SPUN HIGHLY-BIREFRINGENT FIBER FOR CURRENT SENSING WITH INHERENT INSENSITIVITY TO TEMPERATURE

Abstract

It is proposed to use a spun birefringent fiber for a current sensor or magnetic field sensor. The fiber has a birefringence that increases with temperature. In this case, the temperature dependence of the fiber's sensitivity to magnetic fields counteracts the temperature dependence of the fiber's Verdet constant, which allows to design current and field sensors that have reduced temperature dependence.

Claims

1-17. (canceled)

18. A spun birefringent fiber having a local linear birefringence B ≠O, wherein a relative temperature dependence (1/B).Math.dB/dT of said birefringence is larger than zero for at least one wavelength λ and for at least one temperature T between −60° C. and 120° C., and this positive relative temperature dependence leads to an inherent at least partial compensation of the temperature-dependence of the Verdet constant V of the fiber material.

19. The fiber of claim 18, wherein, at least at said wavelength λ and said temperature T, a spin rate α of said fiber is such that 2.Math.α/η is between 1 and 10, wherein ηis a birefringent phase shift per unit length associated with birefringence B and is given by η=2π.Math.B/λ.

20. The fiber of claim 18, wherein said fiber is a microstructured fiber, which comprises cavities extending along said fiber.

21. The fiber of claim 20, wherein said cavities are air-filled cavities.

22. The fiber of claim 18, wherein said fiber is of a glass having a refractive index n.sub.glass and wherein, at least at said temperature T and wavelength λ, (a) dB/dn.sub.glass<0 and dn.sub.glass/dT<0 or (b) dB/dn.sub.glass>0 and dn.sub.glass/dT>0.

23. The fiber of claim 18, comprising an elliptical core and a cladding, wherein at least at said wavelength and temperature a stress-induced birefringence B.sub.geo: |B.sub.stress|<|B.sub.geo|.

24. The fiber of claim 18, wherein said local linear birefringence B is composed of two distinct birefringence contributions B1 and B2 having orthogonal slow axes at least at said wavelength and temperature with |B1|>|B2| and dB2/dT<dB1/dT.

25. The fiber of claim 18, comprising an elliptical core surrounded by a cladding, wherein said elliptical core has a refractive index larger than said cladding, and wherein at least two stress bodies are located in said cladding along a direction perpendicular to an elongate axis direction of said core, and said stress bodies are formed by regions having a higher dopant level than a rest of said cladding.

26. The fiber of claim 18, wherein the temperature dependence of the birefringence is such that (1/B).Math.dB/dT is between 0.05.Math.10.sup.−4/° C. and 5.10.Math.10.sup.−3/° C.

27. The fiber of claim 18, wherein the temperature dependence of the birefringence is such that (1/B).Math.dB/dT is between 0.1.Math.10.sup.−4/° C. and 2.Math.10.sup.−3/° C.

28. The fiber of claim 18, wherein the said fiber is of a diamagnetic glass.

29. The fiber of claim 28, wherein said diamagnetic glass is fused silica.

30. The fiber of claim 18, wherein said at least one wavelength is in a range between 400 nm and 2000 nm.

31. The fiber of claim 18, wherein said at least one wavelength is in a range between 1000 nm and 1700 nm.

32. The fiber of claim 18, wherein said fiber is, at least at said at least one wavelength λ, a single-mode fiber.

33. The fiber of claim 18, wherein said relative temperature dependence (1/B).Math.dB/dT of said birefringence is larger than zero for all temperatures between −40° C. and 85° C.

34. The fiber of claim 18, wherein at said temperature T and wavelength λ, the fiber has a local linear birefringence of at least 1.3.Math.10.sup.−5.

35. The fiber of claim 18, wherein at said temperature T and wavelength λ, the fiber has a local linear birefringence of at least 2.6.Math.10.sup.−5.

36. The fiber of claim 18, wherein the local linear birefringence B=n.sub.1-n.sub.2 is given by the difference of refractive indices n.sub.1, n.sub.2 for two orthogonal linear polarization states.

37. A current sensor comprising: a spun birefringent fiber of claim 18, a light source generating light at said at least one wavelength λ, and a detection unit measuring a phase shift Δφ between two polarization modes having passed through said fiber.

38. The current sensor of claim 37, wherein said phase shift is given by Δφ˜K.Math.V.Math.I, with K being a spun fiber scale factor, V being the Verdet constant and I being the current to be measured, and wherein |(1/K).Math.dK/dT+(1/V).Math.dV/dT|<|(1/V).Math.dV/dT|.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The invention will be better understood and objects other than those set forth above will become apparent from the following detailed description thereof. Such description refers to the annexed drawings, wherein:

[0026] FIG. 1 shows a schematic diagram of a current sensor.

[0027] FIG. 2 shows the design of a microstructured fused silica fiber with positive temperature dependence of the birefringence for Λ=1 nm and d=0.4 nm (with the black dots representing air holes while the white area are representing fused silica),

[0028] FIG. 3 shows the temperature dependent contributions to the sensor signal stemming from Verdet constant and fiber birefringence and overall temperature dependence of a sensor signal for the example given in FIG. 2,

[0029] FIG. 4 shows a schematic cross section of a fiber with two orthogonal contributions to the total linear birefringence—the slow axis of the birefringence from the bow-tie structure is along the horizontal line x2, the slow axis of the birefringence from the elliptical core along the vertical line x1.

MODES FOR CARRYING OUT THE INVENTION

[0030] The new type of spun highly-birefringent sensing fiber described here can be used in fiber-optic current sensors of magnetic field sensors of different kinds, such as polarimetric sensors [1], [10], Sagnac-interferometer based current sensors [4], and in-line interferometric current sensors with phase-modulation-based or passive interrogation schemes.

[0031] FIG. 1 shows a fiber-optic current sensor e.g. as described in [5] or [13]. It comprises a light source 1 generating light of at least one wavelength λ. Two polarization modes of the light pass a detection unit 2 and then enter a polarization maintaining fiber 3 and a coupler 4 to arrive at a sensing fiber 5. In sensing fiber 5, the polarization modes propagate to a mirror 6, where they are reflected in order to return through sensing fiber 5 and polarization maintaining fiber 3 into detection unit 2. In detection unit 2, the phase shift Φ that the two polarization modes have obtained in their trip through sensing fiber 5 is measured.

[0032] Sensing fiber 5 is wound at least once around a current conductor 7, with a current-dependent magnetic field around it. Sensing fiber 5 consists of a magneto-optic material (typically fused silica) with non-zero Verdet constant V.

[0033] The skilled person is aware of various designs of such fiber-optic current sensors. The sensors can work in reflection, as shown in FIG. 1, i.e. the light is reflected from a mirror at the end of the sensing fiber, or they can work in transmission, i.e. the light passes the fiber only once and is analysed after exiting from the end of the fiber.

[0034] In the present invention, sensing fiber 5 is a birefringent spun fiber. Advantageously, sensing fiber 5 is a highly-birefringent spun fiber, i.e. a fiber having a linear birefringence B=n1-n2 of at least 1.3-10.sup.−5 of even at least 2.6.Math.10.sup.−5.

[0035] In fiber-optic current sensors employing spun highly-birefringent fiber of prior art [1], [10], the current (or, equivalently, the magnetic field to be measured) induces a phase shift Φ between two Eigenmodes of the fiber. This phase shift Φ is typically linearly dependent on the current I to be measured. In general

Φ˜K V .Math.I,  (1)

with the tilde (˜) expressing proportionality and K being a spun fiber scale factor, which is 1 for a fiber free of linear birefringence, but K<1 for a spun birefringent fiber. V is the Verdet constant of the fiber and I the current to be measured running through the conductor 7.

[0036] For a variety of current sensors employing spun birefringent fiber, the spun fiber scale factor K takes the form

K=(2+x.sup.−2).sup.−2,  (2)

with x being defined as x=2α/η, wherein a is the spin rate (i.e. the angular rotation in radians over length) and η is the differential modal phase shift per unit length of the orthogonal polarization modes of an equivalent unspun, i.e. linearly birefringent fiber. The exponent a is positive (a ≧0), e.g. a=2 as in Ref. [1].

[0037] The term K leads to an extra temperature dependence of the sensor signal S (Φ), in addition to the temperature dependence of the Faraday effect (Verdet constant v), which is known to be positive in diamagnetic glasses such as fused silica. The relative temperature dependence of the scale factor K is given by

(1/K).Math.dK/dT=−2a-x.sup.2.Math.(1+x.sup.−2).sup.−1.Math.(1/η).Math.(dn/dT  (3)

[0038] Note that thermal elongation of the fiber equally changes the spin rate and the birefringent phase shift term η and therefore leaves x and K unchanged.

[0039] According to Eq. (3), the spun fiber contribution to the temperature dependence of the current sensor can be tuned by tuning the quantity B. The local linear birefringence in a spun fiber, i.e. the difference of refractive indices for two orthogonal linear polarization states, is given by B=n1-n2. The linear phase shift per unit length amounts to η=2≠.Math.B/λ. The temperature dependence of n is given by (1/n) −dη/dT=(1/B).Math.dB/dT.

[0040] From Eq. (1) it follows that the phase shift per current, Φ/1, is proportional to the product of K-V. The relative temperature-dependence of Φ/1 and therefore the temperature dependence of the sensor signal S (Φ) is given by the relative temperature dependences of the spun fiber scale factor K and the Verdet constant V. In particular, it follows from Eq. (1) that

(1/Φ).Math.d(A<p/I)/dT=[(1/K).Math.dK/dT+(1/V).Math.dV/dT]  (4)

[0041] Hence, in order to reduce the temperature dependence, the relative temperature dependence of the scale factor k, i.e. (1/K).Math.dK/dT, should have opposite sign and similar magnitude as the relative temperature dependence of the Verdet constant V, i.e. (1/V).Math.dV/dT. In that case, the absolute value of the relative temperature dependence of K-V is smaller than the absolute value of the relative temperature dependence of V alone, i.e.

I (1/K).Math.dK/dT+(1/V).Math.dV/dT |<|(1/V).Math.dV/dT|

[0042] Since (1/V).Math.dV/dT is positive, (1/K).Math.dK/dT should be negative. Hence, it follows from Eq. (3) that (1/n) −dη/dT=(1/B) −dB/dT should be positive.

[0043] In some types of spun fiber, the local birefringence is achieved by stress bodies (e.g. panda or bow-tie type stress regions) in the fiber cladding. This stress results from the different temperature expansion coefficients of the undoped fiber cladding regions and the typically ÊĈ-doped stress bodies, i.e. a permanent stress field is generated in the fiber while cooling down from the drawing temperature. With increasing temperature, stress is released. Accordingly, the birefringence of such types of fibers typically decreases with increasing temperature (see e.g. [7] for the case of unspun linear birefringent fiber), i.e. a spun fiber with birefringence induced by stress bodies in the cladding typically adds, according to Eqs. (3) and (4), a positive contribution to the temperature dependence of the signal 3, in addition to the positive contribution of the Verdet constant.

[0044] The temperature dependence of the birefringence of unspun linear birefringent fiber has been studied intensively. Especially, the dominance of the negative temperature dependence of stress-induced birefringence was shown [7], [8].

[0045] In elliptical core fibers, the birefringence results from the elliptical shape of the core, and the temperature dependence is determined by the variation of core and cladding refractive indices with temperature as well as from relaxation of residual stress. Commonly, the latter typically dominates [3]. Correspondingly, spun elliptical core fibers also show a decrease of their linear birefringence with temperature and thus also add a positive contribution to the temperature dependence to the sensor signal [Eqs. (3) and (4)].

[0046] In contrast, it is well known that in pure silica, micro-structured linear birefringent fibers stress-induced birefringence, which typically shows a negative temperature dependence, is strongly reduced and, accordingly, the birefringence is mostly due to the particular fiber geometry. The temperature dependence can then be significantly smaller than in fibers with stress-induced birefringence. In Ref. 11, micro-structured spun highly birefringent sensing fiber was used in order to minimize the temperature dependence, i.e. the term |(1/n) −dn/dT|, compared to spun fiber with stress-induced birefringence.

[0047] Hence, a micro-structured spun highly birefringent fiber can be used for current sensing designed such that the term (1/K) −dK/dT becomes negative (i.e. (1/n) −dn/dT and (1/B) −dB/dT are positive) and balances at least partially the positive temperature dependence of the Verdet constant V. As a result, the detected magneto-optic phase shift (sensor signal) becomes less dependent on temperature.

[0048] A transfer of the detailed understanding of the linear birefringence in unspun microstructured fiber [8] helps to engineer the temperature dependence of the birefringence of spun micro-structured fiber to achieve this goal.

[0049] FIG. 2 shows an embodiment of a holey sensing fiber 5 comprising a glass body 8 with longitudinally extending cavities 9. Further possible fiber designs are given in [9].

[0050] The linear birefringence B of a given spun holey fiber design is a function of the wavelength λ, the refractive indices n.sub.glass of the glass and k.sub.ole of the cavity filling material (typically air), respectively, the cavity pitch A and the cavity diameter d: B (λ, n.sub.glass, k.sub.ole, A, d/A). The temperature dependence of the linear birefringence in the spun fiber is:

dB/dn.sub.glass×dn.sub.glass/dT+dB/dn.sub.hole×dn.sub.hole/dT+dB/dA×(Yglass A),  (4)

where Yglass is the thermal expansion coefficient of the glass. The thermo-optic coefficient of air “air/dT” −10.sup.−6 k.sup.−1 is typically small so that the first term in Eq. (4) dominates the other two [9]. The sign of the first term is determined by the signs of the terms dB/dn.sub.glass and dn.sub.glass/dT. The quantity dB/dn.sub.glass depends on the geometrical design of the fiber. The quantity dn.sub.glass/dT strongly depends of the type of glass used, e.g. dn SiO.sup.2/dT −10.sup.−8 K.sup.−1 for fused silica [9] and the majority of available optical glasses shown dn.sub.glass/dT<0. There is also a range of commercial optical glasses with dn.sub.glass/dT<0 such as N-FK5, N-FK51A, N-PK52A, P-PK53, N-PSK53A, N-LAK12, N-SE6, N-SF10, N-SF14, N-SF57, N-SF66, P-SF8 [12].

[0051] Hence, in the embodiment, the fibers can exhibit a combination of values dB/dn.sub.glass<0 and dn.sub.glass/dT<0, while in another embodiment, the fiber can exhibit a combination of values dB/dn.sub.glass<0 and dn.sub.glass/dT>0. In both cases, it follows from Eq. (4) that dB/dT will be positive.

[0052] The temperature dependence of the linear birefringence in unspun holey fibers was studied in detail showing that, by designing the glass composition and/or the structure of the fiber, a positive, zero, or negative temperature dependence of the birefringence in the unspun fiber can be achieved [9].

[0053] It was shown in Ref. [9] that the birefringence B as function of n.sub.glass attains for a design as in FIG. 2 a local maximum at a given n.sub.glass max. Accordingly, the sign of the first term in Eq. (4) can be positive (for n.sub.glass<n.sub.glass.sup.max and d.sub.glass/dT>0 or for n.sub.glass >n.sub.glassmax and dn.sub.glass/dT<0, or negative (for n.sub.glass>n.sub.glassmax and dn.sub.glass/dT>0 or for n.sub.glass<n.sub.glass.sup.max and dn.sub.glass/dT<0).

[0054] The value for n.sub.glass.sup.max where B attains a maximum depends on the fiber structure, whereas n.sub.glass can be tuned by the glass composition. Accordingly, dB/dT can be designed to be positive by proper choice of the fiber geometry as well as of the glass composition. The fiber structure given in FIG. 2 shows a positive temperature dependence of the birefringence over a wide wavelength range 0.5 nm-1.5 nm when using fused silica; for example, at a wavelength k=1.3 nm, this structure shows a relative temperature dependence of roughly (1/n) −dn/dT ≠+0.4×10.sup.−4 ° C..sup.−1 and η≠4.8 mm.sup.−1 [9]. Setting the spin rate of the spun fiber to a=0.75 η results for a=2 in a temperature dependence of the spun fiber current sensor signal [see Eq. (3)] of

[0055] (1/K) −dK/dT≈−1.2.Math.(1/n) .Math.dn/dT ≈−0.5×10.sup.−4 ° C..sup.−1, i.e. the temperature dependence of the Verdet constant V (+0.7×10.sup.−4 ° C..sup.−1) is compensated to within ±0.1% over 100° C. as illustrated in FIG. 3.

[0056] Advantageously, the temperature dependence of the birefringence is such that (1/B) −dB/dT is between 0.1-10.sup.−4/° C. and 2-10.sup.−3/° C. or between 0.05-10.sup.−4/° C. and 5-10.sup.−3/° C., in which case the typical temperature dependence of the Verdet constant is compensated most effectively.

[0057] Further embodiments for a spun highly-birefringent fiber with the desired property of (1/B) −dB/dT >0 comprise—as described in the following—(i) spun fibers with two orthogonal contributions to the total local linear birefringence and (ii) spun elliptical core fibers with negligible birefringence contribution from mechanical stress.

[0058] (i) The birefringence B of this embodiment of a fiber is generated by a superposition of two orthogonal birefringence contributions B1 and B2. In the designated temperature range, the contribution B1 exceeds the contribution B2 so that fast and slow axes of the overall birefringence B are aligned with the birefringence contribution B1 and amounts to B=B1-B2. The temperature dependence of the smaller birefringence contribution dB2/dT is negative, e.g. because it originates from stress induced birefringence, and is chosen such that it is smaller than dB1/dT, i.e. dB2/dT <dB1/dT, in the designated temperature range. The temperature dependence of the overall birefringence accordingly calculated to: dB/dT=dB1/dT-dB2/dT >0. The quantities B1, B2, dB1/dT, dB2/dT can properly be chosen to design B and dB/dT in the designated temperature range.

[0059] FIG. 4 shows an exemplary cross section of such a fiber. It comprises a core 10 enclosed by a cladding 12, with core 10 having higher refractive index than cladding 12. Typically, core 10 is formed by a region that has a higher dopant level than cladding 12.

[0060] Core 10 has an elongate axis direction XI, which designates the direction of the largest radial extension of core 10. In a direction X2 perpendicular to elongate axis direction XI, two stress bodies 11 formed e.g. by a bow-tie structure are embedded in cladding 12. The stress bodies can be formed by regions having higher dopant level than the rest of cladding 12. No such stress bodies are present along elongate axis direction XI. In this context, the term “stress body” designates a region that, at the at least one temperature T, gives rise to mechanical stress within cladding 12.

[0061] In this embodiment, birefringences B1 is given by elliptical core 10 and B2 originates from the stress bodies, i.e. from the bow-tie structure 11. Here, both terms dB1/dT and dB2/dT are typically negative, but the absolute value of dB2/dT can be, as an example, five times larger than the absolute value of dB1/dT, so that the total birefringence B increases with increasing temperature. The two birefringence contributions can in general be introduced by all design means mentioned above, i.e. elliptical cladding, elliptical core, microstructuring, stress bodies in the fiber cladding (e.g. panda or bow-tie).

[0062] (ii) The interplay of the stress and geometry induced birefringence in unspun elliptical core fibers is well described in prior art [8]. While typically the stress-induced contribution dominates and accordingly the overall birefringence shows a negative dependence on temperature, i.e. dB/dT <0, there are parameter ranges where the stress-induced contribution B.sub.stress becomes negligible (see FIG. 5 in Ref. [8]). The temperature dependence of the geometry induced contribution B.sub.geo depends on the temperature dependence of the refractive index difference between core and cladding, d(n.sub.core-n.sub.cladding)/dT, and in case of the most common glass composition for fibers (cladding SiO2, core SiO2/GeO2) the geometry induced birefringence is positive (see FIG. 6 in Ref. [8]). Accordingly, spinning the preform of an accordingly designed elliptical core fiber during the drawing process results in a spun fiber with dB/dT >0.

[0063] While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

REFERENCES

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[0067] 4. I. G. Clarke, “Temperature-stable spun elliptical-core optical-fiber current transducer,” Optics Letters 18, 158-160 (1993).

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[0077] Reference Numbers:

[0078] 1: light source

[0079] 2: detection unit

[0080] 3: polarization maintaining fiber

[0081] 4: polarization converter

[0082] 5: sensing fiber

[0083] 6: mirror

[0084] 7: current conductor

[0085] 8: glass body

[0086] 9: cavities

[0087] 10: elliptical core

[0088] 11: bow-tie structure

[0089] 12: cladding

[0090] XI, X2: core axis directions