Fibre optic cable responsive to microbending forces

A fibre-optic cable responsive to microbending and forming part of a device for measuring pressure in accordance with the principle of creating a periodic mechanical disturbance in the fibre. The cable comprises a fibre (F) having a core (C) and a core-sheathing (M). An elongated element in the form of a twisted filament (T1, T2) or a twisted band (B) extends along the fibre, and a primary shield (H) encases the fibre and the twisted element such that the twisted element contacts the sheathing at periodically spaced separate regions.

FIELD OF THE INVENTION 
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. 
BACKGROUND 
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## 
SUMMARY OF THE INVENTION 
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.

DETAILED DESCRIPTION OF BEST MODES FOR CARRYING OUT 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.