JPH0361926B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a low-loss, high-strength fiber fusion splicing method.

Conventionally, a fusion splicing method has been used to connect fibers to each other. As a heat source for fusing, electric discharge generated by applying a high voltage between opposing electrodes, fire caused by a gas burner, or other high-power lasers are used. On the other hand, when connecting fibers with small core diameters (optical fibers), such as single-mode fibers, in order to reduce loss, the amount of misalignment between the centers of the cores of the two fibers being connected (the amount of axial misalignment) ) must be minimized.

For this reason, conventionally, the core axes of the fibers are aligned to match each other before fusion splicing, and heating conditions are selected to prevent core axes from shifting as much as possible during the fusion splicing process. . This conventional fusion splicing method is the first
This will be explained with reference to the figures.

One side 2 of the fibers to be connected is fixed to a fixed base 3, and the other fiber 1 is fixed to a fixed base 4 that can be moved slightly in each of the z, y, and z directions shown in the figure, and these fibers 1 and 2 are connected to each other. Match with. Then, in order to align the cores as a pre-connection step, the output light from the light source 5 is input from the fiber 2 to the fiber 1, and the power of the light transmitted through the fibers 1 and 2 and reaching the optical receiver 6 is monitored. Then, adjust the power by moving the fixed base 4 in two directions, x and y, so that this power becomes maximum. After that, a high voltage is applied between the discharge electrodes 7 and 8 to cause a discharge, and the end faces of the fibers 1 and 2 are melted by this heat, and the fixing table 4 is slightly moved in the z direction to remove an appropriate amount of the fibers 1 and 2. into fiber 2 to complete the connection. Here, a method of fusing using electric discharge has been explained as an example, but as mentioned earlier, a method of heating with a fire from a gas burner is also used for the same purpose.

When performing such fusion splicing, if the fibers have core eccentricity, the centers of the fibers will not coincide even if the cores coincide with each other. FIG. 2a shows the positional relationship in the cross section between the core centers of the two fibers and the fiber centers before such connection. Core center 11 and fiber center 13 relate to one fiber 2, core center 12 and fiber center 14 relate to the other fiber 1.

The core centers 11 and 12 are perfectly aligned due to alignment. However, the core eccentricity of the fibers is
When d ₁ and d ₂ , the fiber centers 13 and 14 do not necessarily coincide. When fusion is performed in this state, surface tension acts between the ends of the fused fibers, causing the fibers to move so as to bring their centers into alignment.
The direction of movement is the direction connecting the fiber centers 13 and 14, and the amount of shaft movement D is generally proportional to the initial distance d ₁₂ between the fiber centers, given by D=kd ₁₂ . k is determined by the temperature during discharge (fusion) and the time of discharge (fusion), and is a substantially constant value. As a result of this movement of the fiber axes during fusion, the fiber centers are brought closer to each other, as shown in FIG.
1 moves to 11'. As a result, after fusion splicing, an axis deviation of D occurs between core centers 11' and 12,
This will degrade the connection loss.

In general, the connection loss α (unit: dB) that occurs when the axis of D is misaligned is given by α=4.34·2D ² /W ₁ ² +W ₂ ² . W ₁ and W ₂ are connected fibers 1 and 2
The respective spot size is approximately 5 μm for a typical single mode fiber.

The above-mentioned movement of the axis during fusion occurs when the core is eccentric, and the size of the movement is a small amount of 2 μm at most, but still.
The connection loss value calculated from the above formula is when the axis misalignment D is 1μ
It is 0.2 dB at m and 0.7 dB at 2 μm, which is a non-negligible connection loss.

If you reduce the power by shortening the discharge time or lowering the temperature during discharge in order to reduce the effect of surface tension (lower the k value), the connection strength will decrease, melting will be insufficient, and the gap between the end faces will decrease. This causes problems as air bubbles get mixed in and conversely increase connection loss. Therefore, it is only possible to reduce the discharge time and discharge power within a range that satisfies the strength to a certain extent and provides a stable connection, and when connecting fibers with an eccentricity of about 2 to 3%, the surface tension It was difficult to avoid the effects of this, and it was difficult to obtain low connection loss.

In particular, when a high-strength connection is desired, it is necessary to increase the fusion time, and it has been extremely difficult to reduce the effects of surface tension.

In order to eliminate the core axis misalignment that remains after connection due to the influence of surface tension, the present invention observes the eccentricity of the core and the distance between fiber centers during the alignment stage, and measures the surface tension. Therefore, the cores are aligned by giving a mutual offset in advance by the amount of axis deviation that will be moved, and the cores are aligned using the axis deviation during fusion,
The fiber fusion splicing method is characterized in that no axial misalignment remains after splicing and low-loss splicing can be performed, and the object thereof is to provide a fiber fusion splicing method in which no axial misalignment of the core remains. Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.

In order to carry out the present invention, it is necessary to determine the state of the cores between the fibers to be connected, as well as the distance and direction of the fiber centers. Figure 3 shows how to align while observing the core in such a fiber.
This figure shows an example of a fusion device that performs the connection.

The state of the fibers in the horizontal direction (x-axis direction) is determined by illuminating the fibers 1 and 2 from directly below with a light source 21.
The image is observed with a microscope 23. On the other hand, to determine the state of the fibers in the vertical direction (y-axis direction), the light source 22 illuminates the fibers 1 and 2 from directly beside them, and the mirror 24
The image is observed through a microscope 23.

As in FIG. 1, the fibers 1 and 2 are connected to a fixed base 3,
They are each fixed to a fixed base 4 that can be slightly moved along the x, y, and z axes.

Figures 4a and 4b are diagrams showing the observed images,
Figure a is an image observed from the vertical direction and shows the deviation in the horizontal direction, and Figure b is an image observed from the horizontal direction and shows the deviation in the vertical direction. As shown in these figures, the images are dark at the fiber surface and at the center of the core. Therefore, from these images, the center position 31, 3 of each fiber can be determined.
2, 33, 34 and core center position 35, 3
6, 37, 38 are found. In reality, due to the lens effect of the fiber cladding, the core observation position 3
5 to 38 are actually different, and the true position cannot be determined unless this is corrected. However, to simplify the explanation, 35 to 38 are assumed to be the true core positions. First, during alignment, the fixing base 4 is moved in the x-axis direction so that the core centers 35 and 36 are aligned in the horizontal direction. Next, it is moved in the y-axis direction so that the core centers 37 and 38 are aligned in the vertical direction.

As a result, the core axes coincide in both the horizontal and vertical directions, as shown in FIGS. 5a and 5b. FIG. 5a is an image showing the shift in the horizontal direction, and FIG. 5b is an image showing the shift in the vertical direction. Conventionally, this completes the alignment and connects the connection, but in this state, Fig. 5 a and b
As shown in , the fiber center position is 3.
1', 32', 33', and 34', and are shifted by d _1x , d _2x , d _1y , and d _2y from the center of the core, respectively. If we look at this in terms of the positional relationship within the cross section of the fiber, it will be exactly the same as that shown in Figure 2a, and there will be a deviation of d ₁₂ between the fiber centers. If they are connected in this state, the shaft will move D (=kd ₁₂ ) due to surface tension determined by the temperature and time of fusion, resulting in deterioration of connection loss.

Figure 6 shows an example of the loss distribution before and after fusion splicing when the cores are matched by core observation and then fusion spliced.The eccentricity of the fiber used is approximately 3%. . Because there is an error in the observation of the core itself, there is a slight axis misalignment even before welding.
It can be seen that the loss is further deteriorated due to axis misalignment during the fusion process. Moreover, FIG. 7 shows the value of k determined from the change in the position of the fiber center at that time, and large frequencies are distributed from 0.2 to 0.35, with the center value being 0.28.

Therefore, in the present invention, we do not immediately fuse the fibers with the cores aligned, but rather the deviation d _1x of the fiber center position at that time,
Find d _2x , d _1y , and d _2y as shown in Figure 8.
d ₁₂ /1-k [here d ₁₂ =√( _2x − _1x ) ² + ( _2y
−1y ) ² ], move the fixing base 4 in the x-axis and y-axis directions so that the fiber centers are separated from each other. FIG. 8 shows a case in which one fiber whose fiber center is at position 43 is fixed, and the other fiber whose fiber center is at position 44 is moved so that its fiber center is at position 45. That is, when the core centers of the two fibers are aligned with position 41, the center of the other fiber is located at position 45, which is an extension of the line connecting the center position 44 of the other fiber and the center position 43 of one fiber. Move the other fiber so that As a result, the core center of the other fiber moves from position 41 to position 42.
The distance traveled is k/1- _kd12 , k/1-k( _d2x - _d1x ) in the x-axis direction, and k/1-k( _d2x ) in the y-axis direction.
_y −d _1y ).

The amount of movement at this time is 4 to 5 μm at most as long as d ₁₂ uses a normal fiber, so it is up to 2 μm at most, although it also depends on the value of k.

If fusion is performed in this state, movement will occur between the fiber centers 45 and 43 due to the effect of surface tension, but
The amount of movement at that time is d ₁₂ /1-k×k, which exactly cancels out the axis deviation caused by the later shift. Therefore, the fiber center position after fusion splicing is again from 45 to 44
Further, the core center position is moved from 42 to 41, and the core axes are connected in perfect alignment, resulting in a low-loss connection with less axis misalignment loss.

In reality, the value of R is not a completely constant value as shown in Figure 7, so the destination of the center of the core will also vary around 41, but in any case, the loss is low with little axis misalignment loss. Connection is now possible.

In addition, in the case of the example shown earlier, the value of R should be approximately the central value of 0.28, but as explained earlier, this value depends on the conditions, temperature, and time during fusion. , which may differ depending on the device, it is desirable to make several connections for each device in advance and measure and determine the movement of the fiber center at that time in the same manner as shown in FIG.

In this way, if the amount of axial deviation that moves due to surface tension is given in advance for alignment, even if the influence of surface tension is quite large, such as when the welding time is long, the core after joining can be adjusted. The axes can be aligned, and low-loss connections can be made without causing problems such as unstable connections when shortening the fusion time or weakening of the welding strength.

The core observation method has been explained here using visual observation using a microscope as an example, but in reality, it is difficult to accurately detect the amount of deviation of the fiber center and how to give the core axis deviation accordingly. It is necessary to As shown in FIG. 9, this method uses an imaging device 51 such as a TV camera on a microscope, detects the center of the core and the center of the fiber from the imaging signal, and calculates the relative positional relationship between the two fibers to be connected. This can be easily realized by providing the desired image processing device 52 and a control device 53 that controls the movement of the fixed base 4 based on the result, and thereby automatically completes the alignment. It is also possible to perform fusion splicing.

Regarding the fiber axis alignment and movement procedure, in FIGS. 2 and 8, for ease of explanation, first align the two core centers,
Next, we explained the method of moving the correction amount k/1 - kd ₁₂ to account for the axis misalignment due to surface tension, but as shown in Figure 4, two fibers are installed, and the core eccentricity of each fiber is Then, measure the deviation of the core centers between the two fibers, find the deviation d ₁₂ of the fiber center from the core eccentricity state (d _1x , d _1y , d _2x , d _2y ) when the core centers are aligned, and calculate the following: It is also possible to calculate the vector sum of the above-mentioned k/1-kd ₁₂ and the deviation amount of the core center, and move the fiber in one step based on the calculation result.

Furthermore, regarding the observation of the fiber, in addition to the method using a light source and microscope as shown in Figure 3, any method that allows the identification of the core center and fiber center may be used. Needless to say.

As explained above, according to the present invention, core alignment is performed using a device that can detect the core center position of the fibers to be connected and the fiber center position, and furthermore, the core centering is performed using a device that can detect the core center position of the fiber to be connected and the fiber center position. Since the axis misalignment that occurs is given to the mutual cores in advance, the core axes can be perfectly aligned by fusion splicing, and even if the fiber has a large core eccentricity, the fusion time can be sufficiently long. It is possible to adopt fusion bonding conditions, and a low-loss and high-strength connection is possible.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram explaining the alignment and fusion splicing method using the conventional optical power monitoring method, and Figure 2 is an illustration of the axis during fusion splicing when core alignment is performed using the conventional alignment method. FIG. 3 is a schematic configuration diagram showing an example of a core observation type fusion splicing device implementing the connection method of the present invention, FIGS. 4 a, b, and 5 a, b are diagrams of observed fibers. Figure 6 shows an example of fiber loss and deterioration due to axis misalignment caused by surface tension during fusion. Figure 7 shows the position of the core center and fiber center of the image. A diagram showing an example of the distribution of the axis movement coefficient k due to surface tension, FIG. 8 is an explanatory diagram showing the mutual relationship between the core and fiber center when performing the alignment method according to the present invention, and FIG. 9 is an automation diagram of FIG. 3. 1 is a schematic configuration diagram of a device that embodies this. 1, 2...Fiber, 3, 4...Fixing base, 7,
8... Discharge electrode, 21, 22... Light source for illumination, 2
3... Microscope, 31-34, 31'-34'... Fiber center line, 35-38... Core center line, 4
1, 42... core center, 43, 44, 45... fiber center, 51... imaging device, 52... image processing device, 53... fixed base control device.

Claims (1)

[Scope of Claims] 1. Detect the deviation of the fiber centers that occurs when the fiber ends of two fibers to be connected are aligned in a straight line and their mutual core centers coincide, and detect the mutual deviation of the fiber centers at that time. After applying an extra misalignment in the direction in which the misalignment becomes larger on the extension line of the connecting line by the amount of axis misalignment expected between the fiber centers that will occur in accordance with the misalignment in the subsequent fusion process, using a heat source. A fiber fusion splicing method characterized in that the fiber ends are fused and spliced to each other.