Sensor system - Patent Review 6137573

What is claimed is:

1. A sensor system (10) including interferometric means incorporating sensor and reference elements (12, 58) arranged to receive broadband radiation, means for introducing a variation in optical path difference (56) between a first optical path (140) incorporating the reference element and a second optical path (120) incorporating the sensor element, means for combining the light from the first and second optical paths (120, 140) to form interferograms and signal processing means (84) to process interferogram information

characterised in that

the sensor and reference elements (12, 58) are each arranged to support at least two different eigenmodes of radiation; the sensor system (10) is arranged to generate at least two interferograms therefrom; and the signal processing means (84) are arranged to extract optical group delay information from the interferograms and to derive temperature and/or strain data therefrom.

2. A sensor system (10) including interferometric means incorporating sensor and reference elements (12, 58) arranged to receive broadband radiation, means for introducing a variation in optical path difference (56) between a first optical (14) incorporating the reference element and a second optical path (120) incorporating the sensor element, means for combining the light from the first and second optical paths (120, 140) to form interferograms and signal processing means (84) to process interferogram information

characterised in that

the sensor and reference elements (12, 58) are each arranged to support at least two different eigenmodes of radiation; the sensor system (10) is arranged to generate at least two interferograms therefrom; and the signal processing means (84) are arranged to extract optical group delay information from the interferograms and to derive temperature and/or strain data from mean optical group delay and differential optical group

delay obtained from the extracted interferogram information.

3. A sensor system (10) according to claim 1 characterised in that the sensor and reference elements (12, 58) each comprise an optical fibre (38, 58) which provides an optical path within each element.

4. A sensor system (10) according to claim 3 characterised in that the sensor and reference optical fibres (38, 58) are birefringent thereby constraining radiation to propagate in not more than two polarisation eigenmodes of the fibre.

5. A sensor system (10) according to claim 4 characterised in that the birefringence of the sensor and reference optical fibres (38, 58) is stress induced such that the birefringence is responsive to environmental temperature but is substantially unaffected by further application of stress.

6. A sensor system (10) according to claim 5 characterised in that the optical fibres (38. 58) are stress-induced highly birefringent (HiBi) fibres which substantially maintain the polarisation state of radiation propagating therein regardless of moderate external environmental variation.

7. A sensor system (10) according to claim 3, characterised in that the sensor element (12) comprises the sensor optical fibre (38) embedded within a composite material (40).

8. A sensor system (10) according to claim 1 characterised in that the reference element (58) and means for introducing a variation in optical path difference (56) are located in separate arms (50, 52) of an unbalanced Michelson interferometer (14) and further characterised in that the sensor element (38) can be considered as located in a third arm of a Michelson-style interferometer which also incorporates the unbalanced interferometer (14).

9. A sensor system (10) according to claim 8 characterised in that an output broadband interferogram from a Michelson-style interferometer is detected by a first detector (78) and communicated to signal processing means (84).

10. A sensor system (10) according to claim 3 characterised in that the sensor fibre (38) is bounded at light input end by a partially reflecting end (32) and at the opposing end by a mirror (36) arranged for reflection of light back into the fibre (38).

11. A sensor system (10) according to claim 10 characterised in that the reference fibre (58) is bounded at a light input end by a partially reflecting end (62) and at the opposing end by a mirror (64) arranged for reflection of light back into the fibre (58).

12. A sensor system (10) according to claim 4 characterised in that a polarisation controller (42) is located between the sensor element (12) and the reference element (58), the sensor fibre (38) and reference fibre (58) each support a fast eigenmode and a slow eigenmode of propagating radiation wherein the fast eigenmode propagates with a smaller index of refraction than that experienced by the slow eigenmode and the polarisation controller (42) is arranged to discriminate against the formation of any interferogram by light coupled into the fast eigenmode of one fibre (38, 58) interacting with light coupled into the slow eigenmode of the other fibre (58, 38).

13. A sensor system (10) according to claim 11 characterised in that the system (10) is arranged to form an OPD.sub.0 broadband interferogram and further arranged for communication of the OPD.sub.0 interferogram to a first detector (78), the aforesaid OPD.sub.0 interferogram being associated with interference between light rays whose path difference is variable and controlled by the means for introducing a variation in optical path difference (56) and is independent of the environments surrounding the sensor and reference elements.

14. A sensor system (10) according to claim 13 characterised in that the means for introducing a variation in optical path difference (56) is a translatable mirror.

15. A sensor system (10) according to claim 13 characterised in that the means for introducing a variation in optical path difference (56) is an optical fibre of variable length.

16. A sensor system (10) according to claim 14 characterised in that the OPD.sub.0 interferogram is formed from one component reflecting from the translatable mirror (56) and a second component reflecting from a partially reflecting end (62) of the reference fibre (58), said two components having zero path difference when the translatable mirror (56) is located in a central region of its translation range.

17. A sensor system (10) including an interferometer according to claim 13, characterised in that the interferometer is arranged to use the OPD.sub.0 broadband interferogram to provide a reference group delay with respect to which group delays of further broadband interferograms are measured.

18. A sensor system (10) according to claim 1 characterised in that the means for introducing a variation in optical path difference (56) is calibrated with respect to the variation in optical path difference it provides.

19. A sensor system (10) according to claim 18 characterised in that a substantially monochromatic light source (28) is arranged such that narrowband radiation is coupled into the unbalanced Michelson interferometer (14) and thereby forms a fringe interference pattern which is detected by a second detector (82) and analysed by signal processing means (84).

20. A sensor system (10) according to claim 19 characterised in that the narrowband radiation interference fringes provide a calibration scale against which the variation in optical path difference is calibrated with respect to a physical variation of the means for introducing a variation in optical path difference (56).

21. A sensor system (10) according to claim 1 characterised in that it includes extracting means (Box 3) arranged to calculate the Fourier Transform of each interferogram profile and to extract the respective interferogram group delay therefrom.

22. A sensor system (10) according to claim 21 characterised in that the extracting means (Box 3) is arranged to extract interferogram phase information by Fast Fourier Transform and Dispersive Fourier Transform Spectroscopy and to calculate the group delay of each interferogram from said phase information.

23. A sensor system (10) according to claim 1 characterized in that the system (10) is arranged to form interferograms between light coupled into the fast eigenmode of the sensor element (12) and light coupled into the fast eigenmode of the reference element (58) and between light coupled into the slow eigenmode of the sensor element (12) and light coupled into the slow eigenmode of the reference element (58); the sensor element (12) is arranged to be subject to a plurality of predetermined conditions of strain and temperature and thereby provide data from which to derive a transformation relating mean and differential group delays of the aforesaid interferograms to applied conditions of strain and temperature.

24. A sensor system (10) according to claim 23 characterised in that the system (10) is arranged to apply the transformation relating mean and differential group delays to applied conditions of strain and temperature to determination of unknown conditions of strain and temperature by measurement of mean and differential group delays.

25. A sensor system (10) according to claim 1 characterised in that the signal processing means (84) is arranged to:

(1) extract (Box 3) the group delay of each broadband interferogram from the signals received by the detector (78);

(2) apply (Boxes 2, 4 and 5) calibration and zero optical path difference corrections in order to calculate mean group delay and differential group delay of the broadband interferograms arising from optical path differences between ray paths through the reference and sensor elements (58, 12);

(3) derive (Box 6) a relationship between mean and differential group delays experimentally obtained from the sensor system (10) and predetermined conditions of temperature and strain;

(4) apply (Box 7) the derived relationship to experimentally obtained mean and differential group delays and thereby render the sensor system (10) capable of simultaneous measurement of strain and temperature in unknown environmental conditions.

26. A sensor system (10) according to claim 1 characterised in that the sensor element (12) is mounted in a structure for measurement of strain and temperature within the structure.

27. A sensor system (10) according to claim 1 characterised in that the sensor element (12) and reference element (58) are detachable from each other and capable of attachment to other sensor elements or reference elements.

28. A sensor system (10) according to claim 27 wherein the sensor element (12) is embedded within a structure characterised in that the other sensor elements are embedded within different structures or at other positions within the same structure.

29. A sensor system (10) according to claim 1 characterised in that the sensor element (12) is subdivided into sub-elements by a series of partially reflecting mirrors along its length, the second optical path (120) comprises a series of optical paths each corresponding to reflection at a different partially reflecting mirror and the means for introducing a variation in optical path difference (56) provides for formation of a series of interferogram pairs each pair corresponding to a difference between the first optical path (140) and a respective optical path of said series such that temperature and strain information derived therefrom corresponds to environmental conditions at each sensor sub-element.

30. A method of simultaneous measurement of strain and temperature comprising the steps of:

(a) arranging for a dual-eigenmode optical sensor element (12) incorporating a high birefringence optical fibre situated in one arm of a Michelson-style interferometer to be in an environment of unknown strain and temperature;

(b) forming at least two branched interferograms between light travelling a first optical path (120) incorporating the dual-eigenmode sensor element (12) and light travelling a second optical path (140) incorporating a dual-eigenmode reference element (58), one of the optical paths (120, 140) being variable relative to the other;

(c) determining the variation of optical path difference within each broadband interferogram;

(d) extracting values of group delay from the broadband interferograms formed in Step (b) relative to a predetermined reference group delay;

(e) calculating mean group delay and differential group delay from the extracted values of group delay;

(f) deriving deconvolved values for the unknown strain and temperature conditions of the environment of the sensor element (12) by applying a predetermined transformation of the mean group delay and differential group delay.

31. A method according to claim 30 wherein the predetermined transformation is determined by:

(a) arranging for the dual-eigenmode optical sensor element (12) to be located in an environment of predetermined strain and temperature;

(b) following Steps (b) to (e) of claim 30;

(c) deriving a relationship between the predetermined values of strain and temperature of the environment of the sensor element (12) and the values of mean group delay and differential group delay obtained experimentally in Step (b) and thereby deriving the predetermined transformation of claim 30, Step (f).

32. A method according to claim 30 wherein the predetermined reference group delay is determined by forming a further broadband interferogram having a characteristic optical path difference which is independent of the environments of the sensor and reference elements (12, 58) and dependent on variations in the optical path.

33. A method according to claim 30, wherein the variation of optical path difference within each broadband interferogram is determined by:

(a) forming a narrowband interferogram having a characteristic optical path difference which is dependent on the variable optical path;

(b) calibrating the variable optical path with respect to the narrowband interferogram in order to determine variation in characteristic optical path difference of broadband interferograms.

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This invention relates to a sensor system for measuring strain and temperature.

Optical fibre sensors are known in the prior art. They employ optical fibres to guide light which becomes modulated in response to external influences such as strain and temperature acting on the fibres. Information contained in these modulations may be extracted by interferometric means. This is particularly attractive for accurate applications as the phase sensitivity of optical fibres to physical influences such as strain and temperature is high.

The fundamental problem of strain and temperature measurement by optical means is one of deconvolving the contributions from each parameter. Moreover, a further problem may arise in monochromatic interferometry because phase differences of multiplies of 2.pi. are lost. This latter situation is known as phase ambiguity.

Light propagating along a fibre will traverse it in a time .DELTA..tau. which is dependent on the optical path length (OPL) of the fibre. The OPL is defined as the physical length of the fibre multiplied by the refractive index at the wavelength of light propagating therein. Thus physical influences acting to change the OPL of a fibre may affect one or both of the fibre length or refractive index.

Strain in an optical fibre may arise from, among other factors, stress (elasticity) or an electric field (piezoelectricity). In any case the external influence causes displacement of points in a body with magnitude and orientation dependent on position within the body. In general, the strain induced is determined by the magnitude and orientation of the influence as well as by the physical properties and symmetry of the material. The physical distortion within a strained material alters both its physical and electronic structure and hence affects its optical properties. In particular a longitudinal strain component, defined to be parallel to the length of an optical fibre (and hence propagation direction of light therein), will lengthen the fibre inducing a change in OPL via physical length. Transverse components however can only affect the OPL through a change in electronic structure and hence birefringence of the material.

The refractive index of an unconstrained, bare fibre is temperature dependent and so heating such a fibre will result in a change in OPL. The thermal expansion of optical fibre material (typically fused silica) is low and any consequent change in OPL through a change in fibre length will be negligible. The cross-sensitivity of temperature and strain both affecting the OPL in a bare-fibre case is of the order 10 microstrain per Kelvin. An optical fibre embedded within a host material will however be affected by the thermal expansion of the host material. Differing thermal expansivities of host and embedded fibre will result in the host extending a stress on the fibre. Such a fibre used as a sensor will therefore indicate a fictitious strain, due to temperature change, in the absence of an applied stress. This indicated strain is known as the thermal-apparent strain of the sensor.

Despite allowance made for host thermal expansion, a sensor constructed from an embedded fibre will still exhibit temperature cross-sensitivity which restricts its use to environments with known temperatures or to situations of time-varying stresses.

Temperature-independent strain measurement has important applications in the construction and engineering industries. Civil engineering requires strains within buildings to be monitored over time and higher accuracy sensors are required for the high tech construction industry e.g. aeroplanes, helicopters and space equipment. For example, it is important in the aircraft industry to determine the operating strains experienced by panels of aircraft fuselage. One fibre arm of an interferometer embedded in a panel enables strains in that panel to be monitored over its life. Furthermore a sensor arm subdivided into a series of sensor elements by a series of partially-reflective mirrors enables the strain experienced by each sensor element to be determined.

However there exists a fundamental problem in deconvolving the contributions to a measured optical effect from strain and temperature, including the indirect effects of stress exerted via thermal expansion of a host material.

The prior art records a number of temperature-independent strain sensors. Such sensors suffer from various disadvantages: they may only be capable of measuring changes in strain and temperature, they may not be sufficiently sensitive for high accuracy applications or they may suffer from phase ambiguity because of the need to use monochromatic light in forming interference fringes.

Temperature-insensitive strain sensors are reported by R. O. Claus et al., Smart Materials and Structures; 1, 237-242, (1992) but these do not compensate for thermal expansion of the host material. Electrical strain gauges are manufactured to be temperature-compensated for a specific host material, but this approach is unattractive for composite materials for which the large range of thermal expansion coefficients would require a correspondingly large range of matching strain gauges.

Composite materials may be used if strain and temperature are measured simultaneously. In this way, knowledge of the host's thermal expansion coefficient allows thermal cross-sensitivity and thermal-apparent strain to be cancelled out. The relationship between the fibre strain .epsilon..sub.f and the host strain .epsilon..sub.h can be assumed to be a linear relationship for small changes in temperature: ##EQU1## where T.sub.f and T.sub.h are the fibre and host temperatures respectively and good strain coupling is assumed between sensor and host material. The parameter a.sub.T is due to the difference in thermal coefficients of the two materials and can be measured for a specific host material. If thermal

cross-sensitivity is calibrated out by a simultaneous measurement of .epsilon..sub.f and T.sub.f, then the parameters of the sensor environment, .epsilon..sub.h and T.sub.h, can be derived from the relationship above which accounts for thermal-apparent strain. The principal problem in measuring .epsilon..sub.f and T.sub.f lies in finding optical effects for which the ratio of strain-dependent changes to temperature-dependent changes is sufficiently different to enable satisfactory discrimination between strain and temperature to be made.

The prior art records a number of attempts to achieve an acceptable degree of discrimination between temperature and strain.

F. Farahi et al., Journal of Lightwave Technology 8(2), 138-142 (1990) measured the phases at the combined output of a polarimetric and interferometric fibre device for two polarisation modes of an optical fibre sensor. A. M. Vengsarkar et al., Proc SPIE 1367, 249-260, (1990) disclose a sensor comprising a dual-mode elliptical core fibre guiding light beams of two different wavelengths. In these cases the effects of strain and temperature on the light as it passes through the optical fibre sensor are different for the two polarisation modes or at the two different wavelengths and so the relative contributions can be deconvolved. However discrimination was found to be poor and the relative insensitivity of polarisation to environmental changes makes device based on polarisation less sensitive than interferometric devices.

Dunphy and Meltz in "Optical Fibre Sensor Development for Turbine Applications" A1AA-89-2914, 25th Joint Propulsion Conference p3156, disclose discrimination provided by two fundamental modes of a twin-core optical fibre. Temperature and strain change the length of the fibre and also the beat length. Simultaneous operation of the calibrated device at two different wavelengths enables the strain and temperature experienced by the sensor to be calculated. However twin-cored fibres are non-standard and consequently expensive. This device also suffers from phase ambiguity over temperature ranges greater than its dynamic range.

U.S. Pat. No. 5,399,854 discloses mean and differential wavelength reflected from a birefringent reflective element such as a Bragg grating in an optical fibre as discriminators. They are arranged however to provide only a measure of temperature and strain variation, reference conditions are required to derive absolute values. The fibre is embedded in an arrangement of layers comprising carbon fibre filaments woven through a resin. This ensures that a general applied strain will have unequal transverse components which induces birefringence at the grating region in the optical fibre. The longitudinal strain component changes the periodicity of the grating. Thus in a general stressed environment at a particular temperature two characteristic wavelengths from a white light source simultaneously satisfy the grating equation and are reflected. The mean reflected wavelength and wavelength separation are affected differently by variations in strain and temperature. This method is however less accurate than interferometric methods--the shifts observed are relatively insensitive to changes in environmental conditions and so uncertainties are correspondingly large.

A method of providing simultaneous measurement of variations in temperature and strain is disclosed in European Patent Application 0 564 034. Interferograms are developed centred on three different wavelengths filtered from a broadband source. Each interferogram is dependent on the optical path imbalance between the two arms of an interferometer. Neighbouring interferograms are then compared to yield two phase differences containing information concerning the physical length difference, elongation and temperature difference between sensor and reference fibres. Wavelength and dispersion can be derived from these phase differences and fibre elongation and temperature change extracted. Elongation and temperature are measured at the sensor fibre (in one interferometer arm) relative to conditions at the reference fibre (in the second interferometer arm). However dispersion measurements require radiation from a broadband source to be coupled into a single fibre mode. This is difficult to achieve with sufficient optical power to allow accurate measurement.

PCT patent application PCT/GB94/01388 discloses an interferometric sensor based on discrimination by measurement of group delay and dispersion. A broadband interferogram is generated as the optical path length of a reference arm is scanned across that of a sensor arm. The variation of phase with frequency across the interferogram is extracted by Fast Fourier Transform. From this phase relationship, the change in group velocity and dispersion relative to a previous interferogram can be deduced. If the one interferogram is generated under known conditions of temperature and strain then the measured modifications to group velocity and dispersion can be used to derive unique values of the change in temperature and strain relative to the previous known conditions. However, this technique requires a source of very broad spectral width (at least 100 nm) in order to measure dispersion.

A non-interferometric approach is disclosed in patent application GB 2 224 566A. The resonance condition in a fibre loop is monitored in order to derive OPL information. Two coincident optical paths of different OPL are maintained in the loop by the use of a high birefringence fibre which supports two modes of light propagation termed "fast" and "slow". The frequency of a laser source is swept across a range covering several free spectral ranges of the resonator. Dips in the transmitted intensity will occur whenever one or other of the optical paths satisfies the resonance condition. OPLs can be extracted from the resonance information and changes in temperature and strain are reflected in modifications to the two optical paths. Temperature and strain can be extracted from the changes in OPLs and so deduced relative to previous known conditions.

It is an object of this invention to provide an alternative form of optical fibre sensor system.

The present invention provides a sensor system including interferometric means incorporating sensor and reference elements arranged to receive broadband radiation, means for introducing a variation in optical path difference between a first optical path incorporating the reference element and a second optical path incorporating the sensor element, means for combining the light from the first and second optical paths to form interferograms and signal processing means to process interferogram information characterised in that the sensor and reference elements are each arranged to support at least two different eigenmodes of radiation; the sensor system is arranged to generate at least two interferograms therefrom; and the signal processing means are arranged to extract optical group delay information from the interferograms and to derive temperature and/or strain data therefrom.

The invention provides the advantage of capability for discriminating between temperature and strain. This is because the different eigenmodes supported by the sensor element are affected differently by temperature and stress in the sensor environment. It overcomes a limitation of many prior art interferometric strain sensors in that such discrimination is achievable only by extracting a time-varying component or by subtracting a determinable thermal effect from an overall optical signal. This limits such prior art sensors to use in environments with time-varying stresses or with known temperatures. Furthermore the discrimination achievable by group delay measurement improves on that provided by prior art sensors which extract a simultaneous measurement of temperature and strain from other quantities.

In a preferred embodiment the signal processing means is arranged to extract optical group delay information from the interferograms and to derive temperature and/or strain data from mean optical group delay and differential optical group delay obtained from the extracted interferogram information.

This provides the advantage of ease of operation in that optical group delay is a readily extractable quantity from interferometric data. It also provides for exploitation of a consequence of using a broadband source in an interferometer with more than two distinct optical paths. Not only does a broadband source enable elimination of the phase ambiguity inherent in monochromatic fringes but its short coherence length also encourage formation of discrete interferograms generated from the different combinations of optical paths. Such discrete interferograms simplify the measurement of each interferogram group delay. Strain and temperature produce different effects on the individual sensor (and reference) eigenmodes, which are evidenced in optical group delays of interferograms formed from light propagating in individual eigenmodes. Extracting mean and differential group delay, as opposed to just group delay, enables the extracted information to be limited to group delay and not derivatives thereof e.g. dispersion. A prior art device depended on the more complex derivation of group delay and dispersion from a single interferogram, with consequent stringent requirements on source spectral width.

The sensor system is preferably arranged such that the sensor and reference elements each comprise an optical fibre which provides an optical path therein. Moreover each optical fibre is preferably birefringent to establish two eigenmodes per fibre and to constrain radiation to propagate in not more than two polarisation eigenmodes of the fibre.

The optical fibres preferably exhibit high, stress-induced birefringence (HiBi). This provides a means for sensitive discrimination between the effects of strain and temperature on fibre optical properties. A change in temperature modifies induced stresses in a HiBi fibre and so affects its birefringence. High birefringence shields the fibre from transverse stresses and so is substantially unaffected by stress. Thus both strain and temperature modify the length of the fibre, the latter primarily through thermal-apparent strain, whereas only temperature affects the birefringence. Determination of these unequally-affected parameters allows discrimination between the effects of temperature and strain. A further advantage of using HiBi fibres is that the high birefringence substantially maintains the polarisation state of radiation propagating therein regardless of external environmental variation (within reasonable limits).

The sensor optical fibre may be embedded within a composite material, in order to communicate strain within the composite material to the optical fibre.

The sensor system is preferably arranged such that the reference element and means for introducing a variation in optical path difference are located in separate arms of an unbalanced Michelson interferometer. The sensor element may be located in a third arm of a Michelson-style interferometer which also incorporates the unbalanced interferometer. This provides for multiple optical paths through the apparatus of the invention, neglecting any arising from multiple fibre eigenmodes, into which variation in optical path length can be introduced either via the means for introducing a variation in optical path difference or by changing the strain or temperature of the reference or sensor environments. This enables selection of interferograms containing information relating to different causes of optical path difference.

An output broadband interferogram from the Michelson-style interferometer is preferably detected by a first detector and communicated to signal processing means. This allows information to be extracted from interferograms by accurate signal processing techniques.

Both the sensor and reference fibres may have respective partially reflecting ends for light input and at opposing ends respective retroreflecting mirrors. This provides for realising light division into components which traverse different optical paths within the system. Variation in optical path length can be introduced either via the means for introducing a variation in optical path difference or by changing the strain or temperature of the reference or sensor environments and this optical path arrangement provides a capability for isolating the causes of optical path difference in individual interferograms and thus simplifies information extraction.

The sensor and reference fibres may each support two polarisation eigenmodes of propagating radiation, termed the fast and slow eigenmodes. The expression "fast" and "slow" are references to the fact that the different eigenmodes experience different refractive indices and thus propagate at different speeds. Interferograms are referred to as fast-fast, slow-slow, fast-slow and slow-fast to indicate the eigenmodes which produced them. In a preferred embodiment a polarisation controller is located between the sensor element and the reference element and arranged to discriminate against the information of any interferogram by light coupled into the fast eigenmode of one fibre interacting with light coupled into the slow eigenmode of the other fiber. These slow-fast interferograms could be retained and used to extract optical group delay information but the calculations are more complex because an optical path difference will exist for each interferogram regardless of the accuracy with which the physical fibre lengths can be made equal. Furthermore the dispersion exhibited by these interferograms is higher than that shown by fast-fast and slow-slow interferograms. For the purposes of this invention Differential Group Delay (.tau..sub.DGD), i.e. the difference between the group delays of two interferograms, is a convenient parameter to use in the measurement of strain and temperature. The use of interferograms formed by like-mode interactions (i.e. fast-fast or slow-slow) avoids any dependence of .tau..sub.DGD on temperature changes which are common to both sensor and reference fibres. With unlike-mode interferograms .tau..sub.DGD would exhibit a strong dependence on such common-mode temperature changes. Thus interferograms formed by like-mode interactions are preferred because of the advantage of simplifying the information extraction by eliminating any beat length imbalance, avoiding the dependence of .tau..sub.DGD on common temperature changes and by reducing the need for any dispersion compensation.

In a further embodiment of the invention, the system is also arranged to form a broadband interferogram (OPD.sub.0 interferogram) whose path difference is controlled by the means for introducing a variation in optical path difference and is dependent of the environments surrounding the sensor and reference elements. This interferogram is detected at a first detector. The group delay associated with the OPD.sub.0 interferogram is thus capable of acting as an absolute reference position which can be defined to have zero group delay, from which the group delays of interferograms affected by the sensor element environment can be measured. This ensures that group delays extracted by the system are referred to an absolute optical path difference of zero. If the means for introducing a variation in optical path difference had a well-defined zero point then this refinement would be unnecessary. However, for practical purposes, it is envisaged that an OPD.sub.0 interferogram will be used.

More specifically, the means for introducing a variation in optical path difference may be a translatable mirror. The OPD.sub.0 interferogram may thus be formed from one component reflecting from the translatable mirror and a second component reflecting from a partially reflecting end of the reference fibre, the two components having zero path difference when the translatable mirror is located in a central region of its translation range. Alternatively, the means for introducing a variation in optical path difference may comprise an elongatable optical fibre. This provides for the optical path difference to be varied by stretching or compressing the elongatable fibre. The sensor system of the invention is thus capable of being realised in the form of an all-fibre interferometer. In either case, the interferometer may then be arranged to use the OPD.sub.0 broadband interferogram to provide a reference group delay with respect to which group delays of further broadband interferograms are measured. This provides an uncomplicated means for realising the formation of an OPD.sub.0 interferogram which is independent of variations in the sensor environment.

In a preferred embodiment of the invention, the means for introducing a variation in optical path difference is calibrated with respect to the variation in optical path difference it provides. This is advantageous to increasing the accuracy of group delay measurements as the relevant interferogram is scanned by varying the optical path difference. This

calibration may be achieved by active control of the scan speed or by use of a substantially monochromatic light source arranged such that narrowband radiation is coupled into the unbalanced Michelson interferometer to form a fringe interference pattern which is detected by a second detector and analysed by signal processing means. This fringe pattern is capable of providing a calibration scale against which the variation in optical path difference is calibrated with respect to a physical variation of the means for introducing a variation in optical path difference.

Varying the optical path difference effectively scans the relevant interferogram profile across the appropriate detector. The group delay of each interferogram may then be extracted from the Fourier Transform of this profile. The phase variation across each interferogram is preferably extracted by Dispersive Fourier Transform Spectroscopy (DFTS). Curve fitting may then be used to calculate group delay, the first order derivative of this phase variation. This assists the accurate measurement of group delay. Some prior art techniques rely on location of the central fringe of the interferogram. However, noise can significantly reduce the visibility of this peak to the detriment of resolution accuracy; dispersion may alter interferogram shape and complicate the process of centroid location and signal quality can be impaired by a number of external factors. Dispersive Fourier Transform Spectroscopy uses the entire interferogram profile to provide a more accurate tool for determination of group delay.

The system may be arranged to form fast-fast and slow-slow interferograms while the sensor element is subject to a plurality of predetermined conditions of strain and temperature and thereby provide data from which to derive a transformation relating mean and differential group delays of the aforesaid interferograms to applied conditions of strain and temperature. This provides an empirical means with which to equip the system with the capability of relating group delay measurements to particular values of temperature and strain present in the sensor environment. It is advantageous in regard to the simplicity and accuracy provided over theoretical models. Using this empirical data, the system may then be arranged to apply the transformation relating mean and differential group delays to applied conditions of strain and temperature to determination of unknown conditions of strain and temperature by measurement of mean and differential group delays.

In a preferred embodiment the signal processing means may be arranged to

(1) extract the group delay of each broadband interferogram from the signals received by the detector;

(2) apply calibration and zero optical path difference corrections in order to calculate mean group delay and differential group delay of the broadband interferograms arising from optical path differences between ray paths through the reference and sensor elements;

(3) derive a relationship between mean and differential group delays experimentally obtained from the sensor system and predetermined conditions of temperature and strain;

(4) apply the derived relationship to experimentally obtained mean and differential group delays and thereby render the sensor system capable of simultaneous measurement of strain and temperature in unknown environmental conditions.

This embodiment provides a means for realising the determination of strain and temperature in the sensor environment from interferometric data received at the Michelson detectors using signal processing means. This provides the advantages of speed and accuracy generally to be had with signal processing power.

The sensor element is preferably mounted in a structure for measurement of strain and temperature within the structure, which promises a number of useful applications. If the structure concerned is one in which the development of strain can weaken the structure but not affect performance until a catastrophic collapse then constant monitoring of internal strain enables the timing of installing a replacement part to be finely tuned to economic and safety criteria.

The use of Michelson-style interferometer lends itself to construction of an embodiment in which the sensor element and reference element are detachable from each other and capable of attachment to other sensor elements or reference elements. This embodiment facilitates portability. A small sensor element can be permanently embedded in a panel without noticeably affecting the panel's performance. The reference section and attendant communicating wires and signal processing means can then be attached to a number of embedded sensor elements sequentially, thereby enabling monitoring of the strain within many structural elements using only one reference section.

The sensor element may be subdivided into sub-elements by a series of partially reflecting mirrors along its length. The second optical path then comprises a series of optical paths each corresponding to reflection at a different partially reflecting mirror. This embodiment also requires that the means for introducing a variation in optical path difference is arranged to provide for formation of a series of interferograms each corresponding to a difference between the first optical path and a respective optical path of said series such that temperature and strain information derived therefrom corresponds to environmental conditions at each sensor sub-element.

In a further aspect the present invention provides a method of simultaneous measurement of strain and temperature comprising the steps of:

(a) arranging for a dual-eigenmode optical sensor element incorporating a high birefringence optical fibre situated in one arm of a Michelson-style interferometer to be in an environment of unknown strain and temperature;

(b) forming at least two broadband interferograms between light travelling a first optical path incorporating the dual-eigenmode sensor element and light travelling a second optical path incorporating a dual-eigenmode reference element, one of the optical paths being variable relative to the other;

(c) determining the variation of optical path difference within each broadband interferogram;

(d) extracting values of group delay from the broadband interferograms formed in Step (b) relative to a predetermined reference group delay;

(e) calculating mean group delay and differential group delay from the extracted values of group delay;

(f) deriving deconvolved values for the unknown strain and temperature conditions of the environment of the sensor element by applying a predetermined transformation to the mean group delay and differential group delay.

The method of the invention exploits the fact that a powerful discrimination between temperature and strain is provided by the effects of such environmental influences on the two polarisation eigenmodes supported by a birefringent optical fibre. There is therefore a great advantage conferred by this method of using the different effects to simultaneously and accurately measure temperature and strain in the environment of a sensor element.

In a further aspect, the predetermined transformation may be determined by:

(a) arranging for the dual-eigenmode optical sensor element to be located in an environment of predetermined strain and temperature;

(b) following Steps (b) to (e) listed above;

(c) deriving a relationship between the predetermined values of strain and temperature of the environment of the sensor element and the values of mean group delay and differential group delay obtained experimentally in Step (b) and thereby deriving the predetermined transformation of Step (h) of the previous aspect.

This aspect provides the advantage that calibration and measurement steps can be carried out using the same apparatus and techniques.

The predetermined reference group delay is preferably determined by forming a further broadband interferogram having a characteristic optical path difference which is independent of the environments of the sensor and reference elements and dependent on variations in the optical path. This provides an accurate method with which to establish a zero reference for the physical variation in the optical path.

The variation of optical path difference within each broadband interferogram may be determined by:

(a) forming a narrowband interferogram having a characteristic optical path difference which is dependent on the variable optical path;

(b) calibrating the variable optical path with respect to the narrowband interferogram in order to determine variation in characteristic optical path difference of broadband interferograms.

This provides a convenient and accurate way with which to calibrate the variation in optical path difference if the physical scan is not accurately linear.

In order that the invention might be more fully understood, an embodiment thereof will now be described with reference to the accompanying drawings in which:

FIG. 1 is a schematic illustration of the sensor scheme of the invention.

FIGS. 2 to 6 illustrate schematically various alternative optical paths through the apparatus of FIG. 1.

FIG. 7 is a flowchart of the software signal processing steps for determining strain and temperature from the output of the device of FIG. 1.

FIGS. 8(a) and 8(b) illustrate graphically applied and measured strain and temperature conditions present in a system.

With reference to FIG. 1 a sensor system of the invention for simultaneous measurement of strain and temperature is indicated generally by 10. The sensor system 10 incorporates sensing element 12 and an unbalanced scanning Michelson interferometer 14. A four-way directional coupler 16 comprises first, second, third and fourth fibre arms 18, 20, 22 and 24 arranged such that light input along either first arm 18 or fourth arm 24 is output to second arm 20 and third arm 22 and vice versa. The fibre arms 18, 20, 22 and 24 are circular cored. A broadband light source (1.3 .mu.m light emitting diode, .DELTA..lambda.=80 nm FWHM) 26 is spliced to the first arm 18. Light from a monochromatic source (e.g. a helium neon laser) 28 is coupled via a first lens 30 into the third arm 22. The second arm 20 is connected to a partially reflective spliced end 32 of the sensing element 12. At the other end of the sensing element 12 is a first mirror 36. The sensing element 12 itself comprises a length of highly birefringent (HiBi) optical fibre 38, a 0.3 m section of which is enclosed within a carbon fibre composite panel 40 which can be heated and strained. The fourth arm 24 of the directional coupler 16 contains a polarisation controller 42 the output of which is connected to the Michelson interferometer 14 and thereby constitutes an input to it.

The Michelson interferometer 14 comprises a second lens 44 which collimates light exiting fourth fibre arm 24 into a cube beamsplitter 4