The present invention relates to apparatus and a method for using optical fibres as sensors.

Optical fibres, because of their light weight, strength and ability to carry a light wave over a long distance, have been used as sensors, particularly where fire or explosion hazards or electromagnetic interference restricts the use of electronic sensors.

Where large scale sensor arrays are required to provide multi-variable monitoring over a large area or a complex structure, such as in a "Smart Structure", optical fibre sensors have limitations. Although optical fibres are relatively cheap, lightweight and flexible and able to be embedded or attached to a structure with a negligible effect on their functionality, the optical fibre devices such as sources, couplers, connectors, modulators, etc. are relatively expensive, heavy and rigid. Also, the polarisation status of the light waves in an optical fibre can easily be affected by the surrounding environment which can cause the signal to fade. Attempted solutions to this problem have included active polarisation control, use of polarisation maintaining fibres and use of Farady-rotator mirrors. However, these add to the expense and complexity of the use of optical fibres as sensors, particularly when a large sensor array is required.

We have now invented apparatus and equipment for using optical fibres in sensor arrays which reduce other technical difficulties in multiplexing.

According to the invention there is provided a sensor system which system comprises a plurality of optical fibres, each of which can cause the phase, intensity or spectrum of light passing down it to change in response to changes in a parameter which is to be measured, there being a laser adapted to send light down the optical fibres and a receiver adapted to receive light passing down the optical fibres and there being means to sample the image formed on the receiver by light passing down the optical fibres and means to process the said image so as to detect changes in the said parameter.

The invention also provides a method for sensing changes in a parameter in a structure which method comprises passing coherent or partially coherent light down a plurality of optical fibres, causing changes in the phase, intensity or spectrum of light passing down the optic fibres by changes in the said parameter, combining the light from each of the optical fibres to form a pattern, sampling and digitising the said pattern and processing the said digitised pattern to detect the changes in the said parameter.

Although patterns such as interference patterns, formed by the combination of two beams of coherent light of different phases, have been used to detect changes affecting the transmission of one of the beams, combinations of more than two beams of light have not been used for this purpose.

There are two basic systems utilising the device of the present invention which have different types of architecture, transmission and reflection architecture. In transmission architecture light passes down the sensing fibre and is transmitted out of the end of the fibre, in the reflection architecture, light is reflected from the end of the fibre. In general transmission architecture is of a simpler structure, has fewer components and a better power budget, however reflection architecture can be more convenient for some applications.

In the present invention, combination of light from more than two optical fibres and therefore from more than two sensing points can be used. The larger the number of different sensing points the larger the number of optical fibres that are required and the system of the present invention enables a large number of sensing points to be utilised, e.g. greater than 10, more preferably greater than SO and, if necessary, greater than 100.

In one embodiment of the invention, light from the laser is guided into one or a number of reference arms with light in different polarisation states and a number of optical fibre sensing arms, e.g. by a star coupler. Each of the sensing arms responds to the parameter to be measured by inducing a phase, intensity or spectrum change in the light passing down the sensing arm, e.g. if the optical path is affected by a rise

in temperature or movement of the structure to which the sensing arm is attached, then there will be a phase delay, an intensity change or a spectrum variation in that arm relative to the unaffected reference arms.

Preferably the ends of the optic fibres are all placed together with a preset pattern, e.g. to form a bundle with a plane end where all the optical fibres terminate and the positions of the reference fibre and sensing fibres on that plane should follow a pattern designed according to the processing algorithm used in the computer.

Preferably the receiver is a one or two dimensional charge coupled device (CCD) array placed to receive the optical field from the optical bundle directly or through a lens, a grating or other bulk optical devices and the image on this array will be in the form of an interferometric speckle field. The image of the speckle field can then be sampled by the array.

The scheme in which the end faces of the sensing fibres which detect the change in a parameter are arranged is called the encoding scheme. Each combination of two fibres will produce a set of sinusoid Young's fringe on the receiver with its spatial frequency determined by the separation between the two fibres and its position indicating the relative phase of light in them. Since the spatial frequency of a Young's fringe depends on the separation of the ends of the two fibres as well as the wavelength of the light, it is possible to pass light with two or more different wavelengths into the system and down each fibre ar o arrange the ends of the fibre so that the phase signals associated with different wavelengths can be separated on a digital spatial domain and retrieved simultaneously. Thus, for example it is possible to separate the phase signal of one parameter e.g. strain from that of a different parameter e.g. temperature using the dispersion characteristics of the fibre and to increase the absolute measurement range.

For example the end faces of the fibres can be arranged into a two-dimensional pattern for a further increased multiplexing capacity with the speckles recorded by a two-dimensional CCD array with the phases retrieved using a two-dimensional Fourier transform algorithm.

The change in the phase, intensity or spectrum of the light passing down a fibre can be affected at one or more sensing points along the fibre. Structure monitoring applications require the measurement of a physical parameter at a pre-determined position or on a pre-defined length along a fibre and, because of the one dimensional nature of optical fibre, it is necessary to mark out one or more positions along the fibre where the fibre interacts with the parameter to be measured and generates a signal. To achieve this markers can be set up along the fibre to define sensing positions. Types of markers that can be used include fibre grating markers, in-fibre filter markers, in-fibre mirror markers and virtual markers defined by fibre layout.

With fibre grating markers one or more fibre gratings with different resonant wavelengths can be built along each sensing fibre in a network.

With in-fibre filter markers at least two light sources with slightly different wavelengths can be passed into each sensing arm along a sensing fibre e.g. by a star coupler. Along each sensing fibre there are at least two in-fibre filters such as band reflection in-fibre filters, each reflecting one of the wavelengths respectively. Preferably the number of light sources is the same as the number of filters. The effective sensors are the sections of fibre between the two filters.

With in-fibre mirror markers there is a part of the fibre which is to be the sensing fibre and which is partially reflective at both ends and each sensing section is preferably of the same length. In a sensing fibre there will be two mutually incoherent light elements. One element passing through the reflectors and the other reflects once off each reflector.

With virtual markers the marker is defined by fibre layout and that section of the fibre attached to the structure is the sensing section of the fibre, so that by design of the layout of the fibres in the structure the sensing positions can be defined.

In another embodiment of the invention light with multiple wavelength elements or a broad band spectrum can be passed into an interferometer where the light beam is split and a controlled changing optical path difference (OPD) is introduced between

the split beams. The beams are then combined and passed into a star coupler. This embodiment can be used with transmission architecture or with reflection architecture. The marker types referred to above can be used, for example each fibre channel can have a plurality of fibre gratings. In-fibre gratings have a different Bragg wavelength and the light is finally received in one-to-one by a detector array. The output of each detector is sampled in accordance with changing OPD and data sent to a computer for processing. Signals derived from the physical parameters which alter the gratings along the fibre can be retrieved using suitable algorithms.

The system and method of the present invention can be used when a large number of sensing points are needed, e.g. in stress/strain monitoring of large structures such as in the aerospace and specialist construction industries where, when glass fibre composite material are used, the optical fibres are compatible with such materials.

In applications where continuous, precision measurement is required two reference arms with orthogonal status of polarisation can be used which can be created using two high birefringence fibres or free space pin holes to reduce polarisation induced fading.

Any conventional laser producing cohere™ or partially coherent light can be used, depending on the application.

The processing of the image can be achieved using suitable algorithms.

As well as sensing changes in large scale structures, the present invention provides a convenient way of detecting changes at a variety of locations widely spaced apart. The system can be used, for example, for monitoring the integrity of large structures such as buildings, bridges etc. and for monitoring and for use in "Smart Structures" where structures detect and react to changing environmental conditions. It can be used for detecting fire or explosions or structural failure at different points in a building. In this case, optical fibres can be fed from a wide range of locations and easily and relatively inexpensively monitored at a single location.

The invention can also be used for acoustic emission monitoring in which short bursts of broad band strain waves caused by micro seismic activity such as crack formation and propagation are formed. These signals are scattered throughout a structure and are masked, which makes location difficult. With the present invention it is possible to have sensing arms and markers spread throughout and deep within a structure which reduces the problems due to scatter and masking.

The invention with its different system architectures is now described with reference to the accompanying drawings in which:-

Fig. 1 shows a schematic circuit diagram of a system utilising the invention with one sensing unit on each sensing fibre.

Fig. 2a shows a diagram of a system utilising the invention with many in-fibre gratings as sensing units along each sensing fibre

Fig.2b shows another embodiment of the configuration of 2a

Fig.3 shows a schematic circuit diagram with reflection architecture

Fig.4 shows a schematic circuit diagram with two light sources

Fig. 5 shows a schematic circuit diagram using partially coherent light source

Fig 6 shows a different embodiment using transmission architecture

Fig 7 shows a different embodiment using reflection architecture and

Fig. 8 shows a diagram of a system utilising the invention with an architecture suitable for detecting a large change of a physical parameter in a localised area.

In figs. 1 to 7 only a few sensing channels are shown for clarity, there will be a large number of optical sensing arms or channels depending on the number of points to be monitored. The optical fibres can be embedded in a structure or attached to the structure.

Referring to Fig. 1, a laser (1) sends light through star coupler (2) to an array of optical fibres, four of which are schematically shown as (3a), (3b), (3c) and (3d). Light passing down the optical fibres will have its phase or intensity altered by changes in a parameter which affects the optical properties of the optical paths.

With or without the lens (4), light from the end of the optical fibres forms an interferometric speckle field which is received by CCD array (5) controlled by CCD driver (6). The output from CCD array is digitised and passed to computer (7) where it is processed to retrieve the relative phases and intensities of light in the fibre array using designed algorithms.

In use, when a change in the parameter takes place, the phase or intensity change of light passing down the optical fibres takes place and a changed speckle is sampled, digitised and processed to monitor the changes in the parameter.

Referring to Fig. 2a, many in-fibre gratings (10) are built along each sensing fibre as distributed sensing units. The gratings on each fibre are designed to reflect a narrow band of light at different wavelengths back along the fibre. On the plane where all fibres terminate, the sensing fibres are arranged into a linear array. Light from the array is projected onto a bulk line-grating (11), with or without the cylinder lens (12). The grooves of the grating (11) are positioned to be parallel to the fibre array. The diffracted light field from the grating (11) is recorded by a two dimensional CCD array (12) and the digitised image is passed to the computer (14) where it is processed to retrieve the spectrum distribution of the light in each fibre.

Referring to fig. 2b several fibre gratings (20) with different resonant wavelengths are built along each sensing arm (21) in the network. The end faces of these fibres are arranged in a linear array. The light passes through cylinder lens (22) to a bulk line grating (23) with its grooves parallel to the fibre array. The refracted light is

projected to a two dimensional CCD array (24) which has its pixel rows positioned parallel to the fibre array as well as the grating grooves and records a 2-D matrix of the speckle field. The rows (x direction) of the matrix provides spatial encoding for the fibre array while the columns (y direction) spreads the power along the wavelength. By performing linear FFT on each row of the matrix the light intensity in the sensing fibres (21) at a particular wavelength can be retrieved.

Referring to fig.3 light from laser (50) passes through star coupler (51) along sensing arms (52 a,b,c) and to reflectors (53 a,b,c,d) The light is reflected from the reflectors and passes to CCD (54). Arm (52d) is a reference arm.

Referring to fig. 4 two light sources (31) and (32) with slightly different wavelengths λ _j and λ ₂ pass light down sensing arms (33 a,b,c) and reference arm (33d) via star coupler (34). Along each of the sensing arms are two band reflecting in-fibre filters (35a,b,c) and (36a,b,c) each reflecting one of the wavelengths λ _j and λ ₂ - The effective sensors are the sections of the sensing fibres between the filters (35a,b,c) and (36a,b,c).There is a broad band mirror (37) at the tip of the reference fibre (33d). Using a dual wavelength encoding scheme relative phases associated with wavelength _j and λ ₂ in any fibre channel (i) can be retrieved as φ ⁷ , and φ ⁷ respectively. The signal related to the measure and can be calculated as φ ⁷ = λ- λ y 2~ φ' _} - Based on the same principle, more wavelengths can be launched into the system allowing multiple filter markers to be arranged along each sensing channel.

Referring to fig. 5 a partially coherent light from source (40) is guided into single- mode lead fibres (42a,b,c) of equal length. Each of the lead fibres is connected to a section of sensing fibre (43a,b,c) which is partially reflective at both ends. Each of the sensing sections is of the same length which is greater than the coherence of the light. The reference arm (42d) is split into two. The shorter one S equals the length of the lead fibres plus that of the sensing section and the longer one L equals the length of S plus three times that of the sensing section. In any sensing fibre (i) there are two mutually incoherent light elements. One denoted as (s) takes the shorter

optical path by going straight through the two reflectors and the other one (1) reflects once off each reflector before going on, (s) and (1) can only interfere with the light in reference arm S and L which have a relative phase φ ⁷ * and φ ⁷ _s respectively. By adapting an encoding scheme similar to the orthogonal encoding, φ _j and φ ⁷ _s can be retrieved simultaneously in the digital spatial domain at (43). Then the phase change in the sensing section in channel (i) of the network can be obtained as φ^ φ' _j - φ',..

Referring to fig. 6 light from laser (61) passes through optical interferometer (64) where it is split and a controlled changing optical path difference (OPD) is introduced between the split be: :s. The beams are recombined in star coupler (62) and launched into sensing chan. .s (63a,b,c,d). There are fibre gratings (65) along the sensing channels. In fibre gratings along each channel have different Bragg wavelength. The light is finally received one-to-one by a detector array and passed to computer (66) for processing.

Referring to fig. 7 light from laser (73) passes through optical interferometer (70) where it is split and a controlled changing optical path difference (OPD) is introduced between the split beams. The beams are recombined in star coupler (71) and launched into sensing channels (72 a,b,c,d). There are fibre gratings (76) along the sensing channels. In fibre gratings along each channel have different Bragg wavelength. The light is reflected at the end of each of the sensing channels and is finally received one-to-one by a detector array and passed to computer (74) for processing.

In each of the above illustrated architectures changes in a parameter which can affect the sensing arms cause a change which will be detected as a pattern change and thus will enable changes in that parameter to be monitored.

The various architectures can be mixed together to adapt the sensor system to different sensing applications.

Referring to Fig. 8 light istransmitted from laser (1) to star coupler (2) and 32 sensing fibres are divided into 'x' and 'y' axes and are numbered from 0 to 15, the sensing fibres can be structured as in any of figs 1 to 7. These fibres can be considered as forming a 16 x 16 grid with 256 "crossing" points and each crossing point can be placed at a particular location, e.g. a room in a building. As the system of the present invention can detect a phase change in any of the optical fibres, a sudden phase change would indicate, for example, a fire. Thus, if there is a sudden phase change in both fibres 2(x) and 5(y), there must be a fire at position (2), (5). If x fibres (10), (11) and (12) and y fibres (7) and (8) all show sudden phase changes, the fire is in the shaded area in Fig. 8.

The system of the present invention can thus provide a simple, robust and flexible monitoring system for a wide range of applications.