Method of Fusion Splicing Single-Mode Optical Fibers Using an Arc Discharge

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 12.sub.1 and 12.sub.2 of optical fibers 10.sub.1 and 10.sub.2 to be spliced for low loss splicing (FIG. 1). Thereafter, optical fibers 10.sub.1 and 10.sub.2 are subjected to fusion splicing with an arc discharge. However, cores 12.sub.1 and 12.sub.2 of optical fibers 10.sub.1 and 10.sub.2 to be spliced are generally displaced from the axes 14.sub.1 and 14.sub.2 of optical fibers 10.sub.1 and 10.sub.2, respectively, as shown in FIG. 1. Therefore, when cores 12.sub.1 and 12.sub.2 of optical fibers 10.sub.1 and 10.sub.2 are centered to each other, axes 14.sub.1 and 14.sub.2 of optical fibers 10.sub.1 and 10.sub.2 have therebetween displacement or distance D, as shown in FIG. 1. When optical fibers 10.sub.1 and 10.sub.2 are spliced using an arc discharge, surface tension occurs at the spliced portion of optical fibers 10.sub.1 and 10.sub.2 to reduce displacement D. Therefore, cores 12.sub.1 and 12.sub.2 of optical fibers 10.sub.1 and 10.sub.2 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 12.sub.1 and 12.sub.2 makes it impossible to obtain a low loss splicing of spliced optical fibers 10.sub.1 and 10.sub.2. Also it is required for the spliced optical fibers to have at least a certain 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 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 two single-mode optical fibers to be subjected to fusion splicing opposite one another on a line; aligning the axes of 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 aligned; determining conditions for 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 determined arc discharge conditions.

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 discharge 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 as seen through a microscope;

FIG. 9 shows images of optical fibers oppositely arranged, as 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 10.sub.1 and 10.sub.2 subjected to fusion splicing are supported on the V grooves of V groove members 20.sub.1 and 20.sub.2 in such a manner that optical fibers 10.sub.1 and 10.sub.2 are arranged to oppose each other on a Z axis (FIG. 3). Thereafter, core 12.sub.1 of optical fiber 10.sub.1 and core 12.sub.2 of optical fiber 10.sub.2 are centered to each other, using a known technique (refer to FIG. 1). After cores 12.sub.1 and 12.sub.2 are centered, distance or displacement D between axis 14.sub.1 of optical fiber 10.sub.1 and axis 14.sub.2 of optical fiber 10.sub.2 are measured. Thereafter, optical fibers 10.sub.1 and 10.sub.2 are subjected to fusion splicing using an arc discharge through discharge electrodes 30.sub.1 and 30.sub.2. 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 10.sub.1 and 10.sub.2 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 10.sub.1 and 10.sub.2 is made of pure silica (refer to FIG. 8), the arc discharge time and/or arc discharge current is determined as described below, according to the measurement or distance D. (In FIG. 8, 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 .mu.m, an arc discharge time of 1 sec is selected.

(ii) When distance D is larger than 4 .mu.m, an arc discharge time of 0.5 sec is selected.

(iii) When distance D is between 1 .mu.m to 4 .mu.m, 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 .mu.m to 4 .mu.m, 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 .mu.m, a long arc discharge time of up to 1 sec is selected. The long discharge time ensures the spliced portion of optical fibers 10.sub.1 and 10.sub.2 a sufficient mechanical strength. While, the obliquity or slope of cores 12.sub.1 and 12.sub.2 at the spliced portion of optical fibers 10.sub.1 and 10.sub.2 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 10.sub.1 and 10.sub.2 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-(ii), when distance D is larger than 4 .mu.m, 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 10.sub.1 and 10.sub.2 is given the required degree of mechanical strength of 600 g. While, the obliquity of cores 12.sub.1 and 12.sub.2 at the spliced portion of optical fiber 10.sub.1 and 10.sub.2 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 10.sub.1 and 10.sub.2 is provided, as seen in FIG. 5.

As described in I-(iii), when distance D varies from 1 .mu.m to 4 .mu.m, 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 10.sub.1 and 10.sub.2 the required degree of mechanical strength. Further, the obliquity of cores 12.sub.1 and 12.sub.2 at the spliced portion of optical fibers 10.sub.1 and 10.sub.2 is small, since the arc discharge time is linearly reduced from 1 sec to 0.5 sec, as distance D increases from 1 .mu.m to 4 .mu.m. In other words, the low-loss splicing of optical fibers 10.sub.1 and 10.sub.2 is provided, as seen in FIG. 5.

When the discharge time is so selected as described above, the obliquity of cores 12.sub.1 and 12.sub.2 at the spliced portion of optical fibers 10.sub.1 and 10.sub.2 is small. In other words, a low-loss splicing of optical fibers 10.sub.1 and 10.sub.2 is provided, as seen in FIG. 5. Therefore, the light transmission efficiency of an optical fiber cable formed of spliced optical fibers 10.sub.1 and 10.sub.2 is enhanced, even when distance D is large. Further, the spliced portion of optical fibers 10.sub.1 and 10.sub.2 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 1 .mu.m, an arc discharge current of 17 mA is selected.

(ii) When distance D is larger than 4 .mu.m, an arc discharge current of 15 mA is selected.

(iii) When distance D is between 1 .mu.m to 4 .mu.m, 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 .mu.m to 4 .mu.m, 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 .mu.m, a large arc discharge current of up to 17 mA is selected. The large discharge current ensures the spliced portion of optical fibers 10.sub.1 and 10.sub.2 have sufficient mechanical strength. The obliquity or slope of cores 12.sub.1 and 12.sub.2 at the spliced portion of optical fibers 10.sub.1 and 10.sub.2 is small, since distance D is small and the surface tension which occurs is small. In other words, a low-loss splicing of optical fibers 10.sub.1 and 10.sub.2 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 .mu.m, a small arc discharge current of up to 15 mA is selected. Though the discharge current is so small, the spliced portion of optical fibers 10.sub.1 and 10.sub.2 still has the required mechanical strength of 600 g. Also, the obliquity of cores 12.sub.1 and 12.sub.2 at the spliced portion of optical fibers 10.sub.1 and 10.sub.2 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 10.sub.1 and 10.sub.2 is provided, as seen in FIG. 7.

As described in II-(iii), when distance D varies from 1 .mu.m to 4 .mu.m, 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 10.sub.1 and 10.sub.2 a required mechanical strength. The obliquity of cores 12.sub.1 and 12.sub.2 at the spliced portion of optical fibers 10.sub.1 and 10.sub.2 is small, since the arc discharge current is linearly reduced from 17 mA to 15 mA, as distance D increases from 1 .mu.m to 4 .mu.m. In other words, a low-loss splicing of optical fibers 10.sub.1 and 10.sub.2 is provided, as seen in FIG. 7.

When the discharge current is selected as described above, the obliquity of cores 12.sub.1 and 12.sub.2 at the spliced portion of optical fibers 10.sub.1 and 10.sub.2 is small. In other words, a low-loss splicing of optical fibers 10.sub.1 and 10.sub.2 is provided, as seen in FIG. 7. Therefore, the light transmission efficiency of an optical fiber cable formed of spliced optical fibers 10.sub.1 and 10.sub.2 is enhanced, even when distance D is large. Moreover, the spliced portion of optical fiber 10.sub.1 and 10.sub.2 is always 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) monitor 31 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 10.sub.1 and 10.sub.2 subjected to fusion splicing arranged in opposed relation on the Z axis. Microscope 22 is positioned above the opposite ends of optical fibers 10.sub.1 and 10.sub.2. Light source 24 is on the Y axis. Light source 24 is positioned below the opposite ends of optical fibers 10.sub.1 and 10.sub.2. 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 10.sub.1 and 10.sub.2 and objective 23. Light source 26 is on the X axis which passes through the center of the opposite ends of optical fibers 10.sub.1 and 10.sub.2. 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 10.sub.1 x and 10.sub.2 x (FIG. 8) of optical fibers 10.sub.1 and 10.sub.2 taken in X-Z plane, based on light beam Lxx emitted from light source 24. Also, one can see images 10.sub.1 y and 10.sub.2 y (FIG. 8) of optical fibers 10.sub.1 and 10.sub.2 taken in Y-Z plane, based on light beam Lxy emitted from light source 24 and deflected by deflector 28. Cores 12.sub.1 and 12.sub.2 in the images look dark.

Images 10.sub.1 x, 10.sub.2 x, 10.sub.1 y and 10.sub.2 y 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 10.sub.1, of the axis of core 12.sub.1 on image 10.sub.1 x; and a position C3 on scanning line 32 orthogonal to the axis of optical fiber 10.sub.1, of axis of core 12.sub.1 on image 10.sub.1 y, are measured (step 1 in FIG. 10). Thereafter, a position F1 on scanning line 32, of the axis of optical fiber 10.sub.1 on image 10.sub.1 x; and a position F3 on scanning line 32, of the axis of optical fiber 10.sub.1 on image 10.sub.1 y are measured (step 2). After then, a position C2, on scanning line 34, of the axis of core 12.sub.2 on image 10.sub.2 x, and a position C4 on scanning line 34, of the axis of core 12.sub.2 on image 10.sub.2 y are measured (step 3). Thereafter, a position F2, on scanning line 34, of the axis of optical fiber 10.sub.2 on image 10.sub.2 x, and a position F4 on scanning line 34, of the axis of optical fiber 10.sub.2 on image 10.sub.2 y 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 12.sub.1 and 12.sub.2 of optical fibers 10.sub.1 and 10.sub.2 are centered (step 5). Thereafter, equations d1=F1-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 then, ##EQU1## 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 axes 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. Further, the spliced portion of the optical fibers has at least the required mechanical strength.