Optical fiber splicing method

Abstract: A method of splicing optical fibers having different mode field diameters (MFDs), (D1) and (D2), is provided to reduce the splicing loss. In a pre-fusion heating step, the MFD at an end face of the optical fiber (1) having larger MFD is enlarged by heating a portion including the end face thereof so as to diffuse a dopant. After the pre-fusion heating step, fusion-splicing of the first (1) and the second (2) optical fibers is performed. Thereafter, during a post-fusion heating step, the dopant is further diffused by heating a portion (3) including the fusion-spliced parts of the two optical fibers.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of connecting, by fusion-splicing, two optical fibers having different mode field diameter (hereinafter, referred to as MFD) from each other. Description of the Background Art A dispersion-compensating fiber has been used to compensate chromatic dispersion of an optical transmission line comprising optical fibers. An optical amplifier having an optical fiber whose optical waveguide region is doped with a rare earth element has been used to compensate loss in the optical transmission line. In such cases, two optical fibers having different MFD have had to be connected together. Two connecting methods have been known, namely, a method of using a connector and fusion-splicing, the latter causing small connecting loss. To connect two optical fibers by fusion-splicing, the coating on each optical fiber is removed at the adjacent end thereof, then the adjacent end faces of the two optical fibers are butted together, and the end faces are softened and fusion-spliced by heating with an arc discharge or the like. When the MFDs of the two fibers are equal to each other at the fusion-splicing portions, the splicing loss is small. When the MFDs are different, the larger is the difference in diameters, the larger becomes the splicing loss. An optical fiber splicing method has been disclosed in Japanese Patent Application Laid Open No. 04-118607, which is intended to reduce the splicing loss when two optical fibers having different MFDs has been fusion-spliced. Two optical fibers are referred to as a first optical fiber having a larger MFD and a second optical fiber having a smaller MFD, respectively. According to this splicing method, the MFD of the second optical fiber is enlarged by heating a portion including the adjacent end face thereof and by diffusing a dopant therein, and then the first and second optical fibers are fusion-spliced. Thus, the difference between the MFDs of the first and the second optical fibers can be reduced at the fusion-splicing portion, thereby leading to a reduction in the splicing loss. EP 0 340,042 relates to a method of splicing optical fibres in which at least one of the fibers has a high numerical aperture (NA) and the two fibers have different mode spot sizes. The method comprises fusing the ends of the fibers together with their cores in alignment and heating the fused junction between the fibers so as to cause dopant to migrate out of the core. The heating step is carried out for a time sufficient to achieve a predetermined level of matching between propagation modes in the two fibers. EP 1,063,544 describes an exposed part of at least one of two optical fibers with different mode field distributions to be fusion spliced is heated by a burner, to thereby continuously vary its mode field distribution in the longitudinal direction of the optical fiber, and the exposed end part of at least one of the optical fibers is cleaved so that the mode field distributions at the splice end faces of the two optical fibers to be fusion spliced are substantially coincident with each other in configuration, the two optical fibers are fusion spliced, whereby low splice loss is realized. Thereafter, the heated portion of the optical fiber is subjected to a surface treatment by etching process using a hydrofluoric acid solution, whereby a deterioration of its mechanical strength is prevented. SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical fiber splicing method for reducing splicing loss of a first optical fiber and a second optical fiber whose MFDs are different from each other. According to one aspect of the present invention there is provided a method of connecting by fusion-splicing a first optical fiber, having a first Mode Field Diameter which increases with heating, and a second optical fiber, having a second Mode Field Diameter which first increases and next decreases with heating, said second Mode Field Diameter being smaller than the first Mode Field Diameter, said method comprising: a step of heating a portion including an adjacent end face of the first optical fiber so as to diffuse a dopant; and a step of connecting the first and the second optical fibers by fusion-splicing. The method comprises a pre-fusion heating step of heating a portion including the adjacent end face of the first optical fiber so as to diffuse a dopant, and a fusion-splicing step of connecting the first and the second optical fibers. The method further comprises a post-fusion heating step of heating a portion including the fusion-spliced part between the first and the second optical fibers after the fusion-splicing step so as to diffuse the dopant contained therein. The dopant may be diffused during pre-fusion heating step until the Mode Field Diameter 2W 1 defined by Petermann I at the adjacent end face of the first optical fiber is enlarged by at least 1 um. Mode Field Diameter 2W 1 can be obtained by the following formula: , where E(r) represents the distribution of the LP01 mode. According to another aspect of the present invention there is provided an optical transmission line including a first optical fiber having a first Mode Field Diameter which increases with heating and a second optical fiber having a second Mode Field Diameter which first increases and next decreases with heating, said second Mode Field Diameter being smaller than the first Mode Field Diameter, fabricated by a process comprising: a step of heating a portion including an adjacent end face of the first optical fiber so as to diffuse a dopant; and a step of connecting the first and the second optical fibers by fusion-splicing. The above and further object and novel features of the invention will be more fully clarified from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustrating only and are not intended as a definition of the limits of the invention. BRIEF DESCRIPTION OF THE DRAWING In order to more fully understand the drawings used in the detailed description of the present invention, a brief description of each drawing is provided: Figure 1 is an illustration of a method of splicing optical fibers according to an embodiment of the present invention; Figure 2 is an illustration of the refractive index profile of a first optical fiber (Optical Fiber 1); Figure 3 is an illustration of the refractive index profile of a second optical fiber (Optical Fiber 2); Figures 4A is a graph showing variation of MFDs at the adjacent end faces of the Optical Fiber 1 and 2 and Figure 4B showing a change of splicing loss, respectively, versus heating time, with the method of splicing optical fibers according to an embodiment of the present invention; Figure 5A is a graph showing variation of MFDs at the adjacent end faces of the Optical Fiber 1 and 2 and Figure 5B showing a change of splicing loss, respectively, versus heating time, with the method of splicing optical fibers similar to the embodiment of the present invention, but without a pre-fusion heating step; and Figure 6 shows an optical transmission line. DETAILED DESCRIPTION OF THE INVENTION In the following, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. To facilitate the comprehension of the explanation, the same reference numerals denote the same parts, where possible, throughout the drawings, and a repeated explanation will be omitted. The dimensions in the drawings are partly exaggerated and do not always correspond to actual ratios of the dimensions. Referring to Fig. 1, an embodiment of a method for splicing optical fibers according to the present invention will be described. At first, a first optical fiber (Optical Fiber 1) and a second optical fiber (Optical Fiber 2) are prepared. The MFD D 2 of the Optical Fiber 2 is smaller than the MFD D 1 of the Optical Fiber 1 in the range of operating wavelength, e.g., at the wavelength of 1.55 µm. Then, coatings of the Optical Fiber 1 and 2 are removed at the adjacent end thereof to be fusion spliced, and a glass 4 and a glass 5 are exposed. The pair of lines drawn inside each optical fiber in the figure indicates the MFD at each position along the longitudinal direction of each optical fiber. Optical Fiber 1 is, e.g., a typical single mode optical fiber having zero dispersion wavelength around 1.3 µm and about 17 ps/nm/km at 1.55 µm. The MFD D 1 is about 10 µm at the wavelength of 1.55 µm. The Optical Fiber 1 has a step index profile doped with germania (GeO 2 ) in a region including the optical axis. On the other hand, the Optical Fiber 2 is, e.g., a dispersion-compensating optical fiber having a chromatic dispersion value ranging from about -20 to -250 ps/nm/km at the wavelength of 1.55 µm. The MFD D 2 is in the order of several micrometers at the wavelength of 1.55 µm. The Optical Fiber 2 has a more complicated refractive index profile than the Optical Fiber 1. The Optical Fiber 2 is doped with an appropriate amount of, for example, GeO 2 or fluorine (F) in a predetermined region thereof so as to increase or decrease, respectively, the refractive index thereof. In the pre-fusion heating step, the MFD at the adjacent end of the Optical Fiber 1, which is larger than that of the Optical Fiber 2, is enlarged by heating a portion including the adjacent end so as to diffuse the dopant. The MFD at the adjacent end face of the Optical Fiber 1, as defined by Petermann I, is preferably enlarged by at least 1 µm. A burner flame, an arc discharge to be generated by a pair of opposing electrodes flanking the Optical Fiber 1, and a high-power CO 2 laser beam can be used as a heating source in the pre-fusion heating step. The heating temperature in this step is determined such that the dopant contained in the Optical Fiber 1 can be diffused, but this fiber itself is not softened. A fusion-splicing step is performed after the pre-fusion heating step. In the fusion-splicing step, the Optical Fiber 1 and 2 are connected together by fusion-splicing. More specifically, the adjacent end faces of the Optical Fiber 1 and 2 are butted together, and then the portion including the adjacent end faces is heated and softened by an arc discharge generated by a pair of opposing electrodes flanking the butted end faces, thereby fusion-splicing of the two optical fibers can be performed. In the ordinary fusion-splicing step, there is little variation in the MFD of the Optical Fiber 1, substantially keeping the diameter as enlarged in the pre-fusion heating step. There is also little variation in the MFD of the Optical Fiber 2 in this fusion-splicing step. A post-fusion heating step is performed after the fusion-splicing step. In this step, the dopant is diffused by heating a portion including the fusion-spliced part 3 between the Optical Fiber 1 and 2. The MFD at the adjacent end face of the Optical Fiber 2, having been initially small, enlarges rapidly during the post-fusion heating step, becomes larger than that of the adjacent end face of the Optical Fiber 1, reaches peak after a period of time, and subsequently becomes smaller and closer to that of the adjacent end face of the Optical Fiber 1. The MFD at the adjacent end face of the Optical Fiber 2 is accordingly adjusted so as to reduce the difference between the MFDs at the fusion-spliced part 3 of the two fibers. Thus, the splicing loss gradually decreases as the MFD at the adjacent end face of the Optical Fiber 2 decreases as time passes. The same heating source as in the pre-fusion heating step can be used as a heating source in the post-fusion heating step. The heating temperature for this process is determined such that the dopant having been added both optical fibers can be diffused, but neither of the optical fibers are softened. Figure 6 shows an optical transmission line 6 comprising Optical Fiber 1 and 2 connected together in accordance with the method as described above. The fusion splicing loss of the fusion-spliced part 3 can be reduced to 0.2 dB or less. The optical transmission line 6 is provided between a transmitter 7 and a receiver 8. The embodiment will now be described more in detail. A single mode optical fiber 21 having the refractive index profile shown in Fig. 2 is prepared here as the Optical Fiber 1. The optical fiber 21 comprises a core region 22 having a high refractive index and including the optical axis, and a cladding region 23 having a low refractive index and surrounding the core region 22. Both regions together form a simple step index profile. The outer diameter of the core region 22 is 8.2 µm and the outer diameter of the cladding region 23, i.e., the outer diameter of the fiber is 125 µm. The relative refractive index difference of the core region 22 is 0.34% compared to that of the cladding region 23. At the wavelength of 1.55 µm, the optical fiber 21 has a chromatic dispersion of 17 ps/nm/km, a dispersion slope of 0.057 ps/nm 2 /km, a MFD 2W 1 of 10.7 µm defined by Petermann I, a MFD 2W 2 of 10.3 µm defined by Petermann II, and an effective core area of 80 µm 2 . The MFD 2W 2 can be obtained by the following formula: , where E (r) represents the distribution of the LP01 mode. On the other hand, a dispersion compensating optical fiber 31 having the refractive index profile shown in Fig. 3 is prepared as the Optical Fiber 2. The optical fiber 31 comprises a core region 32 including the optical axis, a trench region 33 surrounding the core region 32, a ridge region 34 surrounding the trench region 33, and a cladding region 35 surrounding the ridge region 34, which together form the refractive index profile. The outer diameters of the core region 32, the trench region 33, and the ridge region 34 are 4 µm, 10 µm, and 17 µm, respectively. The outer diameter of the cladding region 35, i.e., the outer diameter of the fiber, is 125 µm. The relative refractive index differences of the core region 32, the trench region 33, and the ridge region 34 are 1.6%, -0.5%, and 0.2%, respectively, compared to that of the cladding region 35. At the wavelength of 1.55 µm, the second optical fiber 31 has a chromatic dispersion of -96 ps/nm/km, a dispersion slope of -0.75 ps/nm 2 /km, an MFD 2W 1 of 7.7 µm defined by Petermann I, an MFD 2W 2 of 4.9 µm defined by Petermann II, and an effective core area of 19 µm 2 . During the pre-fusion heating step in this embodiment, the MFD 2W 1 at the adjacent end face of the optical fiber 21 is enlarged from 10.7 µm to 14.9 µm, the MFD 2W 2 from 10.3 µm to 13.2 µm, and the effective core area from 80 µm 2 to 131 µm 2 . Figs. 4A and 4B show variation of MFDs defined by Petermann I at the adjacent end faces of the Optical fiber 1 and 2 and splicing loss, respectively as heating time passes as shown in the axis of abscissas during the post-fusion heating step. As shown in Fig. 4A, the MFD at the adjacent end face of the Optical Fiber 1 varies little as time passes, substantially keeping the diameter as enlarged during the pre-fusion heating step. On the other hand, the MFD at the adjacent end face of Optical Fiber 2, initially having been smaller, becomes rapidly larger than that at the adjacent end face of the Optical Fiber 1, reaches a peak after a period of time, and subsequently becomes smaller and closer to that at the adjacent end face of the Optical Fiber 1. As shown in Fig. 4B, as the MFD at the adjacent end face of the Optical Fiber 2 varies as time passes, the splicing loss decreases from about 1.7 dB to about 0.08 dB after 450 seconds, and thereafter remaines as low as this value. Figures 5A and 5B show variation of the MFDs defined by Petermann I at the adjacent end faces of the Optical Fiber 1 and 2 and the splicing loss, respectively as heating time passes with the method of splicing two optical fibers similar to the above mentioned embodiment, but without the pre fusion heating step as shown in the axis of abscissas during the post-fusion heating step. As shown in Fig. 5A, the MFD at the adjacent end face of the Optical Fiber 1 varies little as time passes, substantially keeping the initial diameter. On the other hand, the MFD at the adjacent end face of the Optical Fiber 2, initially having been smaller, becomes rapidly larger than that at the adjacent end face of the Optical Fiber 1, reaches a peak after a period of time, and subsequently becomes smaller. The MFD at the adjacent end face of the Optical Fiber 2, however, does not decrease to approximately same MFD at the adjacent end face of the Optical Fiber 1, and the difference between MFD of the two fibers remains about 4 µm, unlike the case of the embodiment. As shown in Fig. 5B, as the MFD at the adjacent end face of the Optical Fiber 2 varies as time passes, the splicing loss decreases from about 1.3 dB to about 0.4 dB after 450 seconds, and thereafter, does not decrease any further. When the fusion-splicing step has been performed after enlarging MFD at the adjacent end face of the Optical Fiber 2 whose MFD is smaller than that of Optical Fiber 1, as disclosed in Japanese Patent Application Laid Open No. 04-118607, the MFDs of the two optical fibers do not become close to each other, thus resulting in failure to reduce the splicing loss, similar to the case as shown in Fig. 5A and Fig. B.

Claims: A method of connecting by fusion splicing a first optical fiber (1), comprising a core region having a high refractive index and including the optical axis, and a cladding region having a low refractive index and surrounding the core region, and having a first Mode Field Diameter which increases with heating time, and a second optical fiber (2), comprising a core region including the optical axis, a trench region surrounding the core region, a ridge region surrounding the trench region, and a cladding region surrounding the .ridge region, and having a second Mode Field Diameter which first increases and next decreases with heating time, said second Mode Field Diameter being smaller than the first Mode Field Diameter, said method comprising: a step of heating a portion including an adjacent end face of the first optical fiber (1) so as to diffuse a dopant, a step of connecting the first (1) and the second (2) optical fibers by fusion-splicing; and a step of heating a portion including the fusion-spliced part between the first and the second optical fibers so as to diffuse the dopant contained therein and reduce the difference between the mode field diameters at the fusion spliced part (3) of the two fibers. The method of connecting the optical fibers by fusion-splicing according to Claim 1, wherein the Mode Field Diameter defined by Petermann I at the adjacent end face of the first optical fiber (1) is enlarged by at least 1µm by heating the portion including the adjacent end face thereof so as to diffuse the dopant during the heating step before fusion-splicing.

Inventors: FUKUDA KEIICHIRO, IWATA NORIKO, KASHIWADA TOMONORI, KATO TAKATOSHI, FUKUDA, KEIICHIRO, IWATA, NORIKO, KASHIWADA, TOMONORI, KATO, TAKATOSHI
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 2001
Main classification: G02B6
IPC codes: G02B6