JPH04219706A - Structure and method for fusingly connecting dissimilar optical fibers - Google Patents

Abstract

PURPOSE:To reduce connection loss between fibers when two optical fibers with different mode field diameters are connected, in the optical fiber communication and the optical fiber sensing engineering. CONSTITUTION:Elongation is applied to the fusingly connected part in fusingly connecting the first optical fiber where a mode field diameter is relatively small and the mode field diameter is increased relative to the tapering of the core diameter, and the second optical fibers where the mode field diameter is relatively large and the mode field diameter is decreased or stays unchanged relative to the tapering of the core diameter. This constitution allows the difference between these mode field diameters to be reduced and the connection loss between the dissimilar optical fibers with different mode field diameters to be reduced.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] This invention relates to fusion splicing to reduce connection loss between fibers when connecting two optical fibers with different mode field diameters in optical fiber communication and optical fiber sensing technology. Regarding the connection structure and fusion splicing method, it is particularly effective when fusion splicing an optical fiber, such as an optical amplification fiber, whose parameters are significantly different from that of an optical fiber used in a transmission line, to a transmission line fiber.

0002

2. Description of the Related Art Among optical fibers used in optical fiber communications, optical fibers intended for substantially single-mode transmission are called single-mode fibers. This single mode fiber is, for example, one with a step type refractive index distribution, and one in which the normalized frequency V shown by the following equation 1 is 2.4 or less is called a single mode fiber.

0003

[Math 1]

In the above formula, λ is the wavelength of light (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 cladding (relative refractive index difference, %). However, in reality, even if V is about 3.2, the second
Since the next mode does not propagate over long distances and is essentially a single mode transmission, fibers having a V value of this level are considered to be single mode fibers.

The size of the fundamental mode propagating through the single mode fiber is called the mode field diameter. The international standard defines the mode field diameter (MFDp) as follows:
It is given by the following number 2.

[0006]

[Math 2]

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

[0008] When two optical fibers with different mode field diameters are connected, the connection loss SL (dB)
is given by the following equation 3.

[0009]

[Math 3]

[0010] Normally, optical fibers having greatly different mode field diameters are not connected to each other, and optical fibers having a difference in mode field diameter within a few percent at most are connected to each other. From Equation 3 above, assuming a connection between optical fibers whose mode field diameters differ by 10%, the resulting connection loss is about 0.05 dB, which is within the allowable range.

[0011]

As an optical fiber technology that has recently attracted attention, there is direct amplification of light using rare earth doped optical fibers. FIG. 4 shows an example of the amplification technology.
This is an erbium-doped single mode fiber aimed at optical amplification in the μm band. ), the excitation light from the excitation light source 3 and the signal light 4 are incident on 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 It is configured to be transmitted to a fiber communication path 5.

[0012] Here, the core diameter of the rare earth-doped optical fiber 1 is often set to be quite small. The reason is,
In order to excite the rare earth ions in the core of the rare earth doped optical fiber evenly and sufficiently in the radial direction of the core, the relative refractive index difference between the core and the cladding is set to a large value as a parameter of the rare earth doped optical fiber. This is because it is desirable to set the core diameter small and to set the intensity of the light electromagnetic field within the core to be high.

As a result, the 1.55μ of normal optical fiber
The mode field diameter at m is set as large as about 10.5 μm, whereas in rare earth doped optical fibers, a very small mode field diameter of about 5 to 6 μm is often set.

[0014] Due to this large difference in mode field diameter, the splice loss when these two types of fibers are spliced becomes extremely large. For example, when an optical fiber with a mode field diameter of 10.5 μm and an optical fiber with a mode field diameter of 5.5 μm are connected, the connection loss will be 1.7 dB. Furthermore, in the optical fiber amplifier 1,
As shown in FIG. 4, a normal optical fiber is connected to both ends of the rare earth-doped optical fiber 1, and the total loss is It becomes 3.4dB, which is twice as high.

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 that can reduce loss in splicing dissimilar fibers having different mode field diameters. It is an object.

[0016]

[Means for Solving the Problems] The present invention provides a first optical fiber having a relatively small mode field diameter and an increasing mode field diameter as the core diameter narrows; The second optical fiber is large and has a mode field diameter that decreases or does not change as the core diameter decreases, and each end face to be connected is fusion spliced, and the fusion spliced portion is heated and stretched. This solved the above problem.

Furthermore, as the first optical fiber, an optical fiber containing a rare earth element can be used. Furthermore, single mode fibers can be used for the first optical fiber and the second optical fiber. In addition, as a method for forming the above fusion splicing structure, a first optical fiber having a relatively small mode field diameter and an increasing mode field diameter as the core diameter narrows, and a first optical fiber having a relatively large mode field diameter. , and when fusion splicing the end faces of the second optical fibers whose mode field diameters decrease or do not change as the core diameter decreases, after fusion splicing the respective optical fibers,
A fusion splicing method is desirable in which the fusion spliced portion is heated and stretched, or the ends of each optical fiber are heated and stretched before fusion splicing, and the stretched ends are fusion spliced.

[0018]

[Operation] The mode field diameter of a single mode fiber takes a fixed value at a certain predetermined core diameter value with respect to changes in the core diameter. In the case of an optical fiber having a core diameter larger than this retention value, decreasing the core diameter also decreases the mode field diameter. On the other hand, in an optical fiber having a core diameter smaller than the above-mentioned retention value, the mode field diameter increases as the core diameter decreases. Therefore, an optical fiber with a core diameter smaller than the stop value (first optical fiber) and a larger optical fiber (second optical fiber)
By fusion splicing the ends of the optical fibers (optical fibers) and stretching the fused portion, the mode field diameter difference between the two is reduced, and splice loss can be reduced.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing an embodiment of a fusion splicing structure for dissimilar 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 It is constructed by heating and stretching the connecting portion 12.

As shown in FIG. 2, the first optical fiber 10 has a core diameter (A) of 3.2 μm and a mode field diameter of 4.4 μm before being drawn. The mode field diameter of the first optical fiber 10 is increased by drawing it to narrow the core diameter.

Further, as shown in FIG. 2, the second optical fiber 11 has a core diameter (B) of 9.5 μm before being drawn.
The mode field diameter is 9.2 μm. The mode field diameter of the second optical fiber 11 is reduced by drawing it to narrow the core diameter. Note that in this second optical fiber 11, if the core diameter is reduced from the state before drawing to about 6 to 7 μm, the mode field diameter will decrease compared to before drawing, but if the core diameter is further reduced, the mode field diameter will decrease. begins to increase.

Single mode fibers are used for these optical fibers 10 and 11, and the first fiber has a smaller core diameter.
As the optical fiber 10, a rare earth doped optical fiber can be used.

By fusion splicing the end faces of these optical fibers 10 and 11 and stretching the fused portion, the diameter of each fiber (core diameter) is reduced to, for example, about 70 to 75% of that before stretching. In the first optical fiber 10, as shown in FIG. 2, the core diameter (C) decreases and the mode field diameter increases. Further, the second optical fiber 11 has a core diameter (D)
decreases, and the mode field diameter also decreases. the result,
The mode field diameters of the first optical fiber 10 and the second optical fiber 11 become close to each other.

[0024] As a result, in the fusion splicing portion 12 between the first optical fiber 10 and the second optical fiber 11, the mode field diameters of both fibers become close, and according to the equation 3 described above, the splice loss is reduced. Become.

Next, a method for forming the above fusion spliced structure will be explained. FIG. 3 is a diagram showing an example of the method for fusion splicing different types of 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 splicer, the respective end faces are abutted at a position between the pair of electrodes 13, and the respective end faces are fused by arc discharge heating. Connect. Subsequently, while heating this fused portion, a tensile force is applied to both optical fibers 10 and 11, and the heated fused portion is stretched.

When a suitable amount of stretching is reached, the stretching and heating of both optical fibers is stopped. In this way, the fusion splicing structure shown in FIG. 1 is formed.

In the above stretching operation, as a means to obtain an appropriate amount of stretching, for example, the outer diameter of the fusion spliced portion is observed and precisely measured using a microscope, and the minimum diameter of the stretched portion is set in advance. There is a method to match the set value, or to monitor the loss at the spliced part by inputting light from one end of one of the optical fibers and measuring the output intensity at the other end, and then stretching the fusion spliced part to determine the splice loss. It is preferable to stretch under the best conditions, such as by a method of stretching such that the

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

Next, the stretched end faces of both optical fibers are placed between the electrodes 13 of a fusion splicer as shown in FIG. 3, abutted against each other, and fused and spliced by arc discharge heating. In this way, the fusion splicing structure shown in FIG. 1 is formed.

In this fusion splicing structure of different types of optical fibers,
Significantly reduces splice loss in fusion splicing between two optical fibers with significantly different mode field diameters intended for substantially single-mode transmission, such as a rare-earth-doped optical fiber and a normal single-mode fiber. can be done.

[Experimental Example] Fiber 1 and fiber 2 having the parameters shown in Table 1 were prepared.

[0032]

[Table 1]

The ends of these fibers were attached to a conventional optical fiber fusion splicer, and their end faces were butted together to perform fusion splicing. Thereafter, both fibers were stretched while heating the fusion spliced portion, and the outer diameter of the fiber at the stretched portion was reduced from 125 μm to approximately 90 μm. Further, another fiber 1 was similarly fusion spliced to the other end of the fiber 2 and stretched.

In this way, the fiber 1 was connected to both ends of the fiber 2, and the connection loss was measured. When the fiber 1 was simply fusion-spliced to both ends of the fiber 2 (without being stretched), the loss was approximately 3.8 dB at the two joints, but by the method of the present invention described above, the loss was reduced to approximately 0.
It decreased to 5dB.

[0035]

As explained above, in the fusion splicing structure of different types of optical fibers of the present invention, two optical fibers having substantially different mode field diameters for the purpose of substantially single mode transmission, for example rare earth doped Splice loss can be significantly reduced in fusion splicing between optical fibers and ordinary single mode fibers.

[Brief explanation of the drawing]

FIG. 1 is an enlarged view of essential parts showing an embodiment of a fusion splicing structure according to the present invention.

FIG. 2 is a graph showing the relationship between the core diameter and mode field diameter of each of different types of optical fibers preferably 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 amplification 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 splicing part

Claims (4)

[Claims] Claim 1: A first optical fiber having a relatively small mode field diameter and an increasing mode field diameter as the core diameter decreases; On the other hand, the fusion splicing of dissimilar optical fibers is characterized in that the respective end faces of the second optical fibers to be spliced are fusion spliced and the fusion spliced portions are heated and stretched. connection structure. 2. The fusion splicing structure for dissimilar optical fibers according to claim 1, wherein an optical fiber containing a rare earth element is used as the first optical fiber. 3. The fusion splicing 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 having a relatively small mode field diameter and an increasing mode field diameter as the core diameter narrows; On the other hand, when fusion splicing the respective end faces of second optical fibers to be spliced whose mode field diameters decrease or do not change, after fusion splicing each optical fiber, the fusion spliced portion is heated and stretched. 1. A method for fusion splicing dissimilar optical fibers, characterized by heating and stretching the ends of each optical fiber before fusion splicing, and fusion splicing the stretched ends.