Cable for distributed sensing
This application relates to a fibre optic cable ( 104, 300 ) suitable for use with a distributed fibre optic sensor apparatus ( 106 ). The fibre optic cable includes at least one optical fibre ( 301 ) and a force transformer ( 304 ) mechanically coupled to the at least one optical fibre. The cable may also include at least one cladding later ( 302 ) and/or a compliant material ( 303 ). The cable may be surrounded by a jacket layer ( 306 ). The force transformer ( 304 ) is configured to transform transverse forces due to dimension changes of the cable arising from a temperature variation of the cable into longitudinal forces to counteract the longitudinal component of said dimension change over a tuned temperature range. In this way optical path length changes due to a change of temperature can be reduced or eliminated providing a cable which is insensitive to temperature.
1. A fibre optic cable comprising;
at least one optical fibre, and
a force transformer mechanically coupled to the at least one optical fibre;
wherein the force transformer is configured to transform transverse forces due to dimension changes of the cable arising from a temperature variation of the cable into longitudinal forces to counteract a longitudinal component of said dimension change over a tuned temperature range, where the stiffness of the force transformer is configured with respect to a thermal coefficient of expansion of the fibre optic cable such that a temperature variation within the tuned temperature range results in a change of effective optical path length of the at least one optical fibre of the fibre optic cable of less than or equal to 5%.
2. The fibre optic cable as claimed in claim 1 wherein the fibre optic cable comprises a compliant core material mechanically coupled to the at least one optical fibre such that a longitudinal force acting on the compliant core material induces a longitudinal strain in the at least one optical fibre.
3. The fibre optic cable as claimed in claim 2 wherein the compliant core material has a positive thermal coefficient of expansion.
4. The fibre optic cable as claimed in claim 2 wherein the at least one optical fibre is coupled to a buffer material and the buffer material is coupled to the compliant core material.
5. The fibre optic cable as claimed in claim 2 where the force transformer is coupled to the at least one optical fibre via the compliant core material.
6. The fibre optic cable as claimed in claim 2 wherein the force transformer is anchored to the compliant core material at a plurality of anchor points.
7. The fibre optic cable as claimed in claim 2 wherein the force transformer is at least partly embedded with the compliant core material.
8. The fibre optic cable as claimed in claim 2 wherein the force transformer is stiffer than the compliant core material.
9. The fibre optic cable as claimed in claim 2 wherein the compliant core material comprises extruded nylon.
10. The fibre optic cable as claimed in claim 1 where the stiffness of the force transformer is configured with respect to a thermal coefficient of expansion of the fibre optic cable such that a temperature variation within the tuned temperature range results in substantially no change of effective optical path length of the fibre optic cable.
11. The fibre optic cable as claimed in claim 1 where the force transformer comprises a braid member.
12. The fibre optic cable as claimed in claim 1 where the force transformer comprises at least one helically coiled member.
13. The fibre optic cable as claimed in claim 12 wherein the helix angle of said helically coiled member is of the order of 50 to 70 degrees.
14. The fibre optic cable as claimed in claim 12 wherein at least one portion of the fibre optic cable comprises a plurality of force transformers, each force transformer comprising a helically wound coiled member.
15. The fibre optic cable as claimed in claim 14 wherein at least one helically wound coiled member is wound in the opposite direction to another helically wound coiled member.
16. The fibre optic cable as claimed in claim 14 wherein at least one helically wound coiled member is interleaved with another helically wound coiled member.
17. The fibre optic cable as claimed in claim 1 wherein the force transformer comprises a shaped steel member.
18. The fibre optic cable as claimed in claim 17 wherein the volume percentage of steel of the fibre optic cable is substantially 10%.
19. A fibre optic cable having a longitudinal axis comprising;
at least one optical fibre; and
a force transformer mechanically coupled to the at least one optical fibre, such that a transverse force acting on the force transformer induces longitudinal forces in the at least one optical fibre,
wherein the force transformer is configured such that, in response to dimension changes of the fibre optic cable transverse to the longitudinal axis due to temperature variations, the force transformer induces counteractive forces on the fibre optic cable along the longitudinal axis, such that variation of an effective optical path length of the at least one optical fibre of the fibre optic cable is less than or equal to 5%.