Specifically, the invention relates to a fibre optics sensor for detecting and measuring physical stresses and/or strains in a flat and/or tubular and/or irregularly shaped surface, being of a type which comprises at least one sensing optical fibre and a back-up member for said at least one fibre.

The invention further relates to an optical apparatus for detecting and measuring mechanical stresses and/or strains in flat or tubular surfaces, being of a type which comprises at least one fibre optics sensor according to the invention, and a plurality of transmitters and receivers connected according to the following: each sensor fibre is supplied a constant optical power from an transmitter; a receiver is connected to each fibre, on the opposite side from the transmitter side thereof.

The invention relates, particularly but not exclusively, to a fibre optics sensor for detecting and measuring mechanical stresses and/or strains in flat or tubular surfaces, and the following description is given with reference to this field of application for convenience of explanation only.

Prior Art As is well known, the use of optical fibres as sensors of mechanical stresses and strains in civil constructions is a long standing subject of investigation.

Sensors employing optical fibres have been developed which have specialized structures according to application, and can detect and locate mechanical stresses, or stresses of another nature (e. g., temperature).

Existing fibre optics sensors (FOS) are based on two types of technologies, namely: Fabry-Perot technology (FOS-FP); Fibre Bragg Grating technology (FOS-FBG).

Both technologies employ very short optical fibre sections having peculiar geometric configurations. These technologies differ from each other by the structure of the fibre used.

Known sensors of this type are essentially local sensors arranged to detect physical stresses or mechanical loads at given spots on a structure corresponding to the positions where the sensors have been placed.

A major drawback with such sensors resides in the limited spatial spread of the stresses that they are able to detect.

The underlying technical problem of this invention is to provide a fibre optics sensor adapted to detect mechanical stresses, even when these are spread over a large area, the sensor having such structural and functional features as to overcome the limitations of similar sensors provided in the prior art.

Summary of the Invention

The concept behind this invention provides for the use, as the sensors, of a plurality of optical fibres having a determined geometric structure, and for the fibre optics sensor to include a plastic back-up member, in particular a tape, wherein the fibres are embedded.

Based on this concept, the technical problem is solved by a fibre optics sensor as previously indicated and further defined in the characterizing portion of Claim 1.

The problem is also solved by an optical apparatus for detecting and measuring mechanical stresses and/or strains in flat or tubular surfaces, as previously indicated and further defined in the characterizing portion of Claim 17.

Furthermore, the problem is solved by a method of locating a strained spot on a surface or structure subjected to mechanical or thermal stresses, as previously indicated and further defined in the characterizing portion of Claim 26.

The features and advantages of a fibre optics sensor and an optical apparatus according to the invention will be apparent from the following description of embodiments thereof, given by way of non-limitative examples with reference to the accompanying drawings.

Brief Description of the Drawings In the drawings: Figure 1 shows schematically a possible embodiment of a fibre optics sensor according to the invention ; Figure 2 shows an embodiment of the sensor of Figure 1; Figure 3 shows a second embodiment of the fibre optics sensor according to the invention; Figure 4 shows a further embodiment of the sensor of Figure 1;

Figure 5 schematically illustrates the phenomenon of the light power of an optical fibre becoming attenuated through bends in the optical fibre; Figure 6 is an exemplary plot of the qualitative evolution of the light power attenuation through an optical fibre versus its radius of curvature; Figures 7A and 7B are respective schematic views of an optical fibre, under normal conditions and in the presence of mechanical strain; Figures 8A and 8B are respective schematic views of an optical fibre, under normal conditions and in the presence of mechanical strain due to its component parts having different coefficients of thermal expansion; Figure 9 is a mathematical schematic of the sensor of this invention as formed of two bundles of optical fibres; Figure 10 is a three-dimensional view of a surface to which a sensor according to the invention can be mounted; Figure 11 shows schematically an optical apparatus for measuring strain which incorporates a sensor according to the invention; Figure 12 illustrates an application of the strain measuring optical apparatus of Figure 11 to a tubular structure; Figure 13 illustrates an application of the strain measuring optical apparatus of Figure 11 to a tubular structure; Figure 14 illustrates an application to a flat structure with scan-laid optical fibres of the strain measuring optical apparatus shown in Figure 11; Figure 15 illustrates an application to a flat structure, with spiral-laid optical fibres of the strain measuring

optical apparatus shown in Figure 11.

Detailed Description Referring to the drawing views, a fibre optics sensor according to this invention is generally shown schematically at 1.

For simplicity of illustration, reference will be made hereinafter to the instance of a flat surface which may either be subjected to mechanical or thermal strains, or be intruded upon and attacked from the outside.

Advantageously in this invention, the presence of physical stresses or strains can be detected by measuring the attenuation in light power through the optical fibres of the sensor, while the points of application of such stresses/strains can be located in two ways, namely: by an appropriate geometric layout of two sets of fibres over the surface or inside the structure to be monitored; by measuring the reflected power in fibres laid along a single direction.

Sensors can be considered which comprise a plurality of optical fibres in a layout shaped as a grid, solenoid, twisting, spiral or another type of geometry.

In the example of Figure 1, the optical fibres used in the making of this sensor are laid over a bi-dimensional surface and divided basically into two sets, as follows: 1st set: fibres extending along a first direction; 2nd set: fibres extending along a second direction across the first.

Both fibre sets lie close to the surface, so that the fibres will follow any deformations produced in the surface

by mechanical stresses.

In particular, the fibre optics sensor 1 comprises, as <BR> <BR> <BR> shown schematically in Figure 1, NH fibres laid along a first direction (FO), and Nv fibres laid along a second direction (FV) across the first, above a surface 2 which may be of very large area.

One embodiment of a fibre optics sensor 1 according to the invention is shown in Figure 2. It comprises a"tape"3 formed from a very thin, e. g. about 1 mm thick, pliable plastic material having plural optical fibres embedded therein which extend in the longitudinal direction of the tape (corresponding to the FO fibres) and in the transverse direction of the tape (corresponding to the FV fibres).

Advantageously in this invention, the sensor 1 uses optical fibres which are evenly distributed over a large surface.

For example, single mode fibres may be used, such as those normally employed for telecommunications and hence available at reasonable prices.

For fibres lying along the second transverse direction FV within the plastic tape 3, however, a solution must be provided to the problem of measuring the attenuation through each fibre section. This problem can be solved, for example, by having two sets of auxiliary fibres FS laid along the tape edges to pick up the optical power from each section of the fibres FV, as depicted in Figure 3.

Thus, the auxiliary fibres FA can be terminated at one end of the tape 3 for measuring the optical power attenuation therethrough.

It should be further noted that, inside the plastic tape 3, the fibres may be laid in a straight-line pattern, as has been assumed above, or a sinusoidal pattern, as shown in Figure 4 for the fibre section 4, or other more complex patterns.

In this way, the responsiveness of the sensor 1 to straining forces in the surface 2, or mere pressure forces acting on the surface 2, is improved.

It should be reminded that the attenuation of optical power, or optical attenuation, through an optical fibre is tied to external physical actions, such as mechanical stresses, temperature, pressure, etc., to different extents according to the composition of the optical material and the geometric structure of the fibre.

For simplicity of calculation, the dependence of optical attenuation on mechanical straining forces will be considered herein below.

When a fibre F is subjected to mechanical deformation, its optical attenuation increases as a function of the radius R of curvature of the deformation. The phe-nomenon is known by the term"bending", and categorized as micro-bending and macro-bending according to whether the radius of curvature is comparable with the core diameter of the fibre.

Figure 5 illustrates how the attenuation increase originates in consequence of the deformation undergone by a fibre F, and highlights points A, B and C where a light loss is experienced. Losses can be of two types, namely: a loss by reflection, PR, at the transition point of the curvature, and a loss by absorption, PA (bending absorption) in the region where the fibre is subject to deformation.

These combined effects produce an increase in the overall attenuation whose qualitative evolution is shown in Figure 6.

The change in optical attenuation as the fibre deformation varies can be evaluated by means of appropriate algorithms, a most widely used of such algorithms being that known as FD-BPM (Finite Difference Beam Propagation Method) and described, for example, in Yevick, D. and Hermansson, B,

"Spli-Step Finite Difference Analysis of Rib Waveguides", Electronic Letters, No. 7,1989.

Advantageously, according to the invention, it has been thought of applying this same algorithm, but working in the reverse direction to derive the geometric configuration of the fibre F along its axis from the variations in overall attenuation through the fibre, thereby to derive the presence of either spread or concentrated mechanical stresses or strains.

A simplified mathematic model of an optical fibre F has been used for the purpose.

In this model, the optical attenuation, as expressed in dB's, of a fibre is a linear function of the fibre length and can increase, all the other conditions being the same, depending on deformations coming under the"bending" heading.

The variations in attenuation of a fibre extending along a longitudinal axis can be regarded to be a function of the geometrical configuration of the fibre itself. Figure 7A shows a fibre F laid parallel to the abscissa axis, while Figure 7B shows the same fibre F in a deformed configuration represented by the following function: y = F (s)(1) The variation in optical attenuation, with respect to the original configuration, will be a function of F (s), as follows: hAIdB=3[F(s)](2) where AdB is the overall attenuation of the fibre F, considered to have a constant length.

From a formal standpoint, the inverse function may also be considered:

F(s)=3''[AA,](3) <BR> <BR> <BR> <BR> <BR> This inverse function S [+] is not, however, a univocal function.

Advantageously in this invention, a most probable value is picked from the aggregate of the values provided by the <BR> <BR> inverse function Z-1, once a value for the variable AAldB is given The above-described bending phenomenon would occur, for example, in a fibre F which is subjected to thermal variations and has different coefficients of thermal expansion between its core 5 and the core-enclosing space or cladding 6, as shown in Figures 8A and 8B.

In particular, the fibre F shown in these Figures has a cylindrical configuration with a core, having a diamater Dco, which is surrounded by a cladding having a diameter Dcl and an outer protective layer 7.

Shown schematically in Figure 8B is a possible deformation induced in the fibre F as a result of a temperature gradient T1-T2 being applied to the fibre, in particular with T1>T2.

If in normal conditions the fibre F has an optical attenuation of Adb, then this attenuation will increase by AAdb in the presence of a deformation, as defined-for example-by a function F (S).

The increase in optical attenuation due to bending can be calculated with reference to a coefficient of attenuation given as:

where: Am is the coefficient of attenuation due to bending; Dco is the core diameter; NA is a numerical aperture; R is the bending radius of curvature.

The coefficient of attenuation, Aa, can be used to calculate the loss along the longitudinal axis, s, by the following expression: dP/ds = AmP (5) where, P is the optical power within the fibre, at an abscissa s.

The expressions (4) and (5) above yallow the overall attenuation of the fibre F to be obtained by integration for a given bending characterized by the function F (s).

Consider now an area of the plane (x, y) which is confined between: Y1 < y < Y2 (6) Xl < x < X2 (7) and two sets, one horizontal and one vertical, of fibres inside this area, as shown in Figure 9; then there will be: a set of fibres: Fibre Hl,..., Fibre Hn, arranged horizontally; a set of fibres: Fibre Vl,..., Fibre VM, arranged vertically.

Thus, for each fibre, an overall optical attenuation can be determined in accordance with the following formal correspondence:

Fibre Hi: corresponding attenuation AHi; Fibre Vj: corresponding attenuation Avj.

Suppose that the fibres are bonded to stay parallel to the axis X and the axis Y, respectively; in the presence of a stress acting parallel to the z axis, each fibre will undergo bending, assuming a geometric configuration defined by a function FHi (z) or FVj(z), according to whether the fibre is horizontal or vertical.

Possible deformations of the surface in the plane (x, y) are illustrated qualitatively by Figure 10.

The original rectangle, defined by the limits (6) and (7) in the plane (x, y), will be changed by a deformation into a surface: z = S (x, y) (8) and, therefore, there will be variations in attenuations which can be represented in the following manner: for horizontal fibres: hAHi = 3 [Fhi (x)] (9) with i=1,..., HN for vertical fibres: NAVj = #[Fvj(x)] (10) with j=1,..., VM From the attenuation variations, the surface deformations are given by the following relationships: for horizontal fibres: FHI (X) = 3-1 [AAhil with i=l..-/HN for vertical fibres: Fvj(x)=3'[DAvj](12) with j=1,..., VM taking in due account that, as mentioned before, the

function 3-1 [.] is not a univocal function and requires, therefore, a suitable algorithm to determine the actual corresponding function FHi (x) or Fvj (x). This algorithm can be based upon relations existing between the optical attenuation measurements made on the whole of the vertical FV and horizontal FO fibres.

In general, it can be stated that, for a set of measured values [AAHI, AAvj 1 (13) there will be a corresponding set of surfaces S (x, y). Of this set, the most probable value should be determined by mathematic and experimental considerations.

For example, if the surface z=F (x, y) behaves variably over time, but still conforms with a"similarity"law, the surface at a time t+T will be: S (x, y, t+T) = K x S (x, y, t) (14) where K is a proportionality constant. In this case, the actual deformation can be found fairly easily by making attenuation measurements continually over time.

Another instance of the strained surface being immediate to determine is that of a concentrated strain for, in this case, variations in attenuation will only occur at some of the vertical fibres and some of the horizontal fibres.

Figure 11 illustrates schematically an embodiment of an optical apparatus for measuring strains in a flat surface by a sensor according to the invention, and using transmitters and receivers connected in the following manner: each fibre is supplied a constant optical power, from an transmitter TX connected through a transmit system 8 which comprises a splitter and plural distribution fibres;

on the far side from the transmission side, a receiver RX is connected to each fibre through a receive system 9 which comprises essentially a plurality of pick-up fibres; the optical power values measured by the receivers RX are reported to a central optical system (OS) which will calculate the attenuation through each fibre, considering that the transmitted powers are constant.

A second embodiment of the optical apparatus for strain measurements according to the invention, shown in Figure 12, concerns the application of this sensor for controlling stresses and strains occurring in a tubular structure 11, e. g. a gas pipeline. In this case, the horizontal fibres FO of the sensor would be laid along the tubular structure 11, and the vertical fibres FV would be wound ring-like around the tubular structure 11. A raceway 12 placed on top of the tubular structure 11, for example, contains an optical fibre 13 adapted to distribute the optical power to the individual vertical fibres, and a bus 14 for supplying the receivers and picking up measurement data of the power received on each wound fibre (vertical fibre FV).

From a functional standpoint, the arrangement is equivalent to that shown in Figure 11, but for a different physical positioning of the transmitters TX and the receivers RX on the ends of the vertical fibres FV.

Another possible form of this optical apparatus for measuring strain and/or intrusion can be embodied to either monitor a flat surface or a pipeline 15, using a single sensing optical fibre 16, which may include sets of horizontal fibres FO, for example, and measuring the reflected power by means of an OTDR 17.

In this case, the strains can be located from anomalies detected by the OTDR 17.

This embodiment of the optical apparatus for strain measurements is useful especially where the strains do not

vary sharply over time. In fact, the measurements then can be made cyclically by the OTDR 17, and the OTDR can be shared by several sensing optical fibres 16.

To check for the presence of mechanical stresses all along a laid pipeline 15, a sensing optical fibre 16 may be wound spirally around the structure and connected through an optical switch 18 at one end of the OTDR 17, as shown in Figure 13. The spiral-wound optical fibre 16 may comprise a single fibre, or alternatively a set of fibres embedded, for example, in a plastic tape, to provide a substantially solenoid-like configuration.

The reflected power measurement made by the OTDR 17 connected to the spiral-wound sensing optical fibre 16 via the optical switch 18 enables the detection of mechanical stresses, producing a concentrated deformation in the pipeline 15, to be detected even a distance away from the point where the measurement is taken.

The strain-measuring optical apparatus of this invention may further comprise a sensing optical fibre 19 laid scan- like over a flat surface 20 to be monitored, as shown in Figure 14, into a substantially serpentine configuration.

Here again, a measurement of reflected optical power would be made by the OTDR 17.

Finally, a modification of the above application is illustrated by Figure 15, wherein a sensing optical fibre 21 is laid into a concentric coil over the flat surface 20 to be monitored, and connected to one end of the OTDR 17.

Advantageously in this invention, the fibre optics sensor and optical apparatus for measuring strains can be used in many situations where the occurrence of mechanical stresses, abnormal displacements, pressure from intrusions, etc. in a structure however long is to be monitored.