of spectroscopic sensor may be used: transmission-based chemical sensor; reflection-based chemical sensor; total internal reflection-based chemical sensor; fibre optic transmission type chemical sensor; fibre optic reflection type chemical sensor; fibre optic evanescent type chemical sensor. Such spectroscopic sensors 7 may be used, for example, for any of the following purposes: determining the concentrations of chemical species, for example substances with active infrared absorption bands, such as molecules containing N-H, C-H, and/or O-H bonds; measuring pH in solution, for example using colour dye indicators; measuring temperature, by looking at the peak absorption wavelength shift of a molecule, for example the peak absorption band of O-H in water; monitoring the kinetics of chemical reactions, for example the cure state of amine-epoxy based thermosets.

In the embodiment of Figure 10 a variety of fibre optic interferometric physical and chemical sensors and spectroscopic chemical sensors are multiplexed along optical fibres 1. Sensors 21 and 22 are extrinsic Fabry-Perot strain sensors, sensor 23 is a fibre optic pressure sensor having a diaphragm 23a, sensor 8 is a temperature sensor, and spectroscopic sensor 71 is a fibre optic evanescent cure sensor.

Claims

CLAIMS 1.. A method of interrogating individual fibre optic i. nterferometric physical and/or chemical sensors multiplexed in series and/or in parallel along one or more optical fibres, each sensor having a Fabry-Perot type etalon structure with an optical pathlength difference which is different from that of the other fibre optic interferometric sensors, in which method the absolute value of the optical pathlength difference of each sensor is determined using a Fourier transform spectrometer, the output of any such physical sensor being read in the time domain and the output of any such chemical sensor being read in the spatial domain, and values for at least one measurand of interest are derived from the absolute values of the optical pathlength difference so obtained.
2. A method of simultaneously interrogating one or more fibre optic interferometric physical and/or chemical sensors and one or more spectroscopy-based chemical sensors using a Fourier transform spectrometer, in which method the absolute value of the optical pathlength difference of the or each interferometric sensor is determined in the time domain, in the case of a physical sensor, or in the spatial domain, in the case of a chemical sensor, whilst the or each spectroscopy-based chemical sensor is interrogated in the frequency domain of the same spectrum.
3. A method as claimed in claim 1 or 2, wherein an interferogram is produced by the Fourier transform spectrometer and the positions of the peaks of each first-order side burst relative to the position of the peak of the centre-burst are determined, from which positional information the said absolute values of the optical pathlength difference of the said fibre optic interferometric sensors are derived.
4. A method as claimed in claim 1,2 or 3, wherein the said sensors are interrogated simultaneously for more than one measurand of interest.
5. A method as claimed in any preceding claim, wherein the measurand, or one of the measurands, is one of the following: quasi-static strain; dynamic strain; quasi-static temperature; dynamic temperature; periodic vibrations; random vibrations; acoustic emission; static pressure; dynamic pressure; presence and/or concentration of specified organic and inorganic substance (s).
6. A method of interrogating sensors multiplexed along one or more optical fibres substantially as hereinbefore described with reference to the accompanying drawings.
7. Fibre optic sensing apparatus, for sensing at least one measurand of interest, characterised by individual fibre optic interferometric physical and/or chemical sensors which are multiplexed in series and/or in parallel at desired positions along one or more optical fibres to be deployed in a region of interest, each such sensor having a Fabry-Perot type etalon structure with an optical pathlength difference which is different from that of the other fibre optic interferometric sensors of the apparatus, and by a sensor interrogation system, comprising a Fourier transform spectrometer, which is operable to read the output of any such physical sensor using time domain analysis and to read the output of any such chemical sensor using spatial domain analysis, from which output values for the said at least one measurand are derived.
8. Fibre optic sensing apparatus characterised by at least one fibre optic interferometric sensor, having a Fabry-Perot type etalon structure, at least one spectroscopy-based chemical sensor, and a sensor interrogation system, comprising a Fourier transform spectrometer, which is operable to interrogate said sensors simultaneously such that the absolute value of the optical pathlength difference of the said at least one interferometric sensor is determined in the time domain, in the case of a physical sensor, or in the spatial domain, in the case of a chemical sensor, and the said at least one spectroscopy-based chemical sensor is interrogated in the frequency domain of the same spectrum.
9. Apparatus as claimed in claim 8, wherein the said at least one spectroscopy-based chemical sensor is one of the following sensor types: transmission-based chemical sensor; reflection-based chemical sensor; total internal reflection-based chemical sensor; fibre optic transmission type chemical sensor; fibre optic reflection type chemical sensor; fibre optic evanescent type chemical sensor.
10. Apparatus as claimed in claim 8 or 9, wherein the said at least one spectroscopy-based chemical sensor is suitable for use in carrying out at least one of the following: determining the concentrations of chemical species; measuring pH in solution; measuring temperature; monitoring the kinetics of chemical reactions.
11. Apparatus as claimed in any one of claims 7 to 10, wherein at least one of the said fibre optic interferometric sensors is one of the following types: intrinsic fibre Fabry-Perot interferometer; extrinsic Fabry-Perot interferometer; in-line fibre Fabry-Perot etalon; Michelson interferometer; Mach-Zehnder.
12. Apparatus as claimed in any one of claims 7 to 11, wherein the or each of the said fibre optic interferometric sensors consists of one of the following elements: (i) perpendicularly cleaved fibre ends; (ii) polished fibre ends; (iii) fibre ends coated with thin metallic or dielectric films acting as partial reflectors; (iv) reflectors comprising a poor fusion splicing joint between two sections of fibre; (v) narrow optical band pass reflecting surfaces including reflectors made of fibre Bragg gratings; (iv) fabricated, micro-machined and/or etched cavities; (vii) reflectors comprising any combination of elements (i) to (vi).
13. Apparatus as claimed in any one of claims 7 to 12, wherein the said Fourier transform spectrometer is one of the following types: moving mirror; spatial domain imaging; CCD imaging.
14. Apparatus as claimed in any one of claims 7 to 12, wherein the said Fourier transform spectrometer comprises one of the following types of scanning unit: mechanically moving mirror or mirrors; Charge Coupled Devices (CCD) as a spatial imager; one or more photodiode detector arrays as a spatial imager; integrated optical phase modulators.
15. Apparatus as claimed in any one of claims 7 to 14, wherein the said Fourier transform spectrometer employs an interferometer of one of the following types: Michelson; Mach-Zehnder; Sagnac.
16. Apparatus as claimed in any one of claims 7 to 15, wherein the said sensor interrogation system is operable to interrogate the said sensors simultaneously for more than one measurand of interest.
17. Apparatus as claimed in any one of claims 7 to 16, wherein the measurand, or one of the measurands, is one of the following: quasi-static strain; dynamic strain; quasi-static temperature; dynamic temperature; periodic vibrations; random vibrations; acoustic emission; static pressure; dynamic pressure; presence and/or concentration of specified organic and/or inorganic substances.
18. Fibre optic sensing apparatus substantially as hereinbefore described with reference to the accompanying drawings.