Method for connecting optical fibers

Abstract: Connection of hermetic optical fibers (1) having a hermetic layer coated on optical fiber glass prepared by positioning the optical fibers on respective electrically nonconductive V-shaped grooves (21), aligning cores of the optical fibers (1) and fusing and connecting the optical fibers (1) through an aerial discharge gives a small break strength decrease after connection without removing the hermetic layer.

Description: BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a method for fusing and connecting so-called hermetic optical fibers which have a thin film coating layer of, for example, a metal or carbon on a surface of an optical fiber glass. Description of related art Hitherto, a method for fusing and connecting optical fibers by an aerial discharge heat is used to permanently connect the optical fibers. The reasons therefor are chat (1) a connection loss is small, (2) excellent reliability for a long term is obtained because the connected part is reinforced with a heat-shrink tube and the like after the connection. A hermetic optical fiber has (1) improved water resistance and (2) improved reliability for a long term by coating a thin film of a metal, carbon or the like on an optical fiber surface. Since the surface of the hermetic optical fiber is electrically conductive, the strength after the connection is decreased. The reason therefor has been thought that the hermetic layer material is incorporated in the fused and connected part of the optical fiber. In order to solve this problem, we proposed a method for connecting carbon hermetic optical fibers, which comprises removing a carbon layer immediately before setting the fibers on a fusing instrument, and fusing and connecting the hermetic optical fibers through an aerial discharge as disclosed in EP-A-0 379 938, corresponding to JP-A-2 195 304. A sample connected by this method and a sample fused and connected without removal of the hermetic layer were subjected to a break strength test. The results of the break strength are shown in Table and Fig. 3. The sample prepared by removing the carbon layer and fusing and connecting the optical fibers has the average break strength of almost twice that of the sample without removing the carbon layer. Characteristically, the breakage always occurred on a V-shaped groove made of a metal. No. Steps number of samples Average value Maximum value Minimum value Break number at V groove 1 coverage removal → cutting → fusion 20 0.7 1.1 0.5 16 2 coverage removal → cutting → carbon removal → fusion 20 1.4 1.8 0.8 8 As stated above, the method for fusing and connecting the optical fibers after removing the carbon layer gives a connection having a slight decrease of strength. But, the removal of the carbon layer needs to remove the carbon layer through a thermal decomposition by heating the carbon layer to higher than 500°C, preferably 500 to 1,000°C, thereby requiring labor and time. Similar methods are disclosed in JP-A-2 087 106 and in a publication relating to 16th European Conference on Optical communication, 16-20 September 1990, Amsterdam, pages ECOC'90-601 to ECOC'90-604. In JP-A-1 046 708, there is described a method of connecting pairs of optical fibers on V-shaped grooves formed in a ceramic base, wherein the optical fibers are not disclosed to be hermetic. Non-prepublished EP-A-0 462 893 discloses a method of fusion splicing hermetic optical fibers supported by V-shaped grooves in an inert gas atmosphere having an oxygen concentration of less than 0.5 vol% without removal of the hermetic coating in order to avoid connection strength degradation and optical transmission loss. SUMMARY OF THE INVENTION An object of the present invention is to provide a method for connecting hermetic optical fibers without removing a hermetic layer, which method forms a high strength connection between the hermetic optical fibers. This and other objects of the present invention are achieved by a method of connecting a pair of hermetic optical fibers having an electrically conductive hermetic layer coated on optical fiber glass, which comprises positioning the optical fibers on respective electrically nonconductive V-shaped grooves, aligning the cores of the optical fibers and fusing and connecting the optical fibers through an aerial discharge without removing the hermetic layer previous to said aligning and fusing and connecting steps and without passing an electric current in the optical fiber surface during the fusion and connection. Preferably, the material forming the electrically nonconductive V-shaped grooves may have a resistivity of larger than 10 8 ohmm. The aerial discharge peferably may be conducted so that the fusion temperature is from 1600 to 2000°C. Preferred examples of the method according to the present invention are described hereinbelow in more detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is a perspective view of an embodiment of a method according to the present invention. Fig. 1B is a cross-sectional view along the X-X line of Fig. 1A. Fig. 2 shows a splice loss characteristic histogram of hermetic optical fibers connected according to the method of the present invention. Fig. 3 shows the break strength of carbon coated hermetic optical fibers after splicing with and without removing a carbon layer according to conventional methods. Fig. 4 shows a grounding test procedure conducted as Comparative Example. DETAILED DESCRIPTION OF THE INVENTION Fig. 1 shows an embodiment of the method according to the present invention. Fig 1A is a perspective view as a whole and Fig. 1B is a cross-sectional view along the X-X line. A hermetic optical fiber 1 has a hermetic layer 12 made of, for example, a metal and carbon, coated on the optical fiber glass surface. A pedestal 2 has a V-shaped groove 21 in a part made of an electrically nonconductive material which contacts with the optical fiber 1. The pedestal 2 may be made of an electrically nonconductive material as a whole. A pair of discharge electrodes 3 fuses the hermetic optical fibers by the aerial discharge heat. As shown in the figure, the hermetic optical fibers are located on the electrically nonconductive V-shaped groove 21 of the pedestal 2 without removing the hermetic layer, the cores 11 are aligned and then the optical fibers are fused and connected. A nonconductive material forming the V-shaped groove has a resistivity of larger than 10 +8 ohm•m and specific examples thereof are ceramics, glass, zirconia and alumina. A diameter of the hermetic optical fiber is usually from 5 to 1000 µm, preferably from 50 to 300 µm, more preferably from 120 to 130 µm. A thickness of the hermetic layer is usually from 10 to 100 nm (100 to 1000 Å). The aerial discharge is conducted so that a fusion temperature is usually from 1000 to 2500°C, preferably from 1600 to 2000°C. Specific examples of the material hermetically coated on the fiber are nickel, copper, aluminum, gold, carbon, boron, titanium and magnesium. PREFERRED EMBODIMENT OF THE INVENTION The present invention will be illustrated by the following Example. Example As shown in Fig. 1, hermetic optical fibers each of which had a carbon layer of about 50 nm (500 Å) in thickness coated on an outer surface of a single mode optical fiber having a diameter of 125 µm were positioned on the electrically nonconductive V-shaped groove. Cores of the fibers were aligned and then the optical fibers were fused and connected. An average break strength was 1.4 kg. A connection loss was as shown in Fig. 2 and an average loss was 0.04 dB. A comparison test was conducted as shown in Fig. 4. The hermetic optical fibers were positioned on the electrically nonconductive V-shaped groove. With contacting a grounded copper wire 4 with the hermetic optical fiber at the point between the electrode 3 and the pedestal 2, the discharge was conducted to fuse and connect the optical fibers. An average break strength of the connected part decreased to 0.6 kg. According to the present invention, the connected part having a small break strength decrease after the connection can be obtained without passing a current in the optical fiber surface during the fusion and connection.

Claims: A method of connecting a pair of hermetic optical fibers having an electrically conductive hermetic layer coated on optical fiber glass, which comprises positioning the optical fibers on respective electrically nonconductive V-shaped grooves, aligning the cores of the optical fibers and fusing and connecting the optical fibers through an aerial discharge without removing the hermetic layer previous to said aligning and fusing and connecting steps and without passing an electric current in the optical fiber surface during the fusion and connection. The method according to claim 1, wherein the material forming the electrically nonconductive V-shaped grooves has a resistivity of larger than 10 8 ohm·m. The method according to claim 1 or 2, wherein the aerial discharge is conducted so that the fusion temperature is from 1600 to 2000°C.

Inventors: OSAKA KEIJI, YANAGI TORU, OSAKA, KEIJI, YANAGI, TORU, Osaka, Keiji, c/o Yokohama Works Sumitomo, Yanagi, Toru, c/o Yokohama Works Sumitomo
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD
Application year: 1992
Main classification: G02B6
IPC codes: G02B6