Patent
1986-03-05
1989-01-03
Lee, John D.
350 9629, G02B 644
Patent
active
047952326
DESCRIPTION:
BRIEF SUMMARY
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.
BRIEF DESCRIPTION OF DRAWINGS
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.
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
REFERENCES:
patent: 3750058 (1973-07-01), Bankert, Jr. et al.
patent: 4226504 (1980-10-01), Bellino
patent: 4488040 (1984-12-01), Rowe
Heartney Phan
Lee John D.
Telefonaktiebolaget LM Ericsson
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