Method for manufacturing a fiber type coupler

Abstract: The invention relates to a method for manufacturing a fiber type coupler by extending at least two optical fibers (1a; 2a), having their coatings removed, along an optical axis while fusing the optical fibers (1a; 2a) to form a coupler member. The method is improved by the step of adding a tension to the coupler member (A) along the optical axis before the coupler member (A) is fixed to a protective member (13).

Description: Background of the Invention (Field of the Invention) The present invention relates to a method for manufacturing a fiber type coupler for splitting or merging a propagated light in an optical fiber, and more particularly to a method for manufacturing the fiber type coupler including a step of fusing and extending a plurality of optical fibers (see e.g. EP-A-0 234 325). (Related Background Art) The fiber type coupler has a function to split and couple single wavelength lights or split and merge multi-wavelength lights. The manufacturing method thereof includes a fuse extension method in which fibers are fused and extended, and a grind method in which two optical fibers having sides thereof that are ground face each other. Fig. 1 is a flow chart which shows a related background art of fiber type coupler manufacturing method (fuse and extend method). (Electronics Letters, 15th March 1984, Vol. 20, No. 6, PP. 230 - 232) A fiber type coupler which uses a single mode fiber is now explained. Coatings of two optical fibers are partially removed and the optical fibers are contacted to each other (step 101). Then, the contacted areas are fused parallelly by heating them with a burner and then the optical fibers are extended along the optical axis (step 102). In this case, a light applied to one end of the optical fiber is measured at the other end to detect a light split ratio (step 103). If a predetermined split ratio is obtained, the extension of the optical fiber is stopped and a coupler member is formed (step 104). Then, the coupler member is fixed and bonded to a protective member to form the fiber type coupler (step 105). In another related background art method, two optical fibers are not fused and extended but are twisted, heated and extended ("a Discussion on A Manufacturing Method of A Biconical Tapered Distributor", 1984 National Conference of Light-Electromagnetic Wave Section of the Institute of Electronics and Communication Engineers of Japan) The related background art fiber type coupler is fixed to a protective material by a bonding material. The coupler whose diameter has been reduced by the extension has a diameter of less than 20µm which is almost one tenth of the diameter before the extension. As a result, the tensile strength at the coupler is remarkably reduced, and it may be broken if subjected to a heat cycle. Thus, the related background art fiber type coupler exhibits a low reliability in an environment where heat cycles exist. A further prior art is that disclosed in WO-A-88/03 661 that teaches the disposing of two or more segments of optical fibers under longitudinal tension; preheating the tensioned segments to a temperature sufficient to soften the segments and thereby to substantially relieve said tension by inelastic stretching of the segments; and that after cooling the segments are heated while in intimate side-by-side contact to a temperature sufficient to cause the fiber segments to fuse together. This prior art uses an apparatus comprising: retaining means for disposing two or mor segments of optical fibre under longitudinal tension; means for heating said tensioned fibre segments; and control means connected for activating said heating means and said retaining means to preheat said tensioned fibre segments to a temperature sufficient to soften the segments and thereby substanially relieve said tension by inelastic stretching of the segments, and, after the segments have cooled, to re-tension the segments and heat them while in intimate side-by-side contact to a temperature sufficient to cause the fibre segments to fuse together. For this purpose the apparatus uses a pair of clamp units for said segments, said clamp units being movable along slideways by lead screws. Means for rotating the clamps are provided as well as means for measuring fiber tension. Between said clamps is a furnace accommodating the optical fibers where they are to be twisted and fused together without twisting the individual fibers. Summary of the Invention It is, therefore, an object of the present invention to provide a method for manufacturing a fiber type coupler which hardly ever fractures in an enviromnent of varying temperature so that the long term reliability of the fiber type coupler is improved. It is a further object of the present invention to provide an apparatus for carrying out said method. According to one aspect of the present invention a fiber type coupler is made by fusing at least two optical fibers extending under tension along an optical axis together to form a coupler member and fixing the coupler member to a protective member, a predetermined tension being applied to the coupler member along the optical axis before the coupler member is fixed to said protective member, and this predetermined tension is maintained while said coupler member is fixed to said protective member. According to a further aspect of the present invention a fiber type coupler is made by fusing at least two optical fibers extending under tension along an optical axis together to form a coupler member and fixing the coupler member to a protective member; and in that a predetermined tension is applied to the coupler member along the optical axis before the coupler member is fixed to said protective member, for checking any break in said coupler member before fixing a break-free coupler member to said protective member. An apparatus for manufacturing a fiber type coupler according to said further aspect of the present invention is as defined in the accompanying Claim 9. The scope of applicability of the present invention will become apparent from the detailed discription given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only. Brief Description of the Drawings Fig. 1 shows a manufacturing method for a fiber type coupler in accordance with the related background art. Fig. 2 shows a flow chart of a manufacturing method for a fiber type coupler in accordance with one embodiment of the present invention, Figs. 3A to 3D show a process chart of a manufacturing process of the fiber type coupler, Fig. 4 shows a perspective view of a manufacturing apparatus for the fiber type coupler, Figs. 5A and 5B show a process chart for fixing the coupler member, Fig. 6 shows a result of experiment relating to a relationship between an applied tension and a break time, Fig. 7 shows a flow chart of a manufacturing method of a fiber type coupler in accordance with one embodiment of the present invention, Fig. 8A to 8E show a process chart of a manufacturing process of the fiber type coupler, and Fig. 9A and 9B show a result of experiment for the present invention. Description of the Preferred Embodiment Fig. 2 shows a flow chart of the manufacturing process for the fiber type coupler in accordance with the embodiment of the present invention, Figs. 3A to 3D show a process chart of the method for manufacturing the fiber type coupler, and Fig. 4 shows a perspective view of a fiber type coupler manufacturing apparatus. The optical fiber wires 1 and 2 are gripped by first clamp members 5 and 6 which are fixed on fine movement stages 3 and 4 which are finely movable along the optical axis. A heating device 7, such as a burner, is arranged below the portions from which the coatings have been removed. Second clamp members 8 and 9 are arranged on the both sides of the clamp members 5 and 6 along the optical axis of the optical fiber wires 1 and 2. The second clamp members 8 and 9 are mounted through a tension application mechanism 10 on a 3-way movable stage 11 which is movable in three directions (optical axis direction and two directions which are orthogonal to the optical axis). The tension application mechanism 10 includes a micrometer so that it can drive the second clamp member 9 by a unit of micron. A tension measurement device 12 is arranged between the first clamp member 6 and the second clamp member 9 to measure the tension applied to the optical fiber. The method for manufacturing the fiber type coupler is now explained with reference to Fig. 2. In a step 201, the coatings of the optical fiber wires 1 and 2 are partially removed, and the optical fibers 1a and 2a having the coatings thus removed are contacted to each other (see Fig. 3A) and gripped by the first clamp member 5, 6. In this case, the optical fibers 1a and 2a may be twisted with each other. In a step 202, the contact area is fused by a heating device 3 such as a burner and it is extended by the first clamp members 5 and 6 along the optical axis. (see Fig. 3B and 4). In a step 203, a light split ratio is detected in the step of fusion and extension by measuring the light applied to one end of the optical fiber, at the other end. In a step 204, when a predetermined light split ratio is obtained, the fusion and extension (heating) of the optical fiber are stopped, and a coupler member having a coupler A is formed (see Fig. 3C). After the coupler member has been formed, the first clamp members 5 and 6 are released and the optical fiber wires 1 and 2 are gripped by the second clamp members 8 and 9. In a step 205, the optical fiber wires 1 and 2 are pulled along the optical axis by the second clamp members 8 and 9, and the coupler member A is fixed to the protective member 13 while the tension is applied to the optical fibers 1a and 2a (see Fig. 3D). A method for fixing the coupler member is now explained with reference to Figs. 5A and 5B. The second clamp members 8 and 9 are finely driven along the optical axis to apply a predetermined tension to the coupler member (see Fig. 5A). In this case, the tension applied may be adjusted because the tension is applied while it is monitored by the tension measurement device 12. Then, with the tension being applied, the coupler member is fixed to the protective member 13 such as ceramic by the bonding material such as epoxy resin or ultraviolet ray cured resin (see Fig. 5B). Referring now to Fig. 6, a result of experiment for the fiber type coupler of the present embodiment is explained. The experiment shows a relationship between the tension applied to the fiber type coupler and the break time in the extension process to apply the tension to the fiber type coupler. In the experiment, 25, 20 and 10 samples were prepared for the tension of 22 grams, 25 grams and 30 grams, respectively. An LED light source (wavelength of 1.3µm) and a photo-diode are connected to one of the wires to measure a time to the break of the wire. If the wire is broken, the light power from the LED is not propagated and the photo-diode does not detect the light power from the LED. The fiber type coupler used in the experiment has the split ratio of 50% and it can split the transmitted light having the wavelength of 1.3µm at 50-50. According to the experiment, in order to assure non-break for 20 years, the tension applied should be less than 14 grams. However, if the tension is too low, it is not possible to fix the coupler to the protective material without degrading the characteristic of the split ratio. Accordingly, it is desirable that the applied tension is set between 4 grams and 14 grams. The present invention is not limited to the above embodiment. For example, the split ratio is not limited to 50%, and the present invention is applicable not only to the single wavelength light propagation fiber type coupler but also the multi-wavelength one. A tapered optical fiber wire may be used as the optical fiber wire of the coupler member. Fig. 7 shows a flow chart of the manufacturing process for the fiber type coupler in accordance with the other embodiment of the present invention, and Fig. 8A to 8E show a process chart of the method for the manufacturing the fiber type coupler. In a step 301, the coatings of the optical fiber wires 1 and 2 are partially removed, and the optical fibers 1a and 2a are contacted to each other (see Fig. 8A). In a step 302, the contact area is fused by a heating device 7 such as a burner and it is extended along the optical axis. (see Fig. 8B). In a step 303, a light split ratio is detected in the step of fusion and extension by measurement of the light applied to one end of the optical fiber, at the other end. In a step 304, when a predetermined light split ratio is obtained, the fusion and extension of the optical fiber are stopped, and a coupler member having a coupler A is formed (see Fig. 8C). In a step 305, a predetermined tension is applied to the coupler member along the optical axis to check any break in the coupler member (see Fig. 8D). In a step 306, only the coupler member which has not been broken in the test is fixed to the protective material 4 by the bonding material such as epoxy resin or ultraviolet ray cured resin, to form the fiber type coupler (see Fig. 8E). Referring now to Figs. 9A and 9B, a result of experiment for the fiber type coupler of the present embodiment is explained. In the experiment, the fiber type coupler is extended until the predetermined split ratio is reached in the extension step. Then, it is cooled to a room temperature and pulled along the optical axis to apply the tension to the coupler A (having the diameter of approximately 20 µm). In this manner, a probability of break is examined for 70 fiber type couplers. In this case, a distance L between the fiber grip members was 25mm, and the displacements ΔL were 50µm, 100µm and 150µm, and the tension was applied for ten seconds. In Fig. 9a, the break probability for the newly manufactured fiber type couplers is shown by a solid line. Accordingly to the experiment, the break probability rapidly increases as the displacement ΔL increases. Then, the heat cycle test shown in Fig. 9B was done 20 times for the fiber type couplers whose break probability had been examined. In the heat cycle test, the manufactured fiber type coupler is left at -20 °C for one hour, heated to 100 °C in one hour, left at 100 °C for one hour, cooled to -40 °C in one hour, left at -40 °C for one hour and heated to 20 °C in one hour to complete the cycle. In Fig. 9A, the result of the heat cycle test is shown by a broken line. It is seen from the result of experiment that the fiber type coupler whose displacement ΔL is longer than 100µm shows a particularly low break probability (almost 0%). It is considered that this is because the fiber type couplers having low break strength have been removed in the experiment applying increased displacement ΔL. In general, it is effective to pull the coupler by the displacement which is longer than five times of the outer diameter of the coupler at the velocity of 10µm(half of the diameter)/sec. The fiber type coupler used in the experiment has the split ratio of 50% which splits the transmitted light having the wavelength of 1.3µm at 50-50. The excess loss is less than 0.1 dB, and the protective material (mold case) is made of ceramic (MgO₃). The stage for fixing the fiber grip member to extend the fiber type coupler along the optical axis is controlled by a pulse motor and a computer, and the extension and the application of the tension are accurately controlled in the order of micron. From the invention thus described, it will be obvious that the invention may be varied within the scope of the accompanying claims.

Claims: A method of manufacturing a fiber type coupler by fusing at least two optical fibers (1,2) extending under tension along an optical axis together to form a coupler member (A) and fixing the coupler member to a protective member (13), characterized in that a predetermined tension is applied to the coupler member along the optical axis before the coupler member is fixed to said protective member, and this predetermined tension is maintained while said coupler member is fixed to said protective member. A method of manufacturing a fiber type coupler by fusing at least two optical fibers (1,2) extending under tension along an optical axis together to form a coupler member (A) and fixing the coupler member to a protective member (13), characterized in that a predetermined tension is applied to the coupler member along the optical axis before the coupler member is fixed to said protective member, for checking any break in said coupler member before fixing a break-free coupler member to said protective member. A method of manufacturing a fiber type coupler according to claim 1 or 2, wherein said at least two optical fibers (1,2) are arranged in twisted state before fusing. A method of manufacturing a fiber type coupler according to claim 1 or 2, wherein said at least two optical fibers (1,2) are arranged in parallel contact state before fusing. A method of manufacturing a fiber type coupler according to any of claims 1 to 4, characterized in that the coating of said at least two optical fibers (1,2) is removed A method of manufacturing a fiber type coupler according to claim 1 or any of claims 3 to 5 when dependent on claim 1, wherein the tension is between 4 grams to 14 grams. A method of manufacturing a fiber type optical coupler according to claim 2 or any of claims 3 to 6 when dependent on claim 2, including detecting a monitoring light applied to one end of said coupler member (A) at the other end while said tension is applied thereto to check any break in said coupler member. A method of manufacturing a fiber type coupler according to claim 2 or any of claims 3 to 5 when dependent on claim 2, wherein said predetermined tension is created by pulling said coupler member (A) by a distance equal to five times of an outer diameter of said coupler member in ten seconds. A manufacturing apparatus for a fiber type coupler for fusing at least two optical fibers (1,2), having coatings thereof removed, together along an optical axis while extending the optical fibers under tension to form a coupler member, comprising: first fiber grip means (5,6) for gripping said at least two fibers at two intermediate positions along the optical axis; fiber heating means (7) arranged between said first fiber grip means for heating said at least two optical fibers; first tension application means (3,4) for increasing the distance between said first fiber grip means for applying a tension to said at least two optical fibers while they are heated by said fiber heating means, along the optical axis, characterized by: second fiber grip means (8,9) that are spaced such that the first fiber grip means are therebetween; second tension application means (8,9,10,11) for increasing the distance between said second fiber grip means for applying a tension to the cooled optical coupler member formed, said second tension application means including a micrometer so that it can drive one (9) of the second fiber grip members by units of a micron; and tension measurement means (12) for measuring the tension applied by said second tension application means. A manufacturing apparatus for a fiber type coupler according to claim 9 , wherein said first fiber grip means (5,6) are mounted on fine movable stages (3,4) movable along the optical axis. A manufacturing apparatus for a fiber type coupler according to claim 9 or 10 wherein said second tension application means (8,9,10,11) is mounted on a 3-way movable stage (11) movable along the optical axis and two directions orthogonal to the optical axis.

Inventors: FUKUMA MASUMI YOKOHAMA WORKS S, FUKUMA, MASUMI YOKOHAMA WORKS SUMITOMO ELECTRICS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1989
Main classification: C03B37
IPC codes: G02B6