Fibre Optic Cable Responsive to Microbending Forces

The present invention relates to a fibre-optic cable responsive to microbending forces, for detecting pressure by mechanically influencing the fibre in the cable by means of a so-called periodic disturbance upon transmission of light through the fibre.

The aforesaid principle of detecting pressure by permitting the pressure to act upon an optic fibre made of glass or a plastics material, so that the transmitted light is periodically disturbed, is described in, for example, U.S. Pat. No. 4,163,397; SE-A-No. 410 521 and EP-A-No. 008 2820. According to EP-A-No. 008 2820 this periodic disturbance is created by winding a filament or wire helically around the optical fibre, whereafter an outer sheathing is placed around the filament and fibre structure. When light is transmitted through the fibre and the sheathing is simultaneously subjected to a pressure force, this pressure force will tend to flatten the wire helix. In this way there is created a series of periodic bends in the fibre, each of which corresponds to half the pitch of the helix. As a result thereof, the light passing through the fibre is attenuated, which can be indicated, for example, with the aid of a photoelectric sensor. Various fibre parameters affect the attenuation of the light. In the case of a fibre which exhibits a parabolic index profile, the periodic disturbance has a critical period length which produces maximum attenuation. This period length can be expressed as ##EQU1##

One problem with the aforesaid known arrangement according to EP-A No. 008 2810 is that when the diameter of the filament is the same as that of the fibre (d.apprxeq.0.3 mm), the filament, because it is wound helically around the fibre, will cause the dimensions of the cable to be unsuitably large. This is undesirable in the case of certain usages, for example when wishing to hide the cable incorporating the prepared fibre from view, or when the cable is to drawn through narrow passages. In addition, it is difficult to obtain an accurately defined disturbance periodicity in those cases when disturbance is determined by the pitch of the filament helix.

The object of the present invention is to provide a cable for determining pressure, in which the periodic disturbance can be obtained with simple elements which afford stability to the periodic disturbance when the cable is subjected to pressure.

The cable according to the invention has the characterizing features set forth in the claims.

The invention will now be described in more detail with reference to the accompanying drawing.

FIG. 1 illustrates a first embodiment of a fibre-optic cable according to the invention;

FIG. 2 illustrates a second embodiment; and

FIG. 3 illustrates a third embodiment of the cable according to the invention.

FIG. 1 is a sectional view of a cable according to the invention for detecting a pressure P applied to the casing H of the cable. The pressure P may be punctiform or distributed uniformly over the casing H. The optical fibre incorporated in the cable comprises a core C, a core-sheathing M and a surrounding casing H, the so-called primary shield, the refractive index of which is chosen to be slightly higher than the refractive index of the core-sheathing.

Located at one end of the fibre cable is a light source (not shown), while at its other end there is arranged a photosensor (not shown), light being transmitted through the fibre F with a given mode distribution. The light source and sensor may also be located at one and the same end of the fibre, and a reflector arranged at the other end thereof. An increase in the pressure P will result in greater attenuation of the modes, due to the fact that part of the power in each mode is coupled to other modes, inter alia radiation modes, which results in reduced transmitted power. This reduction can be indicated in a suitable manner, with the aid of the photosensor.

In order to achieve the aforesaid coupling between the modes, a mechanical disturbance is introduced into the fibre. In the case of a glass fibre, this disturbance can be introduced by permitting the pressure force P to act upon the primary shield H (FIG. 1), while in the case of a plastic fibre it is sufficient for the pressure force P to act directly on the core-sheathing M. The disturbance is intended to create periodic deformation of the fibre. This disturbance is created in the fibre incorporated in the cable according to FIG. 1 with the aid of an elongated deformed filament-element in the form of a double-filament structure comprising two twisted filaments T1 and T2. The filaments are twisted relatively firmly, so as not to be displaced axially in relation to one another to any appreciable extent when the pressure P is applied to the primary shield H. The double-filament structure T1, T2 has small isolated contact surfaces Y.sub.1, Y.sub.2 against the core-sheathing C, which are repeated at a given periodicity d. Accordingly, optimal damping of light through the fibre core is obtained with a fibre of given dimension, namely when d= =2.pi.a/(2D).sup.1/2. In this embodiment, the double-filament structure T1, T2 is placed straight along the fibre and parallel therewith.

In the embodiment of the cable illustrated in FIG. 2, the double-filament structure T1, T2 comprising said twisted filaments is wound helically around the fibre F. As with the FIG. 1 embodiment, there are obtained small contact surfaces Y.sub.1, Y.sub.2 with given periods d along the whole length of the filament structure. In this embodiment the pressure force P may be directed towards the surrounding core-sheathing M from various directions around the fibre, since the given deformation of the filament structure T1, T2 ensures that a pressure force is exerted against the core-sheathing M.

The dimension of the filament structure T1, T2, i.e. the spacing between the casing H and the fibre F should be of the same order as the cross-sectional dimension of the fibre. This enables the filament structure T1, T2 to be wound at a greater pitch around the fibre F, and the critical distance d= will not be contingent on the pitch of the helix.

In the embodiment illustrated in FIG. 3 the longitudinal deforming element comprises a rigid or an elastic twisted band B, which is wound around the fibre. In this way isolated contact surfaces Y.sub.1, Y.sub.2 are formed at the locations at which the band abuts the core-sheathing M, these isolated surfaces producing the periodic disturbance (d= ).

The cable construction according to the present invention distinguishes from the prior art constructions in that the elongated element extended longitudinally around the optical fibre is deformed initially, from the start, so that a periodic disturbance is able to occur upon contact of said element with the fibre. The advantage gained hereby resides in that greater freedeom is obtained in disposing the disturbance-creating element between the cable casing and the fibre, and therewith greater possibility of reducing the cross-sectional dimensions of the whole cable, when it is to be used as a pressure sensing device.