Optical fibre curvature sensor and measurement device comprising said sensor

Abstract

An optical fiber curvature sensor. Two networks (R1, R2) with periodic longitudinal modulation of the refractive index of the optical fiber core are inscribed in the fiber (F) one behind the other or one on top of the other. The networks are configured to respectively reflect wavelengths λ.sub.1 and λ.sub.2 such that λ.sub.1=λ.sub.B+Δλ.sub.B1 and λ.sub.2=λ.sub.B+Δλ.sub.B2, where λ.sub.B is the Bragg wavelength of the networks and where λ.sub.B1 and λ.sub.B2 are shifts sensitive to the temperature, to deformations and to the curvature of the optical fiber. The two networks are defined so that the quantities Δλ.sub.B1 and Δλ.sub.B2 have substantially identical sensitivity to temperature and to deformations and substantially opposite sensitivity to curvature.

Claims

1. Curvature sensor comprising: at least one optical fiber (F) comprising a core and at least one first sheath surrounding said core, said core and said at least one first sheath having different refractive indexes, said at least one optical fiber further comprising an end for receiving polychromatic light, a first grating for periodic longitudinal modulation of the refractive index of the optical fiber core, called first grating (R1), inscribed in the core of said at least one optical fiber and configured to reflect a wavelength λ.sub.1 of the light, said wavelength λ.sub.1 being shifted by a quantity Δλ.sub.B1 with respect to a reference wavelength λ.sub.B and said quantity Δλ.sub.B1 being sensitive to the temperature, to deformations and to the curvature of the optical fiber, at least one second grating for periodic longitudinal modulation of the refractive index of the optical fiber core, called second grating (R2), inscribed in the core of said at least one optical fiber and configured to reflect a wavelength λ.sub.2 of the light, said wavelength λ.sub.2 being shifted by a quantity Δλ.sub.B2 with respect to said reference wavelength λ.sub.B and said quantity Δλ.sub.B2 being sensitive to the temperature, to deformations and to the curvature of the optical fiber, wherein the average effective indexes, pitches and lengths of first and second gratings are configured such that the quantities Δλ.sub.B1 and Δλ.sub.B2 have substantially identical sensitivities to temperature and to deformations and substantially opposite sensitivities to curvature and such that the difference between λ.sub.1 and λ.sub.2 is substantially independent of the temperature and deformations and depends on a radius of the curvature of the optical fiber.

2. Sensor according to claim 1, further comprising a single optical fiber, the first and second gratings (R1, R2) being Bragg gratings inscribed one behind the other in the core of the optical fiber, said first and second gratings having different average effective indexes (δn.sub.dc1,δn.sub.dc2).

3. Sensor according to claim 1, further comprising a single optical fiber and in that the first and second gratings (R1, R2) are inscribed one on top of the other, the first grating being a Bragg grating and the second grating being a long period grating, said first and second gratings having different average effective indexes (δn.sub.dc1,δn.sub.dc2).

4. Sensor according to claim 3, wherein the optical fiber comprises a second sheath surrounding said first sheath, said second sheath having a refractive index less than the refractive index of the first sheath.

5. Sensor according to claim 1, comprising a single optical fiber and a plurality of Bragg gratings inscribed one behind the other in the core of the optical fiber, said plurality of Bragg gratings being arranged in such a way as to behave as the association of a Bragg grating and a long period grating.

6. Sensor according to claim 1, comprising first and second optical fibers in a resin bar having an axis of symmetry, the first and second gratings being inscribed respectively in said first and second optical fibers.

7. Sensor according to claim 6, wherein the first and second gratings (R1, R2) are arranged at substantially identical positions along said axis of symmetry and said first and second optical fibers are placed at equal distances from said axis of symmetry.

8. Sensor according to claim 6, wherein said first and second gratings (R1, R2) are Bragg gratings.

9. Sensor according to claim 6 or, wherein said first and second gratings (R1, R2) are long period gratings.

10. Device for measuring the curvature of a longitudinal element, comprising: a curvature sensor according to claim 1, said at least one optical fiber of the curvature sensor being arranged along said element, a source of polychromatic light for emitting light through said at least one optical fiber, and a circuit (12) for receiving the wavelengths λ.sub.1=λ.sub.B+Δλ.sub.B1 and λ.sub.2=λ.sub.B+Δλ.sub.B2 and determining the curvature of said element from the difference between λ.sub.1=λ.sub.B+Δλ.sub.B1 and λ.sub.2=Δ.sub.B+Δλ.sub.B2.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 diagrammatically shows a curvature sensor from prior art;

(2) FIG. 2 is a diagram showing the change in the shift of the Bragg length Δλ.sub.B according to the radius of curvature R for three average grating effective index values δn.sub.dc;

(3) FIG. 3 diagrammatically shows a curvature sensor in accordance with an embodiment of the invention;

(4) FIG. 4 shows a cross-section view of an optical fiber of the curvature sensor of an embodiment of the invention;

(5) FIG. 5 shows the refractive index profile of the optical fiber of FIG. 4;

(6) FIG. 6 is a diagram showing the change in the shifts Δλ.sub.B1 and Δλ.sub.B2 of the Bragg lengths of the two gratings of the curvature sensor of FIG. 3 according to the curvature 1/R;

(7) FIG. 7 is a diagram showing the change in the shift Δλ=Δλ.sub.B1−Δλ.sub.B2 according to the curvature 1/R;

(8) FIG. 8 diagrammatically shows a device for measuring the curvature comprising the sensor of FIG. 3.

DETAILED DESCRIPTION

(9) Embodiments of the invention are based on the fact that the variation in the resonant length (or Bragg wavelength λ.sub.B) of an index grating such as a Bragg grating or a long period grating is governed by the average effective index n.sub.eff of the grating.

(10) When such a grating is curved, the resonance wavelength is shifted. The shift is given by:
Δλ.sub.B=2.Math.(Δn.sub.eff+δn.sub.dc.Math.Δκ.sub.eff).Math.Λ  (2)
where Λ is the pitch of the grating, Δn.sub.eff is the variation in the effective index n.sub.eff of the core of the fiber due to the curvature, δn.sub.dc is the average effective index of the grating and Δκ.sub.eff is the variation in the coupling coefficient κ.sub.eff of the grating due to the curvature. These two factors depend only on the optical fiber and change in opposite directions: n.sub.eff increases when the radius of curvature decreases while κ.sub.eff decreases when the radius of curvature decreases. It can be seen in the relationship (2) that the variation in the coupling coefficient Δκ.sub.eff is multiplied by the average effective index δn.sub.dc. Therefore, according to this parameter, the variation in the coupling coefficient Δκ.sub.eff can either be negligible compared to the variation in the effective index Δn.sub.eff, or offset it or be much greater than the latter. It can be deduced from the above that the variation in the resonance wavelength Δλ.sub.B can be either negative, or zero or positive, such as is shown in FIG. 2 for a Bragg grating. By varying the average effective index δn.sub.dc of the grating, it is possible to set the shift in the wavelength Δλ.sub.B to a negative, zero or positive value for a given radius of curvature R.

(11) In the example of FIG. 2, a shift Δλ.sub.B is obtained according to the radius of curvature R which is: positive for δn.sub.dc=5.10.sup.−4; zero for δn.sub.dc=1.62.10.sup.−3; negative for δn.sub.dc=5.10.sup.−3.

(12) The idea of embodiments of the invention is therefore to associate two index gratings having the same sensitivity to deformation and to the temperature hut opposite sensitivities according to the curvature.

(13) According to embodiments of the invention, the sensor proposed therefore comprises two fiber index gratings having the same sensitivity to the temperature and to deformations but opposite responses according to the radius of curvature. A block diagram of this sensor is shown in FIG. 3.

(14) In reference to FIG. 3, the sensor comprises two Bragg gratings R1 and R2 arranged in series on an optical fiber F. The two gratings are photo-inscribed in the core of the optical fiber. As these two gratings are made from the same material, they have the same sensitivity to the temperature. The two gratings R1 and R2 are also designed in such a way as to have the same sensitivity to deformations (torsion, compression, tension or elongation) and opposite responses to the curvature.

(15) As such, subjected to the same conditions of temperature, of deformation and of curvature, the two sensors R1 and R2 react in the following way:

(16) $\begin{array}{cc}\{\begin{array}{c}{\mathrm{\Delta \lambda }}_{B1}={\alpha }_{T}T+{\alpha }_{\mathrm{.Math.}}\mathrm{.Math.}+f\left(R\right)\\ {\mathrm{\Delta \lambda }}_{B2}={\alpha }_{T}T+{\alpha }_{\mathrm{.Math.}}\mathrm{.Math.}-f\left(R\right)\end{array}& \left(3\right)\end{array}$
where Δλ.sub.B1 is the variation in the wavelength of the grating R1, Δλ.sub.B1 is the variation in the wavelength of the grating R2, T is the temperature of the optical fiber, α.sub.T is the sensitivity of the grating to the temperature, ε represents the deformation of the fiber, α.sub.ε is the sensitivity to deformation, +f(R) designates the shift in the wavelength due to the curvature in the grating R1 and −f(R) designates the shift in the wavelength due to the curvature in the grating R2.

(17) When the sensor is subjected to a polychromatic light, the grating R1 reflects a light that has a wavelength λ.sub.1=λ.sub.2+Δλ.sub.B1 and the grating R2 reflects a light that has a wavelength λ.sub.2=λ.sub.B+Δλ.sub.B2.

(18) If the reflected wavelengths λ.sub.1 and λ.sub.2 are subtracted, we obtain a magnitude Δλ that is independent of the temperature and of the deformations and which depends only on the radius of curvature R:
Δλ=λ.sub.1−λ.sub.2=λ.sub.B+Δλ.sub.B1−λ.sub.B−Δ.sub.B2=Δλ.sub.B1−Δλ.sub.B2=f(R).

(19) It is therefore possible to directly obtain the radius of curvature R from the shift in the wavelength Δλ.

(20) The optical fiber F is a single-mode and step-index fiber that has the following characteristics: radius of the core: a.sub.1=4.2 μm; index of the core of the fiber: n.sub.1; outer radius of the sheath: a.sub.2=62.5 μm; index of the core of the fiber: n.sub.2.

(21) The dimensions and the index profile of the optical fiber can be seen in FIGS. 4 and 5.

(22) The index of the sheath n.sub.2 is evaluated from the Sellmeier relationship applied to the silica:

(23) $\begin{array}{cc}{n}^2\left(\lambda \right)=A+\frac{B}{1-\frac{C}{{\lambda }^2}}+\frac{D}{1-\frac{E}{{\lambda }^2}}& \left(4\right)\end{array}$
where A, B, C, D and E are the Sellmeier coefficients that depend on the temperature via the relationship X=aT+b, with T the temperature expressed in degrees centigrade.

(24) The coefficients a and b of Sellmeier A, B, C, D and E of the silica are expressed in the following table:

(25) TABLE-US-00001 Coefficient X = aT + b a b A 6.90754 .Math. 10.sup.−6 1.31552 B 2.35835 .Math. 10.sup.−5 0.788404 C 5.84758 .Math. 10.sup.−7 1.10199 .Math. 10.sup.−2 D 5.48368 .Math. 10.sup.−7 0.91316 E 100 0

(26) The index of the core n.sub.1 is deduced from the index of the sheath n.sub.2 by the relationship: n.sub.1=1.0036 n.sub.2.

(27) The grating R1 has a length L1=8.9 mm, a grating pitch Λ.sub.1=541.1 nm and an average effective index δn.sub.dc1=1.Math.10.sup.−4.Math.n.sub.1. The grating R2 has a length L2=250 μm (micrometers), a grating pitch Λ.sub.2=541.4 nm and an average effective index δn.sub.dc2=3.5.Math.10.sup.−3.Math.n.sub.1.

(28) The wavelengths λ.sub.1 and λ.sub.2 reflected respectively by the gratings R1 and R2 (at rest) are then:

(29) λ.sub.1=1565.2 nm and λ.sub.2=1570 nm.

(30) These resonant wavelengths are sufficiently spaced to prevent any superposition of the resonances or inversion in their position in the curvature range 1/R∈[0; 1] cm.sup.−1.

(31) As can be seen in FIG. 6, the shift in the wavelength Δλ.sub.B1 of the grating R1 decreases with the curvature of the fiber while the shift in the wavelength Δλ.sub.B2 of the grating R2 follows an opposite curve.

(32) The sensitivity to axial deformation (α.sub.ε) of the grating R1 is identical to that of the grating R2 and is evaluated at 1.23 pm/με (where 1 με corresponds to a deformation of 10.sup.−6 m/m). Likewise, the sensitivities to the temperature (α.sub.T) of the two gratings R1 and R2 are substantially identical, of about 12.02 pm/° C..sup.cent.

(33) This results in that the subtraction of the two signals of wavelength λ.sub.1 and λ.sub.2, i.e. Δλ=λ.sub.1−λ.sub.2=Δλ.sub.B1−Δλ.sub.B2, is independent of the temperature T and of the deformations ε and depends solely on the curvature of the optical fiber. The curve of FIG. 7 shows the dependency between shift in the wavelength Δλ and the curvature of the sensor of FIG. 3, The dependency is non-linear. Simply measuring the shift Δλ makes it possible to obtain the radius of curvature from this curve.

(34) In the embodiment shown hereinabove, the gratings R1 and R2 are Bragg gratings inscribed one behind the other in the optical fiber F. As indicated hereinabove, these two gratings differ only by their average effective indexes (δn.sub.dc1 and δn.sub.dc2), their pitches (Λ.sub.1 and Λ.sub.2) and their lengths (L.sub.1 and L.sub.2) in such a way that their dependencies on the curvature are opposite.

(35) According to an alternative embodiment, the gratings R1 and R2 are respectively a Bragg grating and a long period grating inscribed in the core of the optical fiber F one on top of the other. The optical fiber F comprises advantageously two sheaths. The second sheath is used to insulate the light that propagates in the first sheath of the outer medium. Its refractive index is less than that of the first sheath. The two gratings advantageously have the same length. The long period grating is designed in such a way as to have only a resonance in the measured spectral range. Moreover, the resonant mode is chosen so as to have the same sensitivity to deformation as the Bragg grating. The average effective indexes of the two gratings are such that the responses of the two gratings to the curvatures are opposite.

(36) According to another embodiment, the sensor comprises a single optical fiber and a plurality of Bragg gratings inscribed one behind the other in the core of the optical fiber, the plurality of Bragg gratings being arranged in such a way as to behave as the association of a Bragg grating and a long period grating.

(37) More particularly, the sensor comprises a superstructured Bragg grating, commonly referred to as SFBG for Superstructured Fiber Bragg Grating. In order to produce this superstructured grating, a hundred or so very short Bragg gratings in series are inscribed in the core of the fiber. All of the Bragg gratings are identical (same pitch, same length, same index modulation). The gratings are regularly spaced by a distance L.sub.LPG. Their length L.sub.FBG is a fraction of L.sub.LPG. The total length of the structure is about one centimeter, as with a conventional grating. This structure behaves as the association of a Bragg grating with a pitch L.sub.FBG and of a long period grating with a pitch L.sub.LPG.

(38) According to another alternative embodiment, the sensor comprises two optical fibers arranged in a resin bar having an axis of symmetry. The two fibers are advantageously placed at equal distances from the axis of symmetry of the bar. The grating R1 is inscribed in the first fiber and the second grating is inscribed in the second fiber. They are advantageously inscribed at substantially identical positions along the axis of symmetry. In this embodiment, the gratings R1 and R2 can be Bragg gratings or long period gratings. In this latter case, the optical fibers advantageously comprise two sheaths. As with the other embodiments, the average effective indexes of the two gratings are selected so that the responses of the two gratings to the curvatures are opposite.

(39) As explained hereinabove, the wavelengths λ.sub.1 and λ.sub.2 coming from the sensor make it possible to determine the radius of curvature. These two wavelengths must therefore be received and processed in order to obtain the radius of curvature. The invention therefore relates to, more globally, a device for measuring the curvature of a longitudinal element comprising: a curvature sensor according to one of the embodiments described hereinabove, with the optical fiber of the curvature sensor being arranged along the element of which the radius of curvature is to be measured, a source of polychromatic light in order to emit light through the optical fiber of the sensor, and a circuit for receiving the wavelengths λ.sub.1=λ.sub.B+Δλ.sub.B1 and λ.sub.2=λ.sub.B+Δλ.sub.B2 coming from the curvature sensor and determining the curvature of the element from said wavelengths.

(40) Such a device is shown diagrammatically in FIG. 8. It comprises a source of white or polychromatic light 10, a curvature sensor 11 such as defined hereinabove for receiving the light emitted by the source 10 and delivering wavelengths λ.sub.1 and λ.sub.2 corresponding to the wavelengths reflected by the gratings R1 and R2 of the sensor, a circuit 12 for determining the radius of curvature R of the element from the wavelengths λ.sub.1 and λ.sub.2. A coupler 13 is used for the transmission of the polychromatic light from the source 10 to the sensor 11 and the transmission of the wavelengths λ.sub.1 and λ.sub.2 from the sensor 11 to the circuit 12. The circuit is for example an interferometer equipped with means for processing in order to perform the subtraction Δλ=λ.sub.1−λ.sub.2 and to deduce therefrom, for example by means of a look-up table, the radius of curvature R of the element.

(41) Of course, it is possible to arrange several curvature sensors in accordance with embodiments of the invention along the element, with offset resonant wavelengths, in order to measure the curvature at several points of the latter.

(42) The sensor and the device presented here have many advantages: easy to manufacture; easy to implement, reduced size of the sensor; obtaining of the radius of curvature directly from a difference in wavelength; insensitivity to the drops in intensity of the light emitted or reflected.

(43) Moreover, as the measurement proposed is independent of the temperature and of the deformation, the invention can be used in many fields, in particular in applications where the sensor can be subjected to temperature gradients, for example in maritime or medical applications.

(44) Embodiments of the invention are described in the above by way of example. It is understood that those skilled in the art are able to produce various alternative embodiments of the invention, by associating for example the various characteristics hereinabove taken alone or in combination, without however leaving the scope of the invention.