JP3149194B2 - Fusion splicing structure of heterogeneous optical fibers and fusion splicing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fusion splicing method for connecting two optical fibers having different mode field diameters in an optical fiber communication or optical fiber sensing technology to reduce a connection loss between the fibers. The connection structure and the fusion splicing method are particularly effective when an optical fiber such as an optical amplification fiber having significantly different parameters from the optical fiber used for the transmission line is fusion spliced to the transmission line fiber.

[0002]

2. Description of the Related Art Among optical fibers used for optical fiber communication and the like, an optical fiber aimed at substantially single mode transmission is called a single mode fiber. This single mode fiber is referred to as a single mode fiber having, for example, a step-type refractive index distribution and a normalized frequency V represented by the following equation 1 of 2.4 or less.

[0003]

(Equation 1)

In the above formula, λ is the light wavelength (μm), a is the core radius (μm), n is the refractive index of the core glass, and Δ is the relative refractive index difference between the core and the clad (specific refractive index difference,%). It is. However, in practice, even if V is about 3.2, the second mode does not propagate over a long distance, and becomes substantially a single mode transmission.
A single mode fiber including a fiber having such a V value is considered.

[0005] The size of the fundamental mode propagating through the single mode fiber is called a mode field diameter. The definition of the mode field diameter (MFDp) is given by the following equation 2 in the international standard.

[0006]

(Equation 2)

In this equation, Φ (r) is the electric field distribution of the light of the core. The mode field diameter (MFDp) given by this equation is approximately the diameter MFDN that gives 1 / e2 of the peak light intensity in a near-field pattern (near field image, abbreviated as NFP) at the end face of the fiber.
And with an error of several% to several tens%.

When two optical fibers having different mode field diameters are connected, the connection loss SL (dB) is obtained.
Is given by the following equation (3).

[0009]

(Equation 3)

[0010] Normally, no connection is made between optical fibers having greatly different mode field diameters, and it is common to connect optical fibers having a mode field diameter difference of at most several% at most. From the above equation (3), assuming a connection between optical fibers having different mode field diameters by 10%, the connection loss due to this is about 0.05 dB, which is within an allowable range.

[0011]

As an optical fiber technology that has been receiving attention recently, there is direct amplification of light by a rare-earth-doped optical fiber. FIG. 4 shows an example of the amplification technique. The amplifier 1 shown in FIG. 4 is a rare earth-doped optical fiber 1 (particularly promising is a wavelength of 1.55 μm).
This is an erbium-doped single mode fiber aiming at m-band optical amplification. ), The excitation light from the excitation light source 3 and the signal light 4 are incident using the optical coupler 2, and the energy of the rare earth ions excited by the excitation light is given to the signal light by stimulated emission, and the amplified light is emitted. The data is transmitted to the fiber communication path 5.

Here, the core diameter of the rare-earth-doped optical fiber 1 is often set quite small. The reason is,
In order to excite the rare-earth ions of the core of the rare-earth-doped optical fiber evenly and sufficiently in the radial direction of the core, a large relative refractive index difference between the core and the clad is set as a parameter of the rare-earth-doped optical fiber. This is because it is desirable to set the core diameter to be small and to set the intensity of the electromagnetic field of light in the core to be high.

As a result, the 1.55 μm
The mode field diameter is set as large as about 10.5 μm, whereas the rare-earth-doped optical fiber
A very small mode field diameter of about 6 μm is often set.

Due to the large difference between the mode field diameters, the connection loss when these two types of fibers are connected becomes very large. For example, when an optical fiber having a mode field diameter of 10.5 μm and an optical fiber having a mode field diameter of 5.5 μm are connected, the connection loss is 1.7 dB. Further, in the optical fiber amplifier 1, FIG.
As shown in (1), a normal optical fiber is connected to both ends of the rare-earth-doped optical fiber 1, and connection loss occurs at both the incident-side connection portion 6 and the outgoing-side connection portion 7, so that the total loss is doubled. It becomes 3.4 dB.

The present invention has been made in view of the above circumstances, and provides a fusion splicing structure and a fusion splicing method for dissimilar optical fibers capable of reducing loss in splicing dissimilar fibers having different mode field diameters. It is an object.

[0016]

According to the present invention, there is provided a first optical fiber having a relatively small mode field diameter and a mode field diameter increasing with decreasing core diameter, and a mode field diameter relatively small. Each of the end faces to be connected to the second optical fiber which is large and whose mode field diameter is reduced with a decrease in the core diameter is fusion-spliced, and the fusion-spliced portion is heated and stretched , so that both optical fibers are The mode
By having a structure with a small difference in field diameter ,
The above problem has been solved.

Further, as the first optical fiber, an optical fiber containing a rare earth element can be used. Furthermore, a single mode fiber can be used for the first optical fiber and the second optical fiber. As a method of forming the fusion splicing structure, a first optical fiber having a relatively small mode field diameter and a mode field diameter increasing with a decrease in core diameter, and a mode field diameter relatively small. When fusion splicing the respective end faces to be connected with the second optical fiber, which is large and the mode field diameter is reduced with the decrease in the core diameter,
After fusion splicing each optical fiber, heat-stretch the fusion spliced part , or before fusion splicing
The ends of these optical fibers are heated and drawn.
A method of fusion splicing the parts is used.

[0018]

The mode field diameter of the single mode fiber takes a stationary value at a certain core diameter value with respect to the change of the core diameter. In the case of an optical fiber having a core diameter larger than the dwell value, a smaller core diameter results in a smaller mode field diameter. On the other hand, in the case of an optical fiber having a core diameter smaller than the stationary value, the mode field diameter increases as the core diameter decreases. Therefore, an optical fiber (first optical fiber) and a large optical fiber (second
When the end portions of the optical fibers are fusion-spliced and the fusion portion is stretched, the difference in the mode field diameter between the two is reduced, and the connection loss can be reduced.

[0019]

FIG. 1 is a view showing one embodiment of a fusion splicing structure for heterogeneous optical fibers according to the present invention. In this fusion splicing structure, the respective end faces of the first optical fiber 10 having a relatively small mode field diameter and the second optical fiber 11 having a relatively large mode field diameter are fusion spliced, and the fusion splicing is performed. The connecting portion 12 is formed by heating and stretching.

As shown in FIG. 2, the first optical fiber 10 has a core diameter (A) of 3.2 μm before stretching, and a mode field diameter of 4.4 μm. The mode field diameter of the first optical fiber 10 is increased by stretching and reducing the core diameter.

As shown in FIG. 2, the core diameter (B) of the second optical fiber 11 before stretching is 9.5 μm, and the mode field diameter is 9.2 μm. The mode field diameter of the second optical fiber 11 is reduced by stretching and reducing the core diameter. This second
The optical fiber 11 has a core diameter of 6 to
When the diameter is reduced to about 7 μm, the mode field diameter decreases as compared with that before stretching, but when the core diameter is further reduced, the mode field diameter starts to increase.

These optical fibers 10 and 11 are single mode fibers and have a smaller core diameter.
As the optical fiber 10, a rare earth-doped optical fiber can be used.

The end faces of the optical fibers 10 and 11 are fusion-spliced, and the fusion portion is stretched to reduce the diameter of each fiber (core diameter) to, for example, about 70 to 75% before stretching. In the first optical fiber 10, the core diameter (C) decreases and the mode field diameter increases as shown in FIG. Further, the core diameter (D) of the second optical fiber 11 decreases, and the mode field diameter also decreases. As a result, the mode field diameters of the first optical fiber 10 and the second optical fiber 11 become closer.

As a result, in the fusion spliced portion 12 between the first optical fiber 10 and the second optical fiber 11, the mode field diameters of both fibers are close to each other, and the connection loss is small according to the above equation (3). Become.

Next, a method of forming the fusion splicing structure will be described. FIG. 3 is a diagram showing an example of the fusion splicing method for dissimilar optical fibers according to the present invention. In this example, first
The respective ends of the optical fiber 10 and the second optical fiber 11 are attached to a fusion splicing device, and the respective end faces are joined at a position between a pair of electrodes 13, and the respective end faces are fused by arc discharge heating. Connect. Subsequently, a tensile force is applied to both the optical fibers 10 and 11 while heating the fusion spliced portion, and the fusion spliced portion in the heated state is drawn.

When an appropriate amount of stretching is reached, the pulling and heating of both optical fibers are stopped. Thus, the fusion splicing structure shown in FIG. 1 is formed.

In the above stretching operation, as a means for obtaining an appropriate amount of stretching, for example, the outer diameter of the fusion spliced portion is observed and observed precisely with a microscope or the like, and the minimum diameter of the stretched portion is set in advance. In order to match the values, the light is incident from one end of one of the optical fibers, and the output intensity is measured on the other side to monitor the loss of the connection part. It is preferred that the film be stretched under the best conditions, for example, by a method of stretching the film so that is minimized.

As another example of the method for forming the fusion splicing structure, first, the first optical fiber 10 and the second optical fiber 1
A portion is heated and pulled to form a stretched portion of each of the pieces. Next, the stretched portion of each fiber is cut so that an end face having a diameter equal to a predetermined minimum diameter of the fused portion is formed.

Next, as shown in FIG. 3, the end faces of the two optical fibers on the extended side are mounted between the electrodes 13 of the fusion splicing apparatus, butted together, and fused by arc discharge heating. Thus, the fusion splicing structure shown in FIG. 1 is formed.

In this fusion splicing structure for different kinds of optical fibers,
Splicing loss between two optical fibers with substantially different mode field diameters for substantially single-mode transmission, for example, fusion splicing between a rare-earth-doped optical fiber and a normal single-mode fiber, greatly reduces splice loss. Can be done.

Experimental Example Fibers 1 and 2 having the parameters shown in Table 1 were prepared.

[0032]

[Table 1]

The ends of these fibers were mounted on a conventional fusion splicer for optical fibers, and their end faces were butted and spliced. Thereafter, both fibers were pulled and drawn while heating the fusion spliced portion, and the fiber outer diameter of the drawn portion was reduced from 125 μm to about 90 μm. Further, another fiber 1 was similarly fusion-spliced to the other end of the fiber 2 and stretched.

The fiber 1 was connected to both ends of the fiber 2 in this way, and the connection loss was measured. In the state where the fiber 1 is simply fusion-spliced to both ends of the fiber 2 (not drawn), the sum of the two connecting portions is about 3.8 dB. However, the loss of about 0.5 dB is obtained by the method of the present invention described above.
B.

[0035]

As described above, in the fusion splicing structure of different kinds of optical fibers according to the present invention, two optical fibers having substantially different mode field diameters for substantially single mode transmission, for example, rare-earth-doped fibers are used. In fusion splicing between an optical fiber and a normal single mode fiber, splice loss can be greatly reduced.

[Brief description of the drawings]

FIG. 1 is an enlarged view of a main part showing one embodiment of a fusion splicing structure according to the present invention.

FIG. 2 is a graph showing a relationship between a core diameter and a mode field diameter of each of different kinds of optical fibers suitably used in the present invention.

FIG. 3 is an enlarged view of a main part for explaining an example of a method for forming a fusion splicing structure according to the present invention.

FIG. 4 is a schematic configuration diagram showing an example of an optical amplifier circuit suitable for applying the fusion splicing structure of the present invention.

[Explanation of symbols]

 10 first optical fiber 11 second optical fiber 12 fusion spliced part

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Tetsuya Sakai 1440 Mutsuzaki, Sakura-shi, Chiba Fujikura Electric Cable Co., Ltd. Sakura Plant (56) References JP-A-1-295208 (JP, A) JP-A-61- 120106 (JP, A)

Claims (5)

(57) [Claims] A first optical fiber having a relatively small mode field diameter and an increased mode field diameter as the core diameter is reduced; and a first optical fiber having a relatively large mode field diameter and a reduced core diameter. On the other hand, the mode field diameter
Each end face to be connected to the decreasing second optical fiber is fusion-spliced, and the fusion-spliced portion is heated and stretched,
The difference in mode field diameter between the two optical fibers is small.
Connection structure different optical fibers, characterized in that it is. 2. The connection structure for heterogeneous optical fibers according to claim 1, wherein an optical fiber containing a rare earth element is used as said first optical fiber. 3. The connection structure for dissimilar optical fibers according to claim 1, wherein the first optical fiber and the second optical fiber are single mode fibers. 4. A first optical fiber whose mode field diameter is relatively small and whose mode field diameter increases with decreasing core diameter, and a first optical fiber whose mode field diameter is relatively large and whose core diameter decreases. On the other hand, the mode field diameter
When each end face to be connected to the decreasing second optical fiber is fusion-spliced, after each optical fiber is fusion-spliced, the fusion-spliced portion is heated and stretched, and the mode field diameter of both of them is reduced. The difference
A fusion splicing method for dissimilar optical fibers, characterized by reducing the size . 5. The method according to claim 1, wherein the mode field diameter is relatively small.
Mode field diameter increases as core diameter decreases
The mode field diameter is relatively large compared to the first optical fiber.
Mode field diameter
Each end to be connected with a decreasing second optical fiber
When fusion splicing surfaces, heat and stretch the end of each optical fiber before fusion splicing
Then, each stretched end is fusion-spliced, and the two
Characteristic of reducing the difference in
Splicing method of seed optical fiber