GB2238613A - Movement sensing process - Google Patents

Abstract

A process for sensing the level of movement comprises passing 7 light through an optical fibre 6, allowing or causing the movement to be sensed to produce microbends in the fibre, and sensing 12 the light level after the microbends have been produced. The fibre may be sandwiched between fixed and movable intermeshing sets of rounded projections 9, 10. The source 7 and detector 12 may instead be at the same end of the fibre, the other end of the fibre being associated with a reflector. One set of the projections 9, 10 may be fixed to a diaphragm adapted to sense gas pressure. <IMAGE>

Description

MOVEMENT TRANSDUCER The invention relates to a transducer and more particularly to a transducer and a method of use or process for sensing small movements. Such a transducer, when calibrated appropriately, may be used for example to measure movements and also pressure, acceleration and direction.

Transducers which convert an external movement into an electrical signal are well known, for example a microphone. Such transducers require the presence of electrical power at the sensing head which can be hazardous in certain environments, for example explosive atmospheres.

It is accordingly one object of the invention to provide a method and means for sensing movement in situations where the presence of electrical power in the vicinity of the movement to be measured is undesirable.

According to a first aspect of the invention there is provided a process for sensing movement comprising passing light through an optical fibre, allowing or causing the movement to be sensed to produce microbends in the fibre and sensing the light level after the microbends have been produced.

In use, the level of movement may be determined from the level of light.

An optical fibre comprises an innermost core having a first refractive index and an outermostcladding having a second refractive index which is less than the first. An optical fibre makes use of the phenomenon of multiple total internal reflections to guide a light source input at one end of the fibre to the other end. A ray of light from the core incident on the boundary between the core and the cladding will either be refracted, that is passed into the cladding, or will be reflected back into the core. Whether the ray is refracted or reflected depends on the angle the ray makes with a normal to the boundary.

If that angle is greater than the Critical Angle the ray will be reflected, if that angle is less, the ray will be refracted. The Critical Angle is, according to Snell's law, dependent on the ratio of the refractive indices of the two media. If the angle is sufficiently large the ray will reflect off -the boundary at the same angle and continue across the core until it is similarly reflected from the opposing boundary of the fibre. The light that propagates in this manner is termed a "core mode". This will continue until the ray emerges at the other end of the fibre.

If a bend of sufficiently small radius is formed in the fibre, say of less than 1 mm, then a ray incident upon that bend may strike the boundary at an angle which is less than the Critical Angle. Such bends are termed "microbends". Accordingly most of the energy in the ray will be refracted and only a small proportion will be reflected. The small proportion of the reflected light will lose most of its small remaining energy upon striking the opposing boundary, and will therefore not travel over long distances. Light that propagates in the core in this manner is termed a "cladding mode". A core mode that strikes a microbend can, therefore, be turned into a cladding mode. In order to make use of this phenomenon, the level of the light transmitted by the fibre is calibrated against the movement which causes microbends in the fibre.The microbends cause part of the core modes to be converted into short lived cladding modes. If the level, say the energy of the light is measured and compared to the value with no microbends, a drop in energy will be noticed, which is dependent on the size of the movement to be measured. The applicants have discovered that by appropriate calibration, movements of up to about 30 or 50 x 10-6m can be accurately measured.

The microbends are preferably formed by sandwiching the fibre between two opposed gratings or sets of rounded intermeshing projections of a radius selected so that when the two sets of projections move together under the influence of the movement to be measured, microbends are produced in the fibre. One of the sets of projections is preferably arranged to be fixed, while the other is arranged to be moved by the movement to be measured.

In one embodiment the method comprises passing light, e.g. from a suitable coherent source such as a laser, into one end of an optic fibre and sensing the power of the light emitted at the other end of the fibre by a suitable receiver. Means, such as the pair of intermeshing gratings, are present to induce microbends in the fibre.

In another embodiment a fibre is used having a reflecting means at one end. This may be in the form of a silvered coating on one end face of the fibre, or the end face may be abutted against a mirror. A light source and a receiver are arranged so that light is caused to travel along the fibre, reflect from the reflecting means and return along the same length of fibre from where the power within the beam is sensed by the receiver. Means are present to induce microbends in the fibre.

Preferably the light source and the receiver are connected to two ports of a means arranged to allow light to travel both from a further port into the two ports, and from the two ports into the further port, the optic fibre including the reflecting means being connected to the further port.

In this way apparatus for use in the method may be provided as a first unit containing the light source and the receiver and the fibre may be connected with a single connection only to the further port which ensures that the apparatus may be compact and easy to use.

The invention is safe to use in hazardous environments because as there is no electrical power of the sensing head, only light, there is little risk of e.g. an explosion.

The invention includes apparatus for use in the method.

In order that the invention may be better understood embodiments thereof will now be described by way of example with reference to the accompanying diagrammatic drawings, in which: Figure l(a) is a schematic longitudinal sectional view of one type of optic fibre; Figure l(b) is a schematic longitudinal sectional view of another type of optic fibre; Figure 2 is a view of the fibre of Figure l(a) in a microbending condition; Figure 3 is a schematic block diagram of one embodiment of the invention; Figure 4 is a schematic block diagram of another embodiment of the invention; and Figure 5 is a schematic block diagram from above of yet another embodiment of the invention.

There are basically two types of optic fibre commonly available; multimode fibres and low or mono mode fibres which are shown schematically in Figures la and Ib respectively. Each of the two types of fibre comprises an innermost core 1 and an outermost cladding 2. The difference between the two types is that the core 1 of the multimode fibre is of much greater diameter than the core of the monomode fibre, say a diameter of 50 x 10-6m compared to 3 x 10-6m for the monomode type. This means that the multimode fibre will accept a much greater number of rays which impinge upon the entrance to the fibre from a wide range of angles, as shown in figure l(a). In this way, the multimode fibre is capable of propagating many thousands of different modes 3, which each differ insofar as the longitudinal spacing between the points of reflection differ. A monomode fibre has a relatively much smaller diameter core so that it is capable of accepting only one or two rays.

The invention may use the phenomenon of microbending applied to both multimode and monomode fibres. If a fibre is bent about a sufficiently small radius, say less than 1 mm, then as shown in Figure 2 as a ray 3 impinges on the boundary 4 between the core and the cladding, the angle that the ray makes to a normal at that boundary is less than the Critical Angle and therefore a large proportion of the energy will be refracted into the cladding (the dotted line 5). A certain proportion will be reflected but will again lose most of its remaining energy upon striking the opposing boundary. In this way microbends are capable of converting core modes into cladding modes and dissipating the energy within the beam.

Figure 3 is a schematic view of one embodiment of the invention and comprises an optic fibre 6 which may be of the multimode or monomode type described above. A light source comprising a laser 7 is operative to shine a beam of light into one end of the fibre 6. Downstream of the laser 7, the optic fibre is bent in a relatively large diameter circle several times to define a mode stripper 8. As discussed previously, the cladding modes cannot propagate over long distances and will therefore not propagate past the long length of fibre 6 present in the mode stripper.

Downstream of the mode stripper 8, the fibre 6 passes between two opposed jaw-like gratings 9, 10. Each grating 9, 10 includes a plurality of intermeshing teeth 11 which in the example are generally sinusoidal. The spacing apart and depth of the teeth 11 are selected so that intermeshing of the teeth produces a plurality of microbends in the fibre when the two gratings are brought together. The depth of the teeth 11 is preferably less than 1 mm, and the teeth are preferably spaced apart by a distance of between 1 and 5 mm. One jaw 9 is arranged to be fixed, while the other jaw 10 is arranged to be moveable and is arranged to be linked to the movement to be sensed. Downstream of the grating 9, 10 the fibre defines a second mode stripper 8.

Downstream of the second mode stripper 8, a receiver 12 of known type is present to sense the level of light, e.g. the energy or power radiated from the free end of the fibre.

In use, light is shone into the fibre 6 from the source 7. The first mode stripper 8 removes any cladding modes present. In the absence of any microbends in the fibre, the receiver 12 senses the level, e.g. the amount of energy or power emitted from the free end of the optic fibre 6. The movement to be measured is linked to one of the gratings 10, so as to cause concomitant movement of grating 10 towards the opposing grating 9 to thereby cause microbends in the fibre. This converts some of the modes in the fibre (which are all core modes) into cladding modes.

Substantially all of the cladding modes are removed by the second mode stripper 8, and the receiver 12 senses only those remaining core modes which have passed through the grating 9, 10 unimpeded.

The reduction in energy received before and after microbending is dependent on the size of the movement, which, by appropriate calibration, can be used to measure movements of up to, say 30 or 50 x 10-6m. The apparatus described above may be used with both multimode and low or monomode fibres.

In a modification, apparatus generally as shown in Figure 3 may be used with a low or monomode fibre. Such a fibre will support two or three propagating modes of light. Each mode will propagate with a slightly different and known wavelength, (or longitudinal spacing between the points of reflection of the rays) and if the teeth of the gratings 9, 10 are suitably spaced apart then one optical mode can be preferentially converted into another. If the wavelength of the two modes is known, the grating periodicity is given by 1 - 1 = n /1 /2 where / 1 and 1 2 are the wavelengths of the two modes and n is an odd integer. With commercially available low mode fibre of diameter 1550 x 10-9m propagating light at 780 x 10-9m, the grating periodicity may be of the order of 0.5 m, when n = 1.

The first mode stripper 8 may be designed with a radius which is selected to remove only the longer wavelength mode, as the critical stripping radius for the longer mode is greater than that for the shorter mode; in this way the first mode stripper 8 can allow only the short mode through. The grating is selected according to the above formula so that a proportion of the short mode is converted to a longer cladding mode when one grating 10 is displaced towards its partner 9. The second mode stripper 8 has a radius similar to that of the first so that the newly created longer cladding mode is removed. As before, the level sensed by the receiver 12 may be calibrated to the movement to be measured.

Figure 4 shows another embodiment of the invention, which has an advantage over the Figure 3 embodiment in that the same fibre 6 is used for both paths to and from the grating. A laser light source 7 shines light into one end of a length of optic fibre which is connected to one port 13 of a four port optic fibre coupler 14 A four port optic fibre coupler is a coupler in which the light entering one of the ports 13, 15 is split and leaves via the two opposing ports 16 and 17. As shown, port 17 is disconnected. A receiver 12 is connected to port 15.

Downstream of port 16 a mode stripper 8 is present to remove any cladding modes present. Downstream of the mode stripper 8, the fibre 6 passes between two intermeshing gratings 9, 10. At the remote free end of the fibre a mirror 20 is present to reflect light exiting the fibre core. The mirror 20 may comprise a layer of silver metal deposited on the fibre end face, or may be a reflector abutted with that end face.

In use, light passes into port 13 and exits via port 16 of the coupler 14. Unwanted cladding modes are removed by passage through the mode stripper 8. The light then passes through the grating 9, 10 and then according to the size of the movement, a proportion of the core modes are converted into cladding modes.

The light is then reflected from the mirror 20 and returns through the mode stripper which removes the newly formed cladding modes, and then enters port 16. The light is split within the coupler 14 and one half emerges on port 15 where it is sensed by the receiver 12.

Figure 5 shows schematically an example of the invention for use as a pressure transducer which was arranged generally as described with reference to Figure 4 using the four port coupler 14 connected at 16 to a single fibre 6 having a mirror 20 at one end. A circular metal diaphragm 21 is located adjacent the fibre 6 located so that the front face 22 is in contact with the air pressure to be measured, e.g. air or gas pressure. The fibre 6 defines a mode stripper 8 downstream of port 16, after which the fibre turns at right angles about a large diameter, e.g. 10 mm, post 23 to pass across the rear face 24 of the diaphragm 21. A grating is defined by two lengths of wire 25 arranged side-byside of 0.5 mm diameter and 2 mm spacing located on the rear face 24 of the diaphragm 21, and an opposing grating comprising a set of three similar wires 26 arranged to intermesh with the first set 25. The grating 26 is located on a support member 27 which is fixed to a micrometer (not shown), whereby the grating 26 may be moved towards or away from the opposing grating 25 and then locked in position. After passing between the gratings 25, 26, the fibre turns a right angle about a large diameter post 28, and terminates with a reflective mirrored surface 20. The fibre was of a multimode (50/125 x 10-6m) type.

In use, light was passed from port 16 and through the mode stripper 8. The grating separation was reduced by appropriate use of the micrometer, in the absence of the pressure to be measured, until a predetermined small amount of microbending was induced in the fibre. The support member 27 was then clamped.

The pressure to be measured was then applied to the front face 22 of the diaphragm. This caused the diaphragm 21 and therefore grating 25, to move towards the opposing grating 26 thereby increasing the amount of microbending applied to the fibre. As described previously, the light, which after microbending is a mixture of core and cladding modes, was then reflected from the mirror 20. The mode stripper 8 removed any remaining cladding modes, so that the receiver 12 was able to measure the power of the core modes in the reflected light. In the example, the diaphragm was arranged to move a distance of 40 x 10-6m when a pressure of 7 x 106Pa (1000 p.s.i.) was applied. With such a pressure, it was found that modulation of the light, or power reduction, of up to 75%, was possible.

Claims (9)

1. A process for sensing movement comprising passing light through an optical fibre, allowing or causing the movement to be sensed to produce microbends in the fibre and sensing the light level after the microbends have been produced.

2. A process according to Claim 1, wherein the microbends are formed by sandwiching the fibre between two opposed intermeshing sets of rounded projections.

3. A process according to Claim 2, wherein one set of projections is arranged to be fixed, while the other set of projections is arranged to be moved by the movement to be sensed, the radius of the intermeshing projections being selected so that when the gratings move together under the influence of the movement to be sensed, microbends are produced in the fibre.

4. A process according to any preceding Claim, wherein the light is produced by a coherent source such as a laser.

5. A process according to any preceding Claim, wherein a reflecting means is present at one end of a length of optical fibre and the process comprises passing light into the fibre from the other end of the optical fibre, allowing or causing the light to be reflected from the reflecting means and to return along the same length of fibre, and sensing the level in the returning light.

6. A process according to Claim 5, wherein the light source and a receiver thereof are each connected to one of two ports of a means arranged to allow light to travel both from a further port into the two ports, and from the two ports into the further port, the optical fibre having the reflecting means at one end being connected to the further port.

7. A process according to any of Claims 2 to 6, wherein one of the sets of projections is fixed to a diaphragm adapted to sense gas pressure.

8. A process for sensing movement substantially as described with reference to any one of the accompanying drawings.

9. Apparatus for use in a process according to any preceding Claim substantially as described with reference to any one of the accompanying drawings.