SPLICED OPTICAL FIBER WITH SPLICE PROTECTION, CURRENT SENSOR WITH SUCH SPLICED OPTICAL FIBER AND METHOD FOR PROTECTING A SPLICED OPTICAL FIBER

Abstract

The invention relates to a spliced optical fiber comprising a first and second polarization-maintaining optical fiber connected at ends by splicing; to fiber optic current sensors; and to a method for protecting the spliced optical fiber against mechanical stress and/or humidity. A protection tube is arranged around the spliced optical fiber in a splice section of the spliced optical fiber. A first and second end of the protection tube is sealed to the spliced optical fiber by first and second sealing arrangement for protecting the splice.

Claims

1. A spliced optical fiber comprising a first and a second polarization-maintaining optical fiber connected to one another at one of their ends by splicing, thereby forming a splice at their connection point, wherein a protection tube is arranged around the spliced optical fiber and surrounds at least an uncoated section of the spliced optical fiber which includes the splice, wherein a first end and a second end of the protection tube are sealed to the spliced optical fiber, by a first and a second sealing arrangement for protecting the splice against mechanical stress and/or moisture, wherein the protection tube has such a length that an uncoated section of the spliced optical fiber comprising the splice and a coated section of the spliced optical fiber at each end of the uncoated section are arranged inside the protection tube.

2. The spliced optical fiber according to claim 1, wherein the length of the protection tube is such that a first distance from the first end of the protection tube and/or from the first sealing arrangement to a position of the splice and a second distance from the position of the splice to the second end of the protection tube and/or to the second sealing arrangement are greater than or equal to a predefined minimum distance L.sub.m, wherein the minimum distance L.sub.m is chosen to satisfy the following equation:
4√{square root over (X.sub.0X.sub.s)}A(L.sub.m)=X.sub.m wherein X.sub.0 is a maximum guaranteed polarization extinction ratio of the splice, X.sub.s is a maximum guaranteed polarization extinction ratio at the sealed ends of the protection tube, A(L) is a coherence function of a length of the polarization-maintaining fiber and a Fourier transform of an optical power spectrum, and X.sub.m is a predetermined maximum allowable polarization extinction ratio variation, in particular wherein X.sub.0≤−25 dB , X.sub.s≤−30 dB, and X.sub.m≤0.1%.

3. The spliced optical fiber according to claim 1, wherein the protection tube is made of a material having a thermal expansion coefficient which is substantially equal to a thermal expansion coefficient of the first and the second polarization-maintaining optical fiber.

4. The spliced optical fiber according to claim 3, wherein the protection tube has a diameter in the range between 1 mm and 5 mm.

5. The spliced optical fiber according to claim 4, wherein at least one of the first and the second sealing arrangements comprises an outer dual-shrink sleeve adapted to shrink radially upon exposure to heat, thereby sealing the protection tube containing the section of the spliced optical fiber which includes the splice against humidity.

6. The spliced optical fiber according to claim 5, wherein a first outer dual-shrink sleeve of the first and sealing arrangement and/or a second outer dual-shrink sleeve of the second sealing arrangement each comprises an outer tube and an inner tube, wherein the first end and the second end of the protection tube are arranged between the inner tube and the outer tube of the respective first and second outer dual-shrink sleeves.

7. The spliced optical fiber according to claim 6, wherein at least one of the first and the second sealing arrangements comprises a capillary sleeve arranged partially inside the protection tube around a section of the spliced optical fiber, wherein the capillary sleeve is sealed to the spliced optical fiber by an inner dual-shrink sleeve at a first end facing the splice and is sealed by the inner tube of the respective outer dual-shrink sleeve at a second end.

8. The spliced optical fiber according to claim 7, wherein said first end of the capillary sleeve is arranged between an inner tube and an outer tube of the respective inner dual-shrink sleeve, wherein the capillary sleeve is chosen to be longer than the inner tube of the second outer dual-shrink sleeve in longitudinal direction.

9. The spliced optical fiber according to claim 8, wherein the outer tubes of at least one of the dual-shrink sleeves are made of a material able to radially shrink upon application of heat and the inner tubes are made of an adhesive material able to melt upon application of heat, wherein the materials of the inner tubes and outer tubes of dual-shrink sleeves have such properties that during a heat-up process by application of heat with a predefined temperature the inner tubes partially melt before the outer tubes begin to shrink.

10. The spliced optical fiber, according to claim 9, wherein the spliced optical fiber is obtainable by a process comprising: connecting a first and a second polarization-maintaining optical fiber at one of their ends by a splice, utilizing a splicing procedure that guarantees a maximum polarization extinction ratio of the splice of X.sub.0; arranging a protection tube around the spliced optical fiber such that the protection tune surrounds at least an uncoated section of the spliced optical fiber which includes the splice, in particular, wherein the protection tube has such a length that an uncoated section of the spliced optical fiber comprising the splice and a coated section of the spliced optical fiber at each end of the uncoated section are arranged inside the protection tube; and sealing a first and a second end of the protection tube to the spliced optical fiber by a first and a second sealing arrangement, utilizing a sealing procedure that guarantees a maximum polarization extinction ratio of the spliced optical fiber at the first and/or the second sealing arrangement of X.sub.s; wherein the length of the protection tube is such that a first distance from the first end of the protection tube and/or from the first sealing arrangement to a position of the splice and a second distance from the position of the splice to the second end of the protection tube and/or to the second sealing arrangement are greater than or equal to a predefined minimum distance L.sub.m, wherein the minimum distance L.sub.m is chosen to satisfy the following equation:
4√{square root over (X.sub.0X.sub.s)}A(L.sub.m)=X.sub.m wherein A(L) is a coherence function of a length of the polarization-maintaining fiber and a Fourier transform of an optical power spectrum, in particular, of a light source connected or to be connected to the spliced optical fiber, and X.sub.m is a predetermined maximum allowable polarization extinction ratio variation.

11. The spliced optical fiber according to claim 10, wherein the splice optical fiber is comprised in an optical fiber communication device or in an optical measurement device.

12. The spliced optical fiber according to claim 12, wherein the spliced optical fiber is comprised in a fiber optic current sensor for measuring a current in a current-carrying conductor, the fiber optic current sensor comprising: a primary converter suitable to be arranged around the conductor, wherein the primary converter is connected to the first polarization-maintaining optical fiber of the spliced optical fiber, and a secondary converter comprising an opto-electronics unit and connected to the second polarization-maintaining optical fiber of the spliced optical fiber for generating light to propagate into the same and detecting light from it.

13. The spliced optical fiber according to claim 12, wherein the fiber optic current sensor is configured for measuring AC currents or DC currents, up to 600 kA in one of a circuit breaker, a substation, or an aluminium production installation.

14. The spliced optical fiber according to claim 10, wherein the spliced optical fiber is formed by connecting two polarization-maintaining optical fibers, against mechanical stress and/or moisture, the splice of the spliced optical fiber protected by a process comprising: step a) positioning the spliced optical fiber into the protection tube, wherein the protection tube has such a length that the splice is located inside it at least at the predefined minimum distance from a first end and a second end of the protection tube; step b) sealing the first end of the protection tube around the spliced optical fiber by applying heat to a first outer dual-shrink sleeve arranged around a portion of the first end of the protection tube and a portion of the spliced optical fiber, thereby shrinking the first outer dual-shrink sleeve onto said portions; and subsequently step c) sealing a capillary sleeve around a portion of the spliced optical fiber in an area of the second end of the protection tube, wherein the capillary sleeve is positioned at least partially inside the protection tube, by applying heat to an inner dual-shrink sleeve arranged around a portion of that end of the capillary sleeve, which is nearest to the splice, and around a portion of the spliced optical fiber; and step d) sealing the second end of the protection tube around the spliced optical fiber by applying heat to a second outer dual-shrink sleeve arranged around a portion of a second end of the protection tube and a portion of the spliced optical fiber, thereby shrinking the second outer dual-shrink sleeve onto said portions; or: leaving out step b) and applying the sealing steps c) and d) at both the first end and the second end of the protection tube, or: leaving out steps b), c) and d) and instead injecting an adhesive material into the protection tube at both the first end and the second end of the protection tube.

15. The spliced optical fiber according to claim 14, wherein a preparation step a′) is carried out before the process step a), consisting of: a′) attaching the first outer dual-shrink sleeve to the first end of the protection tube and the second outer dual-shrink sleeve to the second end of the protection tube by pre-heating the respective outer dual-shrink sleeve such that its free end remains open.

16. The spliced optical fiber according to claim 15, wherein a hydrophobic substance, comprising a silane solution is applied onto an uncoated section of the spliced optical fiber, which comprises the splice, prior to the step a).

17. The spliced optical fiber according to claim 16, wherein the process steps a) to d) are carried out, or the adhesive is applied, only on coated portions of the spliced optical fiber.

18. The spliced optical fiber according to claim 17, wherein the step of sealing the second end of the protection tube around the spliced optical fiber is carried out after a predefined time interval upon completion of the sealing of the first end of the protection tube around the spliced optical fiber, wherein the predefined time interval equals at least a minimum cooling time of the respective outer dual-shrink sleeve or adhesive at the first end of the protection tube.

Description

SHORT DESCRIPTION OF THE DRAWINGS

[0049] Embodiments, further advantages and applications of the invention result from the dependent claims, claim combinations and from the now following description in conjunction with the figures. It is shown in:

[0050] FIG. 1 a schematized view of a fiber optic current sensor according to the invention in an arrangement for measuring a current carried by a current conductor;

[0051] FIG. 2 a schematized sectional view of a spliced optical fiber with a protection according to the invention;

[0052] FIGS. 3 and 4 schematized sectional views of sealing alternatives for splice protection in a spliced optical fiber according to the invention.

[0053] In the drawings same references denote same or similarly acting components.

WAYS OF CARRYING OUT THE INVENTION

[0054] The term “spliced optical fiber” refers to an optical fiber obtained by joining two optical fibers using a splicing method. For simplicity reasons, the spliced optical fiber is regarded as a general term and may therefore include the protection according to the invention but may also refer only to the fact that splicing has already been carried out, depending on the context. Particularly, this term shall also underline that for the purpose of the present invention it is irrelevant, whether the splice section of the spliced optical fiber has been treated in any way before applying the splice protection according to the invention.

[0055] The term “polarization extinction ratio”, PER, refers to the common definition as the ratio of a light intensity in an unwanted polarization state in a polarization-maintaining fiber to a light intensity in an wanted polarization state in the polarization-maintaining fiber; i.e., the smaller the polarization extinction ratio, the better the wanted polarization state is conserved in the polarization-maintainning fiber, e.g., including imperfections such as a splice. A polarization extinction ratio variation is defined as an absolute change of the polarization extinction ratio within specified environmental conditions such as a temperature range.

[0056] FIG. 1 shows a schematized view of a fiber optic current sensor 1 according to the invention in an arrangement for measuring a current C carried by a current conductor 4. A spliced optical fiber with a protection according to the present invention has the reference 2 and is formed by a first and a second polarization-maintaining optical fiber 3a, 3b which are joined together at one end by a splicing process. A splice 3 forms the joint of the two fibers 3a, 3b. On the right side of the figure, the first optical fiber 3a is connected at the other end to a primary converter (PC) 5 and the second optical fiber 3b is connected at its other end to a secondary converter (SC) 6. The secondary converter 6 comprises an opto-electronics unit 6a which optically drives the spliced optical fiber 2, i.e., which sends light for propagation into the spliced optical fiber 2 and receives light from the same. It is understood that the secondary converter 6 also comprises all other necessary equipment for signal processing and signal analysis, and is connected to or comprises a user interface, which is known.

[0057] The primary converter 5 comprises an optical fiber 5a, which is connected to a part of the first polarization-maintaining optical fiber 3a, and which is arranged around the current conductor 4. The measurement is based on the known principles explained at the beginning and is therefore not explained in more detail here.

[0058] FIG. 2 shows a schematized sectional view of a spliced optical fiber 2 with a protection according to the invention. A protection tube 7 is arranged around the spliced optical fiber 2, which is attached to the latter by sealing arrangements 9 at each of its ends, here the first end 11a and the second end 11b. In this figure, the sealing arrangements 9 are meant in an illustrative way and are placeholders for specific embodiments explained in connection with FIGS. 3 and 4.

[0059] On the one hand, the protection tube diameter is chosen to be large enough to allow moderate fiber bending without the bent optical fiber pressing hardly onto the inner wall in case of a small length difference between the spliced optical fiber 2 and the protection tube 7. On the other hand, the protection tube diameter is chosen to be small enough such that the spliced optical fiber 2 can easily be kept centralized during sealing. A preferred protection tube diameter is between 1 mm and 25 mm, more preferred between 1 mm and 5 mm.

[0060] In embodiments, one of the first and the second PM optical fiber 3a, 3b is inserted into the protection tube 7 prior to splicing, such that the protection tube 7 may be shifted freely along the respective PM optical fiber. After splicing, the protection tube 7 is shifted to its designated location where it encloses the splice 3 and the uncoated sections 8a of the spliced optical fiber 2. Alternatively, it is also conceivable to use a protection tube 7 which can be clamped onto the already spliced optical fiber 2, provided the protection tube 7 is made of a material which is suitable for clamping and at the same time satisfies other requirements like a thermal expansion coefficient which is similar to that of the spliced optical fiber 2. In applications where the PM optical fibers 3a, 3b are spliced before connecting them to the primary and secondary converter 5, 6, respectively, the protection tube 7 is inserted onto one of the ends of the spliced optical fiber 2 before connecting this end to the assigned converter 5 or 6.

[0061] Another optional step which may be carried out before sealing the protection tube 7 around the spliced optical fiber 2 is to apply a hydrophobic substance (e.g. silane solution 3M AP115) on the uncoated section 8a of the spliced optical fiber 2. Such treatment is used to prevent moisture attack on glass.

[0062] As can be seen in the figure, the sealing is preferably applied, as mentioned, only on coated portions 8b of the spliced optical fiber 2.

[0063] The splice 3 is located inside the protection tube 7 at a distance L.sub.1 and L.sub.2 from the respective end 11b, 11a of the protection tube 7, with the total length being L.sub.0. In the exemplary embodiment of FIG. 2, these distances L.sub.1 and L.sub.2 are preferably equal, however they may be non-equal as long as the shortest distance of them is not smaller than a minimum length L.sub.m, the determination of which will be derived in detail in the following.

[0064] According to the first aspect of the invention, the length L.sub.0 of the protection tube 7 and the distances L.sub.1, L.sub.2 of the splice 3 to each end 11a, 11b of the protection tube 7 are chosen to satisfy the following equation:

4√{square root over (X.sub.0X.sub.s)}A(L.sub.m)=X.sub.m

wherein X.sub.0 is a maximum guaranteed polarization extinction ratio of the splice 3, X.sub.s is a maximum guaranteed polarization extinction ratio at the respective end 11a, 11b of the protection tube 7, A(L) is a coherence function of a length of the polarization-maintaining fiber 3a, 3b, and X.sub.m is a predetermined maximum allowable polarization extinction ratio variation.

[0065] With a splice protected by the protection tube 7 described herein, polarization crosstalk mentioned at the beginning occurs at three distinctive locations. The first location is the splice 3 itself, where polarization crosstalk occurs due to misalignment of the axes of the PM optical fibers 3a, 3b during the splicing process. The maximum warrantable PER depends on the used splicing equipment. With a mid-range-quality splicing machine, a value up to −30 dB is common without special care, and a value of better than −35 dB is often difficult to guarantee even with care. The PER of the splice itself is stable at all temperatures.

[0066] Polarization crosstalk also occurs in the spliced fiber 3, or the first and second first polarization-maintaining fiber 3a, 3b, at both ends 11a, 11b of the splice protection tube 7, wherein sealing material exerts stress on the coated section 8b of the spliced optical fiber 2. Stress and PER usually increase at low temperatures, as adhesive and coating harden. For dual shrink sleeves (described in connection with FIGS. 3 and 4) applied on a commercial PANDA fiber, experiments have shown that a PER of maximum −50 dB can be expected in the temperature range from −25° C. to 80° C., with even higher PERs at lower temperatures. Tests with common adhesives applied onto a commercial PANDA fiber have shown similar values.

[0067] A maximum PER of −50 dB (of light inside the spliced optical fiber 2) at the sealed ends 11a, 11b of the protection tube 7 (corresponding to maximum 0.002% in scale factor increase of a corresponding current sensor) may seem harmless at first glance; however, when considering the PER of the protected spliced optical fiber 2, one must consider the fiber and the protection assembly as a whole, which particularly means that any interference effects between the individual PER locations must also be taken into account.

[0068] Assuming that the splice 3 has a PER of X.sub.0, and the at the sealing ends 11a, 11b there is a PER of X.sub.1 and X.sub.2, respectively, with the distance being L.sub.1 between the sealing end 11b with X.sub.1 and the splice location with X.sub.0, and the distance being L.sub.2 between the splice location and the sealing end 11a with X.sub.2. The overall PER of the sealed splice protector, taking into account interference, is calculated as

X=X.sub.0+X.sub.1+X.sub.2+2√{square root over (X.sub.0X.sub.1)}A(L.sub.1)cos ϕ.sub.1+2√{square root over (X.sub.0X.sub.2)}A(L.sub.2)cos ϕ.sub.2+2√{square root over (X.sub.1X.sub.2)}A(L.sub.1+L.sub.2) cos(ϕ.sub.1+ϕ.sub.2)

where ϕ.sub.1,2=2πL.sub.1,2/L.sub.B denotes the differential polarization phase shifts in the PM fiber sections between the splice 3 and the sealing ends 11a, 11b, L.sub.B is the PM fiber beat length, and A(L) is the coherence function of PM fiber length L.

[0069] Physically, the coherence function A(L) is the Fourier transform of the optical power spectrum; therefore, its width (known as the coherence length) is inversely proportional to the spectral bandwidth. For a Gaussian spectrum with FWHM bandwidth Δλ.sub.1/2 centered at wavelength λ.sub.0, the coherence function is a Gaussian function

A(L)=exp[−(L/ΔL.sub.c).sup.2/2]

with the coherence length being ΔL.sub.c=KL.sub.bλ.sub.0/Δλ.sub.1/2 , and K=√{square root over (2 ln 2)}/π.

[0070] For example, a superluminescent diode source has a FWHM bandwidth Δλ.sub.1/2 of 35 nm at 1310 nm. A commercial PANDA PM 1300 fiber has a beat length L.sub.B of 3.6 mm. For this system, the coherence length in the PANDA fiber is ΔL.sub.c=50 mm, which means that the degree of coherence A(L) drops to e.sup.−1/2=61% at 50 mm from a polarization crosstalk location in a PANDA fiber.

[0071] For simplicity reasons, it is assumed that L.sub.1=L.sub.2=L, ϕ.sub.1=ϕ.sub.2=ϕ, and X.sub.1=X.sub.2=X.sub.s, such that the overall PER can be written as

X=X.sub.0+2X.sub.s+4√{square root over (X.sub.0X.sub.s)}A(L)cos ϕ+2X.sub.sA(2L) cos 2ϕ

[0072] As an example, we take X.sub.s≤−50 dB=0.001%. Therefore, the temperature variation of X.sub.s and the second interference term |2X.sub.sA(2L)cos 2ϕ|<2X.sub.sA(2L)<2X.sub.s have negligible influence on the overall PER. Furthermore, considering that is X.sub.0 constant, the only important term that contributes to the temperature variation of the overall PER is 4√{square root over (X.sub.0X.sub.s)}A(L)cos ϕ. In order to reduce its temperature variation, one must either minimize the variation of phase shift ϕ=2πL/L.sub.B in the specified temperature range, or suppress the interference amplitude 4√{square root over (X.sub.0X.sub.s)}A(L).

[0073] As to the former approach, it is possible to adjust the distance L between the splice location 3 and the sealing ends 11a, 11b. A commercial PANDA PM 1300 fiber has a beat length temperature coefficient c.sub.b=dL.sub.b/L.sub.bdT of around 6.5×10.sup.−4 K.sup.−1. With the light source and fiber parameters given above and ΔT=90 K (SC temperature range from −25° C. to 65° C.), ϕ(T) variation would already reach π (corresponding to half an oscillation cycle, or, e.g., full amplitude change between minimum and maximum), if L=L.sub.b/(2c.sub.bΔT)=31 mm. Making L a small fraction of that is impractical, because the splicing process requires stripping off the fiber coating, which leaves a bare fiber section 8a on each side with a length up to 25 mm. The sealing material also needs to cover many millimeters of coated fiber to ensure reliable insulation.

[0074] As to the latter approach, the only tuneable variable is also L, but in this case the longer the distance L, the smaller the coherence function A(L). Using the error budget given in the background section as an example, to keep the scale factor variation of the protected splice 3 below ±0.03%, the PER variation must be smaller than ±0.015%. That means 4√{square root over (X.sub.0X.sub.s)}A(L)<0.015%. Using the worst-case values X.sub.0=−30 dB and X.sub.s=−50 dB, to reach the target, the following applies: A(L)<A.sub.m=0.375. Using the light source and fiber parameters given above, this yields a minimum splice-sealing separation L.sub.m=ΔL.sub.c√{square root over (−2 ln A.sub.m)}=71 mm.

[0075] The minimum splice-sealing separation L.sub.m may be reduced, if a lower splice PER can be guaranteed (e.g. with a reliable PER control routine or with a high-quality splicing machine).

[0076] The maximum guaranteed polarization extinction ratio of the splice X.sub.0 depends on the quality of the splicing machine in use and the operating conditions. With a mid-range splicing machine, a value up to −30 dB or −25 dB is common without special care, while a value of better than −35 dB is often difficult to guarantee even with care. With a high-end machine, a value of −40 dB is commonly achievable. The maximum guaranteed polarization extinction ratio of the sealing ends X.sub.s depends on the sealing method or procedure and/or material used as well as on the required temperature range since the material used may contract and/or harden at low temperatures. For some dual shrink sleeves, a value of −50 dB may be achieved over a large temperature range; for other sealing materials or procedures, e.g., adhesive injected into the protection tube ends, a value smaller than or equal to −40 dB or even −30 dB is common. The maximum allowable polarization extinction ratio variation of the entire protected splice section X.sub.m is determined by the required accuracy of the entire fiber-optic current sensor system, the number of splices in the system, and the possible scale factor variation of other components in the system. For example, a 0.2 class current sensor must have an accuracy <±0.2%, which includes the contributions of all components. Depending on the performance of other components, if only one protected splice section is in the system, its maximum allowable polarization extinction ratio variation may be 0.015%. If there are two statistically independent protected splice sections in the same system, the maximum allowable polarization extinction ratio variation of each would be 0.011%. For other applications, X.sub.m maybe smaller or equal to 0.02% or even 0.1%. Typical temperature ranges to which the splice is exposed are a temperature range of −40° C. to 85° c. or −20° C. to 55° C.

[0077] FIGS. 3 and 4 show schematized sectional views of sealing alternatives for splice protection in a spliced optical fiber 2 according to the invention. The sealing arrangement 9 of the two ends of the protection tube 7 shown in both figures reflects the aforementioned first alternative for the method according to the third aspect of the invention, whereas the sealing of FIG. 4 applied at both ends of the protection tube 7 reflects the second alternative for the method according to the third aspect of the invention. The third alternative (sealing with adhesive) is not shown in the figures. It involves injection of the adhesive into the protection tube 7 at both ends 11a, 11b, wherein preferably it is waited until the adhesive at the first end 11a of the protection tube has entirely cooled down before injection of adhesive at the second end llb of the protection tube 7.

[0078] FIG. 3 shows the first end 11a of the protection tube 7 in a sealed state. The first polarization-maintaining optical fiber 3a is shown with a portion 8a which was stripped off its coating for the splicing process and a portion 8b which remained coated. The first outer dual-shrink sleeve 10 is shown in shrunk state, after it has been subjected to heat, and consists of an outer tube 10a and an inner tube 10b. The inner tube 10b has molten during heat application and the outer tube 10a has shrunken around the inner tube 10b and the first end 11a of the protection tube 7, thereby creating a sealing against moisture and/or mechanical stress. Additionally, this type of dual-shrink sleeve improves stability of the PM fiber 3a which has a firm seat after the inner tube 10b has cooled down.

[0079] FIG. 4 shows the second end llb of the protection tube 7 in a sealed state. The second polarization-maintaining optical fiber 3b is shown with a portion 8a which was stripped off its coating for the splicing process and a portion 8b which remained coated. The second outer dual-shrink sleeve 12 is shown in shrunken state, after it has been subjected to heat, and consists of an outer tube 12a and an inner tube 12b. The inner tube 12b has partially molten during heat application and the outer tube 12a has shrunken around the inner tube 12b and the second end 11b of the protection tube 7, thereby creating a sealing against moisture and/or mechanical stress. Additionally, this type of dual-shrink sleeve improves stability of the PM fiber 3b which has a firm seat after the inner tube 12b has cooled down. Beforehand, a capillary sleeve 13 was introduced and fixed inside the second outer dual-shrink sleeve 12, as already described. The inner dual-shrink sleeve 14 seals the splice-facing end of the capillary sleeve 13, which is its left end in the figure, to the second polarization-maintaining optical fiber 3b. Just like in case of the outer dual-shrink sleeve 12, the inner dual-shrink sleeve 14 also has an outer tube 14a and an inner tube 14b, the behavior of which tubes is similar to the one explained for the second outer dual-shrink sleeve 12. Herein, the inner tube melts around a part of the coated section 8b of the PM fiber 3b. It is preferred that the capillary sleeve 13 or capillary sleeves 13 is or are chosen to be longer than the inner tube 12b of the second outer dual-shrink sleeve 12 in longitudinal direction. The capillary sleeve 13 advantageously acts as a reinforcement element for the portion of the PM fiber 3b at the sealing location and also protects it from direct contact of the inner tube 12b with the PM fiber.

[0080] Preferably, the sealing steps b) and d) of the method according to the third aspect of the invention are carried out in such a way that said ends 11a, 11b of the protection tube 7 come to lie between an inner tube 10b; 12b and an outer tube 10a; 12a of the respective first and second outer dual-shrink sleeve 10, 12. Similarly, the sealing step c) of the alternative of FIG. 4 is also carried out in such a way that said end of the capillary sleeve 13 comes to lie between an inner tube 14b and an outer tube 14a of the respective inner dual-shrink sleeve 14.

[0081] The capillary sleeve 13 also has the advantage mentioned at the beginning, which is explained in more detail in the following. Heat shrink tubes normally first shrink radially under heat, and then shrink further longitudinally when cooling. In the protection tube 7 sealed with dual shrink sleeves 10, 12 at both ends, this behavior means that the PM fibers 3a, 3b at the ends 11a, 11b of the protection tube 7 would be drawn into the protection tube 7 by the dual-shrink sleeve 10, 12 during cooling, causing the spliced optical fiber 2 inside the protection tube 7 to bend or even slightly coil, which potentially increases the stress on the splice. To prevent this problem, in a preferred embodiment, only one end (e.g. the first end 11a) of the protection tube 7 is sealed with a dual shrink sleeve 10 of FIG. 3, whereas the second end 11b of the protection tube 7 is sealed with the two inner and outer dual shrink sleeves 14, 12 and the capillary sleeve 13. The second end 11b of the protection tube 7 is sealed with a larger dual shrink sleeve 12 onto the capillary sleeve 13, which contains the PM fiber 3b. The inner end of the capillary sleeve 13 (left end in the figure) is sealed onto the PM fiber 3b with the smaller inner dual shrink sleeve 14, while the outer (right end in the figure) end is open. In application, the first end 11a of the protection tube 7 is heated and sealed first. Next, after the first end 11a has fully cooled down, the second end 11b of the protection tube 7 is heated. During cooling, both the smaller and larger inner and outer shrink sleeves 14, 12, respectively, shrink in length, with the former bringing the sealed fiber (to the right in the figure) into the capillary sleeve 13 and the latter bringing the capillary sleeve 13 (to the left in the figure) into the protection tube 7. The net effects largely cancel one another, thereby keeping the sealed spliced optical fiber 2 at about the same location relatively to the protection tube 7 after the sealing cools down.

[0082] As mentioned, the method steps b) to d) of the method according to the third aspect of the invention are preferably carried out only on coated portions of the spliced optical fiber in order to reduce mechanical stress on the uncoated section of the spliced optical fiber 2.

[0083] Step a) of the method of the invention is preferably carried out in such a way that a longitudinal axis of the spliced optical fiber 2 is substantially identical to a longitudinal axis of the protection tube 7. In other words, the PM fiber 3a, 3b is preferably centered inside the protection tube 7.

[0084] The spliced optical fiber according to the invention has a number of advantages; it provides good protection of the splice against mechanical stress and/or humidity. Particularly the reduction of PER variation due to the different measures of the invention results in a substantial increase in the fidelity of the light polarization state transmitted through the spliced optical fiber, which leads to a substantial increase in measurement accuracy in a FOCS application.

[0085] With regard to the method according to the invention, the first alternative is preferred due to its good compromise between sealing quality and simplicity and cost saving. The second alternative is more complex but may provide better protection at both ends of the tube. The third alternative provides the fastest and most inexpensive solution, at the cost of a lower sealing quality in terms of stress upon the spliced optical fiber.

[0086] 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 otherwise variously be embodied and practised within the scope of the following claims. Therefore, terms like “preferred” or “in particular” or “particularly” or “advantageously”, etc. signify optional and exemplary embodiments only.

REFERENCE LIST

[0087] 1=fiber optic current sensor (FOCS)

[0088] 2=spliced optical fiber

[0089] 3=splice

[0090] 3a=first polarization-maintaining (PM) fiber

[0091] 3b=second polarization-maintaining (PM) fiber

[0092] 4=current conductor

[0093] 5=primary converter of FOCS

[0094] 5a=optical fiber of primary converter

[0095] 6=secondary converter of FOCS

[0096] 7=protection tube

[0097] 8a=uncoated section of spliced optical fiber

[0098] 8b=coated section of spliced optical fiber

[0099] 9=sealing

[0100] 10=first outer dual-shrink sleeve

[0101] 10a=outer tube of first outer dual-shrink sleeve

[0102] 10b=inner tube of first outer dual-shrink sleeve

[0103] 11a=first end of protection tube

[0104] 11b=second end of protection tube

[0105] 12=second outer dual-shrink sleeve

[0106] 12a=outer tube of second outer dual-shrink sleeve

[0107] 12b=inner tube of second outer dual-shrink sleeve

[0108] 13=capillary sleeve

[0109] 14=inner dual-shrink sleeve

[0110] 14a=outer tube of inner dual-shrink sleeve

[0111] 14b=inner tube of inner dual-shrink sleeve

[0112] C=electric current direction

[0113] L0=total length of protection tube

[0114] L1=distance from splice to first end of protection tube

[0115] L2=distance from splice to second end of protection tube