Optical fibre sensor for measuring deformation, said sensor operating in a harsh environment

Abstract

A fiber-optic sensors for measuring deformation, intended to operate in a harsh environment is provided. The sensor comprises a Fabry-Perot-cavity-based optical measurement head, a linking optical fiber and an expansion reserve case, the case comprising a segment of the linking optical fiber. The inside thickness of the case is comprised between one and several millimeters, the case being flat and of shape referred to as bicorne shape, the shape comprising a convex central portion and two concave symmetric ends, the optical fiber forming, inside the bicorne, one and only one arch, the segment of the optical fiber being, in addition, tangent to the internal surfaces of the reserve case, whatever the temperature conditions.

Claims

1. A fiber-optic sensor for measuring deformation, said sensor being intended to operate in a temperature range of a few hundred degrees, said sensor comprising a Fabry-Perot-cavity-based optical measurement head, a linking optical fiber and an expansion reserve case, said case comprising a segment of said linking optical fiber, wherein the inside thickness of the case is similar to the diameter of the linking optical fiber, said case being flat and of shape referred to as bicorne shape, said shape comprising a convex central portion and two concave symmetric ends, the optical fiber being free to form, inside the bicorne, one and only one arch, the segment of the optical fiber being, in addition, tangent to the internal surfaces of the reserve case, whatever the temperature conditions.

2. The fiber-optic sensor as claimed in claim 1, wherein the optical fiber is made of silica, in that it comprises a sleeve made of aluminum and in that the case is made of stainless steel.

3. The fiber-optic sensor as claimed in claim 1, wherein the height of the expansion reserve case is one quarter of its width.

4. The fiber-optic sensor as claimed in claim 3, wherein the height of the expansion reserve case is 20 millimeters.

5. The fiber-optic sensor as claimed in claim 1, wherein at the link end that is furthest from the optical measurement head, the optical fiber is held in proximity to the exit of the case by a high-temperature ceramic adhesive, this high temperature being about a few hundred degrees.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood and other features will become apparent on reading the following non-limiting description, which is given with reference to the appended figures, in which:

(2) FIG. 1 shows a general overview of a Fabry-Perot-cavity-based movement sensor;

(3) FIG. 2 shows an enlarged view of the Fabry-Perot cavity and of the operation thereof;

(4) FIG. 3 shows the spectrum of the emission source before and after passage through the Fabry-Perot cavity;

(5) FIG. 4 shows a cross-sectional view of the optical link;

(6) FIG. 5 shows an embodiment of an expansion reserve according to the prior art;

(7) FIG. 6 shows an embodiment of an expansion reserve according to the invention;

(8) FIG. 7 shows the operation of the expansion reserve according to the invention.

DETAILED DESCRIPTION

(9) The fiber-optic sensor for measuring elongation according to the invention comprises a Fabry-Perot-cavity-based optical measurement head such as described above, a linking optical fiber and a specific fiber-optic expansion reserve. Typically, this sensor is arranged so as to measure elongations of about 150 microns over a gauge length of 15 mm with a precision of one micron.

(10) This optical sensor is most particularly intended to operate in a harsh environment. This environment is characterized by high temperatures, of about several hundred degrees, and high levels of radiation, both with respect to the gamma radiation and neutron doses to be withstood.

(11) FIG. 6 shows a cross-sectional view of this fiber-optic expansion reserve 20. This expansion reserve takes the form of a hollow and flat case of small thickness comprising a segment of the linking optical fiber. More precisely, the internal thickness of the case is similar to the diameter of the linking optical fiber. This optical fiber is not shown in FIG. 6.

(12) This case has a shape referred to as “bicorne” shape, the top portion of this shape comprising a convex central portion 21 and two concave symmetric ends 22, the optical fiber being held at one 24 of the two ends of the case and passing through the second end 25 of the case, the optical fiber forming, inside the bicorne, one and only one arch, the height of which varies with the expansion or compaction of the optical fiber. The bottom portion 23 of the case is slightly concave.

(13) The geometry of this case may be characterized by a certain number of dimensions, namely its length L.sub.U excluding end fittings for assembly purposes, its maximum length L.sub.MAX, its height H at the inflection point separating the concave portions from the convex portion, and the length of the convex portion L.sub.C. By way of non-limiting example, a case according to the invention may have the following geometric features (values rounded to one tenth of a millimeter):

(14) Length L.sub.MAX: 66.0 mm

(15) Length L.sub.U: 60.0 mm

(16) Height H: 9.2 mm

(17) Length L.sub.C: 38.3 mm

(18) Inside thickness: 1.2 mm

(19) Total thickness: 2.5 mm

(20) The material of the case must be chosen to resist harsh sensor environments, such as described above. It may be made of stainless steel, the optical fiber being made of silica and comprising an aluminum coating.

(21) This casing may be made by 3D printing using the technique of selective laser melting (SLM).

(22) At the link end that is furthest from the optical measurement head, the optical fiber is held at 30 mm from the exit of the case by a ceramic adhesive that is resistant to high temperatures of about a few hundred degrees.

(23) The expansion reserve case is located, at a distance of at least 500 millimeters from the optical head so that its own heat does not corrupt the elongation measurement carried out by the optical head, in a position such that, beyond the point of fiber/sheath bonding away from the optical head, compaction and thermal expansion are negligible, and such that the bulk of this case is acceptable at this position. The operation of this case in shown in FIG. 7. This figure shows a cross-section view of the case 20 and the optical fiber 30 at two temperatures. The dashed line is representative of the position of the optical fiber at low temperature and the solid curve is representative of the position of the optical fiber at high temperature. These two curves are of arch shape. When the temperature increases, the optical fiber experiences two effects as has been described, an expansion that is smaller than the sheath of the link and a compaction under the effect of radiation. In the end, its length in the case varies and this variation in length is absorbed by the height of the arch. A case such as defined above is able to absorb a maximum differential variation between the expansion and compaction of 12 millimeters.

(24) The advantage of this arch shape is that the optical fiber moves inside the case without much rubbing so as to prevent any risk of sticking of the optical fiber liable to create excessive mechanical stresses in the optical fiber. In addition, the optical fiber remains tangent to the surfaces with which it makes contact. The sensor may thus undergo a high number of thermal cycles without deterioration, thus significantly increasing its lifetime and its reliability.