METHOD OF FUSION SPLICING SINGLE-MODE OPTICAL FIBERS USING AN ARC DISCHARGE

Abstract: There is disclosed a method of fusion splicing single-mode optical fibers, in which distance D, between the axes (14₁), (14₂) of the optical fibers (10₁) and (10₂) subjected to fusion splicing, is measured when the cores (12₁) and (12₂) of the optical fibers (10₁) and (10₂) are centered, and an arc discharge time is deter­ mined according to the measurements of distance D.

Description: This invention relates to a method of fusion splicing single-mode optical fibers using an arc discharge. In fusion splicing single-mode optical fibers using an arc discharge, it is necessary to center cores 12₁ and 12₂ of optical fibers 10₁ and 10₂ to be spliced for low loss splicing (Fig. 1). Thereafter, optical fibers 10₁ and 10₂ are subjected to fusion splicing with an arc discharge. However, cores 12₁ and 12₂ of optical fibers 10₁ and 10₂ to be spliced are generally displaced from the axes 14₁ and 14₂ of optical fibers 10₁ and 10₂, respectively, as shown in Fig. 1. Therefore, when cores 12₁ and 12₂ of optical fibers 10₁ and 10₂ are centered to each other, axes 14₁ and 14₂ of optical fibers 10₁ and 10₂ have therebetween displacement or distance D, as shown in Fig. 1. When, while, optical fibers 10₁ and 10₂ are spliced using an arc discharge, surface tension occurs at the spliced portion of optical fibers 10₁ and 10₂ to reduce displacement D. Therefore, cores 12₁ and 12₂ of optical fibers 10₁ and 10₂ are sloped at the spliced portion, as seen in Fig. 2. The slope causes a loss in fusion splicing. A large slope in spliced cores 12₁ and 12₂ makes it impossible to obtain a low loss splicing of spliced optical fibers 10₁ and 10₂. While, it is required for the spliced optical fibers to have a limited mechanical strength. This invention has been achieved in consideration of the above-mentioned circumstances, the object being to provide a method of fusion splicing single-mode optical fibers using an arc discharge, which can provide low-loss spliced optical fibers, even when the optical fibers subjected to fusion splicing have the cores displaced from the axes of the optical fibers. According to the invention, there is provided a method of fusion splicing single mode optical fibers using an arc discharge comprising the steps of: arranging oppositely two single-mode optical fibers to be subjected to fusion splicing on a line; centering the cores of the optical fibers to each other; measuring a distance D between the axes of the optical fibers when the cores of the optical fibers are centered; deter­ mining conditions of an arc discharge to be applied to the optical fibers according to the distance D; and applying an arc discharge to the optical fibers according to the arc discharge conditions. This invention can be more fully understood from the following detailed description when taken in con­ junction with the accompanying drawings, in which: Fig. 1 shows two optical fibers subjected to fusion splicing with the cores centered to each other, the cores of the optical fibers being displaced from the axes of the optical fibers. Fig. 2 shows the spliced optical fibers; Fig. 3 is a schematical view of a fusion splicing apparatus to which a method of fusion splicing single mode optical fibers of the invention is applied; Fig. 4 shows a characteristic curve of an arc discharge time and the distance between the axes of the optical fibers when the cores of the optical fibers are centered to each other, the characteristic curve being applied to a fusion splicing method of the invention; Fig. 5 shows a characteristic curve of a loss in the fusion splicing and the distance when the optical fibers are spliced, obtained when an arc dis-charge time is determined, with respect to the distance, as shown in the characteristic curve of Fig. 4; Fig. 6 shows a characteristic curve of an arc discharge current and a distance between the axes of the optical fibers when the cores of the optical fibers are centered to each other, the characteristic curve being applied to a fusion splicing method of the invention; Fig. 7 shows a characteristic curve of a loss in the fusion splicing and the distance when the optical fibers are spliced, obtained when an arc discharge current is determined, with respect to the distance, as shown in the characteristic curve of Fig. 6; Fig. 8 shows images of optical fibers oppositely arranged which is seen through a microscope; and Fig. 9 shows images of optical fibers oppositely arranged, which is displayed on a screen of a television camera through a microscope; and Fig. 10 shows a flow chart with which distance between the axes of oppositely arranged optical fibers are determined. Embodiments of the invention will now be described with reference to the drawings. The same reference numerals are employed for the same parts through all the drawings. In fusion splicing single-mode optical fibers using an arc discharge, two single-mode optical fibers 10₁ and 10₂ subjected to fusion splicing are supported on the V grooves of V groove members 20₁ and 20₂ in such a manner that optical fibers 10₁ and 10₂ are oppositely arranged to each other in Z axis (Fig. 3). Thereafter, core 12₁ of optical fiber 10₁ and core 12₂ of optical fiber 10₂ are centered to each other, using a known technique (refer to Fig. 1). After cores 12₁ and 12₂ are cen­ tered, distance or displacement D between axis 14₁ of optical fiber 10₁ and axis 14₂ of optical fiber 10₂ are measured. Thereafter, optical fibers 10₁ and 10₂ are subjected to fusion splicing using an arc discharge through discharge electrodes 30₁ and 30₂. The arc discharge conditions are determined according to the measurement or distance D. When distance D is large, either or both of the arc discharge time and arc discharge current are selected to be small, while the spliced portion of the cores 10₁ and 10₂ has or attains the required mechanical strength. On the other hand, when distance D is small, either or both of the arc discharge time and arc discharge current are selected to be large. When the cladding 16 of cores 10₁ and 10₂ is made of pure silica (refer to Fig. 10), the arc discharge time and/or arc discharge current is determined as de­ scribed below, according to the measurement or distance D. (In Fig. 10, reference numeral 18 denotes a jacketing.) I. When the discharge current is fixed at 17 mA, the discharge time is determined as shown in Fig. 4, in which: (i) When distance D is smaller than 1 µm, an arc discharge time of 1 sec is selected. (ii) When distance D is larger than 4 µm, an arc discharge time 0.5 sec is selected. (iii) When distance D is between 1 µm to 4µm, an arc discharge time of between 1 sec. and 0.5 sec. is linearly or proportionally selected according to distance D. Namely, when distance D varies from 1 µm to 4µm, the determined arc discharge time linearly varies from 1 sec to 0.5 sec, correspondingly. As described in I-(i) above, when distance D is smaller than 1 µm, a long arc discharge time of up to 1 sec is selected. The long discharge time ensures the spliced portion of optical fibers 10₁ and 10₂ a sufficient mechanical strength. While, the obliquity or slope of cores 12₁ and 12₂ at the spliced portion of optical fibers 10₁ and 10₂ is small, since distance D is small and the surface tension which occurs is also small. In other words, a low-loss splicing of optical fibers 10₁ and 10₂ is provided, as seen in Fig. 5 which shows a characteristic curve of a loss in splicing and distance D, obtained when an arc discharge time is selected as shown in the characteristic curve in Fig. 4. As described in I-(i), when distance D is larger than 4 µm, a short arc discharge time of up to 0.5 sec is selected. Though the discharge time is so short, the spliced portion of optical fibers 10₁ and 10₂ is given the required degree of mechanical strength of 600 g. While, the obliquity of cores 12₁ and 12₂ at the spliced portion of optical fiber 10₁ and 10₂ is small, since such a short arc discharge time is selected and the surface tension which occurs is small. In other words, a low-loss splicing of optical fibers 10₁ and 10₂ is provided, as seen in Fig. 5. As described in I-(iii), when distance D varies from 1 µm to 4 µm the determined arc discharge time linearly varies from 1 sec to 0.5 sec, according to distance D. The linearly determined discharge time gives the spliced portion of optical fibers 10₁ and 10₂ the required degree of mechanical strength. While, the obliquity of cores 12₁ and 12₂ at the spliced portion of optical fibers 10₁ and 10₂ is small, since the arc discharge time is linearly reduced from 1 sec to 0.5 sec, as distance D increases from 1 µm to 4 µm. In other words, the low-loss splicing of optical fibers 10₁ and 10₂ is provided, as seen in Fig. 5. When the discharge time is so selected as described above, the obliquity of cores 12₁ and 12₂ at the spliced portion of optical fiber 10₁ and 10₂ is small. In other words, a low-loss splicing of optical fibers 10₁ and 10₂ is provided, as seen in Fig. 5. Therefore, the light transmission efficiency of an optical fiber cable formed of spliced optical fibers 10₁ and 10₂ is enhanced, even when distance D is large. While, the spliced portion of optical fibers 10₁ and 10₂ is given at least the required degree of mechanical strength. II. When the discharge time is fixed at 1 sec. the discharge current is determined as shown in Fig. 6. in which: (i) When distance D is smaller than 1µm, an arc discharge current 17 mA is selected. (ii) When distance D is larger than 4 µm, an arc discharge of 15 mA is selected. (iii) When distance D is between 1 µm to 4µm, an arc discharge current of between 17 mA and 15 mA is linearly or proportionally selected, according to distance D. Namely, when distance D varies from 1 µm to 4 µm, the determined discharge current linearly varies from 17 mA to 15 mA. As described in II-(i) above, when distance D is smaller than 1 µm, a large arc discharge current of up to 17 mA is selected. The large discharge current ensures the spliced portion of optical fibers 10₁ and 10₂ have sufficient mechanical strength. While, the obliquity or slope of cores 12₁ and 12₂ at the spliced portion of optical fiber 10₁ and 10₂ is small, since distance D is small and the surface tension which occurs is small. In other words, a low-loss splicing of opti­ cal fibers 10₁ and 10₂ is provided, as seen in Fig. 7 which shows a characteristic curve of a loss in splicing and distance D, obtained when an arc discharge time is selected as shown in the characteristic curve in Fig. 6. As described in II-(ii), when distance D is larger than 4 µm, a small arc discharge current of up to 17 mA is selected. Though the discharge current is so small, the spliced portion of optical fibers 10₁ and 10₂ still has the required mechanical strength of 600 g. While, the obliquity of cores 12₁ and 12₂ at the spliced por­ tion of optical fibers 10₁ and 10₂ is small, since such a small arc discharge current is selected and the surface tension which occurs is small. In other words, a low-loss splicing of optical fibers 10₁ and 10₂ is provided, as seen in Fig. 7. As described in II-(iii), when distance D varies from 1 µm to 4 µm, the arc discharge current linearly varies from 17 mA to 15 mA, according to distance D. The linearly determined discharge current gives the spliced portion of optical fibers 10₁ and 10₂ a required mechanical strength. While, the obliquity of cores 12₁ and 12₂ at the spliced portion of optical fibers 10₁ and 10₂ is small, since the arc discharge current is linearly reduced from 17 mA to 15 mA, as distance D increases from 1 µm to 4 µm. In other words, a low-loss splicing of optical fibers 10₁ and 10₂ is provided, as seen in Fig. 7. When the discharge current is selected as described above, the obliquity of cores 12₁ and 12₂ at the spliced portion of optical fibers 10₁ and 10₂ is small. In other words, a low-loss splicing of optical fibers 10₁ and 10₂ is provided, as seen in Fig. 7. Therefore, the light transmission efficiency of an optical fiber cable formed of spliced optical fibers 10₁ and 10₂ is enhanced, even when distance D is large. While, the spliced portion of optical fiber 10₁ and 10₂ is given at least the required mechanical strength. In the embodiments described above, the discharge time or discharge current is selected according to distance D. However, the product of the electrical power (mW) and time (sec.) shows the electrical energy level (mW.sec.). Therefore, both the discharge time and discharge current (i.e. the electrical energy) may be controlled according to distance D. In that case, substantially the same effects as those in the above-­ described embodiments are provided, too. The above-mentioned embodiments refer to an optical fiber whose cladding is made of pure silica. When a cladding of an optical fiber contains an additive such as phosphor or bromine, the melting point of the cladding lowers. Therefore, if such an optical fiber containing any additive is referred to, a modification to the above-mentioned embodiments is necessary. Distance D is measured as follows. For measuring distance D, microscope 22 and TV (television) camera 30 are provided (Fig. 3). Furthermore, light sources 24, 26 and deflector 28 are provided (Fig. 3). Microscope 22 is on the Y axis passing through the center of the opposite ends of optical fibers 10₁ and 10₂ subjected to fusion splicing oppositely arranged in the Z axis. Microscope 22 is positioned above the opposite ends of optical fibers 10₁ and 10₂. Light source 24 is on the Y axis. Light source 24 is positioned below the opposite ends of optical fibers 10₁ and 10₂. A light beam of light source 24 is incident upon microscope 22, in the Y axis, as light beam Lxx, through the opposite ends of optical fibers 10₁ and 10₂ and objective 23. Light source 26 is on the X axis which passes through the center of the opposite ends of optical fibers 10₁ and 10₂. Deflector 28 is on the X axis. Deflector 28 inclines by 45 degrees to the X axis so that a light beam of light source 26 is deflected to be directed in the Y axis, causing the light beam to be incident upon microscope 22 through objective 23 as light beam Lxy. One can see through microscope 22 images 10₁x and 10₂x (Fig. 8) of optical fibers 10₁ and10₂, taken in X-Z plane, based on light beam Lxx emitted from the light source 24 and also can see images 10₁y and 10₂y (Fig. 8) of optical fibers 10₁ and 10₂ taken in Y-Z plane, based on light beam Lxy emitted from light source 24 and deflected by deflector 28. Cores 12₁ and 12₂ in the images look dark. Images 10₁x, 10₂x, 10₁y, and 10₂y are displayed on a screen of TV camera 30, as shown in Fig. 9. In measuring distance D, a position C1, on scanning line 32 orthogonal to the axis of optical fiber 10₁, of the axis of core 12₁ on image 10₁x, and a position C3, on scanning line 32 orthogonal to the axis of optical fiber 10₂, of axis of core 12₁ on image 10₁y are measured (step 1 in Fig. 10). Thereafter, a position F1, on scanning line 32, of the axis of optical fiber 10₁ on image 10₁x, and a position F3, on scanning line 32, of the axis of optical fiber 10₁ on image 10₁y are measured (step 2). After then, a position C2, on scanning line 34, of the axis of core 12₂ on image 12₂x, and a position C4, on scanning line 34, of the axis of core 10₂ on image 10₂y are measured (step 3). Thereafter, a position F2, on scanning line 34, of the axis of optical fiber 10₂ on image 10₂x, and a position F4, on scanning line 34, of the axis of optical fiber 10₂ on image 10₂y are measured (step 4). Following this, positions C1 and C2 are adjusted to be aligned with each other, as are positions C3 and C4, so that cores 12₁ and 12₂ of optical fibers 10₁ and 10₂ are centered (step 5). Thereafter, equations dl=Fl-F2, d2=F3-F4 are carried out to provide a distance d1 between positions F1 and F2 and a distance d2 between positions F3 and F4 (step 6). After than, D=d1²+d2² is carried out to provide distance D (step 7). As can clearly be understood from the above, in the splicing method of the invention, distance D, between the cores of the optical fibers to be fusion spliced, is measured, and according to the measurement, discharge conditions are properly selected. Therefore, the obliquity of the cores at the spliced portion of the optical fibers is small. In other words, a low-loss splicing of the optical fibers is provided. Therefore, the light transmission efficiency of an optical fiber cable formed of the optical fibers is enhanced, even when the distance is large. While, the spliced portion of the optical fibers has at least the required mechani­ cal strength.

Claims: 1. A method of fusion splicing single-mode optical fibers comprising the steps of: arranging oppositely two single-mode optical fibers (10₁), (10₂) subjected to fusion splicing on a line; centering the cores (12₁), (12₂) of the optical fibers (10₁), (10₂) to each other; measuring a distance D between the axes (14₁), (14₂) of the optical fibers (10₁), (10₂) when the cores (12₁), (12₂) of the optical fibers (10₁), (10₂) are cen­ tered; determining the conditions of an arc discharge to be applied to the optical fibers (10₁), (10₂) according to the distance D; and applying an arc discharge to the optical fibers (10₁), (10₂) according to the arc discharge conditions. 2. A method of fusion splicing single-mode optical fibers according to claim 1, in which said conditions include the determination of the arc discharge time according to the distance. 3. A method of fusion splicing single-mode optical fibers according to claim 1, in which said conditions include the determination of the arc discharge current according to the distance. 4. A method of fusion splicing single-mode optical fibers according to claim 1, in which said conditions include the determination of the electric energy for an arc discharge according to distance. 5. A method of fusion splicing single-mode optical fibers according to claim 2, in which, when claddings (16) of the optical fibers (10₁), (10₂) are made of pure silica and the arc discharge current is fixed at substantially 17 mA, said arc discharge time is to determined that, when the distance is smaller than 1 µm, said arc discharge time is determined at substantially 1 sec., when the distance is larger than 4 µm, said arc discharge time is determined at substantially 0.5 sec., and, when the distance is between 1 µm and 4 µm, said arc discharge time is determined at a value linearly varied between 1 sec. and 0.5 sec., according to the distance. 6. A method of fusion splicing single-mode optical fibers according to claim 3, in which when claddings (16) of the optical fibers (10₁), (10₂) are made of pure silica and the arc discharge time is fixed at 1 sec., said arc discharge current is so determined that, when the distance is smaller than 1 µm, said arc discharge current is determined at substantially 17 mA, when the distance is larger than 4 µm, said arc discharge current is determined at substantially 15 mA, and, when the distance is between 1 µm and 4 µm, said arc discharge current is determined at a value linearly varied between 17 mA and 15 mA, according to the distance. 7. A method of fusion splicing single-mode optical fibers according to claim 1, in which said measuring step of the distance D comprises the steps of: displaying, on a screen of a TV camera (30), a first image (10₁x) of one (10₁) of the optical fibers (10₁) and (10₂) taken in the X-Z plane, a second image (10₁y) of said one optical fiber (10₁) taken in the Y-Z plane, a third image (10₂x) of the other (10₂) of the optical fibers (10₁) and (10₂) taken in the X-Z plane, and a fourth image (10₂y) of said other optical fibers (10₂) taken in the Y-Z plane; measuring a position C1, on a first scanning line (32) orthogonal to the axis of said one optical fiber (10₁), of the axis of the core (12₁) on said first image (10₁x), and a position C3, on said first scanning line (32), of the axis of core (12₁) on said second image (10₁y); measuring a position F1, on said first scanning line (32), of the axis of said one optical fiber (10₁) on said first image (10₁x), and a position F3, on said first scanning line (32), of the axis of said first optical fibre (10₁) on said second image (10₁y); measuring a position C2, on a second scanning line (34) orthogonal to the axis of said the other optical fiber (10₂), of the axis of the core (12₂) on said third image (10₂x), and a position C4, on said second scanning line (34), of the axis of core (12₂) on said fourth image (10₂y); measuring a position F2, on said second scanning line (34), of the axis of said other optical fiber (10₂) on said third image (10₂x), and a position F4, on said second scanning line (34), of the axis of said other optical fiber (10₂) on said fourth image (10₁y); adjusting the optical fibers (10₁) and (10₂) so that said position C1 of the axis of the core (12₁) on said first image (10₁x) and said position C2 of the axis of the core (12₂) on said third image (10₂x) are aligned with each other and also said position C3 of axis of the core (12₁) on said second image (10₁y) and said position C4 of the axis of the core (12₂) on said fourth image (10₂y) are aligned with each other, whereby the cores (12₁) and (12₂) of optical fibers (10₁) and (10₂) are centered; carrying out equations d1=F1-F2, d2=F3-F4 to pro­ vide a distance d1 between positions F1 and F2 and a distance d2 between positions F3 and F4; and carrying out equation D=d1²+d2² to provide distance D.

Inventors: KAWATA, OSAMU, OSATO, YASUKUNI, SUZUKI, MASAO, WATANABE, OKOSU, YAMADA, TAKESHI
Applicants: FUJIKURA LTD., NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1985
Main classification: C03B37
IPC codes: G02B6