Abstract
A Bragg grating temperature sensor includes an optical fiber including a core, an optical cladding surrounding the core and a Bragg grating incorporated in the core and extending along a sensitive segment of the optical fiber. The core of the temperature sensor includes a core gap extending along a core gap segment of the optical fiber, the core gap segment being located in the vicinity of the sensitive segment. The optical cladding includes a cladding gap extending along a cladding gap segment of the optical fiber, the cladding gap segment including the sensitive segment.
Claims
1. A Bragg grating temperature sensor comprising: an optical fiber including a core, an optical cladding surrounding the core and a Bragg grating incorporated in the core and extending along a segment of the optical fiber called sensitive segment, wherein the core includes a core gap extending along a segment of the optical fiber called core gap segment, the core gap segment being located in the vicinity of the sensitive segment, and wherein the optical cladding includes a cladding gap extending along a segment of the optical fiber called cladding gap segment, the cladding gap segment including the sensitive segment.
2. The sensor according to claim 1, wherein the optical fiber includes a plurality of Bragg gratings incorporated in the core and each extending along a sensitive segment, the core including, for each Bragg grating, a core gap extending along a core gap segment in the vicinity of the corresponding sensitive segment, the optical cladding including, for each Bragg grating, a cladding gap extending along a cladding gap segment including the corresponding sensitive segment.
3. A method for manufacturing a Bragg grating temperature sensor from an optical fiber including a core, an optical cladding surrounding the core and a Bragg grating incorporated in the core and extending along a segment of the optical fiber called sensitive segment, the method comprising a step of ablating the core along a segment of the optical fiber called core gap segment, the core gap segment being located in the vicinity of the sensitive segment, and a step of ablating the optical cladding along a segment of the optical fiber called cladding gap segment, the cladding gap segment including the sensitive segment.
4. The method according to claim 3, wherein the optical fiber includes a plurality of Bragg gratings incorporated in the core and each extending along a sensitive segment, the step of ablating the core comprising, for each Bragg grating, ablating the core along a core gap segment in the vicinity of the corresponding sensitive segment, the step of ablating the optical cladding comprising, for each Bragg grating, ablating the optical cladding along a cladding gap segment including the corresponding sensitive segment.
5. The method according to claim 3, wherein the step of ablating the core comprises an application of a femtosecond laser beam focused in the vicinity of the core, and/or the step of ablating the optical cladding comprises an application of a femtosecond laser beam focused in the vicinity of the optical cladding.
6. The sensor according to claim 1, wherein each cladding gap segment includes, in addition to a sensitive segment, the corresponding core gap segment.
7. The sensor according to claim 1, wherein the core gap segment extends over a length less than or equal to 10 micrometers.
8. The sensor according to claim 1, wherein the optical cladding is microstructured.
9. The sensor according to claim 8, wherein the optical fiber is a suspended-core optical fiber, the optical cladding comprising an inner ring surrounding the core and an outer ring surrounding the inner ring, the inner ring including a plurality of hollow channels extending longitudinally in the optical fiber and forming walls connecting the core to the outer ring.
10. The sensor according to claim 8, wherein the optical fiber is a photonic crystal optical fiber, the optical cladding comprising a plurality of hollow channels extending longitudinally in the optical fiber and being arranged periodically in a transverse plane of the optical fiber.
11. A Bragg grating temperature and deformation sensor comprising: a temperature sensor according to claim 1, wherein the optical fiber further includes at least one Bragg grating incorporated in the core and extending along a segment of the optical fiber called mechanically sensitive segment, the core being devoid of a core gap in the vicinity of the mechanically sensitive segment and the optical cladding being devoid of a cladding gap in the vicinity of the mechanical sensitive segment.
12. A measurement unit including a Bragg grating temperature sensor according to claim 1 and a Bragg grating temperature and deformation sensor, the temperature and deformation sensor comprising a second optical fiber including a core, an optical cladding surrounding the core and at least one Bragg grating incorporated in the core.
13. The manufacturing method according to claim 3, wherein each cladding gap segment includes, in addition to a sensitive segment, the corresponding core gap segment.
14. The manufacturing method according to claim 3, wherein the core gap segment extends over a length less than or equal to 10 micrometers.
15. The manufacturing method according to claim 3, wherein the optical cladding is microstructured.
16. The manufacturing method according to claim 15, wherein the optical fiber is a suspended-core optical fiber, the optical cladding comprising an inner ring surrounding the core and an outer ring surrounding the inner ring, the inner ring including a plurality of hollow channels extending longitudinally in the optical fiber and forming walls connecting the core to the outer ring.
17. The manufacturing method according to claim 15, wherein the optical fiber is a photonic crystal optical fiber, the optical cladding comprising a plurality of hollow channels extending longitudinally in the optical fiber and being arranged periodically in a transverse plane of the optical fiber.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] Further features, details and advantages of the invention will emerge on reading the following description, given merely by way of non-limiting example and with reference to the appended drawings wherein:
[0028] FIG. 1 represents an example of a method for manufacturing a Bragg grating sensor according to the invention from a suspended-core optical fibre;
[0029] FIG. 2A represents, in a perspective view with a partial cross-section, an example of suspended-core optical fibre capable of being used to embody a temperature sensor according to the invention;
[0030] FIG. 2B represents, in an identical view to that in FIG. 2A, the optical fibre following a step of incorporating a Bragg grating;
[0031] FIG. 2C represents, in an identical view to those in FIGS. 2A and 2B, the optical fibre following a step of ablating the core;
[0032] FIG. 2D represents, in an identical view to those in FIGS. 2A to 2C, the optical fibre following a step of ablating the optical cladding;
[0033] FIG. 3 represents, in a graph, the sensitivity to deformation of a standard Bragg grating sensors and that of an example of Bragg grating sensor according to the invention;
[0034] FIG. 4A represents, in a perspective view with a partial cross-section, an example of photonic crystal optical fibre capable of being used to embody a temperature sensor according to the invention;
[0035] FIG. 4B represents, in an identical view to that in FIG. 4A, the temperature sensor obtained by applying the manufacturing method according to the invention.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0036] FIG. 1 represents an example of a method for manufacturing a Bragg grating sensor according to the invention from a suspended-core optical fibre. FIG. 2A represents, in a perspective view with a partial cross-section, an example of a suspended-core optical fibre 20 from which the manufacturing method 10 can be implemented and FIGS. 2B to 2D illustrate the optical fibre 20 following different steps of this manufacturing method. The optical fibre 20 includes a core 21, an optical cladding 22 surrounding the core 21 and a mechanical cladding 23 surrounding the optical cladding 22. The core 21 is formed from a silica glass. It can be doped in order to modify the refractive index thereof and/or the mechanical properties thereof. It has a revolving cylindrical shape about a longitudinal axis of the optical fibre 20. The optical cladding 22 includes three hollow channels 221, an intermediate surface 222 and an outer surface 223. The outer surface 223 comes into contact with the mechanical cladding 23. The intermediate surface 222 is cylindrical of revolution, centred on the longitudinal axis of the optical fibre 20 and located between the core 21 and the outer surface 223. It defines for the optical cladding 22 an inner ring 224, comprised between the core 21 and this intermediate surface 222, and an outer ring 225, comprised between this intermediate surface 222 and the outer surface 223. Each hollow channel 221 extends longitudinally in the optical fibre 20 and extends radially in the entire inner ring 224, i.e. between the core 21 and the intermediate surface 222. Each hollow channel 221 covers an angular cross-section approximately equal to 120′, so as to form walls 226 extending radially between the core 21 and the intermediate surface 222. The core 21 is thus suspended from the outer ring 225 of the optical cladding 22 via the walls 226. The walls 226 and the outer ring 225 form a one-piece part comprising silica glass. As a general rule, prior to applying the steps of the manufacturing method 10 of a Bragg grating sensor, the optical fibre 20 has a continuous nature along the longitudinal axis thereof.
[0037] The manufacturing method 10 comprises a first step 11 of incorporating one or more Bragg gratings 24 in the core 21 of the optical fibre. Each Bragg grating 24 extends along the longitudinal axis of the optical fibre 20 along a segment called sensitive segment. FIG. 2B represents, in an identical view to that in FIG. 2A, the optical fibre 20 following this step 11. The optical fibre is then also called Bragg grating sensor 200. It should be noted that the Bragg grating 24 is represented schematically by disks extending over the entire cross-section of the core 21. Nevertheless, the Bragg grating sensor according to the invention is not limited to a Bragg grating having patterns in disk form, but is applicable to any type of Bragg grating, particularly to a Bragg grating wherein the patterns are formed by spheres. Moreover, the Bragg grating can include any number of patterns.
[0038] The manufacturing method 10 then comprises a second step 12 of ablating the core 21. FIG. 2C represents, in an identical view to that in FIGS. 2A and 2B, the optical fibre 20 following this step 12. Ablating the core 21 consists of removing the core 21 along a segment, called core gap segment, located in the vicinity of the sensitive segment, i.e. in the vicinity of the Bragg grating 24. The core gap segment can be adjoined to the sensitive segment. Nevertheless, in order to prevent damage of the pattern located at the end of the Bragg grating 24, the core gap segment is advantageously offset, for example by a distance corresponding to the wavelength of the Bragg grating 24. The portion removed from the core 21 is called the core gap 25. The core gap segment has for example a length equal to 1 μm. The length must be sufficiently reduced to enable a usable light signal to cross the core gap 25.
[0039] The manufacturing method then comprises a third step 13 of ablating the optical cladding 22. FIG. 2D represents, in an identical view to that in FIGS. 2A to 2C, the optical fibre 20 following this step 13. Ablating the optical cladding 22 consists of removing the walls 226 of the optical cladding 22 along a segment, called cladding gap segment, including at least the sensitive segment. In the present embodiment example, the cladding gap segment covers both the sensitive segments and the core gap segment. The portion removed from the optical cladding 22 is called cladding gap 26.
[0040] The ablation of the core 21 and the optical cladding 22 can be carried out by applying a femtosecond laser beam in the near infrared or in the ultraviolet range. A femtosecond laser beam is a laser beam formed of pulses, the duration whereof is between a few femtoseconds and a few hundred femtoseconds. The laser beam is focused on the zone to be removed. The pulse power is adapted according to the numerical aperture of the beam. During the ablation steps, the optical fibre 20 can be set in rotation and/or in translation. Rotation facilitates the removal of the walls 226 around the core 21.
[0041] It should be noted that the steps of the manufacturing method 10 can be carried out in any order. In particular, the Bragg grating(s) can be incorporated after ablating the core and the optical cladding. Moreover, the ablation of the optical cladding can be carried out prior to that of the core.
[0042] FIG. 3 represents, in a graph, the sensitivity to deformation of a standard Bragg grating sensors and that of a Bragg grating sensor according to the invention. The two sensors are made from the same hollow-core optical fibre by incorporating in the core two Bragg gratings having Bragg wavelengths of 1510 nm and 1550 nm, respectively. The 1550 nm Bragg grating is mechanically isolated from the rest of the optical fibre by ablating the core and ablating the optical cladding, according to the method according to the invention. The 1510 nm Bragg grating is not mechanically isolated from the rest of the optical fibre. The sensitivity to deformation of the two Bragg gratings is determined by measuring a spectral offset of each of the Bragg wavelengths during a deformation of the optical fibre by stretching along the longitudinal axis thereof. The stretching applied comprises a cycle of constant increases and decreases. In the graph, the x-axis indicates different measurement points, corresponding to different stretching amplitudes, and the y-axis indicates the deformation of the optical fibre, in micrometres per metre, this deformation being determined by each Bragg grating based on the offset of the wavelength thereof. A first curve 31 represents the deformation of the optical fibre determined by the Bragg grating not mechanically isolated and a second curve 32 represents the deformation of the optical fibre determined by the mechanically isolated Bragg grating. The first curve indicates a maximum deformation of 2000με and the second curve indicates a maximum deformation of 40με. Thus, the mechanical isolation of the Bragg grating makes it possible to reduce the sensitivity to deformation of this Bragg grating by a factor of 50.
[0043] FIG. 4A represents, in a perspective view with a partial cross-section, an example of a photonic crystal optical fibre from which the manufacturing method can be implemented and FIG. 4B represents, in an identical view, an example of Bragg grating sensor obtained by applying the method to the optical fibre in FIG. 4A. The optical fibre 40 includes a core 41, an optical cladding 42 surrounding the core 41 and a mechanical cladding 43 surrounding the optical cladding 42. The optical cladding 42 comprises a set of hollow channels 421 arranged periodically in a transverse plane of the optical fibre 40. The hollow channels 421 have a revolving cylinder shape and extend parallel with a longitudinal axis of the optical fibre 40. The optical cladding 42 can be fictitiously delimited radially by an inner ring 424 and an outer ring 425. All the hollow channels 421 are disposed in the inner ring 424 of the optical cladding 42, the outer ring 425 being devoid of such microstructures. The core 41 is optically formed at the centre of the grating of hollow channels 421. It should be noted that the optical fibre 40 has a material continuity between the core 41 and the inner ring 424 of the optical cladding 42, in the same way as between the inner ring 424 and the outer ring 425 of the optical cladding 42. The core 41 and the optical cladding 42 are for example formed of silica glass, optionally doped. Like the optical fibre 20 represented in FIG. 2A, the optical fibre 40 has, prior to applying the steps of the manufacturing method according to the invention, a continuous nature along the longitudinal axis thereof.
[0044] In FIG. 4B, the optical fibre 40 is represented after applying the steps of the manufacturing method according to the invention. It is then also called Bragg grating sensor 400. The Bragg grating sensor 400 comprises a Bragg grating 44 incorporated in the core 41 and extending along a sensitive segment, a core gap 45 extending over a core gap segment extending in the vicinity of the sensitive segment and a cladding gap 46 extending along a cladding gap segment encompassing the sensitive segment and the core gap segment. In this embodiment example, the core 41 is presented in the form of a revolving cylinder along the sensitive segment. The core gap segment has for example a length equal to 1 μm. It should be highlighted that, with a view to facilitating the comprehension of the invention, the proportions of the different elements of the Bragg grating sensor are not necessarily observed. In particular, the length of the core gap segment is preferably less than that represented in FIG. 4B.
[0045] The present invention has been described above with reference to Bragg grating sensors comprising a suspended-core optical fibre or a photonic crystal optical fibre. Nevertheless, the invention is found to be applicable to any Bragg grating optical fibre once it is possible to extract from the optical fibre the material removed from the core and the optical cladding. Moreover, the same optical fibre can include both one or more first Bragg gratings mechanically isolated by the presence of a core gap and a cladding gap according to the invention, and one or more second Bragg gratings not mechanically isolated. The first Bragg gratings are then used as elements sensitive only to temperature and the second Bragg gratings are used as elements sensitive to temperature and deformation. The Bragg grating sensor can thus be called a Bragg grating temperature and deformation sensor.
