JP2002214467A - Optical fiber fusion splicing method - Google Patents

Abstract

(57) [Summary] [Problem] In fusion splicing optical fibers,
To provide a structure in which connection loss is not easily increased. SOLUTION: After fusion-splicing two optical fibers, the joint is heated while applying tension to the joint along the length of the optical fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for fusion splicing an optical fiber.

[0002]

2. Description of the Related Art In recent years, in long-distance wavelength multiplexing transmission performed in a relatively wide wavelength band such as the 1.53-1.63 μm band, values of chromatic dispersion and dispersion slope are small, and a transmission line capable of suppressing a nonlinear effect. Development is underway. Therefore, an effective core area enlarged optical fiber (hereinafter, Aeff enlarged SM) obtained by enlarging the effective core area (hereinafter sometimes abbreviated as Aeff) of a normal single mode optical fiber.
F) and a dispersion compensating optical fiber of dispersion slope compensation type (hereinafter abbreviated as SC-DCF). The dispersion slope is the slope of the curve when the horizontal axis represents wavelength and the vertical axis represents wavelength dispersion.
When performing wavelength division multiplexing transmission, the wavelength band used is wide,
If the dispersion slope is large, the chromatic dispersion generally increases near both ends of the transmission band, and the transmission characteristics deteriorate.

In this composite transmission line, the non-linear effect can be reduced by using the Aeff-expanded SMF, and the chromatic dispersion and dispersion slope inherent in the Aeff-expanded SMF are compensated by the SC-DCF. Although the chromatic dispersion is not zero, the chromatic dispersion of the entire transmission line can be made substantially zero. Good transmission characteristics can be obtained by suppressing the nonlinear effect and making the chromatic dispersion almost zero.

[0004]

However, Aef
When connecting the f-expanded SMF and the SC-DCF, there is a problem that the connection loss increases because the mode fields of the two greatly differ. Thus, the present inventors
A connection structure in which the eff-enlarged SMF and the SC-DCF are connected via an intermediate optical fiber was considered. This intermediate optical fiber is designed so that its mode field has substantially the same value as that of the SC-DCF, and theoretically has a small connection loss with the SC-DCF. Then, an intermediate optical fiber having an appropriate length is inserted between the Aeff-expanded SMF and the SC-DCF, and both ends thereof are fusion-spliced with the Aeff-expanded SMF and the SC-DCF, respectively. Expansion S
By heating the end on the MF side and diffusing the dopant normally added to the core to adjust the refractive index, the mode field in this area is increased by the Aeff expanded SMF.
Expanding to be close to the mode field of Ae
The connection loss with the ff-expanded SMF can also be reduced.

However, the intermediate optical fiber and the SC-D
It has been found that when fusion splicing with CF, splice loss may be larger than expected. It was also found that this connection loss tends to increase particularly on the long wavelength side of 1.53 μm or more. If the loss in this wavelength range is large, it becomes a particular problem in performing long-distance wavelength multiplex transmission using the 1.55 μm band as a used wavelength band.

[0006] The present invention has been made in view of the above circumstances,
It is an object of the present invention to provide a structure in which a splice loss hardly increases in fusion splicing optical fibers. It is still another object of the present invention to provide a structure in which a splice loss is unlikely to increase when fusion splicing optical fibers having a small mode field difference, such as an SC-DCF and an intermediate optical fiber. In particular, it is an object to provide a structure in which the loss is hardly increased particularly on the long wavelength side of 1.53 μm or more.

[0007]

In order to solve the above-mentioned problems, a method for fusion splicing an optical fiber according to the present invention comprises the steps of fusion splicing two optical fibers and then connecting the optical fiber to the joint. The connection portion is heated while applying tension along the length direction. The tension is 100g
As mentioned above, it is preferable that it is 500 g or less. Further, from the value A1 of the effective core area of the optical fiber having the larger effective core area of the two optical fibers, the value A2 of the effective core area of the optical fiber having the smaller effective core area is calculated. It is preferable that the subtracted value be within 45% of A1. Further, it is effective to apply the present invention when one or both of the two optical fibers change the mode field by heating. The above 2
One or both of the optical fibers of the present invention are provided on the outer periphery of the center core, the center core, and a side core having a lower refractive index than the center core, and provided on the outer periphery of the side core, and A cladding having a higher refractive index than the center core, wherein a radius of the side core is 2.0 to 3.0 times a radius of the center core, and a relative refractive index difference between the cladding and the center core. Is +1.2 to + 1.8% when the refractive index of the clad is zero, and the relative refractive index difference between the clad and the side core is −0 when the refractive index of the clad is zero. The present invention is effective when the present invention is applied to a case having a refractive index distribution shape of 0.1 to -0.8%.
In addition, one or both of the two optical fibers are provided on a periphery of the center core, a side core having a lower refractive index than the center core, and a side core provided on the periphery of the side core. A ring core having a higher refractive index than the center core,
A cladding provided on the outer periphery of the ring core and having a lower refractive index than the ring core and a higher refractive index than the side core, wherein a radius of the side core is 2.5 to 3.5 times a radius of the center core; The radius of the ring core is 3.0 to 5.5 times the radius of the center core, and the relative refractive index difference between the cladding and the center core is:
When the refractive index of the cladding is set to zero, +0.9 to +1.
5%, and the relative refractive index difference between the cladding and the side core is -0.3 when the refractive index of the cladding is made zero.
-0.5%, and a relative refractive index difference between the cladding and the ring core is +0.1 to + 1.2% when the refractive index of the cladding is set to zero. In this case, it is effective to apply the present invention. It is effective to apply the present invention when one or both of the two optical fibers are dispersion-slope compensating optical fibers.

[0008]

FIG. 1 is a diagram showing an example of a W-type as an example of a refractive index profile (refractive index profile) of an SC-DCF. In this refractive index distribution shape, a core 3 is constituted by a center core 1 at the center and side cores 2 provided concentrically on its outer periphery, and a clad 5 is provided concentrically on its outer periphery. The relationship between these refractive indices is that the refractive index of the side core 2 is lower than that of the center core 1, the refractive index of the cladding 5 is higher than that of the side core 2,
And it is lower than the center core 1. In the drawings, delta ₁ is in when the cladding 5 to the reference (zero), the cladding 5 and the relative refractive index difference between the center core 1, delta ₂ is when the clad 5 to zero, the cladding 5 and the side core 2 And the relative refractive index difference between the two. R ₁ is the radius of the center core 1, and r ₂ is the radius of the side core 2. Then, the values of Δ ₁ and Δ ₂ and r ₁
By adjusting the ratio of the r _2, it can be adjusted Aeff, wavelength dispersion, the characteristics such as dispersion slope. For example, the r ₂ is 2.0 to 3.0 times the r _1,
The delta ₁ is + 1.2 is + 1.8%, the delta ₂ is -
It is preferable that it is 0.1 to -0.8%. By satisfying these structural parameters, SC-
Preferred characteristics as DCF are obtained.

FIG. 2 is a diagram showing a segment type example as another example of the refractive index distribution shape of the SC-DCF. This refractive index distribution shape is constituted by a core 13 in which a center core 11, a side core 12, and a ring core 14 are sequentially provided concentrically, and a clad 15 provided concentrically on the outer periphery thereof. I have. The refractive indexes of the center core 11 and the ring core 14 are set higher than that of the clad 15, and the refractive index of the side core 12 is set lower than that of the clad 15. In the drawings, delta ₁₁ is based on the cladding 15 (zero)
Relative refractive index difference between the cladding 15 and the center core 11 when it, delta ₁₂ is a relative refractive index difference between the cladding 15 and the side core 12 when based on the cladding 15, delta ₁₃ is when based on the cladding 15 Cladding 15 and ring core 14
And the relative refractive index difference between the two. Further, r ₁₂ is the radius of the center core 11, r ₁₂ is the radius of the side core 12, and r ₁₃ is the radius of the ring core 14. Then, the values of Δ ₁₁ , Δ ₁₂ , Δ ₁₃ and r
By adjusting the ratio of ₁₁ , r ₁₂ and r ₁₃ , Ae
Characteristics such as ff, wavelength dispersion, and dispersion slope can be adjusted. For example, r ₁₂ is a radius of r ₁₁ .
Is from 5 to 3.5 times, the delta ₁₁ is + 0.9 + 1.5%
In and, delta ₁₂ is -0.3 0.5%, the delta ₁₃
Is preferably +0.1 to + 1.2%. By satisfying these structural parameters, preferable characteristics as SC-DCF as described later can be obtained.

The SC-DCF having the W-type or segment-type refractive index distribution shape shown in FIGS. 1 and 2 uses a known method such as VAD method, MCVD method and PCVD method using quartz glass. Can be manufactured. The refractive index of each layer can be adjusted by adding, for example, a dopant such as germanium having a function of improving the refractive index or a dopant such as fluorine having a function of reducing the refractive index.

[0011] It should be noted that the SC-
The DCF has the W-type or segment-type refractive index distribution shape shown in FIG. 1 or FIG. 2 and has a wavelength of 1.55.
In μm, it is preferable that Aeff is 35 μm ² or less, chromatic dispersion is −70 to −40 ps / nm / km, and dispersion slope has a negative value. Depending on the chromatic dispersion of the single-mode optical fiber to be compensated and the value of the dispersion slope, the single-mode optical fiber has such a length that the chromatic dispersion of the single-mode optical fiber such as Aeff-expanded SMF to be compensated can be compensated to zero. It is preferable that the dispersion slope compensation ratio when compensated is 80 to 120%. SC-DCF having a chromatic dispersion greater than -40 ps / nm / km and a dispersion slope having a negative value
Is SC- for compensating single mode optical fiber.
The length of the DCF is increased, which may be disadvantageous in terms of transmission loss and cost. In addition, chromatic dispersion is -40 ps
If the ratio is not more than / nm / km and Aeff exceeds 35 μm ² , the range in which the Aeff expanded SMF can be compensated may be narrowed. Further, the chromatic dispersion is -40 ps / nm / km.
Ae, or when the dispersion slope takes a positive value, Ae
In some cases, it is not possible to compensate for the chromatic dispersion of the ff-expanded SMF. There is also a module-type SC-DCF that is housed in a small housing by making use of the chromatic dispersion smaller and increasing the amount of dispersion compensation per meter to shorten the use length. This type of SC
The DCF has the refractive index distribution shapes shown in FIGS. 1 and 2 and has an Aeff of 25 at a wavelength of 1.55 μm.
μm ² or less, and the chromatic dispersion is −100 to −70 ps /
It is preferable that the wavelength is nm / km and the length is such that the chromatic dispersion of the single-mode optical fiber to be compensated is zero, and the dispersion slope compensation factor is 80 to 120% when the single-mode optical fiber is compensated.

The SC-DCF having the refractive index profile shown in FIG. 1 and FIG. 2 is inserted between the Aeff-enlarged SMF and the SC-DCF to connect the intermediate optical fiber. Can also be used. SC-D
As the CF and the intermediate optical fiber, the same type having the same refractive index distribution shape and structural parameter can be used, or different types having different refractive index distribution shapes and structural parameters can be used. At this time, SC-DC
Of the Aeff of F and the Aeff of the intermediate optical fiber, the larger one is A1 and the smaller one is A2. And
It is preferable that the value of A1-A2 be within 45% of the value of A1. If it exceeds 45%, the connection loss due to the difference in the mode field may increase.

In the fusion splicing of optical fibers having relatively similar Aeffs, the present invention relates to a method of performing fusion splicing by heating and then applying tension to the spliced portion while applying a tension to the spliced portion. Heat. The heating temperature is not particularly limited, and a heating temperature generally used for fusion splicing of a silica-based optical fiber can be applied.
It is set to about 0 to 1500 ° C. The time for applying the tension is not particularly limited. For example, it is preferable to stop the application of the tension when desired characteristics are obtained while monitoring the optical characteristics. The applied tension varies depending on the refractive index distribution shape, structural parameters, outer diameter, etc. of the optical fiber, but is preferably 100 g or more and 500 g or less. 500g
If it exceeds, there is a risk of disconnection. This tension is more preferably 300 g or less, most preferably 200 g.
With the degree, the connection loss can be more reliably reduced. If the amount is less than 100 g, the effect may not be obtained.

FIG. 3 shows an SC-type LED having the segment type refractive index profile shown in FIG. 2 and having the same structural parameters.
After the two DCFs are fusion-spliced at a heating temperature of about 1200 ° C., while further heating, from both sides of the connection portion, a predetermined tension is applied to the connection portion along a length direction of the SC-DCF while applying a predetermined tension. 6 is a graph showing an experimental result of measuring a relationship between a wavelength and a connection loss when heating is performed for one minute. The structural parameters and characteristic values of the used SC-DCF are shown below.

The Δ11: 0.96% Δ12: -0.33% Δ13: 0.17% r 11: 1.875μm r 12: 5.05μm r 13: 9.375μm cladding diameter: 125μm Aeff: 27. 0 μm ² wavelength dispersion: −54 ps / nm / km Dispersion slope: −0.17 ps / nm / km ² cutoff wavelength: 1.55 μm

In the graph, the term “immediately after fusion” indicates the relationship between the connection loss and the wavelength before the application of tension.
The connection loss on the longer wavelength side than 3 μm is larger.
On the other hand, T = 0 (g) indicates the result of heating for a predetermined time without applying tension after fusion splicing. T
= 200 (g), T = 290 (g), T = 390 (g)
Means 200 g, 290 g, 3 g
It shows the result of applying a tension of 90 g. When a tension is applied, the connection loss decreases in any case,
In particular, the connection loss on the longer wavelength side than 1.53 μm is reduced.

The reason why the connection loss increases on the longer wavelength side than 1.53 μm is that a slight fluctuation in the outer diameter of the optical fiber near the connection portion causes perturbation in that portion. Can be considered. Therefore, FIGS. 4A to 6B show the same type of SC-DCF as in the case described above in order to clarify the cause of the connection loss.
Are fused and spliced at a heating temperature of about 1200 ° C., and the SCs are further attached to the joint from both sides of the joint.
Heating for 1 minute while applying a predetermined tension along the length direction of the DCF, and the connecting portion in the longitudinal direction (vertical direction)
5 shows the results of measuring the variation of the outer diameter in the horizontal direction (horizontal direction).

FIGS. 4 (a) and 4 (b) show the results when only fusion splicing is performed, and FIGS. 5 (a) and 5 (b) show that heating is continued without applying tension after fusion splicing. It shows the result in the case of the above. FIGS. 6A and 6B show the results of applying 200 g of tension after fusion splicing. The horizontal axis of the graph indicates the position near the connection portion of the optical fiber. In any case, the variation of the outer diameter is the same. On the other hand, from the results shown in FIG.
When 0 g is applied, connection loss can be reduced.
Therefore, it is clear that the increase in the connection loss is not a perturbation due to the change in the outer diameter of the connection portion.

Another possible reason is that the stress caused by the spinning tension remaining in the core during the spinning process is released at the connection portion, so that the refractive index distribution shape changes and the mode field becomes unnecessarily large. It can be considered. That is, when a tension is applied in the length direction of the optical fiber, the refractive index can be equivalently recovered by the photoelastic effect. As a result, in the core, the confinement of light in a long wavelength region can be strengthened, and the expansion of the mode field can be suppressed. Therefore, the present invention is suitable for use in connection of an optical fiber having a refractive index profile in which a mode field is easily changed by heating. Generally, when a part or the whole of the core is made of a silica-based glass to which a dopant is added, the mode field changes by heating. Note that the W-type or segment-type refractive index distribution shapes shown in FIGS. 1 and 2 are relatively complicated having a core having two or more layers, so that the mode field is easily changed by heating.

FIG. 7 is a graph showing the result of connecting optical fibers having different refractive index distribution shapes and structural parameters in the same manner as described above. In this case, one is S
It is assumed that C-DCF and the other are intermediate optical fibers. The first optical fiber used for the connection had the segment type refractive index distribution shape shown in FIG. 2 and had the following structural parameters and characteristics. Δ11: 0.65% Δ12: 0.04% Δ13: 0.32% r ₁₁ : 3.65 μm r ₁₂ : 9.96 μm r ₁₃ : 14.02 μm Cladding outer diameter: 125 μm Aeff: 90.2 μm ² cut-off Wavelength: 1.32 μm

The second optical fiber had the segment type refractive index distribution shape shown in FIG. 2 and had the following structural parameters and characteristics. Δ11: 0.96% Δ12: −0.33% Δ13: 0.17% r ₁₁ : 1.875 μm r ₁₂ : 5.05 μm r ₁₃ : 9.375 μm Cladding outer diameter: 125 μm Aeff: 27.0 μm ² wavelengths Dispersion: -54 ps / nm / km Dispersion slope: -0.17 ps / nm / km ² Cutoff wavelength: 1.55 μm

As can be seen from FIG. 7, in this case, similarly to the case where the same type of optical fiber is connected, the connection loss on the long wavelength side can be reduced by applying tension. The present invention relates to connecting a single mode optical fiber such as an Aeff expanded SMF to an SC-DCF,
The present invention can be applied not only to the case of connecting the C-DCF and the intermediate optical fiber, but also to various uses.

[0023]

As described above, in the present invention,
By heating while applying tension after the fusion splicing of the optical fiber, the connection loss can be reduced. In addition, it is possible to reduce the connection loss particularly on the long wavelength side.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a W-shaped refractive index distribution shape.

FIG. 2 is a diagram showing an example of a segment type refractive index distribution shape.

FIG. 3 is a graph showing measurement results of connection loss when SC-DCFs having the same structural parameters are connected.

FIGS. 4 (a) and 4 (b) are graphs showing variations in the outer diameter of an optical fiber due to application of tension during fusion splicing.

FIGS. 5 (a) and 5 (b) are graphs showing a change in the outer diameter of an optical fiber due to the application of tension during fusion splicing.

FIGS. 6 (a) and 6 (b) are graphs showing the measurement of the change in the outer diameter of the optical fiber due to the application of tension during fusion splicing.

FIG. 7 is a graph showing the results of connecting optical fibers having different refractive index distribution shapes and structural parameters.

[Explanation of symbols]

1, 11: Center core, 2, 12: Side core, 3, 1
3 core, 14 ring core, 5, 15 clad.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kazuhiko Aikawa 1440, Mukurosaki, Sakura City, Chiba Prefecture Inside Fujikura Sakura Office Co., Ltd. Terms (reference) 2H036 MA15 2H050 AC14 AC38 AD01 AD16

Claims (7)

[Claims] 1. An optical fiber wherein two optical fibers are fusion-spliced to each other, and the connected portion is heated while applying tension to the connected portion along the length of the optical fiber. Fusion splicing method. 2. The method according to claim 1, wherein the tension is 100 g or more and 500 g or less. 3. An effective core area A of an optical fiber having a larger effective core area among the two optical fibers.
3. A value obtained by subtracting a value A2 of the effective core area of the optical fiber having a smaller effective core area from 1 to 45% or less with respect to A1.
3. The fusion splicing method for an optical fiber according to item 1. 4. The fusion splicing of an optical fiber according to claim 1, wherein one or both of the two optical fibers change a mode field by heating. Connection method. 5. One or both of the two optical fibers are provided on a center core and an outer periphery of the center core,
And a side core having a lower refractive index than the center core, and a clad provided on the outer periphery of the side core and having a higher refractive index than the side core and having a lower refractive index than the center core. Is 2.0 of the radius of the center core.
The relative refractive index difference between the clad and the center core is +1.2 to + 1.8% when the refractive index of the clad is zero.
Where the relative refractive index difference between the cladding and the side core is -0.1 to -0.8% when the refractive index of the cladding is zero.
5. The optical fiber fusion splicing method according to claim 4, wherein the optical fiber has a refractive index distribution shape of: 6. One or both of the two optical fibers are provided on a center core and an outer periphery of the center core,
A side core having a lower refractive index than the center core,
A ring core provided on the outer periphery of the side core and having a refractive index higher than the side core and lower than the center core; and a refraction provided on the outer periphery of the ring core and lower than the ring core and higher than the side core. And the radius of the side core is 2.5 times the radius of the center core.
The radius of the ring core is 3.0 times the radius of the center core.
The relative refractive index difference between the clad and the center core is +0.9 to + 1.5% when the refractive index of the clad is set to zero.
Where the relative refractive index difference between the cladding and the side core is -0.3 to -0.5% when the refractive index of the cladding is set to zero.
The relative refractive index difference between the cladding and the ring core is +0.1 to + 1.2% when the refractive index of the cladding is set to zero.
5. The optical fiber fusion splicing method according to claim 4, wherein the optical fiber has a refractive index distribution shape of: 7. The optical fiber fusion device according to claim 1, wherein one or both of the two optical fibers are dispersion slope compensation type dispersion compensation optical fibers. Connection method.