GB2532031A

GB2532031A - System and method for detecting a change in shape of at least one predefined region of an optical fibre

Info

Publication number: GB2532031A
Application number: GB1419732.1A
Authority: GB
Inventors: Edward Brock Martin; Villar Gabriel; Jonathan Finchley Cletheroe Daniel; Lewis Emma
Original assignee: Cambridge Consultants Ltd
Current assignee: Cambridge Consultants Ltd
Priority date: 2014-11-05
Filing date: 2014-11-05
Publication date: 2016-05-11
Also published as: GB2534643A; GB201519586D0; WO2016071700A1; GB201419732D0

Abstract

A method for detecting a change in shape of at least one predefined region of an optical fibre, comprising: transmitting a light pulse from a first end of an optical fibre towards a second end of the optical fibre; receiving, at the first end of the optical fibre, light returned from within the optical fibre selectively during at least one preconfigured time interval, wherein each preconfigured time interval is configured to be a time interval having a known relationship to a time period during which any light returned from a selected predefined region of the optical fibre is expected; measuring the intensity of the light received during the at least one preconfigured time interval; and identifying that a change in shape of the optical fibre has occurred in the at least one predefined region based on the measured intensity of said light returned during the corresponding at least one preconfigured time interval.

Description

System and method for detecting a change in shape of at least one predefined region of an optical fibre The present invention relates to detection of a change in shape of at least one predetermined region of an optical fibre. The present invention relates in particular but not exclusively to the measurement of bending, strain and/or displacement at one or more predefined regions along an optical fibre, which is preferably attached to or embedded within a flexible material.

Optical fibres have been used to sense strain and bending, and systems exist that measure the bend-induced attenuation of light transmitted through an optical fibre. However, known techniques monitor only a single point along a fibre, and are therefore difficult to extend to perform measurements at multiple points along the optical fibre. To obtain measurements at multiple points currently requires the use of multiple independent fibre sensors with associated cost and complexity of electrical multiplexing.

Systems are known that make distributed measurements of strain at all points along a single optical fibre attached to a structure. However, known methods use bulky and expensive interrogation systems, either relying on multiple fibre Bragg gratings and wavelength multiplexing, or frequency-domain measurements of Rayleigh-or Brillion-scattering using tuneable lasers or optical modulators. These techniques are not well suited to a low-cost product where, for example, the measurement equipment is to be attached to a flexible material.

In addition, systems are known that rely on the principle of optical time domain reflectometry (OTDR) to detect deformation along an optical fibre. However, in addition to being expensive and bulky, these systems also operate by taking measurements at all points along a fibre. Such systems are therefore limited in the frequency at which measurements can be made, with a known system currently taking around one second to perform a complete sweep along a 30m optical fibre using a 'fast' OTDR.

It can be seen therefore that there exists a need for an improved fibre-optic sensor and -2 -related improved sensing methods. Of particular benefit would be an improved sensor and associated method that avoids or at least partially ameliorates at least one of the above issues.

Such an improved sensor and associated method would, for example, be suitable for use in determining deformation of a flexible material at one or more defined points along a single optical fibre, and at a higher frequency than is currently possible. Indeed, a fibre-optic sensor capable of measuring the bending of a flexible material at one or more defined points within that material using a single optical fibre would offer a significant

advantage of the prior art.

The ability to measure deformation in a flexible material has numerous conceivable applications, such as: 'motion capture' garments for digitising body motion; sports and fitness aids for providing feedback to users; medical garments for monitoring patient health; and condition monitoring of flexible materials used in buildings and civil engineering. In each of these applications, multiple measurements of the bending or flexing of the flexible material at defined points on its surface are typically needed in order to provide useful feedback to the system's users.

According to the present invention there is provided a method for detecting a change in shape of at least one predefined region of an optical fibre, comprising: transmitting a light pulse from a first end of an optical fibre towards a second end of the optical fibre; receiving, at the first end of the optical fibre, light returned from within the optical fibre selectively during at least one preconfigured time interval, wherein each preconfigured time interval is configured to be a time interval having a known relationship to a time period during which any light returned from a selected predefined region of the optical fibre is expected; measuring the intensity of the light received during the at least one preconfigured time interval; and identifying that a change shape of the optical fibre has occurred in the at least one predefined region based on the measured intensity of said light returned during the corresponding at least one preconfigured time interval. -3 -

Identifying that a change of shape has occurred may further comprise comparing changes in the amplitude of light reflected from the predefined region. Identifying that a change of shape has occurred may further comprise comparing the measured intensity of light received during a time interval with a predetermined intensity of light expected to be returned from within the optical fibre during that time interval, and determining a difference.

Comparing the measured intensity of light may further comprise determining a degree of stretching, bending or flexing of the optical fibre at the predefined region based on the difference between the measured and the predetermined intensities of light.

Identifying that a change of shape has occurred may further comprise performing a calibration operation to determine an intensity of light expected from a predefined region of the optical fibre before a change in shape occurs. Performing a calibration may further comprise taking regular measurements of an intensity of light returned from the predefined region of the optical fibre to determine the predetermined intensity of light expected. Performing a calibration operation may further comprise predetermining an intensity of light expected from a predefined region of the optical fibre for a range of values for radius of curvature of the optical fibre in that predefined region.

Identifying that a change of shape has occurred may further comprise comparing the measured intensity of light to the predetermined range of values of expected intensities 20 of light and thereby determining the radius of curvature of the optical fibre at the predefined region.

The at least one preconfigured time interval ideally has a linear relationship to the distance to the predefined region of the optical fibre.

Receiving light returned from within the optical fibre may further comprise configuring the 25 at least one preconfigured time interval to be a time period within which any light returned from the selected predefined region of the optical fibre is expected, such that the intensity of light being measured is that of backscatter from the predefined region.

The position of the preconfigured time interval may be determined through an automatic -4 -self-calibration process. The position of the preconfigured time interval may be continually optimised through an automatic self-calibration process.

Receiving light returned from within the optical fibre may further comprise: configuring the at least one preconfigured time interval to be a time period within which any light 5 returned from a region of the optical fibre before the selected predefined region of the optical fibre is expected; configuring a further preconfigured time interval to be a time period within which any light returned from a region of the optical fibre after the selected predefined region is expected; and measuring the intensity of the light received during the further preconfigured time interval. Identifying that a change of shape in the optical 10 fibre has occurred may further comprise comparing the intensity of the light received during the at least one preconfigured time interval with an intensity of light received during the further preconfigured time interval to determine an increase in attenuation through the predefined region of the optical fibre.

Receiving light returned from within the optical fibre may further comprise: receiving light returned from within the optical fibre selectively during a plurality of preconfigured time intervals; measuring the intensity of the light received during each preconfigured time interval; and identifying that a change shape of the optical fibre has occurred in each predefined region based on the measured intensity of said light returned during the corresponding preconfigured time interval.

Transmitting a light pulse may further comprise generating light pulses of a length that corresponds with the length of the predefined region such that there is negligible overlap between light reflected from two adjacent regions along the optical fibre.

Receiving light returned may comprise receiving an optical signal from the light returned, and may further comprise converting the optical signal into an electrical signal. Receiving light returned may further comprise using a time gate to configure the at least one preconfigured time interval. Measuring the intensity of the light received may further comprise integrating a plurality of received optical signals to improve he signal-to-noise ratio of the measurements.

The method may further comprise coiling or folding the optical fibre within the predefined -5 -region of optical fibre to increase detection sensitivity. The optical fibre may be attached to, or embedding within, a flexible material. The method may further comprise securing points of the optical fibre that are adjacent each end of the predefined region to the flexible material such that the optical fibre is fixed in both a transverse and a longitudinal direction with respect to the flexible material. Securing the optical fibre may further comprise securing the optical fibre within the predefined region to the flexible material to be fixed only in a transverse direction such that the shape of any bends or loops in the optical fibre can change in response to deformation or stretching of the flexible material.

The method may further comprise modulating the light pulse in amplitude, phase or 10 frequency, with either a smooth or pseudorandom signal, for use in a spread-spectrum system.

According to the present invention there is also provided a system for detecting a change of shape of at least one predefined region of an optical fibre, comprising: an optical fibre; means for transmitting a light pulse from a first end of the optical fibre towards a second end of the optical fibre; means for receiving, at the first end of the optical fibre, light returned from within the optical fibre selectively during at least one preconfigured time interval, wherein each preconfigured time interval is configured to be a time interval having a known relationship to a time period during which any light returned from a selected predefined region of the optical fibre is expected; means for measuring the intensity of the light received during the at least one preconfigured time interval; and means for identifying that a change shape of the optical fibre has occurred in the at least one predefined region based on the measured intensity of said light returned during the corresponding at least one preconfigured time interval.

2The optical fibre may be attached to or embedded in a flexible material such that a deformation of the flexible material causes a change in shape of the optical fibre. The optical fibre may be arranged such that the predefined region of the optical fibre coincides with a region of the flexible material at which a deformation is expected. The optical fibre may be coiled or folded within the predefined region of optical fibre to increase detection sensitivity. Points of the optical fibre that are adjacent each end of the predefined region may be secured to the flexible material such that the optical fibre is -6 -fixed in both a transverse and a longitudinal direction with respect to the flexible material. The optical fibre within the predefined region may be secured to the flexible material in a transverse-only direction such that the shape of any bends or loops in the optical fibre can change in response to deformation or stretching of the flexible material.

There may be a plurality of predefined regions along the optical fibre, each having a known relationship with a respective preconfigured time interval, such that an intensity of light returned from a plurality of predefined regions can be measured and hence a change in shape of the optical fibre can be identified at a plurality of predefined regions.

The position of the preconfigured time interval may be determined through an automatic self-calibration process. The position of the preconfigured time interval may be continually optimised through an automatic self-calibration process.

The system may further comprise means for splitting the light pulses being transmitted from the first end of the optical fibre and the light returned to the first end of the optical fibre. The system may further comprise a pulse generating means arranged to provide a pulsed signal to control the means for transmitting a light pulse to transmit light pulses. The means for transmitting a light pulse may be synchronised with the at least one preconfigured time-interval such that the amount of light returned can be measured as a function of time since the light pulse was transmitted.

The system may further comprise a signal conditioning means arranged to receive an optical signal output from the receiving means and convert the optical signal into an electrical signal. The system may further comprise an analogue-digital converter arranged to receive an electrical signal output from the signal conditioning means and convert it into a digital signal that is output to the processing means.

The means for identifying that a change in shape has occurred may be configured to determine a change in shape of the optical fibre at a predefined region from the light returned from the predefined region and/or the increase in attenuation in the optical fibre 30 after the predefined region. -7 -

The means for transmitting a light pulse, means for receiving returned light, means for measuring and the means for identifying may be combined to form a processing unit that is attachable to the flexible material.

The optical fibre may be a plastic optical fibre. The optical fibre may comprise both a graded refractive index profile and a stepped refractive index profile at regions along its length. The refractive index profile of the optical fibre may vary along its length from a graded index profile at a region proximate to the light source to a stepped index profile at a region distal to the light source.

The optical fibre may be treated to increase its backscattering properties or bend sensitivity. The optical fibre may be treated at one or more predefined sections of the fibre. The treatment may comprise notches, side-windows, mechanical abrasion, exposure to UV light or patterning with a laser.

The system may be arranged to monitor the condition of a building or structure, or 15 deformation in a garment (or other flexible material or textile).

According to the present invention there is also provided an optical fibre sensor unit for use in the above-described system, comprising: said optical fibre; and said means for receiving light returned from within the optical fibre coupled to said end of the optical fibre.

An optical splitter means may be provided at said end of the optical fibre. The optical fibre may be coiled or folded in the one or more predefined regions. The optical fibre may comprise plastic optical fibre. The optical fibre may comprise both a graded refractive index profile and a stepped refractive index profile at regions along its length. The refractive index profile of the optical fibre may vary along its length from a graded index profile at a region proximate to the light source to a stepped index profile at a region distal to the light source.

The optical fibre may be treated to increase its backscattering properties or bend sensitivity. The optical fibre may be treated at one or more predefined regions of the -8 -fibre. The treatment may comprise notches, side-windows, mechanical abrasion, exposure to UV light or patterning with a high-power laser.

The optical fibre sensor may be arranged to be attached to the flexible material at points 5 of the optical fibre that are adjacent each end of the predefined region such that the optical fibre is fixed in both a transverse and a longitudinal direction with respect to the flexible material. The optical fibre within the predefined region may be securable to the flexible material in a transverse-only direction such that the shape of any bends or loops in the optical fibre can change in response to deformation or stretching of the flexible 10 material.

According to the present invention there is also provided a processing unit for use with the above-described system, comprising: a light source arranged to transmit a light pulse into an end of an optical fibre; a light detector arranged to receive light returned from said end of the optical fibre; and processing means arranged to measure the intensity of returned light and identify whether the optical fibre has changed shape in a predefined region based on the measured intensity of light compared to a predetermined intensity of light expected from said predefined region.

The processing may further comprise an optical splitting means arranged to couple with 20 an end of the optical fibre. The optical splitting means may comprise an optical splitter, an optical switch or an optical circulator.

The light detector may be a photo-diode. A pulse generator may be arranged to control the light source to transmit light pulses.

The processing unit may further comprise signal conditioning means for converting an optical signal received from the light detector into an electrical signal and an analoguedigital-converter for converting the electrical signal into a digital signal to be processed by the processing means. The processing means may be a microprocessor.

The processing unit may be arranged to be secured to a flexible material and to couple -9 -with said end of said optical fibre.

According to the present invention there is also provided a computer programme product for implementing the above-described method.

The present invention provides a method of detecting a change in shape of at least one predefined region of an optical fibre and a corresponding system suitable for applications where deformation of the optical fibre must be tracked rapidly and continuously, for example in garments for body motion capture or sports training aids. In such applications, an update rate of 10Hz (and preferably upwards of 100Hz) is required, which is made possible by the present invention.

The present invention may be embedded or otherwise attached to a flexible material. In the simplest embodiment, a single optical fibre traverses the flexible material and 15 crosses each predefined region where flexing is to be measured.

Certain predefined regions of the optical fibre may be designed to have a more sensitive response to local stretching of the material, or to the flexing or bending of the flexible material around an underlying support at defined regions. Multiple folds or coils may be formed in the optical fibre and attached to the material in such a way that stretching the material or flexing it over the underlying support results in a net strain being applied to the material, which in turn changes the shape of the pattern in which the fibre is attached to the material, for example by changing the average radius of curvature of the optical fibre.

When the material is flexed or stretched the resulting change in shape of the fibre is measured in each defined sensing region. For example, this can be done either by measuring the local increase in backscatter which occurs with reducing bend radius at the location of the associated bend or bends, or by measuring the increase in attenuation of the fibre across the associated bend or bends which occurs with reducing bend radius, or a combination of both.

-10 -The present invention provides numerous advantages over known systems. For example, with the present invention, no electrical contacts are exposed, thus rendering the sensor waterproof. Furthermore, the sensing mechanism occurs within a small, flexible and tough fibre, rather than at the intersection of two or more conductive fibres or wires resulting in a more robust sensor as a whole than electronic sensors.

Compact, low-cost light sources and photo-detectors are connected to the fibre, illuminate it and measure the light reflections that occur at various points along its length due to backscattering inherent within the fibre itself and additional backscattering from any bends along the fibre. When the fibre is flexed or stretched the system measures either the change in reflection occurring at the associated bend or bends, or the change in backscattering after the bend or bends due to a change in loss, or both.

Therefore the present invention comprises a method and apparatus for measuring a 15 change of shape in the optical fibre at one or more predefined regions along an optical fibre, which may be attached to or otherwise embedded in a flexible material.

An example of the present invention will now be described, with reference to the accompanying drawings, in which: Figure 1 shows an exemplary embodiment of the present invention; Figure 2 illustrates a signal detected from light reflected from a change in shape occurring at a predefined region of an optical fibre; Figure 3 illustrates a signal detected from light reflected from a change in shape occurring at two distinct predefined regions of an optical fibre; Figure 4 shows an optical fibre incorporated into a garment; Figure 5 is a schematic showing processing optoelectronics; and Figures 6A and 6B show the deformation of an optical fibre attached to a flexible material before and after the material undergoes stretching.

Overview Figure 1 shows a particularly beneficial exemplary embodiment of the present invention in overview.

In the embodiment of Figure 1, an optical fibre is shown attached to a flexible material to 5 provide a sensor at position Po. A pulse of light provides an input signal that is transmitted from a first end of the optical fibre towards a second end of the optical fibre. Light reflected (backscatter) from the optical fibre at position Po is received at the first end of the optical fibre, thereby providing a return signal. The return signal is sampled during a sampling window configured to be a time interval (to -t1) within which any light 10 reflected from position Po is expected.

When the flexible material is bent or otherwise deformed it causes a change in shape of the optical fibre. The measured intensity of light returned from the sensor at position Po is used to identify a change in the shape of the optical fibre. The output illustrated in Figure 2 shows how the intensity of backscattered (reflected) light changes at the location of a bend in an optical fibre, thereby providing a different return signal for a bent optical fibre to the return signal expected for a straight optical fibre. Additional sensors may be placed at further positions (P2, Ps_ Pn) along the optical fibre, being positioned at predefined regions at which a deformation of the material is expected, providing multiple outputs as shown in Figure 3.

In this embodiment, the sampling window is configured to coincide with the time period when any backscatter is expected from the sensor at position Po. However, it will be appreciated that in another embodiment, a sampling window may be configured to occur either side of the time period when any backscatter is expected from the sensor position Po to detect attenuation of light within the optical fibre caused by a change in shape at position Po, as will be described in more detail further on.

Beneficially, therefore, by targeting the sampling window at predefined locations in the 30 optical fibre the arrangement shown in Figure 1 allows changes in the shape of the material to be detected rapidly, at potentially several different locations. This rapid detection has the potential to allow real-time or pseudo-real-time movement analysis that -12 -has a wide range of beneficial applications.

Background principle

All optical fibre inherently scatters a small amount of light back towards the source. Normally, with increasing length of fibre, the amount of light scattered back decays smoothly and exponentially in proportion to the length due to attenuation in the fibre. A bend in an optical fibre generally produces two effects when the fibre is illuminated. Firstly, a portion of light is reflected at the bend location. Secondly, the light in the optical fibre becomes attenuated beyond the bend. The attenuation can be measured by comparing the backscattering properties of the fibre before and after the bend location, since the amount of light backscattered will have a discontinuity at the bend location, as illustrated in Figure 2. The present invention takes advantage of this principle.

The present invention In the present invention, a pulse of light is transmitted into an optical fibre and the intensity of the light returned from within the optical fibre is measured at a preconfigured time interval corresponding to a predefined region of the optical fibre. The reflected light (backscattered light caused mainly by Rayleigh scattering) is recorded as a function of time, wherein the time interval between transmitting the light pulse and receiving the reflected signals is linearly related to the distance to the location (i.e. a predefined region) at which the deformation or change in shape of the optical fibre has occurred.

Figure 3 illustrates an output indicating that deformation has occurred at two distinct points along an optical fibre.

If, for example, we consider a 30m long fibre, the round trip delay of this fibre is of the order of 100ns, i.e. it takes a pulse of light 100ns to travel from the light source to the far end of the fibre and back to a photodiode, which measures the backscattered and reflected light. A pulse length of around 1ns could be used to achieve a spatial resolution of about 20cm, which is the typical length of a defined region. A time gate "window" must -13 -therefore have an opening time of less than 1ns (otherwise the spatial resolution of the system will be limited by the time-gate window and not the pulse length) and be stepped along each point covering the 10Ons delay. This means that a total of at least 100 window positions would therefore need to be sampled. Typically, pulses are be emitted 5 at a repetition frequency of 1MHz, and also typically, 10,000 pulses need to be integrated at each time gate window position to achieve an acceptable signal-to-noise ratio before the time gate window is swept to the next position. Collecting a complete graph of the form of Figure 3 will therefore take around 1s (100 window positions each requiring the integration of 10,000 pulses at a 1MHz frequency). Hence the update rate is limited to 10 around 1Hz.

With the present invention, however, rather than sampling every point along the fibre, only a few predefined points along the fibre need to be sampled. This can be achieved by knowing the positions of the predefined regions along the optical fibre in advance, or by determining the positions through a self-calibration process based on acquiring a graph similar to that in Figure 3. These predetermined points can be chosen to yield the most information about the deformation of the fibre in each defined region.

For example, if the backscatter from bending is to be measured then the time-gate 20 window positions can be set to coincide with the bend positions, while if the attenuation from bending is to be measured, the window positions can be set to be before and after bend positions so the attenuation through the bent region can be determined.

To illustrate the advantage of the present invention over the known prior art systems, consider the above described example with two predefined regions on 30m of fibre. The present invention requires only two window positions to be measured instead of 100, and hence the achievable update rate increases from 1Hz to 50Hz. Thus, it can clearly be seen that the present invention provides a big advantage when compared to looking along the whole length of an equivalent length of optical fibre.

Figure 4 shows a flexible material 100 in the form of a garment 110 into which an optical fibre 120 has been embedded or otherwise attached. The optical fibre 120 traverses the -14 -flexible material 100 and crosses each region 130 where flexing is to be measured. An optoelectronic processing unit 140 arranged to illuminate the fibre 120 and to measure the backscattered and reflected light from points along the length of the fibre 120 is also attached to the garment, and connected to the fibre 120.

An additional length of fibre 120 is arranged in certain defined regions 130 of the garment 110 where increased sensitivity is required. The fibre 120 is formed into multiple folds or coils at the location of the defined regions 130, which in this example correspond to body joints (e.g. shoulder, elbow, etc.) to facilitate the defined of the motion of the body joints. The defined regions 130 are connected by sections of fibre 120 having a larger bend radius so that the backscattering is reduced in these connecting sections 120 where no sensitivity is needed.

The increased sensitivity is achieved by attaching the defined regions 130 to the garment 110 in such a way that any stretching of the garment 110, or flexing it over the underlying support, results in a net strain being applied to the material 100, which in turn changes the shape of the pattern in which the fibre 120 is attached to the material, for example by changing the average radius of curvature of the optical fibre 120. This affects the amount of backscattered and reflected light.

From the measurements of the degree of bending at the various defined regions 130 along the fibre 120, and knowledge of the locations of those defined regions 130, it is then possible to infer and determine information about the shape of the underlying flexible material 100.

Figure 5 is a schematic showing processing optoelectronics 200 used with the present invention. The processing optoelectronics 200 include a pulse generator 210, a light source 220, an optical splitter 230, a photo-diode 240, signal conditioning electronics 250, an analogue-to-digital converter (ADC) 260 and a microprocessor 270. A resonant cavity light emitting diode (RC-LED) may be used, for example, to provide a low-cost, high-bandwidth light source 220.

-15 -In use, the pulse generator 210 is controlled by the microprocessor 270 to produce a pulse of electrical current sufficiently short that when converted to light by the light source 220 it results in negligible overlap between the light reflections from two adjacent regions along the fibre 120. The optical splitter 230 allows backscattered and reflected light to return from the fibre 120 into the photo-diode 240 and from there the signal conditioning electronics 250. The optical splitter 230 may consist or any device able to separate the light transmitted into the fibre 120 from the backscatter and reflected light returned from the fibre 120, for example a passive optical splitter, optical switch or optical circulator.

The optoelectronics 200 convert the optical signal into an electrical signal, use a time-gate to collect selectively only the pulses returning from a variable but defined region of the fibre 120, integrate a number of such pulses and then pass the output to the ADC 260 for digitisation and processing by the microprocessor 270.

The microprocessor 270 synchronises the pulse generator 210 and the time-gate and, by varying the delay before the time-gate opens, is able to measure the amount of light returned as a function of time since the pulse was emitted, resulting in data generally in the form shown in the chart of Figure 2. From such data, the degree of bending at each point of interest along the fibre 120 can be computed using either the direct reflected light from the bend, or the increase in attenuation, or some combination of these. Generally, the signals from many thousands of reflected pulses are integrated to improve the signal-to-noise ratio of the measurements.

The processing optoelectronics 200 may be housed in a compact unit 140, as shown in Figure 4, which is attached to the garment 110 (or other material 100) when coupled to the fibre 120 attached to or embedded therein.

Figures 6A and 6B show the fibre 120 attached to a flexible material 100 and arranged 30 so that bending of the material 100 results in a net strain of the material 110 and that this in turn results in a change in the average radius of curvature of the fibre 120, as described earlier. This result can be achieved by ensuring that the fibre 120 is secured -16 -to the material 100 at certain predefined points such that when the material 100 is stretched the fibre 120 experiences a change in shape and/or an additional strain within a defined "sensitive" region 130. Both a change in shape and a change in strain cause a change in the loss and backscatter. If the additional strain on the fibre 120 is minimised, then the compliance of the material 100 will not be affected, which is often desirable for a garment. Alternatively, the fibre 120 can be arranged so as to increase the stiffness of the material 100 if a more supportive and rigid region of the garment is desired. The material in and around the sensitive region 130 may be elastic and able to compensate for contraction of the fibre pattern along one direction, or it may be inelastic to impose more stretch on the fibre, or inelastic in certain regions where the fibre is attached in order to prevent the fibre from stretching in those regions. The density of restraint points 150,160 may be very high (or continuous in the case of a laminated structure), or relatively low, and the average bend radius of the fibre 120 may increase or decrease in response to stretching of the material 100. For example, the fibre 120 may be arranged as shown in Figures 6A and 6B, where the fibre 120 is secured at each end 150 in the longitudinal direction so that the length of the sensitive region is forced to change in response to a longitudinal stretch of the material 100. Within the sensitive region the attachment 160 of the fibre 120 is looser, so that the fibre can move in the transverse direction, and the average bend radius of the fibre 120 decreases in response to the longitudinal stretching of the material 100.

Although multiple defined regions 130 can be located on a single fibre 120, it may sometimes be advantageous to use multiple fibres (not shown), each with a smaller number of defined regions 130. The signals from these multiple fibres may be combined optically using a passive or electronically controlled splitter (not shown), or processed separately using multiple light sources (not shown) and photo-diodes (not shown). The system may also use multiple processing units, each of which takes measurements using one or more optical fibres.

The fibre may consist of a plastic optical fibre, which may be treated to alter its backscattering properties or bend sensitivity. Such treatment may be applied to selected regions of the fibre or to the entire fibre.

-17 -Greater reflections upon bending can be produced by using an optical fibre having a step change in its radial refractive index profile. Alternatively, a graded-index profile can be used to minimise dispersion within the fibre, and hence to maximise spatial resolution, i.e. the system's ability to distinguish bending at each of two closely separated sensing regions. Depending on the desired spatial resolution and number of sensing regions, the refractive index profile of the optical fibre may (optionally) therefore be stepped, graded, or both. Alternatively, the index profile may be varied along the length of the fibre, for example from a graded profile near the light source (where minimising dispersion is more important to avoid distorting the signals from the multiple sensors located beyond the initial length of fibre) to a stepped profile further from the light source (where maximising the amplitude of the backscattered signal is more important due to the higher losses inherent in traversing a longer length of fibre). In addition, the optical fibre may be treated to increase its backscattering properties or bend sensitivity. For example, it could be notched, side-windowed, mechanically abraded, exposed to UV light or patterned with a high-power laser. The treatment of the fibre may either uniformly increase its scattering, or it may take the form of a fibre Bragg grating (FBG) and be reflective only at a selected wavelength. The treatment may also be applied to either the whole fibre or just those defined regions where increased sensitivity is required.

The processing unit may produce a pulse of light for each measurement. Alternatively, the processing unit may produce an optical signal that is modulated in amplitude, phase or frequency. The light returned from the predefined region of the optical fibre can be characterised by its propagation delay. The optimum propagation delays corresponding to the predefined regions can be determined through a self-calibration process before or during the use of the system.

As an alternative to the pulsed system described above, a spread-spectrum system can be used where the light source is amplitude-, phase-or frequency-modulated, with either a pseudorandom signal, or a smooth signal as in a frequency chirp. The backscattered signals are then detected by cross-correlation using well-known techniques from direct-sequence and pulse-compression radar systems.

-18 -Advantageously, the present invention provides an optical fibre sensor for measuring deformation of a flexible material when attached thereto. The optical fibre may traverse multiple regions of the material where flexing is to be measured. The fibre is so arranged 5 as to be preferentially sensitive to bending or flexing of the material in the desired measurement regions. An optoelectronic processing unit, that uses only compact, low-cost light sources and photo-detectors, connects to the fibre, illuminates it and measures the reflections that occur at each point along its length. The amount of bending in the flexible material is thereby measured at multiple points using reflections of light in a 10 single optical fibre.

Claims

-19 -Claims 1. A method for detecting a change in shape of at least one predefined region of an optical fibre, comprising: transmitting a light pulse from a first end of an optical fibre towards a second end 5 of the optical fibre; receiving, at the first end of the optical fibre, light returned from within the optical fibre selectively during at least one preconfigured time interval, wherein each preconfigured time interval is configured to be a time interval having a known relationship to a time period during which any light returned from a selected predefined region of the optical fibre is expected; measuring the intensity of the light received during the at least one preconfigured time interval; and identifying that a change in shape of the optical fibre has occurred in the at least one predefined region based on the measured intensity of said light returned during the 15 corresponding at least one preconfigured time interval.
2. The method of claim 1, wherein identifying that a change in shape has occurred further comprises comparing changes in the amplitude of light reflected from the predefined region.
3. The method of claim 1 or 2, wherein identifying that a change in shape has occurred further comprises comparing the measured intensity of light received during the at least one time interval with a predetermined intensity of light expected to be returned from within the optical fibre during that time interval.
4. The method of claim 3, wherein comparing the measured intensity of light further comprises determining a degree of stretching, bending or flexing of the optical fibre at the 25 predefined region based on a difference between the measured intensity of light and the predetermined intensity of light.
5. The method of any of claims 1 to 4, wherein identifying that a change in shape has occurred further comprises performing a calibration process to determine a predetermined intensity of light expected from a predefined region of the optical fibre -20 -before a change in shape occurs.
6. The method of claim 5, where performing a calibration further comprises taking regular measurements of an intensity of light returned from the predefined region of the optical fibre to determine the predetermined intensity of light expected.
7. The method of claim 5 or 6, wherein performing a calibration operation further comprises predetermining an intensity of light expected from a predefined region of the optical fibre for a range of values for radius of curvature of the optical fibre in that predefined region.
8. The method of claim 7, wherein identifying that a change of shape has occurred further comprises comparing the measured intensity of light to the predetermined range of values of expected intensities of light and thereby determining the radius of curvature of the optical fibre at the predefined region.
9. The method of any of claims 1 to 8, wherein the at least one preconfigured time interval has a linear relationship to the distance to the predefined region of the optical 15 fibre.
10. The method of any of claims 1 to 9, wherein receiving light returned from within the optical fibre further comprises configuring the at least one preconfigured time interval to be a time period within which any light returned from the selected predefined region of the optical fibre is expected, such that the intensity of light being measured is that of backscatter from the predefined region.
11. The method of any of claims 1 to 10, where the position of the preconfigured time interval is determined through an automatic self-calibration process.
12. The method of claim 11, wherein the position of the preconfigured time interval is continually optimised through an automatic self-calibration process.
13. The method of any of claims 1 to 12, wherein receiving light returned from within the optical fibre further comprises: -21 -configuring the at least one preconfigured time interval to be a time period within which any light returned from a region of the optical fibre before the selected predefined region of the optical fibre is expected; configuring a further preconfigured time interval to be a time period within which 5 any light returned from a region of the optical fibre after the selected predefined region is expected; and measuring the intensity of the light received during the further preconfigured time interval.
14. The method of claim 13, wherein identifying that a change of shape in the optical fibre has occurred further comprises comparing the intensity of the light received during the at least one preconfigured time interval with an intensity of light received during the further preconfigured time interval to determine an increase in attenuation through the predefined region of the optical fibre.
15. The method of any preceding claim, wherein receiving light returned from within the optical fibre further comprises: receiving light returned from within the optical fibre selectively during a plurality of preconfigured time intervals; measuring the intensity of the light received during each preconfigured time interval; and identifying that a change shape of the optical fibre has occurred in each predefined region based on the measured intensity of said light returned during the corresponding preconfigured time interval.
16. The method of any preceding claim, wherein transmitting a light pulse further comprises generating light pulses of a length that corresponds with the length of the 25 predefined region such that there is negligible overlap between light reflected from two adjacent regions along the optical fibre.
17. The method of any preceding claim, wherein receiving light returned comprises receiving an optical signal from the light returned, converting the optical signal into an electrical signal, and using a time gate to configure the at least one preconfigured time -22 -interval.
18. The method of claim 17, wherein measuring the intensity of the light received further comprises integrating a plurality of received optical signals to improve he signal-to-noise ratio of the measurements.
19. The method of any preceding claim, further comprising coiling or folding the optical fibre within the predefined region of optical fibre to increase detection sensitivity.
20. The method of any preceding claim, wherein the optical fibre is attached to, or embedded within, a flexible material.
21. The method of claim 20, further comprising securing points of the optical fibre that 10 are adjacent each end of the predefined region to the flexible material such that the optical fibre is fixed in both a transverse and a longitudinal direction with respect to the flexible material.
22. The method of claim 21, wherein securing the optical fibre further comprises securing the optical fibre within the predefined region to the flexible material to be fixed 15 only in a transverse direction such that the shape of any bends or loops in the optical fibre can change in response to deformation or stretching of the flexible material.
23. The method of any preceding claim, further comprising modulating the light pulse in amplitude, phase or frequency, with either a smooth or pseudorandom signal, for use in a spread-spectrum system.
24. A system for detecting a change of shape of at least one predefined region of an optical fibre, comprising: an optical fibre; means for transmitting a light pulse from a first end of the optical fibre towards a second end of the optical fibre; means for receiving, at the first end of the optical fibre, light returned from within the optical fibre selectively during at least one preconfigured time interval, wherein each preconfigured time interval is configured to be a time interval having a known relationship -23 -to a time period during which any light returned from a selected predefined region of the optical fibre is expected; means for measuring the intensity of the light received during the at least one preconfigured time interval; and means for identifying that a change shape of the optical fibre has occurred in the at least one predefined region based on the measured intensity of said light returned during the corresponding at least one preconfigured time interval.
25. The system of claim 24, wherein the optical fibre is attached to or embedded in a flexible material such that a deformation of the flexible material causes a change in 10 shape of the optical fibre.
26. The system of claim 25, wherein the optical fibre is arranged such that the predefined region of the optical fibre coincides with a region of the flexible material at which a deformation is expected.
27. The system of claim 26, wherein the optical fibre is coiled or folded within the predefined region of optical fibre to increase detection sensitivity.
28. The system of any of claims 24 to 27, wherein points of the optical fibre that are adjacent each end of the predefined region are secured to the flexible material such that the optical fibre is fixed in both a transverse and a longitudinal direction with respect to the flexible material.
29. The system of claim 28, wherein the optical fibre within the predefined region is secured to the flexible material in a transverse-only direction such that the shape of any bends or loops in the optical fibre can change in response to deformation or stretching of the flexible material.
30. The system of any of claims 24 to 29, wherein there are a plurality of predefined regions along the optical fibre, each having a known relationship with a respective preconfigured time interval, such that an intensity of light returned from a plurality of predefined regions can be measured and hence a change in shape of the optical fibre can be identified at a plurality of predefined regions.

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31. The system of any of claims 24 to 30, wherein the position of the preconfigured time interval is determined through an automatic self-calibration process.
32. The system of claim 31, wherein the position of the preconfigured time interval is continually optimised through an automatic self-calibration process.
33. The system of any of claims 24 to 32, further comprising a pulse generating means arranged to provide a pulsed signal to control the means for transmitting a light pulse to transmit light pulses.
34. The system of any of claims 24 to 33, wherein the means for transmitting a light pulse is synchronised with the at least one preconfigured time-interval such that the 10 amount of light returned can be measured as a function of time since the light pulse was transmitted.
35. The system of any of claims 24 to 34, wherein the means for identifying that a change in shape has occurred is further configured to determine a change in shape of the optical fibre at a predefined region from the light returned from the predefined region 15 and/or the increase in attenuation in the optical fibre after the predefined region.
36. The system of any of claims 24 to 35, wherein the means for transmitting a light pulse, means for receiving returned light, means for measuring and the means for identifying are all comprised in a processing unit that is attachable to the flexible material.
37. The system of any of claims 24 to 36, wherein the optical fibre is a plastic optical 20 fibre.
38. The system of any of claims 24 to 37, wherein the optical fibre comprises both a graded refractive index profile and a stepped refractive index profile at regions along its length.
39. The system of claim 38, wherein the refractive index profile of the optical fibre 25 varies along its length from a graded index profile at a region proximate to the light source to a stepped index profile at a region distal to the light source.

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40. The system of any of claims 24 to 39, wherein the optical fibre is treated to increase its backscattering properties or bend sensitivity.
41. The system of claim 40, wherein the optical fibre is treated at one or more predefined sections of the fibre.
42. The system of claim 40 or 41, wherein the treatment comprises notches, side-windows, mechanical abrasion, exposure to UV light or patterning with a laser.
43. The system of any of claims 24 to 42 arranged to monitor the condition of a building or structure.
44. The system of any of claims 24 to 42 arranged to monitor deformation in a 10 garment.
45. An optical fibre sensor unit for use in the system of claim 24, comprising: said optical fibre; and said means for receiving light returned from within the optical fibre coupled to said end of the optical fibre.
46. The optical fibre sensor unit of claim 45, wherein the optical fibre is arranged to be coiled or folded in the one or more predefined regions.
47. The optical fibre sensor unit of claim 46, wherein the optical fibre comprises plastic optical fibre.
48. The optical fibre sensor unit of any of claims 45 to 47, wherein the refractive index 20 profile of the optical fibre varies along its length from a graded index profile at a region proximate to the light source to a stepped index profile at a region distal to the light source.
49. The optical fibre sensor unit of claim 48, wherein the refractive index profile of the optical fibre varies along its length from a graded index profile at a region proximate to 25 the light source to a stepped index profile at a region distal to the light source.
50. The optical fibre sensor unit of any of claims 45 to 49, wherein the optical fibre is -26 -treated to increase its backscattering properties or bend sensitivity.
51. The optical fibre sensor unit of claim 50, wherein the optical fibre is treated at one or more predefined regions of the fibre.
52. The optical fibre sensor unit of claim 50 or 51, wherein the treatment comprises 5 notches, side-windows, mechanical abrasion, exposure to UV light or patterning with a high-power laser.
53. The optical fibre sensor unit of any of claims 45 to 52 arranged to be attached to the flexible material at points of the optical fibre that are adjacent each end of the predefined region such that the optical fibre is fixed in both a transverse and a 10 longitudinal direction with respect to the flexible material.
54. The optical fibre sensor unit of any of claims 45 to 53 further arranged wherein the optical fibre within the predefined region is arranged to be secured to the flexible material in a transverse-only direction such that the shape of any bends or loops in the optical fibre can change in response to deformation or stretching of the flexible material.
55. A processing unit for use with the system of claim 24, comprising: a light source arranged to transmit a light pulse into an end of an optical fibre a light detector arranged to receive light returned from said end of the optical fibre; and processing means arranged to measure the intensity of returned light and identify whether the optical fibre has changed shape in a predefined region based on the measured intensity of light compared to a predetermined intensity of light expected from said predefined region.
56. The processing unit of claim 55, further comprising a pulse generator arranged to control the light source to transmit light pulses.
57. The processing unit of claim 55 or 56, further comprising signal conditioning means for converting an optical signal received from the light detector into an electrical -27 -signal and an analogue-digital-converter for converting the electrical signal into a digital signal to be processed by the processing means.
58. The processing unit of any of claims 55 to 57 arranged to be secured to a flexible material and to couple with said end of said optical fibre.
59. A system substantially as described herein and shown in the accompanying figures.
60. An optical fibre sensor unit substantially as described herein and shown in the accompanying figures.
61. A processing unit substantially as described herein and shown in the 10 accompanying figures.