Composite insulators with built-in optical fibre

Abstract: A composite insulator has a built-in optical fibre (3) extending through a central axial through-hole (2). End portions of the optical fibre (3) are sealed in the through-hole by melting an inorganic glass (5) around them. To achieve better sealing, portions (3-2) of the fibre surrounded by the glass (5) are stripped of their fibre coating. To protect the vulnerable fibre, protective tubes (6-1,6-2) of ceramics or glass are fitted in the through-hole on either side of the sealing glass (5). The protective tubes (6-1,6-2) have an inner diameter closely fitting the coated portions (3-1) of the optical fibre, and surround the transition from stripped to coated fibre outside the sealing glass (5). Adhesive layers (7-1,7-2) are provided to secure the fibre (3) in the protective tubes and the protective tubes in the through-hole (2). Preferably a buffer space (10) is left between this assembly and an elastomer or resin sealant (4) filled into the middle of the through-hole (2), to allow gradual flexion of the optical fibre (3) at that position.

Description: The present invention relates to an optical fiber built-in type composite insulator having an end portion sealed by an inorganic glass. Heretofore, various structures of optical fiber built-in type composite insulator have been known. Among them, the structure of sealing the optical fiber at both ends of a penetration hole of an insulator with inorganic glass requires bonding a cladding of the optical fibre and inorganic glass to each other, so that the portion of the optical fiber which bonds the inorganic glass has to be deprived of its coating before the sealing. An example of such structure is shown in the attached Fig. 3 wherein, when inserting an optical fiber 23 in a penetration hole 22 penetrating through the central axis portion of an insulator 1, the optical fiber 23 having a coating portion 23-1 is fixed at the middle portion of the penetration hole 22 by filling a rubbery elastomer or a resin, such as silicone rubber or epoxy 24. A coating-removed or stripped portion 23-2 of the optical fiber 23 is sealed in both the end portions of the penetration hole 22 by means of an inorganic glass 25 to extend the coating portion 23-1 from the inorganic glass 25. The coating-stripped portion 23-2 and the coated portion 23-1 of the optical fiber 23 extended or projecting from the inorganic glass 25 to the exterior are directly protected by a protective layer 26 made of a rubbery elastomer or a resin, such as silicone rubber or epoxy resin. The coating-stripped portion 23-2 and the coated portion 23-1 of the optical fiber 23 projecting from the inorganic glass 25 to the interior are protected by an inorganic or heat resistant organic adhesive layer 27 filled between the inorganic glass 25 and the middle portion of the penetration hole 22. Further examples can be found in EP-A- 297 728 and EP-A- 355 958. However, the abovementioned structure of the optical fiber built-in type composite insulator has a tendency for the optical fibre 23 to be damaged at the coated portion 23-1 and/or the stripped portion 23-2, as described below. The stresses exerted on the optical fiber 23 from the exterior, include a force unavoidably exerted during the sealing operation or the end-protecting operation, a stress accompanying expansion and shrinkage of the rubbery elastomer or resin 24 in the middle portion and the protective layer 26 of the penetration hole 22, and a force accompanying the handling of the optical fiber 23, such as fusion splicing, etc., after finishing the protection of the end portion of the penetration hole 22. Therefore, there is a problem in that after the finishing of the-sealing by means of the inorganic glass 25 and before the finishing of the protection of the coating-stripped portion 23-2 and the coating portion 23-1 projecting from the sealing portion to the exterior, the optical fiber 23 is damaged due to an unavoidable force generated during the operation of forming the protection. There is also another problem in that, even when the coating-stripped portion 23-2 and the coated portion 23-1 projecting from the sealing portion of the inorganic glass 25 to the exterior is directly protected by the protective layer 26 by a rubbery elastomer or resin, the projecting portion of the optical fiber 23, particularly the coating-stripped portion 23-2 of weak mechanical strength, is damaged by stress generated by expansion and shrinkage of the protective layer 26 due to temperature change in use. There is also a further problem in that, when the coating-stripped portion 23-2 and the coating portion 23-1 projecting from the sealing inorganic glass portion 25 to the interior are protected by the heat resistant organic adhesive layer 27, the optical fiber 23 is similarly damaged. Meanwhile, in case when the extruded portion of the optical fiber 23 is protected by the inorganic adhesive layer 27, there is a problem in that the inorganic adhesive layer 27 is fragile and liable to entrain air bubbles therein, so that it can not firmly fix the optical fiber 23 sufficiently, and the optical fiber 23, particularly the coating-stripped portion 23-2 of weak mechanical strength, is affected by a stress generated by expansion and shrinkage of the rubbery elastomer or resin 24 filled in the middle portion of the penetration hole 22 due to temperature change in the use, so that the optical fiber is also damaged. Furthermore, there is also a problem in that, accompanying the expansion and shrinkage of the rubbery elastomer or resin 24 filled in the middle portion of the penetration hole 22, a bending stress is exerted on the portion of the optical fiber 23 existing on the interface between the adhesive 27 and the rubbery elastomer or resin 24, so that the optical fiber 23 is damaged. It would be desirable to reduce or eliminate one or more of the problems mentioned above. In particular, it would be desirable to decrease damage rate of the optical fiber during operations and use. A preferred aim herein is to provide an optical fiber built-in type composite insulator having improved light transmitting property and prolonged life of the optical fiber. The present invention is defined in claims 1 and 9 and refer to an optical fiber built-in type composite insulator and a method of making such an insulator having an insulator body and a through-hole extending axially in the insulator body, an optical fiber extending in the through-hole, and an inorganic glass arranged at one or both ends of the through-hole for sealing the optical fiber therein, with a protective tube having a bore of an inner diameter substantially equal to the outer diameter of coated portion of the optical fibre, allowing insertion of the coated portion of the optical fiber therethrough and arranged in the through-hole at such a position that a portion of the coated portion of the optical fiber and a portion of the coating-stripped portion of the optical fiber reside in the bore of the protective tube, to protect the portions of the coated portion and the coating-stripped portion of the optical fiber in the end - usually at both ends - of the hole. For a better understanding of the present invention, reference is made to accompanying drawings, in which: Fig. 1 is a schematic cross-sectional view of an end portion of an example of the present composite insulator; Fig. 2 is a schematic cross-sectional view of another example of the protective tube usable in the present invention; and Fig. 3 is a schematic cross-sectional view of an end portion of an example of a conventional composite insulator. As described above, the coated portion and the coating-stripped portion of the optical fiber are protected by inserting them in a suitable protective tube, so that the stripped portion projecting from the sealing inorganic glass portion can be immobilized and hence the optical fiber can be prevented from breakage at the stripped portion due to forces from the exterior. As a result, damage rate of the optical fiber during the production operation of the composite insulator may be decreased, light transmitting property of the composite insulator may be improved, and the life of the optical fiber of the composite insulator may be prolonged. Preferably, the inner bore of the protective tube has a diameter abutting to the outer diameter of the coated portion of the optical fibre, or has a clearance of not more than 0.1 mm from the outer diameter portion of the coated portion of the optical fiber. If the clearance exceeds 0.1 mm, the optical fiber displaces in the inner bore of the protective tube, so that the optical fiber is not sufficiently immobilised and is occasionally broken. If the optical fiber is fixed in the interior or at the end portions of the inner bore of the protective tube by means of an adhesive, such as modified acrylate adhesive or silicone resin, etc., the force exerted from the exterior on the coating-stripped portion projecting from the sealing inorganic glass portion can further be mitigated and the optical fiber can further effectively be protected from the damage at the coating-stripped portion due to force exerted thereon from the exterior. If the protective tube is fixed in the penetration hole of the insulator, the optical fiber can be prevented from damage caused by wobbling of the protective tube in the penetration hole. If an appropriate buffering layer which allows free movement of the optical fiber is arranged between the protective tube and e.g.rubbery elastomer or resin in the middle portion of the penetration hole, transmission loss of optical signal can be reduced or prevented, because the optical fiber can be bent with a larger radius of curvature than the permissible minimum radius of curvature of the optical fibre, between the protective tube and the rubbery elastomer or resin, when the rubbery elastomer or resin is expanded. The buffering layer may be a space. However, preferably an insulative material, such as silicone oil, or SF₆ gas, etc., is incorporated therein to improve insulation property thereof. Referring to Fig. 1, a cross-sectional view of an end portion of an embodiment of the optical fiber built-in type composite insulator of the present invention is shown. In the embodiment shown in Fig. 1, the structure seals an optical fiber 3 consisting of a coated portion 3-1, and a coating-stripped portion 3-2 prepared in a region corresponding to the inorganic glass sealing portion, in the penetration hole 2 in the central axis portion of an insulator 1. The sealing structure is accomplished by sealing the coated portion 3-1 in the middle portion of the penetration hole 2 with a silicone rubber 4, and sealing the coating-stripped portion 3-2 in both the end portions of the penetration hole 2 with an inorganic glass 5. This embodiment has a characteristic feature that the coated portions 3-1 and the coating-stripped portions 3-2, arranged at the upper and lower neighboring portion of the sealing portion of the inorganic glass 5, are fixed by glass or ceramic protective tubes 6-1,6-2. These preferably have an inner diameter abutting to the outer diameter of the coated portion 3-1, or an inner diameter having a clearance of not more than 0.1 mm from the outer diameter of the coated portion 3-1, and an outer diameter substantially abutting to the inner wall of the penetration hole 2 and capable of being inserted therein. An adhesive layer 7-2, a holder 9 fixed on a stepped portion 8 of the penetration hole 2, and a buffering space 10 are arranged between the lower protective tube 6-2 and the silicone rubber 4. A protective layer 11 made of silicone rubber is arranged on the upper protective tube 6-1 via an adhesive layer 7-1. In this embodiment, the coated portion 3-1 of the optical fiber 3 is fixed in the inner bore or the end portion of the protective tube 6-1 (6-2) by means of an adhesive, and the outer circumferential portion of the protective tube 6-1 (6-2) and the penetration hole 2 are similarly fixed together by an adhesive. Referring to Fig. 2, a structure of another embodiment of the protective tube is shown, which may be used in the present invention. The protective tube 12 used in this embodiment has an inner diameter abutting to the outer diameter of the coated portion 3-1, or an inner diameter having a clearance of not more than 0.1 mm from the outer diameter of the coated portion 3-1, as in the embodiment described above. This protective tube, however, has an outer diameter smaller than the inner diameter of the penetration hole 2, differently from the abovementioned protective tube 6-1 (6-2). The protective tube 12 can also achieve the same effects as those of the abovementioned protective tube 6-1 (6-2), since the end portion of the protective tube 12 is embedded and fixed in the inorganic glass 5. In producing the present composite insulator of the abovementioned structures, at first the optical fiber 3 having the preliminarily stripped coating corresponding to the sealing inorganic glass portion 5 is inserted in the penetration hole 2. The optical fiber 3 has a primer treatment on the coating thereof beforehand for improving the adhesivity of the coating to the silicone rubber 4. Next, the silicone rubber 4 is filled and cured in the middle portion of the penetration hole 2. The silicone rubber 4 has a desired tensile strength and sufficient elongation at breakage so as not to deteriorate by thermal expansion and shrinkage caused by temperature change in use environment. Then, in order to provide the buffering layer 10 between the silicone rubber 4 and the protective tube 6-2, a support 9 is arranged on the step portion 8 of the penetration hole 2, an adhesive is poured on the support 9, the protective tube 6-2 is mounted on the adhesive, and the adhesive is cured to form the adhesive layer 7-2. The buffering layer 10 preferably has a thickness of at least 10 mm. The adhesive is a modified acrylate resin or a silicone resin. Thereafter, a calcined body of an inorganic glass having a throughhole for the optical fibre, and an appropriate shape suited to insertion thereof in the penetration hole, is arranged on the protective tube 6-2. The other protective tube 6-1 is arranged on the calcined body of the inorganic glass, and then they are heated to melt the calcined body of the inorganic glass. The inorganic glass is a glass which has a lower coefficient of thermal expansion than that of the ceramic insulator and which can melt at a low temperature as far as possible, in order to decrease the residual stress by means of sealing and to maintain the mechanical strength of the sealing glass portion against the thermal stress accompanying temperature change in use environment. Finally, an adhesive is applied or poured and cured on the protective tube 6-1 in the penetration hole 2 to form the adhesive layer 7-1, and then the protective layer 11 consisting of a rubbery elastomer, such as silicone rubber, etc., is formed on the adhesive layer 7-1. The adhesive is a modified acrylate resin or a silicone resin. Hereinafter, the present invention will be explained in more detail with reference to examples. Examples According to the aforementioned production method, optical fiber built-in type composite insulators of test Nos. 1-8 embodying the invention and test Nos. 9-11 of conventional structures are prepared having the structures as shown in the following Table 1. The prepared composite insulators are measured on the damage rate and the light signal transmitting property. The damage rate (%) of the optical fiber is the determined ratio of the number of damaged optical fibres (during the sealing operation or the handling of the optical fiber after finishing the protecting operation of the end portion of the penetration hole) to the whole number of tested optical fiber of the composite insulator. The light signal transmitting property is determined by an initial loss of light signal transmission (to be referred to as "LLST", hereinafter) at the time of assembling or preparing the composite insulator and LLST change accompanying temperature change in a range of between -20°C and 80°C each by an average value of each 10 samples at each level of measurements of LLST by means of interpolation method. In addition, for expressing trouble rate after thermal shock test, thermal shock tests are effected using 1 cycle of a high temperature of 80°C for 30 min and a low temperature of -20°C for 30 min with each 10 samples at each level, and expressed trouble rate of 0% with a symbol o ○, 10-20% with a symbol Δ, and 30% or more with a symbol × in the following Table 3. The word "trouble" used herein means the breakage of the optical fiber or LLST of not less than 50%. Trouble rates of the optical fiber are shown in the following Table 2, and the light signal transmission property of the optical fiber is shown in the following Table 3. As seen from the results shown in Tables 2 and 3, the test Nos. 1-8 embodying the invention as compared with the test Nos. 9-11 of the conventional examples have reduced damage rate both in the producing operation and in the handling after the finishing of the protection of the end portion of the penetration hole of the insulator, and remarkably reduced initial LLST and LLST in a desired temperature range as well as noticeably reduced trouble rate after thermal shock test. Though a sealing structure at only one end portion of the penetration hole of the insulator is shown in the above embodiments, the other end portion of the penetration hole of the insulator may of course similarly be sealed by the same sealing structure. As explained in detail in the foregoing explanation, in the preferred optical fibre type composite insulator of the present invention, the coated portion and the coating-stripped portion of the optical fiber are inserted and protected in a suitable protective tube to immobilize the stripped portion projecting from the sealing inorganic glass portion, and an adhesive layer is provided to fix the optical fiber and the protective tube to the inner wall of the penetration hole thereby to prevent the stripped portion of the optical fiber projecting from the sealing inorganic glass portion from being directly affected by a force from the exterior, so that tendency for damage of the fiber at the stripped portion caused by the force exerted from the exterior can be reduced or even eliminated. As a result, the damage rate of the optical fiber during the producing operation and the handling operation can be reduced, the light signal transmitting property can be improved, and the life of the optical fiber can be prolonged. By the provision of the buffering layer which allows free movement of the optical fiber between the sealing inorganic glass portion and the rubbery elastomer or resin in the middle portion of the penetration hole, the loss of light signal transmission accompanying thermal expansion and shrinkage of the rubbery elastomer or resin can be prevented from increasing. Although the present invention has been explained with specific examples and numeral values, it is of course apparent to those skilled in the art that various changes and modifications thereof are possible within the scope of the claims.

Claims: A composite insulator comprising an insulator body having a built-in coated optical fibre (3) which extends through an axial through-hole (2) of the insulator body and is sealed into the through-hole (2) at end portions thereof by inorganic glass (5) in the through-hole (2), in which a portion (3-2) of the optical fibre (3) which passes through the inorganic glass (5) is stripped of its coating, the coating-stripped portion (3-2) emerging from the glass (5), whereby the point of transition to coated fibre (3-1) is outside the glass (5), characterised in that a protective tube (6-1,6-2,12) having a bore of diameter substantially equal to that of the coated fibre (3-1) is disposed in the through-hole (2) adjacent to the inorganic glass (5) and around the optical fibre (3) at the point of transition, whereby both coated fibre (3-1) and stripped fibre (3-2) are in the bore of the protective tube (6-1,6-2,12) to protect the optical fibre (3) at the end portion. A composite insulator according to claim 1, in which a said protective tube (6-1,6-2,12) is provided for the inorganic glass (5) seal at both ends of the insulator. A composite insulator according to claim 1 or claim 2 in which there are two said protective tubes at the end of the insulator, an outer protective tube (6-1) and an inner protective tube (6-2) on the axially outer side and the axially inner side of the inorganic glass (5), respectively. A composite insulator according to any one of claims 1 to 3 in which an adhesive (7-1,7-2) fixes the optical fibre (3) in the protective tube (6-1,6-2) and preferably also fixes the protective tube in the through-hole. A composite insulator according to any one of the preceding claims in which the protective tube (6-1,6-2,12) contacts the inorganic glass (5). A composite insulator according to any one of the preceding claims in which the protective tube (6-1,6-2,12) is of glass or ceramics material. A composite insulator according to any one of the preceding claims, in which a central axial extent of the through-hole (2) is filled with a sealing elastomer or resin (4), which stops before the inorganic glass (5) to provide a buffer space (10) between the inorganic glass (5) and the sealing elastomer or resin to allow relatively free movement of the optical fibre (3) in said space (10). A composite insulator according to any one of the preceding claims, in which the diameter difference between the coated fibre (3-1) and the bore of the protective tube (6-1,6-2,12) in which it fits is not more than 0.1mm. A method of making a composite insulator having a built-in optical fibre (3) extending through an axial through-hole (2) thereof, in which portions (3-2) of the optical fibre towards the ends of the insulator are stripped of coating and sealed in the through-hole by melting inorganic glass (5) around them to leave stripped optical fibre (3-2) projecting from the inorganic glass (5) with a transition to coated optical fibre (3-1) outside the glass (5), characterised by arranging a preformed protective tube (6-1,6-2,12) having a bore of diameter substantially equal to that of the coated fibre (3-1) around the optical fibre (3) at said transition and in the through-hole (2), whereby both coated fibre (3-1) and stripped fibre (3-2) are in the bore of the protective tube (6-1,6-2,12) to protect the optical fibre (3). A method according to claim 9 in which said protective tube (6-1,6-2,12) is of glass or ceramics and is inserted pre-formed into the through-hole (2) without subsequently being melted.

Inventors: IKEDA MITSUJI, NISHIOKA MASAO, NOZAKI MASAYUKI, IKEDA, MITSUJI, NISHIOKA, MASAO, NOZAKI, MASAYUKI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1991
Main classification: G02B6
IPC codes: G02B6, H01B17