GB2419401A

GB2419401A - Polarimetric differential optical fibre sensor

Info

Publication number: GB2419401A
Application number: GB0423246A
Authority: GB
Inventors: Arthur Harold Hartog; Didier Dominguez
Original assignee: Sensor Highway Ltd
Current assignee: Sensor Highway Ltd
Priority date: 2004-10-20
Filing date: 2004-10-20
Publication date: 2006-04-26
Anticipated expiration: 2024-10-20
Also published as: GB0423246D0; GB2419401B

Abstract

An optical fibre differential sensor 20 comprises two sensing fibre portions 22, 24 having equal but opposite birefringent sensitivities to a parameter of interest, and a biassing fibre portion 26 having a fixed birefringence. Application of a first value of the parameter to one of the sensing fibre portions and a second value to the other sensing fibre portion gives an overall birefringence related to the difference in the values, from which a differential value can be calculated upon optical interrogation of the sensor to measure the birefringence. The overall birefringence is offset by the birefringence of the biassing portion which allows the sign of the differential measurement to be determined and a meaningful result to be obtained in the event of a nil differential. The parameter may be pressure. The fibre portions may comprise side-hole fibres or photonic crystal optical fibres.

Description

DIFFERENTIAL OPTICAL FIBRE SENSOR

BACKGROUND OF THE INVENTION

Field of the Invention

100011 The present invention relates to a differential sensor utilising optical fibre, and a method of making differential measurements using optical fibre.

Description of Related Art

2] Optical fibres form the basis of a wide range of sensors for measuring numerous parameters. The light propagating characteristics of the fibre are modified by an external parameter acting on the fibre, so that changes in the parameter can be deduced from changes in light transmitted through the fibre. Fibre sensors are advantageous for many applications; they can respond to a variety of parameters, are robust, durable and compact, and do not rely on electrical current so are safe for use in explosive environments.

100031 An example is a polarimetric sensor, based on polarisationmaintaining fibre.

An external parameter such as temperature or strain alters the length of the fibre. This affects the polarisation of light propagating in the fibre, so that a value of the parameter can be determined from differences in the polarization of light entering and exiting the fibre. A problem with this apparently simple arrangement is the difference between the effective optical path lengths of the two orthogonal polarization modes of the fibre arising from the inherent birefringence of the polarisation-maintaining fibre, which introduces a phase delay between the two modes. This difference accentuates any phase noise arising from the optical source used to generate the light, so that errors may result in analysis of the detected polarization, which is particularly important when sources with low coherence are used.

4] An alternative fibre sensor proposed to address this problem is described in US Patent 4,638,153. Two equal lengths of identical polarization maintaining fibre are spliced together with their axes rotated at 90 to one another. Thus, the fast mode of one piece of fibre couples into the slow mode in the other piece of fibre, and vice versa. Overall, therefore, light launched into each mode of the first piece experiences the same effective optical path by the time it exits from the second piece, so that the effective path length difference between the polarization modes is cancelled.

[00051 It has been recognised that such an arrangement can be used for differential measurements if the two lengths of fibre are exposed to different levels of the parameter being measured. In particular, one half of the sensor is used to compensate for ambient changes affecting the whole sensor while the other half is exposed to the parameter (See J.P. Dakin and C.A. Wade, "Compensated Polarimetric Sensor Using Polarisationmaintaining Fibre in a Differential Configuration", Electronics Letters, Vol. 20, No. 1, pages 51-53, 5 January 1984). In contrast to the previous example, this approach depends on the birefringence of the fibre and consideration of its effects on the emitted light. The birefringence of each half of the sensor is affected by an amount depending on the level of the parameter to which that half is exposed, so that the difference in birefringence, apparent as a difference in phase velocity between light in the two modes, is related to the difference in the parameter between the two halves. In fact, a useful signal is only obtained if the two fibre halves experience different parameter values, because otherwise the birefringence is cancelled by the matching of the effective path lengths as discussed above. Thus, to obtain an absolute measurement with this arrangement, isolation of the two halves of the sensor is required together with knowledge of the ambient environment around the compensating half, using which the parameter value at the measurement half can be determined from the differential measurement. Problems may arise from the necessary isolation, however, if the isolating arrangement acts on the fibre in a way that affects the birefringence.

Also, in the event that there is no parameter differential and hence zero total birefringence, no meaningful measurement can be obtained to indicate this situation.

[00061 An alternative approach that seeks to avoid problems with differential arrangements has been suggested for obtaining measurements of pressure (See US Patent 5,515,459). The sensor again comprises two lengths of polarization maintaining birefringent fibre spliced at 90 . However, the fibre is specifically side-hole fibre, in which the birefringence is engineered using a pair of air holes run along the fibre cladding, one on each side of the fibre core. The birefringence of the fibre depends on the difference in pressure between the outer surface of the fibre and the inside of the holes. In the sensor, the holes in one half are open and in the other half are sealed shut, but in use, both halves are exposed to the same pressure. The pressure difference between outer surface and holes is thus different for the two halves since the holes of one half are sealed against the applied pressure. Thus, the birefringence of the two halves is differentially altered, affecting the measured phase velocity from which the pressure is calculated. Differential measurements are not possible, however; but the arrangement provides temperature compensation of the pressure measurement.

7] Differential measurements are sometimes desirable, but the prior art devices are either not capable of making such measurements, or else fail when the parameter equalises. There is thus a need for an improved optical fibre sensor for differential measurements.

BRIEF SUMMARY OF THE INVENTION

8] Accordingly, a first aspect of the present invention is directed to a differential optical fibre sensor for sensing a parameter, comprising: a first sensing portion, a second sensing portion and a biassing portion optically coupled in series and arranged such that the first sensing portion can be exposed to a first value of the parameter, the second sensing portion can be exposed to a second value of the parameter, and light can be launched into and subsequently received from the sensor; where the first sensing portion comprises birefringent optical fibre having a birefringence that varies in response to the parameter to give the first sensing portion a first sensitivity to the parameter; the second sensing portion comprises birefringent optical fibre having a birefringence that varies in response to the parameter to give the second sensing portion a second sensitivity to the parameter that is substantially equal in magnitude but opposite in sign to the first sensitivity; and the biassing portion comprises birefringent optical fibre having a birefringence that does not vary substantially in response to the parameter.

9] A sensor according to the first aspect is a simple device comprising three fibre components that can be used to obtain differential measurements of a wide range of parameters using standard fibre sensor interrogation techniques. Selection of appropriate optical fibre with appropriate sensitivity for the sensing portions allows the sensor to be tailored to a particular parameter of interest. The biassing portion provides a known offset to the overall birefringence of the sensor when deployed for measurement, which allows the sign of the differential between the first and second parameter values to be determined, and also offers a meaningful result in the event that the parameter has an equal value on each side of the sensor so that the differential is nil. Further, by choosing appropriately long lengths of fibre, arbitrarily high sensitivity can be provided since the birefringent sensitivity scales with fibre length. Thus, the sensor can be designed to be able to withstand a differential value of the parameter equal to or exceeding the full operating value in the environment for which it is intended.

0] Any technique for engineering the required sign reversal of the sensitivity between the sensing portions can be employed. However, a simple approach uses two separate portions of fibre spliced together with a relative rotation of 90 . Thus, in some embodiments, the first sensitivity and the second sensitivity are arranged to be opposite in sign by the portions being coupled together such that light propagating along a slow polarisation axis of the first sensing portion is coupled into a fast polarisation axis of the second sensing portion and light propagating along a fast polarisation axis of the first sensing portion is coupled into a slow polarization axis of the second sensing portion.

1] The biassing portion can be arranged in any convenient position without detriment to the operation of the sensor. Thus, in one alternative, the biassing portion -s- is coupled between the first sensing portion and the second sensing portion. In a second alternative the biassing portion is coupled at an end of the sensor.

[00121 Although the biassing portion is configured to have no birefringent sensitivity to the parameter of interest, many types of birefringent fibre have a range of sensitivities to various parameters, so that the biassing portion may undesirably respond to a second parameter that is not of interest. This will perturb the offset birefringence. To address this, in some embodiments, the biassing portion may comprise two or more sections of birefringent fibre that each have a birefringence that varies in response to a second parameter to which the sensor will be exposed in use, the two or more sections arranged so that the biassing portion has an overall birefringence that does not vary in response to the second parameter.

3] In one example, the first sensing portion and the second sensing portion comprise side-hole fibre. This fibre type is of particular advantage for making pressure measurements, since it has a birefringence that responds well to changes in pressure.

The pressure may be applied to the outer surface of the fibre, to the inside of the holes in the fibre, or both. Thus, the side holes in the first sensing portion and the side holes in the second sensing portion may be sealed, or the side holes in the first sensing portion and the side holes in the second sensing portion may be open.

[00141 Alternatively, the first sensing portion and the second sensing portion may comprise photonic crystal optical fibre. Photonic crystal fibre may be used for differential pressure measurements, but may also be engineered to respond to one or more of a range of other parameters.

[00151 Therefore, in some embodiments, the parameter is pressure.

100161 The sensor may further comprise an isolating barrier disposed around the sensor between the first sensing portion and the second sensing portion and operable to isolate the first sensing portion from the second value of the parameter and to isolate the second sensing portion from the first value of the parameter when the sensor is in use. This helps to keep the two values of the parameter clearly separated and defined to increase accuracy of the measurement. The isolating barrier may be disposed around the biassing portion; this reduces the risk of the isolating barrier disrupting measurements by impinging on the sensing fibre and altering the sensor birefringence. The isolating barrier may comprise a pressure feedthrough, for example.

100171 The sensor may further comprise a polarising element arranged at an input end of the sensor and positioned with respect to fast and slow polarisation axes of the sensing fibres such that light launched into the sensor via the polarising element is coupled substantially equally into both axes. This is a practical and simple way to couple light into both axes of the sensor.

10018] The sensor is suitable for use in transmissive and reflective modes. In the latter case, the sensor preferably further comprises a reflective element arranged at an end of the sensor opposite an input end of the sensor such that light launched into the sensor at the input end propagates through the sensor, is reflected from the reflective element, and propagates back to the input end where it can be received.

10019] The sensor may additionally comprise a detector operable to receive light from the sensor and determine a total birefringence of the sensor therefrom. Also, the sensor may further comprise a processor operable to determine any difference between the first value of the parameter and the second value of the parameter from the total birefringence of the sensor, including taking account of a contribution to the total birefringence from the biassing portion.

0] A second aspect of the present invention is directed to a method of making a differential measurement of a parameter comprising: disposing into a measurement region an optical fibre sensor according to any of the examples and embodiments of the first aspect of the invention; exposing the first sensing portion of the sensor to a first value of the parameter; exposing the second sensing portion of the sensor to a second value of the parameter; launching light into the sensor; receiving light subsequently emitted from the sensor; determining a total birefringence of the sensor from the received light; adjusting the total birefringence to take account of a contribution from the biassing portion of the sensor; and determining any difference between the frst value of the parameter and the second value of the parameter from the adjusted total birefringence.

BRIEF DESCRIPTION OF THE DRAWINGS

[00211 For a better understanding of the invention and to show how the same may be carried into effect, reference is now made by way of example to the accompanying drawings in which: 100221 Figure 1 shows a schematic side view representation of a differential optical

fibre sensor according to the prior art;

3] Figure 2A shows a schematic side view representation of a first embodiment of a differential optical fibre sensor according to the present invention; [00241 Figure 2B shows a schematic side view representation of a second embodiment of a differential optical fibre sensor according to the present invention; [00251 Figure 3A shows an axial cross-sectional view of a portion of side- hole optical fibre; [0026] Figure 3B shows a schematic side view representation of an embodiment of a differential optical fibre sensor according to the present invention utilising side-hole optical fibre; [0027] Figure 4 shows a schematic side view representation of an embodiment of a differential optical fibre sensor according to the present invention comprising an isolating barrier arranged between portions of the sensor; [00281 Figure 5A shows a schematic side view representation of an example of a differential optical fibre sensor according to the present invention used in transmissive mode; [00291 Figure 5B shows a schematic side view representation of an example of a differential optical fibre sensor according to the present invention used in reflective mode; and [0030] Figure 6 shows a schematic side view representation of a further embodiment of a differential optical fibre sensor according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[00311 Figure 1 shows a schematic representation of a differential optical fibre sensor according to the prior art. The sensor 10 comprises a first fibre portion 12 and a second fibre portion 14 spliced or otherwise optically coupled together in series. The two portions 12, 14 comprise birefringent, polarisation-maintaining optical fibre. Such fibre has two principal, orthogonal propagation axes along which light may propagate.

The axes are defined by different values of phase velocity for light propagating along the axes (usually caused by some asymmetry in the fibre, such as differing refractive indices). Thus, light takes different amounts of time to propagate along the two axes, so that the effective optical path length of one axis is longer than the other. The longer path may be referred to as the "slow" axis (S), whereas the shorter path is referred to as the "fast" axis (F). The difference between the phase velocities defines the birefringence of the fibre. For use in a sensor such as that of Figure 1, the birefringent fibre must have a birefringence that varies in response to an applied parameter of interest, such as temperature or pressure. The phase velocities of the two axes have different responses to the parameter, so that the birefringence is a function of the parameter.

2] The fibre portions 12, 14 of the sensor 10 are coupled together such that the first portion l 2 is rotated about its longitudinal axis by 90 with respect to the second fibre portion 14. The resulting relative positions of the axes F and S are shown in Figure 1. This means that light propagating along the fast axis of the first portion 12 is coupled at the join between the portions into the slow axis of the second portion 14, and vice versa. Also, the two portions are chosen to have substantially equal sensitivity to the parameter of interest. This may be achieved by using equal lengths of the same type of fibre. Alternatively, different lengths of fibre having different sensitivities per unit length may be used if the products of length and unit sensitivity are matched. Although the sensitivities are thus equal, the rotation of one portion with respect to the other introduces a sign change in the sensitivity, so that the two portions have sensitivities of equal magnitude but opposite sign. This allows differential measurements to be made, as follows.

[00331 The sensor 10 is deployed into a measurement region across which a differential measurement of a parameter is required. The sensor 10 is arranged so that a first value (or range of values) of the parameter acts on the first fibre portion 12, and a second value (or range of values) of the parameter acts on the second fibre portion 14. For the sake of example, assume that the parameter is pressure, P. Thus, pressure Pl is applied to the first portion 12 and pressure P2 to the second fibre portion 14. An interrogating light signal is launched in an input end 16 of the sensor 10 such that substantially equal amounts of light are coupled into the two axes. The light propagates along the sensor, switching between fast and slow axes at the join between the two portions, and is received at the far end 18 of the sensor 10.

10034] For both portions 12, 14 a change in pressure P modifies the birefringence by altering the phase velocity difference between the two axes. Assuming that the sensor l O is operated in a regime where an increase in pressure P increases the birefringence in each portion, the pressure Pi will increase the phase difference in the first portion by an amount related to Pl and to the pressure sensitivity of the first portion.

Similarly, the pressure P2 will increase the phase difference in the second portion by an amount related to P2 and the pressure sensitivity of the second portion. However, because the axes are exchanged at the join between the two portion, the total phase slippage between the two axes of the sensor as a whole is the difference between the resulting birefringences of the first portion and the second portion. The sensitivities are equal in the two portions, though, so the overall birefringence of the sensor is proportional only to the difference between the two pressures. Thus, a measurement of the birefringence, determined from the received light, gives a measurement of the differential pressure.

10035] It has been emphasised that the prior art device of Figure 1 should comprise two halves matched in terms of sensitivity to the parameter of interest. A device that is perfectly balanced in this way has zero total birefringence for no differential in the parameter. In practice, this situation is difficult to interrogate optically using known techniques for measuring fibre birefringence. In other words, no useful signal can be obtained in the event that the two fibre portions are exposed to equal values of the parameter. In many cases, however, it is desirable to be able to detect such a condition, for example to distinguish it from some failure of the sensor or other circumstance that results in a meaningless signal.

6] The present invention addresses this problem by providing a fibre sensor for differential measurements that does not demonstrate zero birefringence when there is no pressure differential. It has been recognised that a certain degree of birefringence in the sensor under all conditions is desirable. The invention achieves this by adding a length of high-birefringence fibre to a fibre sensor of the type shown in Figure 1.

10037] Figure 2A shows a schematic representation of a first embodiment of a sensor according to the present invention. The sensor 20 comprises a first sensing portion 22 of birefringent optical fibre and a second sensing portion 24 of birefringent optical fibre. These portions 22, 24 correspond to the first and second portions 12, 14 of the sensor 10 of Figure 1, and are thus arranged with their longitudinal axes rotated by 90 relative to the other portion, and have substantially equal but opposite birefringent sensitivities to a parameter of interest. The sensor 20 further includes a biassing portion 26, which comprises a length of birefringent fibre coupled in series between the first sensing portion 22 and the second sensing portion 24. The biassing portion 26 has an inherent birefringence that does not respond to changes in the parameter of interest, so that its birefringence is fixed with regard to the parameter.

The biassing portion is preferably arranged with its orthogonal polarization axes in line with the axes of the first and second sensing portions, to give the most efficient optical coupling between the various portions. The fibre of the biassing portion is preferably a high birefringence fibre, where high birefringence may be considered to be a birefringence of 5xlO-5 or more. It may consist of a fibre element similar to sensing portion 22 or 24, but isolated from the measurand of either the first or second sensing portion.

8] The primary function of the biassing portion 26 is to provide the sensor 20 with a known fixed component to its parameter-sensitive birefringence. This component acts as an offset to the measured birefringence. In the event that there is no differential in the parameter being measured, the sensor does not return a zero result, so that problems inherent in interrogating the zero birefringence situation are avoided.

Also, the offset allows the sign of the differential to be determined unambiguously, so that it can be ascertained whether the parameter is higher at the first sensing portion 22 or at the second sensing portion 26. The offset is readily accounted for; it is known so can be subtracted from the overall measured birefringence of the sensor to leave the parameter-sensitive birefringence component from which the differential parameter value can be determined.

100391 A further function of the biassing portion 26 is to provide some spectral dependence to the state of polarization of light entering the sensor at 45 to the principal axes. This is a feature required by most methods suitable for interrogating the sensor.

100401 Figure 2B shows a further embodiment of the sensor 20. The position of the biassing portion 26 in the series of fibre portions is immaterial; it will perform its function in any position. Thus, it is not necessary for the biassing portion 26 to be positioned between the sensing portions 22, 24. It may equally usefully be positioned at one end of the sensor 20, either at the proximal or distal end with respect to the direction in which light is launched into the sensor. The embodiment of Figure 2B has the biassing portion 26 positioned at the end of the sensor 20 and adjacent to the second sensing portion 24. In an alternative embodiment (not shown), the biassing portion 26 can be positioned at the opposite end of the sensor 20, adjacent to the first sensing portion 22.

1] Any type of parameter-sensitive birefringent optical fibre may be used for the first and second sensing portions. According to one embodiment, which is well- suited for the measurement of pressure, side-hole fibre is used (See H.M. Xie, P. Dabkiewicz, R. Ulrich, and K. Okamoto, "Side-hole fiber for fiber- optic pressure sensing", Optics Letters, Vol. I l, No. 5, pages 333-335, May 1986).

[00421 Figure 3A shows a schematic axial cross-section through a sidehole fibre 30.

The fibre 30 comprises a waveguiding core 32 surrounded by a cladding 34 in the conventional manner. However, two longitudinal air holes 36 are formed in the cladding 34 which run along the length of the fibre 30, and are positioned one on each side of the core 32. This gives the fibre the necessary asymmetry for birefringence. It has been shown that such sidehole fibre has sensitivity to pressure applied to the inner holes as well as to the outside surface of the cladding (See H.M. Xie, P. Dabkiewicz, R. Ulrich, and K. Okamoto, "Side-hole fiber for fiber-optic pressure sensing", Optics Letters, Vol. 11, No. 5, pages 333-335, May 1986).

3] Figure 3B shows an embodiment of a sensor 20 according to the present invention implemented using side-hole fibre. First and second sensing portions 22, 24 fabricated from side-hole fibre are arranged either side of a biassing portion 26, as in Figure 2A. The second sensing portion 24 is axially rotated by a quarter-turn relative to the first sensing portion 22, as evidenced by the different illustrated positions of the side holes 36. In Figure 3B, the side holes of both sensing portions are sealed against the fluid, so that pressure is applied only to the outer surface. In an alternative embodiment, the fluid can be introduced into the side holes only, so that the pressures to be measured are not applied to the outer surface. For all arrangements, however, the pressures should be applied to both sensing portions in the same way, to avoid differential effects within the individual sensing portions which could disrupt the intended overall differential measurement.

4] Other types of birefringent fibre may be used in place of sidehole fibre for the sensing fibre. The selection of a suitable fibre will depend at least partly on the parameter of interest, with a high sensitivity to that parameter being desirable. For example, optical fibre fabricated from photonic crystal material may be used.

10045] For accurate operation of the sensor, it will be necessary in some cases to provide isolation between the first and second sensing portions. This requirement will depend on the nature of the parameter of interest and whether it is possible to expose the two parts of the sensor to different values of the parameter without interference between the two values. To achieve the isolation, an isolating barrier can be arranged around the axial perimeter of the sensor which has sealing or blocking properties sufficient to allow different values of the parameter to be applied to the two sensing portions without exchange between the two.

l0046l Figure 4 shows a schematic representation of such an arrangement. An isolating barrier 40 is disposed around the sensor 20 to create a divide between the two sensing portions 22 and 24. The isolating barrier 40 will extend sufficiently far from the sensor to provide the required degree of isolation when the sensor is installed for use. In the case of pressure, for example, it will typically be necessary to provide an isolating barrier in the form of a pressure feedthrough (See US Patents 6, 427,046 and 6,526,212). This is an arrangement that allows an optical fibre (or fibre assembly such as a sensor) to extend through a bulkhead between two zones that typically contain different pressures, with sufficient sealing effect to inhibit pressure leakage between the two zones. Thus, the first sensing portion 22 of the sensor 20 can be exposed to a first pressure Pl on one side of the bulkhead while the second sensing portion is exposed to a second pressure P2 on the other side of the bulkhead.

100471 Some pressure feedthroughs may exert a force on the fibre whichwill affect the birefringence. This may also apply to isolating barriers used for the measurement of other parameters. The birefringence caused by the feedthrough is a possible source of drift that is liable to perturb the birefringence measurement, leading to inaccuracies in the deduced parameter differential. This drift should preferably be taken into account when processing measurements obtained using a sensor affected in this way.

Also, if the extraneous birefringence is caused in a fibre section where the birefringence is weak, it can dominate the local birefringence and cause unwanted coupling between the principal modes of the fibre, causing further errors. It is therefore desirable that the section of sensor which passes through the feedthrough should have a significant birefringence compared to any change that may be caused by the feedthrough. This can be achieved by arranging for the biassing portion of the sensor to be fabricated from high birefringence fibre and to be positioned between the first and second sensing portion so that it forms that part of the fibre acted upon by the feedthrough (or other barrier). This arrangement is shown in Figure 4.

lO048] The sensor may be operated in transmission or reflection mode. Figure 5A shows a schematic representation of the sensor 20 in use in transmission mode. Light is launched into a first end of the sensor 20 from an appropriate optical source such as a laser, and propagates along the sensor 20, being emitted as an output 54 at the far end of the sensor 20. The output 54 is received by a detector 56, and the overall birefringence of the sensor 20 is determined from the detected output. The measured birefringence is then analysed to determine the differential measurement. A processor or other suitable signal analysis device may receive a signal from the detector and perform this latter function. Any suitable technique for reading polarimetric fibre sensors may be used to interrogate the sensor of the present invention, such as by means of an optical spectrum analyser to define the spectral response of the sensor output, or by means of a compensating tracking interferometer, or any other appropriate method for determining remotely the overall birefringence of the sensor.

10049] For operation in reflection mode, the sensor further comprises a reflecting element disposed at its distal end opposite to the proximal end into which the light is launched. Figure 5B shows a schematic representation of an example of a sensor of this type, the sensor 20 having a reflector 58 at one end. Any reflector suitable for reflecting light of the wavelength used to interrogate the sensor 20 can be used, such as a Bragg grating written into the fibre, a high reflectivity coating applied to the distal end of the sensor, a bulk mirror arranged adjacent to the fibre end, or a suitably cleaved end to the proximal fibre portion if this gives a high enough level of reflectivity. In operation, the interrogating light is launched into the proximal end of the sensor, propagates through the various fibre portions to the distal end where it is reflected from the reflecting element back along the sensor, and leaves the proximal end as the output 54, for detection by the detector 56. To separate the input and output, any suitable beam separating device may be employed. Figure 5B includes a beam splitter 59 arranged at an angle to the propagation axis of the sensor 20, which transmits the incident input light 50 into the fibre, and reflects the incident output light 54 onto the detector 56. A fibre coupler, an optical circulator, or other device capable of performing a similar function may alternatively be used.

0] The interrogating light needs to be launched into both axes of the optical fibre. Any technique which achieves this can be used, but a simple method uses an input polariser 52 positioned at the proximal end of the sensor (see Figures 5A and 5B). The polariser 52 is arranged with its axis at 45 to the fast and slow axes of the sensor fibres, so that light passing through it becomes polarised in such a way as to couple substantially equally into both the fibre axes. The polariser 52 may take the form of a fibre polariser that is spliced onto the sensor 22 at 45 from the principal axes of the latter. In reflection mode, the same device then, in addition, fulfils the function of analysing the returned optical signal, modulated by the birefringence of the sensor.

1] The biassing portion, made up of birefringent fibre, may have a birefringence that is sensitive to a second parameter different from the parameter of interest. For example, if the sensor is designed for differential pressure measurements, it is possible that the biassing portion may have a temperature sensitivity, since this is a common feature of birefringent fibre. If the sensor is deployed in a location where the temperature varies, the birefringence of the biassing portion is no longer fixed.

Accounting for the birefringence offset provided by the biassing fibre therefore becomes more complex; one needs to know the relationship between temperature and birefringence and also the temperature to which the sensor is exposed (which may perhaps be determined by using a further fibre sensor of a different type) to calculate the offset. Alternatively, the biassing portion may be constructed to have an overall zero sensitivity to temperature (or other second parameter). One way of achieving this using fibre that has a temperature sensitivity is to splice together two or more sections of fibre selected in such a way that the temperature sensitivity of their birefringences, but not the actual birefringences, cancel each other out, so that the temperature sensitivity is eliminated. For example, sections of fibre of different design may be used, such as a section of fibre having a birefringence provided by stress and a section of fibre having a birefringence provided by the shape of the fibre. This approach may also be used to remove sensitivity to other parameters to which the fibre of the biassing portion may otherwise respond.

[00521 Figure 6 shows a schematic representation of an example of a sensor 20 having a biassing portion 26 made up of two sections 60, 62 of different fibre to give an overall null response to a second parameter.

100531 The present invention is applicable to the differential measurement of any parameter to which the birefringence of an optical fibre can be made sensitive. These include, but are not limited to, pressure, temperature, strain, and also electric and magnetic fields, and the presence of chemicals, for which the sensing portions may be provided with electro-strictive, magneto-strictive, or chemically sensitive coatings.

Claims

What is claimed is: 1. A differential optical fibre sensor for sensing a parameter, comprising: a first sensing portion, a second sensing portion, and a biassing portion optically coupled in series and arranged such that the first sensing portion can be exposed to a first value of the parameter, the second sensing portion can be exposed to a second value of the parameter, and light can be launched into and subsequently received from the sensor; wherein the first sensing portion comprises birefringent optical fibre having a birefringence that varies in response to the parameter to give the first sensing portion a first sensitivity to the parameter; the second sensing portion comprises birefringent optical fibre having a birefringence that varies in response to the parameter to give the second sensing portion a second sensitivity to the parameter that is substantially equal in magnitude but opposite in sign to the first sensitivity; and the biassing portion comprises birefringent optical fibre having a birefringence that does not vary substantially in response to the parameter.
2. A differential optical fibre sensor according to claim 1, in which the first sensitivity and the second sensitivity are arranged to be opposite in sign by the portions being coupled together such that light propagating along a slow polarization axis of the first sensing portion is coupled into a fast polarization axis of the second sensing portion and light propagating along a fast polarization axis of the first sensing portion is coupled into a slow polarization axis of the second sensing portion.
3. A differential optical fibre sensor according to claim I or claim 2, in which the biassing portion is coupled between the first sensing portion and the second sensing portion.
4. A differential optical fibre sensor according to claim 1 or claim 2, in which the biassing portion is coupled at an end of the sensor.
5. A differential optical fibre sensor according to any preceding claim, in which the biassing portion comprises two or more sections of birefringent fibre that each have a birefringence that varies in response to a second parameter to which the sensor will be exposed in use, the two or more sections arranged so that the biassing portion has an overall birefringence that does not vary in response to the second parameter.
6. A differential optical fibre sensor according to any one of claims I to 5, in which the first sensing portion and the second sensing portion comprise side-hole fibre.
7. A differential optical fibre sensor according to claim 6, in which the side holes in the first sensing portion and the side holes in the second sensing portion are sealed.
8. A differential optical fibre sensor according to claim 6, in which the side holes in the first sensing portion and the side holes in the second sensing portion are open.
9. A differential optical fibre sensor according to any one of claims 1 to 5, in which the first sensing portion and the second sensing portion comprise photonic crystal optical fibre.
10. A differential optical fibre sensor according to any one of claims 1 to 5, in which the biasing section comprises photonic crystal optical fibre.
11. A differential optical fibre sensor according to any preceding claim, in which the parameter is pressure.
12. A differential optical fibre sensor according to any preceding claim, and further comprising an isolating barrier disposed around the sensor between the first sensing portion and the second sensing portion and operable to isolate the first sensing portion from the second value of the parameter and to isolate the second sensing portion from the first value of the parameter when the sensor is in use.
13. A differential optical fibre sensor according to claim 12 when dependent on claim 3, in which the isolating barrier is disposed around the biassing portion.
14. A differential optical fibre sensor according to claim 12 or claim 13, in which the isolating barrier comprises a pressure feedthrough.
15. A differential optical fibre sensor according to any preceding claim, and further comprising a polarising element arranged at an input end of the sensor and positioned with respect to fast and slow polarization axes of the sensing fibres such that light launched into the sensor via the polarising element is coupled substantially equally into both axes.
16. A differential optical fbre sensor according to any preceding claim, and further comprising a reflective element arranged at an end of the sensor opposite an input end of the sensor such that light launched into the sensor at the input end propagates through the sensor, is reflected from the reflective element, and propagates back to the input end where it can be received.
17. A differential optical fibre sensor according to any preceding claim, and further comprising a detector operable to receive light from the sensor and determine a total birefringence of the sensor therefrom.
18. A differential optical fibre sensor according to claim 17, and further comprising a processor operable to determine any difference between the first value of the parameter and the second value of the parameter from the total birefringence of the sensor, including taking account of a contribution to the total birefringence from the biassing portion.
19. A method of making a differential measurement of a parameter comprising: disposing into a measurement region an optical fibre sensor according to any one ofclaims 1 to 18; exposing the first sensing portion of the sensor to a first value of the parameter; exposing the second sensing portion of the sensor to a second value of the parameter; launching light into the sensor; receiving light subsequently emitted from the sensor; determining a total birefringence of the sensor from the received light; adjusting the total birefringence to take account of a contribution from the biassing portion of the sensor; and determining any difference between the first value of the parameter and the second value of the parameter from the adjusted total birefringence.