Optical waveguides – With splice – Fusion splicing
Reexamination Certificate
2002-05-30
2004-04-27
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With splice
Fusion splicing
C385S099000
Reexamination Certificate
active
06726378
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a connecting method of different kind optical fibers for connecting the different kind optical fibers having different mode field diameters (MFDs) to each other.
BACKGROUND ART
In recent years, an increase in the capacity of an optical communication system is required. A dispersion management line for performing high bit-rate communication in a band of e.g., 1550 nm in wavelength is vigorously considered to satisfy this requirement. For example, this dispersion management line is formed by connecting a single mode optical fiber such as a 1300 nm zero dispersion optical fiber, and a dispersion-compensating optical fiber for compensating dispersion and a dispersion slope of this single mode optical fiber.
For example, the above dispersion-compensating optical fiber is formed by a DCF (Dispersion Compensating Fiber), a DSCF (Dispersion Slope Compensating Fiber), an RDF (Reverse Dispersion Fiber), etc. When such a dispersion management line is used in an optical submarine cable, etc., it is desirable to connect different kind optical fibers with low transmission loss and high strength.
For example, the 1300 nm zero dispersion optical fiber is a single mode optical fiber, and the MFD of the 1300 nm zero dispersion optical fiber at a wavelength of 1550 nm ranges from 9 to 11 &mgr;m. Further, the MFD of an MFD enlarged type single mode optical fiber at the wavelength of 1550 nm is 11 &mgr;m or more.
In contrast to this, the MFD of the dispersion-compensating optical fiber having negative high dispersion characteristics at the wavelength of 1550 nm is about 5 &mgr;m. Accordingly, the MFD of the dispersion-compensating optical fiber is small in comparison with the single mode optical fiber. The dispersion-compensating optical fiber has a high relative refractive index difference A of about 3% and a small core diameter of 2 to 3 &mgr;m.
When the above different kind optical fibers of different MFDs are connected to each other, splice loss is increased by the difference in MFD. For example, the splice loss is about 1.94 dB when the single mode optical fiber having 10 &mgr;m in MFD and the dispersion-compensating optical fiber having 5 &mgr;m in MFD are connected to each other by conforming their optical axes.
For example, processing using a TEC method (Thermally Diffused Expanded Core) is performed to restrain this increase in splice loss after connecting end faces of the different kind optical fibers are connected to each other. For example, the processing using the TEC method is performed by heating connecting portions of the single mode optical fiber and the dispersion-compensating optical fiber. The processing using the TEC method is processing for enlarging the MFD of the dispersion-compensating optical fiber by diffusing Ge (dopant) within a core and conforming this MFD to the MFD of the single mode optical fiber by heating the above connecting portions. The splice loss is greatly reduced by performing this processing.
FIG. 9
is a flow chart showing one example of a conventional connecting process of the different kind optical fibers. For example, the different kind optical fibers such as the single mode optical fiber and the dispersion-compensating optical fiber are connected to each other as follows in accordance with the flow shown in
FIG. 9. A
step
101
of
FIG. 9
is a coating removing process for removing the coating layers of respective connecting end portions of the different kind optical fibers and exposing the connecting end portions of the optical fibers. A step
102
is a fiber cleaning process for cleaning an exposed portion of each of these optical fibers. A step
103
is a process for forming a protecting layer on the surface of the exposed portion of each cleaned optical fiber.
A step
104
is a process for cutting a tip portion of each optical fiber. A step
105
is a process for fusion-splicing the cut different kind optical fibers by butting their connecting end faces. The above cutting processing of the optical fiber and the fusion splice processing of the optical fiber are performed by arranging the exposed portion (tip portion) of the optical fiber in e.g., a V-groove formed in a fixing member.
The exposed portion of the optical fiber arranged in the V-groove is positioned and fixed to the fixing member by pressing this optical fiber from its upper side by a clamp member. In such a gripping fixing state (glass gripping state), the cutting processing and the fusion splice processing of the above optical fiber are performed. The protecting layer formed on the surface of the exposed portion of the optical fiber in the above step
103
is arranged to reduce a bad influence on the optical fiber due to the gripping fixation.
A step
106
is a process for taking heat treatment for conforming the MFDs of the connecting end portions of the optical fibers connected to each other.
A small crack is caused in a surface portion of the connecting portion of the different kind optical fiber by heating of the fusion splice processing and the heat treatment. When there is such a crack, strength of the connecting portion of the different kind optical fiber is reduced. Therefore, an operation for preventing this strength deterioration is performed. This operation is performed by the etching process of a step
107
, and is an operation for removing the crack by etching the surface portion of the connecting portion of the optical fiber after the above heat treatment.
A step
108
is a recoating process for coating the connecting portion of the different kind optical fiber by forming a coating layer on the surface of the exposed portion of the optical fiber. A step
109
is a process for inspecting whether or not the different kind optical fiber is connected in accordance with a standard by a proof test.
However, in a conventional method, the connecting portions of the single mode optical fiber and the dispersion-compensating optical fiber were about 1 GPa in average breaking strength, and no satisfactory strength was obtained. Further, this strength was greatly dispersed.
Therefore, the present inventors examined the cause of the strength deterioration of the connecting portion of the different kind optical fiber. As a result, it has been found that one of causes of the strength deterioration of the connecting portion of the different kind optical fiber resides in an environment in the heat treatment for conforming the mode field diameters of the respective optical fibers in the connecting portions.
Namely, when the heat treatment is taken, foreign matters floated around the connecting portion of the different kind optical fiber and foreign matters attached to the surface of the connecting portion are burned onto the surface of the connecting portion of the different kind optical fiber by the heat treatment. Therefore, there is a case in which a small crack is caused in the connecting portion of the different kind optical fiber by these burned foreign matters. Thus, it has been found that the strength of the connecting portion of the different kind optical fiber is deteriorated by this crack.
In the conventional method, processing for removing the crack from a surface portion of the connecting portion of the different kind optical fiber by etching this surface portion is performed after the heat treatment. However, it is confirmed that no crack caused by the burned foreign matters due to the heat treatment can be almost removed by this etching processing.
Further, the present inventors have also found the following matters by examining the cause of the strength deterioration of the connecting portion of the different kind optical fiber. The heat treatment of the connecting portion of the different kind optical fiber was conventionally taken after the protecting layer was formed on the surface of an exposed portion of the optical fiber. Therefore, the protecting layer formed in the connecting portion of the different kind optical fiber was burned and a large amount of foreign matters was generated at a heat treatment time of the
Knobbe Martens Olson & Bear LLP
Rojas Omar
Sanghavi Hemang
The Furukawa Electric Co. Ltd.
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