Patent Publication Number: US-2003223711-A1

Title: Method of repairing optical submarine cable and repair cable used in the method

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The invention relates to a method of repairing a broken optical transmission cable, and further to a repair cable used in the method.  
       [0003] 2. Description of the Related Art  
       [0004] An optical transmission cable is recently often laid on the sea bottom as a cable for transmitting data therethrough. FIG. 1 is a conceptual view of a conventional system for transmitting data through an optical transmission cable.  
       [0005] The conventional optical submarine transmission system illustrated in FIG. 1 is comprised of a plurality of optical submarine repeaters  52 , and optical cables  51  each optically connecting adjacent optical submarine repeaters  52  to each other.  
       [0006] The optical cable  51  in the conventional optical submarine transmission system illustrated in FIG. 1 is comprised of a single kind of fiber  53 . Since the optical cable  51  is laid on the sea bottom, a fishing craft often hooks the optical cable  51  with a fishing net, and resultingly, damages the optical cable  51 , in which case, the damaged optical cable  51  is pulled up onto a repair ship to be repaired.  
       [0007]FIGS. 2A and 2B illustrate a conventional method of repairing the damaged optical cable  51 .  
       [0008] It is assumed that the optical cable  51  is broken at a breakage point  55 , as illustrated in FIG. 2A.  
       [0009] First, a repair area  56  including the breakage point  55  and having a predetermined length is determined, and then, a portion of the optical cable  51  corresponding to the repair area  56  is cut out. Then, a spare cable having the same fiber arrangement as that of the optical cable  51  is cut to make a repair cable  57  having a length equal to or slightly greater than the length of the repair area  56 . Then, the repair cable  57  is inserted into the repair area  56 , as illustrated in FIG. 2B. Thus, the optical cable  51  is repaired for the breakage.  
       [0010] Since the repair cable  57  to be inserted into the repair area  56  has the same fiber arrangement as that of the optical fiber  51 , there are not caused such problems as mentioned later, specifically, an increase in splicing loss caused by splicing cables having different fiber arrangements from each other, and necessity of preparing a plurality of spare cables.  
       [0011] Recently, optical data transmission made through an optical transmission cable is made in wavelength division multiplexing (WDM) system. In wavelength division multiplexing (WDM) system, signals having different wavelengths from one another are multiplexed into a single optical fiber, and transmitted through the optical fiber.  
       [0012] With recent popularization of internet, there is necessity of transmitting and receiving a mass of signals in optical data transmission. As a result, the number of wavelengths to be multiplexed into a fiber was four (4) at the start of the wavelength division multiplexing system, and then, was increased to eight (8), sixteen (16), thirty two (32) and sixty four (64). The number is now being increased to a hundred (100) to two hundreds (200) or greater.  
       [0013] In addition, a bit rate per a wavelength was mainly 2.5 Gb/s, but a present bit rate per a wavelength is mainly 10 Gb/s. A bit rate per a wavelength is expected to be 40 Gb/s in the near future.  
       [0014] A channel spacing between adjacent optical signals was 125 GHz (1.0 nm) or wider, but is presently 100 GHz (0.8 nm) or 50 GHz (0.4 nm). A channel spacing between adjacent optical signals is expected to be about 25 GHz (0.2 nm) or narrower in near future.  
       [0015] As mentioned above, data to transmit or receive at a time in optical transmission has been remarkably increased in the past ten years.  
       [0016] In the wavelength division multiplexing (WDM) system, linearity is likely to be degraded in comparison with a single-wavelength transmission system. The degradation in linearity is caused mainly by self-phase modulation (SPM+GVD) wherein a waveform of a signal is deformed due to accumulated dispersion, and cross phase modulation (XPM) wherein a waveform of a signal is deformed due to interference between adjacent wavelengths.  
       [0017] Thus, in accordance with a distance by which an optical signal is transmitted, a harmful influence is exerted on the optical signal by dispersion, that is, a waveform of the optical signal is deformed. In order to avoid the deformation of a waveform, the optical signal is generally designed to periodically reset the accumulated dispersion, as illustrated in FIG. 3 which is a map showing dispersion.  
       [0018] If dispersion in fiber or local dispersion is reset or reduced to zero for removing the accumulated dispersion, a relative speed between adjacent channels also becomes zero. This results in generation of cross phase modulation (XPM) and deformation of a wavelength of the optical signal, and accordingly, transmission performance is much lowered.  
       [0019] Accordingly, it is necessary in designing an optical fiber not to set the local dispersion zero, but to set the total dispersion zero.  
       [0020] Dispersion in fiber generally has inclination relative to a wavelength. Hence, the accumulated dispersion is periodically reset in about one channel. With respect to other channels, the total dispersion is reset in a terminal station.  
       [0021] Under the above-mentioned circumstance, the following fibers are presently used, for instance.  
       [0022] (A) Non-Zero Dispersion Shifted Fiber (NZ-DSF)  
       [0023] The non-zero dispersion shifted fiber has dispersion of about −2 ps/nm/km in a wavelength of a transmitted signal to suppress degradation caused by cross phase modulation. The non-zero dispersion shifted fiber was used in a wavelength division multiplexing system, but was accompanied with a problem that the fiber had a small core diameter, and hence, was not suitable for much volume data transmission.  
       [0024] (B) Large Core Fiber (LCF)  
       [0025] The large core fiber is designed to have a greater core diameter than a core diameter of the above-mentioned non-zero dispersion shifted fiber in order to compensate for the shortcoming of the non-zero dispersion shifted fiber. The large core fiber can transmit data in a larger volume than the non-zero dispersion shifted fiber. However, the large core fiber is accompanied with a problem that the large core fiber is likely to be degraded due to self-phase modulation, because the large core fiber includes much accumulated dispersion.  
       [0026] (C) Large Core Fiber (LCF)+Low Dispersion-Slope Fiber (LS)  
       [0027] This fiber has fiber having small dispersion slope, at a latter half of a span, in order to compensate for the shortcoming of the large core fiber. This fiber can suppress the accumulated dispersion as much as possible, and further suppress degradation caused by self-phase modulation. However, this fiber is accompanied with a problem that it is impossible to completely suppress the degradation caused by self-phase modulation, because the accumulated dispersion is reset in a relatively long period.  
       [0028] (D) Dispersion Management Fiber (DMF)  
       [0029] The dispersion management fiber is comprised of a combination of two fibers having dispersion polarity different from each other, in a span. In the dispersion management fiber, the dispersion is frequently reset.  
       [0030] As mentioned above, in order to solve the problems of deformation of a waveform and degradation in transmission performance in the wavelength division multiplexing system, it is necessary to reset the accumulated dispersion to zero by using the fiber having a dispersion value which is not zero. To this end, it would be necessary to use a hybrid cable comprised of a plurality of kinds of fibers in place of the single kind fiber  53  illustrated in FIG. 1, in a span between the adjacent optical submarine repeaters  52 .  
       [0031] When a hybrid cable is to be repaired, if a repair cable to be inserted into the hybrid cable at a breakage point has different fiber arrangement from the hybrid cable, splicing loss would be increased with the result of a problem of degradation in transmission performance.  
       [0032] Hereinbelow is explained the problem with reference to an example case.  
       [0033]FIG. 4A is a conceptual view of a system for optically transmitting data, including an optical cable  61  comprised of a hybrid cable.  
       [0034] As illustrated in FIG. 4A, an optical cable  61  extending in a span between the adjacent optical submarine repeaters  52  is comprised of a first optical fiber  63  and a second optical fiber  64  overlapping each other. The first optical fiber  63  is comprised of a first fiber  63   a  and a second fiber  63   b  both extending in a length-wise direction of the optical fiber  61 , and the second optical fiber  64  is comprised of a third fiber  64   a  and a fourth fiber  64   b  both extending in a length-wise direction of the optical fiber  61 . The first fiber  63   a  in the first optical fiber  63  is the same as the fourth fiber  64   b  in the second optical fiber  64 , and the second fiber  63   b  in the first optical fiber  63  is the same as the third fiber  64   a  in the second optical fiber  64 .  
       [0035] The first fiber  63   a  in the first optical fiber  63  partially overlaps the fourth fiber  64   b  in the second optical fiber  64 .  
       [0036] It is assumed herein that the optical cable  61  is broken at a point  55 , as illustrated in FIG. 4A.  
       [0037] First, a repair area  56  including the breakage point  55  and having a predetermined length is determined, and then, a portion of the optical cable  61  corresponding to the repair area  56  is cut out. As illustrated in FIG. 4A, the repair area  56  extends across both the first fiber  63   a  of the first optical fiber  63  and the fourth fiber  64   b  of the second optical fiber  64  at a left end thereof, and further extends across the second fiber  63   b  of the first optical fiber  63  and the fourth fiber  64   b  of the second optical fiber  64  at a right end  61   b  thereof.  
       [0038] After removal of a portion corresponding to the repair area  56 , the first fiber  63   a  of the first optical fiber  63  and the fourth fiber  64   b  of the second optical fiber  64  appear at an exposed left surface  61   a  of the optical cable  61 , and the second fiber  63   b  of the first optical fiber  63  and the fourth fiber  64   b  of the second optical fiber  64  appear at an exposed right surface  61   b  of the optical cable  61 .  
       [0039] In a conventional method of repairing the broken optical cable  61 , a repair cable  65  having a length equal to or slightly greater than a length of the repair area  56  is first fabricated, and then, the repair cable  65  is inserted into the repair area  56 , as illustrated in FIG. 4B. Thus, the optical cable  61  is repaired for the breakage  55 .  
       [0040] The repair cable  65  is comprised at a left half thereof of the first fiber  63   a  of the first optical fiber  63  or the fourth fiber  64   b  of the second optical fiber  64  such that the repair cable  65  has the same fiber arrangement as that of the exposed left surface  61   a  of the optical cable  61 , and at a right half thereof of the second fiber  63   b  of the first optical fiber  63  and the fourth fiber  64   b  of the second optical fiber  64  such that the repair cable  65  has the same fiber arrangement as that of the exposed right surface  61   b  of the optical cable  61 . Accordingly, the repair cable  65  has the same fiber arrangement at its opposite ends as those of the optical cable  61 , and hence, there are caused no problems caused by the connection between different fiber arrangements.  
       [0041] However, the first fiber  63   a  and the second fiber  63   b  are spliced directly to each other at a splicing plane  66  in the repair cable  65 . That is, fibers having different fiber arrangements from each other are spliced at the splicing plane  66 . As a result, the optical cable  61  into which the repair cable  65  was inserted is accompanied with a problem that splicing loss is increased at the splicing plane  66 , and hence, data-transmission quality is deteriorated.  
       [0042] Japanese Patent Application Publication No. 4-319904 published on Nov. 10, 1992 has suggested a method of repairing an optical transmission cable system including a plurality of optical repeaters each having functions of equalizing amplification, retiming and reproduction, and an optical transmission cable connecting adjacent optical repeaters to each other. The method includes the steps of cutting the optical transmission cable across a breakage point, and optically coupling a spare optical repeater having only a function of equalizing amplification, to the optical cable through a spare optical transmission cable.  
       [0043] Japanese Patent No. 2867586 (Japanese Patent Application Publication No. 3-296703 published on Dec. 27, 1991) has suggested a method of fabricating a cable used for repairing breakage of an optical cable. The method includes the steps of (a) preparing a plurality of kinds of pipe cables each comprised of pipes, and a plurality of kinds of optical fiber units each comprised of optical fibers, (b) identifying an optical fiber necessary for repairing the optical cable, based on the number of cores in the broken optical fiber at a breakage point, (c) determining an optical fiber unit among the prepared optical fiber units such that the determined optical fiber has cores in the greater number than the optical fiber identified in the step (b), (d) determining a pipe cable among the prepared pipe cables such that the optical fiber unit determined in the step (c) can be inserted into the determined pipe cable, (e) cutting the pipe cable determined in the step (d) in a necessary length, and (f) inserting the optical fiber unit into the pipe cable by means of pressurized fluid before transferring the pipe cable to a breakage point.  
       [0044] Japanese Patent Application Publication No. 60-73506 published on Apr. 25, 1985, based on the U.S. patent application Ser. No.  529 , 297  filed on Sep. 6, 1983, has suggested a method of repairing an optical fiber cable, including the steps of preparing at least two optical fiber cables spaced away from each other, each of the optical fiber cables being comprised of an optical fiber and a metal tube into which the optical fiber is inserted, extending the optical fibers at one or opposite end(s) thereof beyond the metal tube, splicing the optical fibers to each other, forming a cylinder around the spliced ends of the optical fibers, the cylinder having an inner diameter equal to an outer diameter of the metal tube, inserting cushion into the cylinder such that the cushion surrounds the spliced ends of the optical fibers and the cylinder is filled with the cushion.  
       SUMMARY OF THE INVENTION  
       [0045] In view of the above-mentioned problems in the conventional optical cable and the conventional method of repairing a broken optical cable, it is an object of the present invention to provide a method of repairing an optical cable comprised of a hybrid cable, without an increase in splicing loss caused by splicing cables having different fiber arrangements to each other, and degradation in data-transmission quality.  
       [0046] In one aspect of the present invention, there is provided a method of repairing an optical transmission cable which is broken at a breakage point, including the steps of (a) preparing at least one kind of a spare cable which includes a first area comprised of a bridge fiber and a second area comprised of at least one kind of fiber such that the first and second areas are arranged in a length-wise direction of each of the spare cable, (b) fabricating a repair cable through the use of the even number of the spare cable such that the repair cable has opposite end surfaces having the same fiber arrangement as that of exposed surfaces of the optical transmission cable which are caused by a breakage of the optical transmission cable and that the first areas in the spare cables are spliced to each other and the second areas in the spare cables are spliced to each other in the repair cable, and (c) inserting the repair cable into the optical transmission cable at the breakage point such that the broken optical transmission cable is spliced to each other through the repair cable.  
       [0047] There is further provided a method of repairing an optical transmission cable which is broken at a breakage point, including the steps of (a) preparing at least one kind of a spare cable which includes a first area comprised of a bridge fiber and a second area comprised of at least one kind of fiber such that the first and second areas are arranged in a length-wise direction of each of the spare cable, (b) defining a repair area by cutting the optical transmission cable around the breakage point by a predetermined length, (c) fabricating a repair cable through the use of the even number of the spare cables such that the repair cable has opposite end surfaces having the same fiber arrangement as that of exposed surfaces of the repair area which are caused by a breakage of the optical transmission cable and that first areas in the spare cables are spliced to each other and the second areas in the spare cables are spliced to each other in the repair cable, and (d) inserting the repair cable into the repair area for splicing the broken optical transmission cable to each other through the repair cable.  
       [0048] For instance, the optical transmission cable is an optical transmission cable.  
       [0049] It is preferable that the repair cable has a length equal to or greater than 2L wherein L indicates a depth of the breakage point from a sea level.  
       [0050] In another aspect of the present invention, there is provided a method of fabricating a repair cable which is to be inserted into a breakage point of an optical transmission cable to repair the optical transmission cable, including the steps of (a) preparing the even number of spare cables each of which includes a first area comprised of a bridge fiber and a second area comprised of at least one kind of fiber such that the first and second areas are arranged in a length-wise direction of each of the spare cables, and (b) forming the repair cable such that the repair cable has opposite end surfaces having the same fiber arrangement as that of exposed surfaces of the optical transmission cable which are caused by a breakage of the optical transmission cable and that the first areas in the spare cables are spliced to each other and the second areas in the spare cables are spliced to each other in the repair cable.  
       [0051] In still another aspect of the present invention, there is provided a repair cable which is to be inserted into a breakage point of an optical transmission cable to repair the optical transmission cable, the repair cable being comprised of the even number of spare cables each of which includes a first area comprised of a bridge fiber and a second area comprised of at least one kind of fiber such that the first and second areas are arranged in a length-wise direction of each of the spare cables, the repair cable having opposite end surfaces having the same fiber arrangement as that of exposed surfaces of the optical transmission cable which are caused by a breakage of the optical transmission cable, and the repair cable having at least one are at which the bridge fiber of a spare cable is spliced to the bridge fiber of another spare cable.  
       [0052] It is preferable that the second area is comprised of a single kind of fiber, and the bridge fiber has a core diameter equal to a core diameter of the fiber.  
       [0053] It is preferable that the second area is comprised of two or more kinds of fibers, and the bridge fiber has a core diameter smaller than a maximum core diameter among core diameters of the fibers, but greater than a minimum core diameter among core diameters of the fibers.  
       [0054] For instance, the bridge fiber may be comprised of a non-zero dispersion shifted fiber (NZ-DSF) or a dispersion shifted fiber (DSF), and the second area is comprised of a large core fiber (LCF).  
       [0055] For instance, the bridge fiber may be comprised of a non-zero dispersion shifted fiber (NZ-DSF) or a dispersion shifted fiber (DSF), and the second area is comprised of a large core fiber (LCF) and a low dispersion-slope fiber (LS).  
       [0056] For instance, the bridge fiber may be comprised of a low dispersion-slope fiber (LS) or a non-zero dispersion shifted fiber (NZ-DSF), and the second area is comprised of a single mode fiber (SMF).  
       [0057] For instance, the bridge fiber may be comprised of a low dispersion-slope fiber (LS) or a non-zero dispersion shifted fiber (NZ-DSF), and the second area is comprised of a single mode fiber (SMF) and a dispersion compensation fiber (DCF).  
       [0058] In yet another aspect of the present invention, there is provided a spare cable used for fabricating a repair cable which is to be inserted into a breakage point of an optical transmission cable to repair the optical transmission cable, the spare cable including a first area comprised of a bridge fiber and a second area comprised of at least one kind of fiber such that the first and second areas are arranged in a length-wise direction of the spare cable.  
       [0059] The advantages obtained by the aforementioned present invention will be described hereinbelow.  
       [0060] The first advantage is that splicing loss can be minimized, and hence, optical SNR (signal to noise ratio) is prevented from being degraded, and gain flatness can be maintained.  
       [0061] In accordance with the present invention, the repair cable is designed to have opposite surfaces having the same fiber arrangements as fiber arrangements of exposed surfaces of the optical transmission cable from which a portion corresponding to a repair area has been removed. Accordingly, the repair cable and the optical transmission cable are spliced to each other through the common fiber arrangement. Hence, it is possible to repair an optical transmission cable without an increase in splicing loss caused by slicing fibers to each other through different fiber arrangements, and degradation in quality in transmission performance.  
       [0062] Specifically, whereas splicing loss in the conventional method of repairing an optical transmission cable was about 1 dB, splicing loss in the method in accordance with the present invention is about 0.3 dB. That is, the present invention can reduce splicing loss by about 10% in comparison with the conventional method.  
       [0063] The second advantage is that cost reduction can be accomplished because the number of specific spare cables to be prepared in advance is minimized.  
       [0064] For instance, an optical transmission cable can be repaired through the use of two spare cables in the method in accordance with the present invention. In addition, an optical transmission cable can be repaired in a longer range by using four or more spare cables. That is, the number of spare cables is determined in accordance with a length of an optical transmission cable to be repaired, ensuring that it is not necessary to prepare spare cables in the number more than necessary.  
       [0065] The third advantage is that the repair cable is designed to have a necessary length, ensuring that there is no much difference in wavelength dispersion. Thus, it is possible to prevent degradation in transmission performance.  
       [0066] The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0067]FIG. 1 is a conceptual view of a conventional system for transmitting data through an optical transmission cable.  
     [0068]FIG. 2A is a conceptual view of a step of determining a repair area in a damaged optical cable, in a conventional method of repairing a damaged optical cable.  
     [0069]FIG. 2B is a conceptual view of a step of inserting a repair cable into a damaged optical cable, in a conventional method of repairing a damaged optical cable.  
     [0070]FIG. 3 is a map showing dispersion which generates in optical signal transmission.  
     [0071]FIG. 4A is a conceptual view of a step of determining a repair area in a damaged optical cable, in another conventional method of repairing a damaged optical cable.  
     [0072]FIG. 4B is a conceptual view of a step of inserting a repair cable into a damaged optical cable, in another conventional method of repairing a damaged optical cable.  
     [0073]FIG. 5 is a conceptual view of a system for transmitting data through an optical transmission cable which is to be repaired by the method in accordance with the first embodiment of the present invention.  
     [0074]FIG. 6 is a conceptual view of a breakage in an optical transmission cable which is to be repaired by the method in accordance with the first embodiment of the present invention.  
     [0075]FIG. 7A is a conceptual view of a step of determining a repair area in a damaged optical transmission cable, in the method in accordance with the first embodiment of the present invention.  
     [0076]FIG. 7B is a conceptual view of a step of inserting a repair cable into a damaged optical transmission cable, in the method in accordance with the first embodiment of the present invention.  
     [0077]FIG. 8A is a cross-sectional view of a spare cable used in the method in accordance with the first embodiment of the present invention.  
     [0078]FIG. 8B is a cross-sectional view of a spare cable used in the method in accordance with the first embodiment of the present invention.  
     [0079]FIG. 8C is a cross-sectional view of a cable fabricated from the spare cables illustrated in FIGS. 8A and 8B.  
     [0080]FIG. 9A is a level diagram showing optical power of a signal transmitted through an optical transmission cable before the optical transmission cable is broken.  
     [0081]FIG. 9B is a level diagram showing optical power of a signal transmitted through an optical transmission cable after the optical transmission cable has been repaired by a conventional method.  
     [0082]FIG. 10 is a level diagram showing optical power of a signal transmitted through an optical transmission cable after the optical transmission cable has been repaired by the method in accordance with the first embodiment of the present invention.  
     [0083]FIGS. 11A to  11 G are conceptual views illustrating respective steps of a method of repairing an optical transmission cable.  
     [0084]FIG. 12A is a conceptual view of a step of determining a repair area in a damaged optical transmission cable, in the method in accordance with the second embodiment of the present invention.  
     [0085]FIG. 12B is a conceptual view of a step of inserting a repair cable into a damaged optical transmission cable, in the method in accordance with the second embodiment of the present invention.  
     [0086]FIGS. 13A to  13 D are cross-sectional views of a spare cable used in the method in accordance with the second embodiment of the present invention.  
     [0087]FIG. 13E is a cross-sectional view of a cable fabricated from the spare cables illustrated in FIGS. 13A to  13 D.  
     [0088]FIGS. 14A and 14B show relation between a gain and a frequency of an optical signal. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0089] Preferred embodiments in accordance with the present invention will be explained hereinbelow with reference to drawings.  
     [0090] [First Embodiment] 
     [0091] A method of repairing an optical transmission cable, in accordance with the first embodiment of the present invention is explained hereinbelow with reference to FIGS.  5  to  10 . In the first embodiment, an optical transmission cable is used as an optical submarine cable.  
     [0092]FIG. 5 illustrates a system for transmission of data through an optical submarine cable  1  which is to be repaired by the method in accordance with the first embodiment. The optical submarine cable  1  optically connects adjacent optical submarine repeaters  2  to each other. The optical submarine cable  1  is comprised of four fiber pairs.  
     [0093] The optical submarine cable  1  is comprised of a first optical fiber  3  and a second optical fiber  4  overlapping each other. The first optical fiber  3  is comprised of a first fiber  3   a  and a second fiber  3   b  both extending in a length-wise direction of the optical submarine cable  1 , and the second optical fiber  4  is comprised of a third fiber  4   a  and a fourth fiber  4   b  both extending in a length-wise direction of the optical submarine cable  1 . The first fiber  3   a  in the first optical fiber  3  is the same as the fourth fiber  4   b  in the second optical fiber  4 , and the second fiber  3   b  in the first optical fiber  3  is the same as the third fiber  4   a  in the second optical fiber  4 .  
     [0094] The first fiber  3   a  in the first optical fiber  3  partially overlaps the fourth fiber  4   b  in the second optical fiber  4 .  
     [0095] As illustrated in FIG. 5, in the one-span optical submarine cable  1 , a direction towards the second fiber  3   b  from the first fiber  3   a  in the first optical fiber  3  is a forward direction in up-link, and a direction towards the fourth fiber  4   b  from the third fiber  4   a  in the second optical fiber  4  is a forward direction in down-link.  
     [0096] It is assumed herein that the optical submarine cable  1  is broken at a point  5 , as illustrated in FIG. 6.  
     [0097] The method of repairing an optical submarine cable, in accordance with the first embodiment of the present invention is reduced into practice as follows.  
     [0098] First, a repair area  6  including the breakage point  5  therein and having a predetermined length is determined, as illustrated in FIG. 7A. The repair area  6  extends across both the first fiber  3   a  of the first optical fiber  3  and the fourth fiber  4   b  of the second optical fiber  4  at a left end thereof, and further extends across the second fiber  3   b  of the first optical fiber  3  and the fourth fiber  4   b  of the second optical fiber  4  at a right end thereof.  
     [0099] Then, a portion of the optical submarine cable  1  corresponding to the repair area  6  is cut out, as illustrated in FIG. 7B.  
     [0100] After removal of the portion of the optical submarine cable  1  corresponding to the repair area  6 , the first fiber  3   a  of the first optical fiber  3  and the fourth fiber  4   b  of the second optical fiber  4  appear at a left exposed surface  1   a  of the optical submarine cable  1 , and the second fiber  3   b  of the first optical fiber  3  and the fourth fiber  4   b  of the second optical fiber  4  appear at a right exposed surface  1   b  of the optical submarine cable  1 .  
     [0101]FIG. 8A is a cross-sectional view of a first spare cable  11  used in the method in accordance with the first embodiment, and FIG. 8B is a cross-sectional view of a second spare cable  12  used in the method in accordance with the first embodiment.  
     [0102] The first spare cable  11  illustrated in FIG. 8A is designed to include a first area comprised of a bridge fiber  11   a , and a second area comprised of eight first fibers  3   a . The first and second areas are sliced to each other through a splicing plane  11   b.    
     [0103] The second spare cable  12  illustrated in FIG. 8B is designed to include a first area comprised of a bridge fiber  12   a , and a second area comprised of four first fibers  3   a  and four second fibers  3   b . The first and second areas are sliced to each other through a splicing plane  12   b.    
     [0104] Thus, the second area of the first spare cable  11  has the same fiber arrangement as that of either the first fiber  3   a  in the first optical fiber  3  or the fourth fiber  4   b  in the second optical fiber  4 , and the second area of the second spare cable  12  has the same fiber arrangement as a combination of a fiber arrangement of the second fiber  3   b  in the first optical fiber  3  and a fiber arrangement of the fourth fiber  4   b  in the second optical fiber  4 .  
     [0105] The bridge fiber  11   a  in the first spare cable  11  has a core diameter equal to a core diameter of the first fiber  3   a.    
     [0106] The bridge fiber  12   a  in the second spare cable  12  has a core diameter intermediate between core diameters of the first fiber  3   a  and the second fiber  3   b.    
     [0107] Specifically, assuming that the bridge fiber  11   a  has a core diameter D11a, the bridge fiber  12   a  has a core diameter D12a, the first fiber has a core diameter D3a, and the second fiber  3   b  has a core diameter D3b greater than the core diameter D3a, the following relation is established.  
     [0108] D11a=D3a  
     [0109] D3a&lt;D12a&lt;D3b  
     [0110] If the second area in the second spare cable  12  is comprised of three or more fibers, the bridge fiber  12   a  is designed to have a core diameter smaller than a maximum core diameter among core diameters of the fibers, and greater than a minimum core diameter among core diameters of the fibers.  
     [0111] That is, a core diameter of the bridge fiber  12   a  is defined as follows.  
     [0112] Dmin&lt;D12a&lt;Dmax  
     [0113] Dmin indicates a minimum core diameter among core diameters of the fibers, and Dmax indicates a maximum core diameter among core diameters of the fibers.  
     [0114] The bridge fiber  11   a  and the first fiber  3   a  in the first spare cable  11  are spliced to each other through different fiber arrangements, and the bridge fiber  12   a  and the first and second fibers  3   a  and  3   b  in the second spare cable  12  are spliced to each other through different fiber arrangements, resulting in splicing loss to some degree. However, since the bridge fibers  11   a  and  12   a  can be fabricated on the land, for instance, in a factory, splicing loss could be relatively readily controlled. For instance, it would be possible to minimize the splicing loss by slicing the bridge fibers  11   a  and  12   a  to the first and second fibers  3   a  and  3   b  with the splicing loss being measured while they are being spliced to each other.  
     [0115] In addition, since the bridge fiber  12   a  in the second spare cable  12  is designed to have a core diameter gradually decreasing towards a core diameter of the first fiber  3   a  from a core diameter of the second fiber  3   b , the bridge fiber  12   a  would cause small splicing loss.  
     [0116] A repair cable  15  to be inserted into the repair area  6  illustrated in FIG. 7B is fabricated as follows.  
     [0117] A portion is cut out of the first spare cable  11  such that the portion includes the splicing plane  11   b  and has a predetermined length L. Similarly, a portion is cut out of the second spare cable  12  such that the portion includes the splicing plane  12   b  and has a predetermined length L.  
     [0118] Then, as illustrated in FIG. 8C, the portion of the first spare cable  11  and the portion of the second spare cable  12  are spliced to each other through a splicing plane  13   a  into a cable  13  such that the bridge fiber  11   a  of the first spare cable  11  and the bridge fiber  12   a  of the second spare cable  12  are spliced to each other and that the first fiber  3   a  of the second spare cable  12  is located below the second fiber  3   b  of the second spare cable  12 .  
     [0119] The thus fabricated cable  13  has at its left end surface the same fiber arrangement as that of the left exposed surface  1   a  of the optical submarine cable  1 , and at its right end surface the same fiber arrangement as that of the right exposed surface  1   b  of the optical submarine cable  1 .  
     [0120] Then, the cable  13  illustrated in FIG. 8C is cut into a length equal to or slightly greater than the length of the repair area  6 . The cable  13  is cut out such that the first fiber  3   a  of the first optical fiber  3  or the fourth fiber  4   b  of the second optical fiber  4  is exposed at a left end surface of the repair cable  15 , and the first fiber  3   a  of the first optical fiber  3  and the second fiber  3   b  of the first optical fiber  3  are exposed at a right end surface of the repair cable  15 . Thus, there is fabricated the repair cable  15 .  
     [0121] Then, as illustrated in FIG. 7B, the thus fabricated repair cable  15  is inserted into the repair area  6 .  
     [0122] Hereinbelow is explained splicing loss in the optical submarine cable  1  caused when the optical submarine cable  1  is repaired in accordance with the above-mentioned method.  
     [0123]FIG. 9A is a level diagram showing optical power of a signal transmitted through an optical submarine cable before the optical submarine cable is broken, and FIG. 9B is a level diagram showing optical power of a signal transmitted through an optical submarine cable after the optical submarine cable has been repaired by a conventional method.  
     [0124] As shown in FIG. 9A, optical power of a transmitted signal is raised generally at the optical submarine repeater  2 , and attenuates due to loss in the optical submarine cable  1  until the signal reaches the next optical submarine repeater  2 . There is no remarkable loss in optical power.  
     [0125] When the optical submarine cable  1  has been repaired at the breakage point  5  in accordance with the conventional method, there is caused large loss due to different fiber arrangements being spliced to each other, as illustrated in FIG. 9B. As a result, a reduced optical power is input to the next optical submarine cable  1 , and thus, the optical signal having the reduced power is input into the next submarine repeater  52 . This causes reduction in an optical signal to noise ratio (SNR).  
     [0126]FIGS. 14A and 14B show relation between a gain and a wavelength of an optical signal.  
     [0127] If there is little or almost no loss in the optical submarine cable  1 , a gain is kept at a constant relative to a wavelength of an optical signal, as illustrated in FIG. 14A. In other words, it is possible to have gain flatness.  
     [0128] However, if there is non-ignorable loss in the optical submarine cable  1 , a gain is higher for a shorter wavelength, but lower for a longer wavelength, as illustrated in FIG. 14B. That is, a gain would have an inclination relative to a wavelength, and hence, it is no longer possible to have gain flatness. This causes degradation in transmission performance.  
     [0129]FIG. 10 is a level diagram showing optical power of a signal transmitted through the optical submarine cable  1  after the optical submarine cable  1  has been repaired by the method in accordance with the first embodiment.  
     [0130] As is obvious in view of FIG. 10, loss in optical power is quite small at the breakage point  5  of the optical submarine cable  1  which has been repaired by the method in accordance with the first embodiment. The method in accordance with the first embodiment makes it possible to minimize loss in optical power caused by splicing optical cables to each other, ensuring that an optical signal to noise ratio (SNR) can be minimized, and gain flatness can be maintained.  
     [0131] Hereinbelow are explained respective steps of repairing the optical submarine cable  1  by the above-mentioned method in accordance with the first embodiment, with reference to FIGS. 11A to  11 G.  
     [0132] As illustrated in FIG. 11A, it is assumed that the optical submarine cable  1  is broken at a point  5 , and a repair ship  16  monitors whether the optical submarine cable  1  is broken at any point, by means of a detector  17 .  
     [0133] When the repair ship  16  detects the breakage point  5 , as illustrated in FIG. 11B, the repair ship  16  pulls up the optical submarine cable  1  before reaching the breakage point  5 .  
     [0134] Then, as illustrated in FIG. 11C, the repair ship  16  cuts the optical submarine cable  1  at a point where the optical submarine cable  1  is pulled up, and couples a sign buoy  18  to a half of the optical submarine cable  1  not including the breakage point  5  to allow the optical submarine cable  1  to flow on the sea level.  
     [0135] Then, as illustrated in FIG. 11D, the repair ship  16  pulls up the breakage point  5 , and then, repairs the optical submarine cable  1  by the method in accordance with the above-mentioned first embodiment. A repaired portion of the optical submarine cable  1  is inserted into and covered with a protection box  19  to avoid corrosion, as illustrated in FIG. 11E.  
     [0136] After the optical submarine cable  1  has been repaired, as illustrated in FIG. 11E, the repair ship  16  goes towards the sign buoy  18 , holding an end of the repaired optical submarine cable  1 .  
     [0137] Then, as illustrated in FIG. 11F, the repaired optical submarine cable  1  is spliced to the optical submarine cable  1  to which the sign buoy  18  has been attached, by the method in accordance with the first embodiment.  
     [0138] Thereafter, as illustrated in FIG. 11G, the optical submarine cable  1  is caused to sink to the sea bottom. Thus, the optical submarine cable  1  has been repaired at the breakage point  5  by the method in accordance with the first embodiment.  
     [0139] In the example having been explained with reference to FIGS. 11A to  11 G, a minimum length of the repair cable  15  is set equal to 2L wherein L indicates a depth from the sea level at the breakage point  5 .  
     [0140] Hereinbelow are explained examples of the first and second spare cables  11  and  12  used in the method in accordance with the first embodiment.  
     FIRST EXAMPLE  
     [0141] Bridge fiber  11   a  of the first spare cable  11 : Non-zero dispersion shifted fiber (NZ-DSF) or Dispersion shifted fiber (DSF)  
     [0142] First fiber  3   a : Large core fiber (LCF)  
     [0143] Second fiber  3   b : Low dispersion-slope fiber (LS)  
     SECOND EXAMPLE  
     [0144] Bridge fiber  11   a  of the first spare cable  11 : Low dispersion-slope fiber (LS) or Non-zero dispersion shifted fiber (NZ-DSF)  
     [0145] First fiber  3   a : Single mode fiber (SMF)  
     [0146] Second fiber  3   b : Dispersion compensation fiber (DCF)  
     [0147] Though the optical submarine cable is used as an optical submarine cable in the first embodiment, it should be noted that the method in accordance with the first embodiment may be applied to an optical cable lying on the land.  
     [0148] The above-mentioned method of repairing an optical submarine cable, in accordance with the first embodiment provides the following advantages.  
     [0149] The first advantage is that splicing loss can be minimized, and hence, optical SNR (signal to noise ratio) is prevented from being degraded, and gain flatness can be maintained.  
     [0150] In accordance with the first embodiment, the repair cable  15  is designed to have opposite surfaces having the same fiber arrangements as fiber arrangements of the exposed surfaces  1   a  and  1   b  of the optical submarine cable  1  from which a portion corresponding to the repair area  6  has been removed. Accordingly, the repair cable  15  and the optical submarine cable  1  are spliced to each other through the common fiber arrangement. Hence, it is possible to repair the optical submarine cable  1  without an increase in splicing loss caused by slicing fibers to each other through different fiber arrangements, and degradation in quality in transmission.  
     [0151] The second advantage is that cost reduction can be accomplished because the number of specific spare cables to be prepared in advance is minimized.  
     [0152] For instance, the optical submarine cable  1  can be repaired through the use of the two spare cables  11  and  12  in the method in accordance with the first embodiment, ensuring that it is not necessary to prepare spare cables in the number more than necessary.  
     [0153] The third advantage is that the repair cable  15  is designed to have a necessary length, ensuring that there is no much difference in wavelength dispersion. Thus, it is possible to prevent degradation in transmission performance.  
     [0154] [Second Embodiment] 
     [0155] A method of repairing an optical submarine cable, in accordance with the second embodiment of the present invention is explained hereinbelow with reference to FIGS. 12A, 12B an  13 A to  13 E. In the second embodiment, an optical submarine cable is used as an optical submarine cable, similarly to the first embodiment.  
     [0156] An optical submarine cable  21  to be repaired by the method in accordance with the second embodiment, illustrated in FIG. 12A, has the same structure as the structure of the optical submarine cable  1  illustrated in FIG. 7A.  
     [0157] As illustrated in FIG. 12A, it is assumed that the optical submarine cable  21  is broken at a point  25 .  
     [0158] The method of repairing an optical submarine cable, in accordance with the second embodiment is reduced into practice as follows.  
     [0159] First, a repair area  26  including the breakage point  25  therein and having a predetermined length is determined, as illustrated in FIG. 12A. The repair area  26  extends across both the first fiber  3   a  of the first optical fiber  3  and the third fiber  4   a  of the second optical fiber  4  at a left end thereof, and further extends across the second fiber  3   b  of the first optical fiber  3  and the fourth fiber  4   b  of the second optical fiber  4  at a right end thereof.  
     [0160] Then, a portion of the optical submarine cable  21  corresponding to the repair area  26  is cut out, as illustrated in FIG. 12B.  
     [0161] After removal of the portion of the optical submarine cable  21  corresponding to the repair area  26 , the first fiber  3   a  of the first optical fiber  3  and the third fiber  4   a  of the second optical fiber  4  appear at a left exposed surface  21   a  of the optical submarine cable  21 , and the second fiber  3   b  of the first optical fiber  3  and the fourth fiber  4   b  of the second optical fiber  4  appear at a right exposed surface  21   b  of the optical submarine cable  21 .  
     [0162]FIGS. 13A to  13 D are cross-sectional views of first to fourth spare cables  21  to  24  used in the method in accordance with the second embodiment.  
     [0163] The first and fourth spare cables  21  and  24  illustrated in FIGS. 13A and 13D are designed to include a first area comprised of a bridge fiber  21   a , and a second area comprised of four first fibers  3   a  and four second fibers  3   b . The first and second areas are sliced to each other through a splicing plane  21   b.    
     [0164] The second and third spare cables  22  and  23  illustrated in FIGS. 13B and 13C are designed to include a first area comprised of a bridge fiber  22   a , and a second area comprised of eight first fibers  3   a . The first and second areas are sliced to each other through a splicing plane  22   b.    
     [0165] Thus, the second area of the first and fourth spare cables  21  and  24  has the same fiber arrangement as that of an area where the second fiber  3   b  of the first optical fiber  3  and the fourth fiber  4   b  of the second optical fiber  4  overlaps each other, and the second area of the second and third spare cables  22  and  23  has the same fiber arrangement as that of an area where the first fiber  3   a  of the first optical fiber  3  and the fourth fiber  4   b  of the second optical fiber  4  overlaps each other  
     [0166] The bridge fiber  22   a  in the second and third spare cables  22  and  23  has a core diameter equal to a core diameter of the first fiber  3   a.    
     [0167] The bridge fiber  21   a  in the first and fourth spare cables  21  and  24  has a core diameter intermediate between core diameters of the first fiber  3   a  and the second fiber  3   b.    
     [0168] Specifically, assuming that the bridge fiber  21   a  has a core diameter D21a, the bridge fiber  22   a  has a core diameter D22a, the first fiber has a core diameter D3a, and the second fiber  3   b  has a core diameter D3b greater than the core diameter D3a, the following relation is established.  
     [0169] D22a=D3a  
     [0170] D3a&lt;D21a&lt;D3b  
     [0171] If the second area in the first and fourth spare cables  21  and  24  is comprised of three or more fibers, the bridge fiber  22   a  is designed to have a core diameter smaller than a maximum core diameter among core diameters of the fibers, and greater than a minimum core diameter among core diameters of the fibers.  
     [0172] That is, a core diameter of the bridge fiber  22   a  is defined as follows.  
     [0173] Dmin&lt;D22a&lt;Dmax  
     [0174] Dmin indicates a minimum core diameter among core diameters of the fibers, and Dmax indicates a maximum core diameter among core diameters of the fibers.  
     [0175] The bridge fiber  21   a  and the first fiber  3   a  in the second and third spare cables  22  and  23  are spliced to each other through different fiber arrangements, and the bridge fiber  22   a  and the first and second fibers  3   a  and  3   b  in the second and third spare cables  22  and  23  are spliced to each other through different fiber arrangements, resulting in splicing loss to some degree. However, since the bridge fibers  21   a  and  22   a  can be fabricated on the land, for instance, in a factory, splicing loss could be relatively readily controlled. For instance, it would be possible to minimize the splicing loss by slicing the bridge fibers  21   a  and  22   a  to the first and second fibers  3   a  and  3   b  with the splicing loss being measured while they are being spliced to each other.  
     [0176] In addition, since the bridge fiber  22   a  in the second and third spare cables  22  and  23  is designed to have a core diameter gradually decreasing towards a core diameter of the first fiber  3   a  from a core diameter of the second fiber  3   b , the bridge fiber  22   a  would cause small splicing loss.  
     [0177] A repair cable  35  to be inserted into the repair area  26  illustrated in FIG. 12A is fabricated as follows.  
     [0178] A portion is cut out of the first and fourth spare cables  21  and  24  such that the portion includes the splicing plane  21   b  and has a predetermined length L. Similarly, a portion is cut out of the second and third spare cables  22  and  23  such that the portion includes the splicing plane  22   b  and has a predetermined length L.  
     [0179] Then, as illustrated in FIG. 13E, the portion of the second spare cable  22  and the portion of the third spare cable  23  are spliced to each other into a cable  33  such that the second areas of them comprised of the first fiber  3   a  are spliced to each other. The thus fabricated cable  33  has opposite surfaces at which the bridge fiber  22   a  of the second spare cable  22  and the bridge fiber  22   a  of the third spare cable  23  are exposed.  
     [0180] Then, the portion of the first spare cable  21  having the length L and the second spare cable  22  of the cable  33  are spliced to each other such that the first areas of them comprised of the bridge fibers  21   a  and  22   a  are spliced to each other and that the first fiber  3   a  of the first spare cable  21  is located below the second fiber  3   b  of the first spare cable  21 .  
     [0181] Similarly, the portion of the fourth spare cable  24  having the length L and the third spare cable  23  of the cable  33  are spliced to each other such that the first areas of them comprised of the bridge fibers  21   a  and  22   a  are spliced to each other.  
     [0182] Thus, there is fabricated such a cable  34  as illustrated in FIG. 13E.  
     [0183] Then, the cable  34  is cut into the repair cable  35  (see FIG. 12B) such that the first and second fibers  3   a  and  3   b  in this order from upward are exposed at a left end surface of the repair cable  35 , and the second and first fibers  3   b  and  3   a  in this order from upward are exposed at a right end surface of the repair cable  35 , and that the repair cable  35  has a length equal to a length of the repair area  26 .  
     [0184] Then, as illustrated in FIG. 12B, the thus fabricated repair cable  35  is inserted into the repair area  26 .  
     [0185] The optical submarine cable  1  is actually repaired in accordance with the method having been explained with reference to FIGS. 11A to  11 G, similarly to the first embodiment.  
     [0186] The method in accordance with the second embodiment provides the same advantages as those provided by the method in accordance with the first embodiment.  
     [0187] Though the greater number of the spare cables are used in the method in accordance with the second embodiment than in the method in accordance with the first embodiment (specifically, the four spare cables  21 ,  22 ,  23  and  24  are used in the second embodiment whereas the two spare cables  11  and  12  are used in the first embodiment), it is possible in the second embodiment to set the repair area  26  having a greater length than a length of the repair area  6  in the first embodiment. For instance, if the optical submarine cable  1  is broken at a plurality of points within a certain length, those breakage points can be repaired at a time by the method in accordance with the second embodiment.  
     [0188] The two spare cables  11  and  12  are used in the first embodiment, and the four spare cables  21 ,  22 ,  23  and  24  are used in the second embodiment. However, the number of the spare cables used for fabricating a repair cable is not to be limited to two or four. There may be used N spare cables for fabricating a repair cable such as the repair cable  15  or  35  wherein N is an even integer equal to six or greater.  
     [0189] While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by way of the present invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the invention to include all alternatives, modifications and equivalents as can be included within the spirit and scope of the following claims.  
     [0190] The entire disclosure of Japanese Patent Application No. 2002-155147 filed on May 29, 2002 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.