JP2000206361A - Optical fiber type multi-branch coupler, and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture at a high yield a coupler with an excellent characteristic for multi-branching signal light in a lump. SOLUTION: An optical fiber bundle of which N numbers (n>5) of outer circumferential fibers are arranged with substantially equal intervals is formed in a periphery of a center fiber having a clad and a core, and a cross-section of the optical fiber bundle has gaps between the optical fibers and is arranged in a slightly shifted condition shifted slightly from closest packing constitution, (a) a diameter D of the center fiber is larger than a Do specified by do/Do=sin(π/N)/(1-sin(π/N)), and is within a range of 1.7×10-3<(D-Do)/Do<2.0×10-2, or (b) a diameter (d) of the outer circumferential fiber is smaller than the (do) specified by the expression hereinbefore, and is within a range of 1.7×10-3<(do-d)/do<2.0×10-2, the optical fiber bundle is pressed by clamps at least two portions in its longitudinal direction to be fixed, a portion between the clamps is heated to fuse the respective optical fibers, and the fixing clamps are separated each other concurrently to form an optical coupling part of a biconical tapered form in a fused part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical fiber type multi-branch coupler used for optical communication and a method for manufacturing the same.

[0002]

2. Description of the Related Art With the development of optical communication using optical fibers, high functions are required for a large number of optical components. Among them, an optical signal transmitted through one optical fiber is converted into a plurality of optical components. Multi-branch couplers that split into fibers are indispensable components for optical communications, and require low insertion loss, uniform distribution ratio, small size, compact size, high reliability, etc., and are vigorously developed along this direction. Is being promoted.

[0003] The easiest method for uniformly distributing a signal from one optical fiber to a plurality of optical fibers is to use a 3 dB coupler that equally distributes the signal into two, using two branches to four branches.
In this method, the branch is divided into eight branches, and the eight branches are further divided into 16 branches. This method is disclosed in detail in, for example, JP-A-63-205616. However, in this method, the entire distributor becomes long, and even if it is housed in a storage case, the bending diameter of the optical fiber is limited, and the storage case becomes as large as about 100 mm x 80 mm.
There is a disadvantage that it cannot be stored compactly.

If instead of such a cascade connection, distribution can be made to a plurality of optical fibers at one place, a compact distributor can be obtained and the manufacturing process can be simplified.
In pursuit of such compactness, development of a distributor capable of collectively branching an optical signal into multiple branches has been advanced. As one of the methods, Japanese Patent Application Laid-Open No. 62-27707 discloses a method in which a plurality of optical fibers are arranged and bundled so that the cross-sectional configuration is axisymmetric and the closest packing is provided. It states that a distributor is formed by heating, fusing and stretching.

[0005] The closest packing is performed when the optical fibers are arranged.
In close packing, all neighbors are
Mating optical fibers are in contact with each other. That is, the diameter
D₀ And the diameter d₀Light of
Around the center fiber with N fibers as outer fibers
When placed at₀And diameter d₀Is the function of
In charge. In addition, the optical fiber may be
What is close packing when fused with a matching optical fiber?
I don't say. d₀/ D₀= Sin (π / N) / (1-sin (π / N)) (4)

In US Pat. No. 4,682,849, this method is further extended, in which a glass tube or a glass rod is placed at the center and a plurality of optical fibers are placed around the glass tube or glass rod, and the fiber is heated, fused and drawn. A plurality of optical fibers and a central glass tube or glass rod are integrated, and this is inserted into another cylindrical glass tube and heated from the outside. In the case of a glass tube, this is collapsed, and the cylindrical glass tube is collapsed. A method for producing a distributor by fusing and stretching the same is disclosed. The arrangement of the outer peripheral fibers before being inserted into the cylindrical glass tube is different from the close-packed structure claimed in the above-mentioned JP-A-62-27707. However, even if there is a gap between adjacent outer fibers (see FIG. 3B in the aforementioned US Patent Publication), or all the adjacent outer fibers are in contact with each other, some of the outer fibers are in the center. The gap between the glass tube or glass rod (here,
This gap is defined as a reverse gap. FIG. 3 of the US patent publication
A)). The reason is that the outer fiber becomes soft during the steps of heating, fusing, and stretching, thereby filling the gap or the reverse gap.

[0007] The close-packing configuration in Japanese Patent Application Laid-Open No. 62-27707 is finely defined from another point of view in US Patent No. 5,408,554, in which a plurality of outer fibers are provided around a central fiber. An optical fiber bundle is formed linearly along the central fiber to form an optical fiber bundle.At this time, all of the peripheral fibers are in contact with the central fiber and are in a non-close-packed configuration having a gap between the peripheral fibers. A method of manufacturing a multi-branch coupler in which the optical fiber bundle is twisted to form a close-packed structure, and then heated, fused, and drawn, and one end of the center fiber is used as an input port, and the other end is multi-branched. Is described.

Here, the reason why the non-close-packed structure is a close-packed structure by torsion is that, in a cross section perpendicular to the longitudinal direction of the central fiber, the cross section of the outer peripheral fiber is a perfect circle in a linear arrangement. However, when the optical fiber bundle is twisted, the ellipse becomes a slightly flat ellipse although the amount is very small. For this reason, in order to obtain a close-packing by twisting, the linear arrangement state before twisting must be a non-close-packing configuration.

The section perpendicular to the longitudinal direction of the outer peripheral fiber is slightly oblique rather than perpendicular to the longitudinal direction of the optical fiber bundle due to twisting, so that the cross section becomes a slightly flat ellipse with a very small amount. At this time, the increase amount Δd of the diameter in the major axis direction of the ellipse can be easily obtained from the following equation. Δd = (1/2) · (Nd ₀ / p) ² · d ₀ (5) where p is the pitch length of the twist of the optical fiber bundle, and d ₀ is the diameter of the outer peripheral fiber. Therefore, in order to twist the optical fiber bundle to achieve the closest packing, the diameter of the cross section of the outer peripheral fiber is obtained by calculation as d ₀ + Δd. U.S. Patent No.
The method described in US Pat. No. 5,408,554 discloses that a close-packed configuration is the preferred method for achieving good equal distribution of optical signals.

[0010]

However, as will be described later, according to our experimental results, even if a close-packing configuration without a gap as specified in JP-A-62-27707 is adopted, However, even if a configuration is adopted in which the close-packing is performed after applying the twist specified in U.S. Pat.No. 5,408,554, the close-packing having a gap or an inverse gap specified in U.S. Pat. Regardless of the configuration, good distribution characteristics could not be realized with any configuration.

As another method of manufacturing a distributor for branching an optical signal in a lump, a hollow glass tube is prepared, and a plurality of optical fibers are inserted therein in a close-packed configuration to form a straight optical fiber bundle. Is formed, and the glass tube is heated from the outside,
There is a method of fusing and stretching optical fiber bundles in each glass tube (Electronic Letters, vol. 25, No. 9,
pp.606-607, (1989), Applied Optics, vol.3
0, No. 6, pp. 650-659, (1991), U.S. Pat.
And U.S. Pat. No. 5,373,572). These publications demonstrate, through detailed experimental data, that the relative positional relationship between optical fibers is strictly defined by inserting an optical fiber bundle into a hollow glass tube, and that excellent distribution characteristics can be obtained. However, according to this method, a distributor having good characteristics can be obtained if a plurality of optical fibers can be accurately arranged at predetermined positions.
Inserting a plurality of, for example, eight, optical fibers into a hollow tube having an inner diameter of about 1 mm in a close-packed configuration requires a fairly highly skilled dexterity and a yield in this process. A decline is inevitable. In addition, U.S. Pat.
No. 782 also discloses a method of collectively branching an optical signal.

Still another method is to form an optical coupling portion by arranging a plurality of optical fibers in parallel, instead of bundling them, and heating, fusing and stretching all adjacent optical fibers at a certain location. This is the 5th Optoelectronic Conference (Fifth Optoe
electronics Conference) (OEC'94) 15B4-6. However, in this method, in the optical coupling section, the optical signal is shifted little by little from the core of the input optical fiber to the adjacent core in order, and the light is shifted to the adjacent core outside the core. Because of the mechanism for optically coupling to the optical fiber, it is difficult to accurately control the coupling length of the light transfer at each stage. For this reason, the number of branches of the distributor having excellent distribution characteristics is naturally limited.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to manufacture a coupler having good characteristics and branching an optical signal in a batch at a high yield.

[0014]

According to the method of manufacturing an optical fiber type multi-branch coupler of the present invention, N (N> 5) outer peripheral fibers are arranged at substantially equal intervals around a central fiber having a cladding and a core. An optical fiber bundle is formed, and the cross section of the optical fiber bundle is located slightly out of the close-packed configuration with a gap between the optical fibers, and (a) the diameter D of the central fiber is defined by the equation d between the diameter _{D 0} and the diameter _{d 0} of the outer peripheral fibers of the central fiber at packing configuration _{0 / D 0 = sin (π} / N) / (1-sin (π / N)) (6) (B) The diameter d of the outer peripheral fiber is larger than D ₀ and is in the range of 1.7 × 10 ⁻³ <(D−D ₀ ) / D ₀ <2.0 × 10 ^−2. Smaller than d ₀ defined by the equation and 1.7 × 10 ⁻³ <(d ₀ −d) / d ₀ <2.0 × 10 ⁻² (8) Satisfy any of (a) and (b) above,
The optical fiber bundle is clamped and fixed in at least two places in the longitudinal direction with a clamp, and a portion between the clamps is heated to fuse the optical fibers, and at the same time, the fixing clamps are separated from each other to form a biconical taper at the fusion portion. It is characterized in that a light coupling portion having a shape is formed.

In the fusion of the portion between the clamps of the optical fiber bundle, the optical fiber bundle is twisted so that the cross-sectional shape perpendicular to the longitudinal direction of the optical fiber bundle before heating, fusion and stretching is adjusted to an appropriate size. It is preferable to form an optical coupling part by fusing after forming a non-close-packed structure having a gap of, and in this optical coupling part, the central fiber and the outer fiber do not have an unfused part, and all the optical fibers are It is fused together.

The optical fiber type multi-branch coupler of the present invention is manufactured in this manner, and the optical signal incident on the central fiber from one end of the optical fiber bundle is coupled to the outer peripheral fiber at the optical coupling section, and the optical fiber is connected to one central fiber. And approximately equal branches into 1 + N of N outer fibers. Further, the optical output from the central fiber can be made substantially zero, and the optical fiber can be branched into N outer peripheral fibers substantially equally. The center fiber and the outer fiber used have substantially the same refractive index between the core and the cladding, and the refractive index distribution at and near the core portion of the center fiber is similar to the refractive index distribution at and near the core portion of the outer fiber. It is preferable to have a distributed shape. Also, the propagation constant between the center fiber and the outer fiber,
The cutoff wavelength or the mode field diameter is substantially the same. The center fiber and the outer fiber are single mode optical fibers, and the cladding surface layer is titania (TiO 2).
An optical fiber doped in ₂ ) can also be used.

Further, in the optical fiber type multi-branch coupler according to the present invention, at one end on the optical input side, N outer fibers are cut, and the cut surface is subjected to non-reflection termination processing. It may be housed in a coupler housing and integrated with the coupler. Also, at one end on the light output side, the center fiber may be cut and subjected to non-reflection termination processing on the cut surface, and then the non-reflection termination processing portion may be housed in a coupler housing to be integrated with the coupler. Further, in the optical fiber type multi-branch coupler of the present invention, at the optical signal input / output end of the central fiber, after the central fiber is fusion-bonded to the same optical fiber as the outer peripheral fiber, the fusion spliced part is housed in the coupler housing. It can also be integrated with a multi-branch coupler.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Optical coupling of a distributor having a plurality of peripheral fibers arranged around a central fiber will be described with reference to FIG. Signal input from one end of the center arranged single-mode optical fiber, as the core of the closer optical fiber to the optical coupling part becomes smaller in the LP ₀₁ mode (mode axisymmetric having a light intensity peak in the core portion), the light intensity distribution spreads leak to the outside of the core portion, evanescent bind to can propagate LP ₀₂ mode at the optical coupling section (mode having a ring-like light intensity with a recess in the core center), further axisymmetric bind to arranged the outer circumferential fiber, in each of the outer peripheral fibers are propagated as LP ₀₁ mode. The light intensity distribution under such conditions is
ol. 2997, pp. 318-325, Feb. 12-14, (1997), detailed calculations have been performed, and in order to facilitate understanding of optical coupling by mode conversion during light propagation, these calculation results are shown in FIG. Added to 1.

As shown in FIG. 1, in order to optically couple optical signals and perform uniform equal distribution, it is necessary to arrange the outer peripheral fibers axially symmetric with respect to the input optical fiber. That is,
As shown in FIG. 2, the distance c (104) between the core (101) of the central fiber and the core (102) of the outer fiber in the optical coupling section (105) is the same for all the outer fiber cores. And the distances b (103) between adjacent outer peripheral fibers are all the same (the cores are indicated by black circles). If at least one core of the outer fiber is far from the core of the center fiber, the amount of light transferred to the outer fiber is smaller than the amount of transfer to the other outer fibers, and as a result, a uniform output is obtained at the output port. Is not obtained and is not evenly distributed.

In addition, since optical coupling is performed not only between the core of the central fiber and the core of the peripheral fiber, but also between the cores of adjacent peripheral fibers, adjacent optical fibers are required for uniform distribution. The distances b between the fibers must all be the same. Further, in the optical coupling section, since the optical signal leaks from the core and spreads to the adjacent core, it is necessary that the cladding between them is uniform as shown in FIG. 2 and a void (FIG. 5E) described later. 114) and Figure 5
If there are gaps (118) as shown in (G), these will be barriers to light transfer, and good optical coupling cannot be obtained.

When a plurality of optical fibers are bundled, if the optical fibers are simply gathered and arranged linearly, the optical fiber bundle is thermally deformed and scattered when heated, and it is difficult to form a coupler without forming a bundle. . Each optical fiber must be fixed by an external force, or when the optical fiber bundle is heated and fused, all the outer fibers must be pressed against the central fiber. The best example of fixing by the external force is the electronic letter
ronics Letters), vol. 25, No. 9, pp. 606-607, (1989), Applied Optics, vol. 30, No. 6, pp. 650-659,
(1991), U.S. Pat.No. 5,175,779, U.S. Pat.
In this method, an optical fiber bundle is inserted into a hollow glass tube as disclosed in U.S. Pat. No. 5,175,782. However, in this case, it is necessary to make no gap between the optical fiber bundle and the inner wall of the hollow glass tube, or to make this gap as small as possible. As a preferable example of the latter, there is a method of applying a tension to the optical fiber bundle by twisting the optical fiber bundle.

That is, as shown in FIG. 3A, when a twist is applied to the optical fiber bundle and both ends thereof are pulled to apply tension in the longitudinal direction, the tension on the outer peripheral fiber (111) is directed toward the central fiber (110). Converted to force. Since the process of producing a coupler always involves stretching, tension is always applied in the longitudinal direction by stretching. Thus, this tension acts to always press the outer fiber against the center fiber. This twisting method is also the most simple, easy and reliable way to form an optical fiber bundle.

The present inventors made an optical fiber bundle by arranging a plurality of optical fibers axially symmetrically as shown in FIG. 3, twisted the optical fiber bundle (FIG. 3A), and clamped both ends (FIG. 3A). (Not shown), apply some force to the clamps to separate them from each other, apply tension to the optical fiber bundle, heat this optical fiber bundle with a torch flame from the outside, and heat it even after the optical fiber bundle softens. The optical fiber bundle is fused while the clamps are further separated from each other, and the optical fiber bundle is stretched to form the biconical tapered portion (116) of the multi-branch coupler shown in FIG. Note that FIG. 3 is greatly enlarged only in the radial direction of the optical fiber bundle for easy understanding). At this time, an optical signal is incident on the center fiber from the left end of the optical fiber bundle, and the output light from the center fiber and the outer fiber is monitored at the right end, and heating is performed so that the optical signal intensity of the center fiber and the outer fiber becomes the same. -Fusion and stretching were performed. 3 (C) to 3 (G) are cross-sectional views at the position in FIG. 3 (B) corresponding to the arrow, and details will be described later in the first embodiment.

According to the results of a series of experiments conducted by the present inventors, it is necessary to accurately adjust the diameters of the central fiber and the outer peripheral fiber to values within a certain range. In order to realize the ideal core arrangement shown in FIG. 2 and obtain good characteristics of the distributor, the close-packing structure described in JP-A-62-27707 and the close-packing structure described in U.S. Pat. It is difficult with the filling configuration, and there is a gap of an appropriate value between the outer fibers,
An optical fiber bundle in a non-close-packed configuration must be used.

The reason for this will be described below with reference to FIGS. 4A is a cross-sectional view of a close-packed configuration of an optical fiber bundle, FIG. 4B is a cross-sectional view of a non-close-packed configuration having a gap between peripheral fibers, and FIG. 4C is a cross-section of FIG. FIG. 3D is a cross-sectional view in a deformation process in which the shape of the cross section of the outer peripheral fiber is flattened by heating in an arrangement, and FIG.
FIG. 4 is a good cross-sectional view obtained after heating, fusion, and stretching when a gap between outer peripheral fibers is appropriate in the cross-sectional arrangement of FIG. 5 (A) to 5 (D) show cross sections of the optical fiber bundle before heating, fusion and stretching, and FIGS. 5 (E) to 5 (G) show optical coupling after heating, fusion and stretching. 2 shows a cross section of a portion.

When the optical fiber bundle is heated, the outer peripheral fiber is heated first, and then the center fiber is not directly heated by the torch flame because it is surrounded by the outer fiber, but the heat is transferred from the heated outer fiber. Heated by Therefore, the outer fiber becomes softer than the center fiber, and a force acts on the outer fiber toward the center of the optical fiber bundle by the tension applied to the optical fiber bundle. The cross-sectional shape is flattened (111-b) as shown in FIG. 4C (the flattening is exaggerated for easy understanding). At the stage before heating the optical fiber bundle, as shown in FIG. 4 (A), if all the optical fibers are in a close-packed configuration in which they are in contact with each other, a gap for taking in all the outer fibers deformed flat by heating is obtained. Therefore, when fusion is performed, one of them is repelled and the core spacing of the outer peripheral fibers is not equal in the circumferential direction, but becomes as shown in FIG. 5 (E), and the core arrangement as shown in FIG. Cannot be obtained. Therefore, the arrangement of the optical fiber may be adjusted before heating, fusion, and stretching, for example, as shown in FIG.
As shown in FIG. 5 (C), the structure should be a non-close-packed structure having an appropriate gap (113) between the outer peripheral fibers, and the cross section after fusion / stretching is as shown in FIG. 5 (F). Good condition is obtained.

In addition, the optical fiber bundle is a non-close-packed structure having an inverted gap (117) as shown in FIG.
When the gap (113) in FIG. 5C is too small, the void (114) is heated, fused and stretched as shown in FIG. ). If the gap (113) is too large as shown in FIG. 5 (D), all the gaps cannot be filled in the circumferential direction even with an optical fiber whose cross section is flattened by heating. In the subsequent cross section, a greatly cracked gap (118) appears as shown in FIG. 5 (G), and the cross section as shown in FIG. 2 cannot be obtained. Therefore, the arrangement of the optical fibers before heating, fusion, and stretching requires a non-close-packed configuration and a gap of an appropriate size. In U.S. Pat.No. 5,408,554, it is described that a non-close-packed configuration is used before twisting and a close-packed configuration after twisting. 5 (E), and the core arrangement shown in FIG. 2 cannot be obtained.

Therefore, the cross-section of the twisted optical fiber bundle requires a non-close-packed structure having an appropriate gap. For this purpose, it is necessary to slightly less than the diameter d ₀ of the closest packing a diameter d of either slightly larger than the diameter D ₀ comprising the diameter D of the central fiber and closest packing, or the outer circumferential fiber . These two methods are geometrically related and they are equivalent. Optimum size of the gap varies depending detailed conditions of each step of heating and fusing and drawing, in the experiments of the present inventors, the diameter D of the central fiber in the latter case taking larger than D _0, below In the first embodiment, 1.7 × 10 ⁻³ <(D−D ₀ ) / D ₀ <2.
When 0 × 10 ⁻² was satisfied, good uniformity was obtained.

In order to distribute equally to a total of 1 + N fibers of one central fiber and N outer fibers, an optical signal is input to the central fiber at one end of the optical fiber bundle, and the steps of heating, fusion and drawing are performed. The output signals of the center fiber and one outer fiber at the multiple ends of the optical fiber bundle are compared, and heating, fusion, and stretching may be stopped so that the two signals become equal. By stopping the heating, fusion and drawing so that the output of the central fiber, which is the monitor signal, is zero or the minimum, an N-branch coupler can be obtained.

In order to increase the efficiency of optical coupling between two adjacent optical fibers, it is preferable that the propagation speeds of optical signals traveling in both optical fibers are substantially equal. That is, if the propagation constant β is substantially the same, good optical coupling is obtained, which is also effective in the multi-branch coupler of the present invention. Similarly,
It is also effective to make the cutoff wavelength or the mode field diameter substantially the same for both optical fibers. In order to increase the optical coupling efficiency, an optical fiber adjusted so that adjacent cores of the optical fibers have substantially the same refractive index between the cores and the claddings and have the same refractive index distribution in the radial direction of the cores. Its use is also effective in producing the multibranched coupler of the present invention.

The transfer of the optical signal between the cores in the optical coupling section is easier as the core diameter is larger. Cladding diameter 125 μm,
Although it is easy to use a multimode optical fiber having a core diameter of 40 to 50 μm, the core diameter is 5 to 10 μm.
A multi-branch coupler can be manufactured even with a single-mode optical fiber of μm if the manufacturing conditions of the present invention are satisfied.

When an optical signal is distributed by the multi-branch coupler manufactured in this manner, the signal is supplied only to the central fiber at the input end. Therefore, the outer peripheral fiber is cut and subjected to an anti-reflection termination process. Is preferred. This is to minimize re-reflection of the optical signal at the optical fiber cutting point. Similarly, in a multi-branch coupler in which the optical signal from the central fiber is substantially zero at the output end, it is preferable to cut the central fiber at the output end and subject it to non-reflection termination processing.

When the multi-branch coupler of the present invention is used for various uses such as an optical communication device, the input and output optical fibers of this device may be the same. Therefore, when the diameters of the optical fibers at the input and output ends are different, it is preferable that the optical fibers are fusion-spliced to another optical fiber having the same diameter, and the fusion-spliced portion is also incorporated in the multi-branch coupler.

The optical fiber type multi-branch coupler of the present invention comprises:
By making the diameter of the optical fiber different between the center fiber and the outer fiber, a good branching ratio can be provided. In order to adjust the outer diameter of the optical fiber to an appropriate value, (1) adjust the thickness of the clad portion in a process of manufacturing a preform as a base material of the optical fiber, (2)
Adjusting the cladding outer diameter of the optical fiber by chemical etching, (3) adjusting the optical fiber by heating and stretching, or (4) adjusting the combination of the above (1), (2) and (3); Etc. may be appropriately adopted.

The first step of producing the optical fiber type multi-branch coupler of the present invention is a step of removing the coating resin since the optical fiber is coated with a polymer resin. May be damaged, which may cause a decrease in the yield of coupler production. As a countermeasure, if an optical fiber doped with titania (TiO ₂ ) is used for the cladding surface layer of the optical fiber to be used, it is hard to be damaged and the yield can be improved. Next, examples of the present invention will be described.

[0036]

(Embodiment 1) This embodiment will be described using an 8-branch coupler as an example with reference to FIGS. Outer fiber (111)
A standard single-mode optical fiber having a cladding outer diameter of 124.1 μm was used. As the center fiber (110), a number of optical fibers were prepared which differed only in the outer diameter of the outer fiber and the cladding. The diameters of the cores of the central fiber and the peripheral fiber are about 9 μm, and the refractive index distributions of the core and the vicinity thereof are almost the same. Seven optical fibers (111) are arranged around the central fiber (110) to form an optical fiber bundle composed of eight optical fibers, and twisted as shown in FIG. 3 (A). Wavelength from left end of optical fiber bundle to center fiber
An optical signal of 1550 nm is incident, and while monitoring the output light from the center fiber and the outer fiber at the right end,
The film was stretched, and the heating, fusing and stretching were stopped so that the two optical signals became the same. Thus, an eight-branch coupler having a biconical tapered portion (116) shown in FIG. 3B was produced.

After measuring and evaluating the characteristics of the eight-branch coupler, the coupler was cut at several places, and the section was observed with a microscope. FIGS. 3C, 3D, 3E show examples of cross sections of a good eight-branch coupler obtained by optimizing the diameter of an optical fiber.
(F) and (G). Figure 3 shows the cutting position of each section.
(B) and arrows indicate the corresponding locations. Optical coupling between the core of the central fiber and the core of the outer peripheral fiber is performed only in the vicinity of the center of the left and right tapered portions in the vicinity of FIG. Voids (114) are naturally observed in (D) between the non-fused (C) and fused (E) of the center fiber and the outer fiber. Since the optical coupling in (D) is interpreted as weak or non-existent, there is no effect on the coupler characteristics. However, if such voids are present in (E) of the optical coupling part, a barrier in the light transfer process will occur. And good optical coupling cannot be obtained.

In evaluating the performance of equal distribution, the uniformity of the output optical signal is the most appropriate parameter.
This is defined by the difference between the maximum value and the minimum value of a plurality of output signals, and is usually expressed in dB. The smaller the value of the uniformity, the more uniform the signal between different output ports.

FIG. 6 shows an embodiment in which the diameter of the outer fiber is fixed and the diameter of the center fiber is changed. The vertical axis in the figure is the uniformity of the insertion loss, and the horizontal axis is the diameter of the central fiber D, which is plotted with a black circle in the figure.
FIG. 6 corresponds to FIG. 5, where the diameter D of the central fiber is D
<161.92 μm (123 in FIG. 6) has the cross section of FIG. 5A at the stage of optical fiber arrangement before heating, fusion and stretching, and D
= 161.92 μm (121 in FIG. 6) has a cross-section of the close-packed configuration of FIG. 5B, and 161.92 μm <D <162.7 μm (FIG. 6).
124) has the cross section of FIG. 5 (C) and 162.7 μm ≦ D
≤164.2 μm (125 in FIG. 6) has the cross section of FIG. 5C, and 164.2 <D (126 in FIG. 6) shows the cross section of FIG.
Has a cross section of D <162.7 μm (121, 12 in FIG. 6)
2, 123, and 124), the cross-sectional shape shown in FIG. 5E was obtained at the optical coupling portion. 162.7 μm ≦ D ≦ 164.2 μm
In the case of (125 in FIG. 6), the cross-sectional shape shown in FIG. 5 (F) is obtained at the optical coupling portion, and further, 164.2 μm <D (12 in FIG. 6).
At the time of 6), the sectional shape of FIG. 5 (G) was obtained.

As is clear from FIG. 6, the diameter of the central fiber must be within a certain range in order to obtain good equal distribution characteristics. In the case of this embodiment, the diameter is indicated by an arrow (125) in the figure. The center fiber diameter is 162.7 μm and 164.2 μm
Good distribution characteristics were obtained only during the period. Regardless of whether the diameter of the central fiber deviated from this range toward a smaller one or a larger one, the value of the uniformity was large and poor in uniformity. In order for the cross section of the optical fiber bundle to be close-packed, when the diameter of the outer peripheral fiber is 124.1 μm, the diameter of the center fiber is 161.92 μm from the close-packed equation (6) (121 in FIG. 6). However, good uniformity was not obtained at this diameter, as is apparent from FIG. In the case of D <161.92 μm, there was an inverted gap as shown in FIG. 5 (A) at the stage of arranging the optical fibers before heating, fusion and stretching, and a good 8-branch coupler could not be obtained with this configuration.

Also, in the close packing after twisting claimed in US Pat. No. 5,408,554, the diameter of the central fiber is increased by a small amount, and the diameter of the central fiber at this time is 162.13 μm (122 in FIG. 6). In this case also, the result was that good distribution characteristics could not be obtained. Observation of the cross section of the optical coupling portion revealed that two voids (114) were observed at D <162.7 μm as shown in FIG. 5 (E). This is apparently because one outer fiber has been flipped out. Also, if the diameter of the central fiber is D> 16
At 4.2 μm, a greatly cracked gap (118) was observed between the two outer fibers as shown in FIG. 5 (G). This is apparently because the gap was too large at the stage of arranging the optical fibers before heating, fusion, and stretching. The diameter of the central fiber is
Only when it is in the range of 162.7 μm ≦ D ≦ 164.2 μm, as shown in FIG.
Was obtained, and the uniformity was small, and an eight-branch coupler with good distribution was obtained.

FIG. 7 is a graph in which the results of the uniformity are plotted collectively in the relationship between the diameter of the central fiber (vertical axis) and the diameter of the outer peripheral fiber (horizontal axis), and a good eight-branch coupler is obtained. Parameter area (132)
Is shown. The diameter of the outer fiber was fixed at 124.1 μm, and only the diameter of the center fiber was changed. In the figure, a circle indicates a data point at which a good branch characteristic was obtained, and a cross indicates a data point at which a good branch characteristic was not obtained. In the figure, a straight line 130 shows the relationship (determined by equation (4)) corresponding to the close-packed configuration claimed in Japanese Patent Application Laid-Open No. 62-27707. The relationship corresponding to the tightly packed configuration after twisting claimed by U.S. Pat. No. 5,408,554 is shown by straight line 131.

FIG. 8 shows the insertion loss at the eight output ends (output ports) when the diameter of the center fiber is 163.5 μm. The insertion loss is the value obtained by dividing the output signal strength by the input signal strength and is usually expressed in dB. Ideally, it is 9.03 dB without excess loss, and a good value close to this is obtained in this experiment. Was. The difference between the maximum value and the minimum value of the variation of the insertion loss was uniformity, which was 1.15 dB, which was a good value. The conditions for obtaining such a good uniformity depend on many manufacturing parameters, but the optimum optical fiber diameter differs depending on the flame intensity of the torch. Table 1 shows the results.
Here, the flame intensity of the torch was adjusted by the gas flow rates of propane and oxygen.

[0044]

[Table 1]

As is clear from Table 1, the optimum central file
The diameter D of the iva varies slightly depending on the flame intensity of the torch.
Was. When the optical fiber bundle is heated by the heat of the torch flame
As described above, the outer fiber comes before the center fiber.
As a result, the cross-section of the outer peripheral fiber is flattened.
However, at this time, if the torch is heated strongly,
The diameter of the central fiber is slightly larger
Because it is more suitable. Therefore, the optimal central
The diameter of the bath depends on the heating conditions and other factors.
In a series of experiments by the lighter, etc., the above equation (6) is specified.
Diameter D₀The diameter D of the central fiber is 1.7 × 10
^-3<(DD₀) / D₀<2.0 × 10 ^-2Satisfy
Good branching characteristics were obtained at a diameter D. 6 to 8
The results described in the above are for the case where the intensity of the torch flame in Table 1 is medium.
It is.

The outer diameter of the outer peripheral fiber used in this example was nominally 125.0 μm, but the measured value by the present inventors was 124.1 ± 0.4 μm. The diameter of the center fiber and the diameter of the outer fiber defined by the present invention are not absolute values, but are relative values based on the above equations (6) to (8).

[0047]

According to the above construction, the optical fiber type multi-branch coupler of the present invention has good branching characteristics, and the optical signal incident on the central fiber from one end of the optical fiber bundle is transmitted to the outer periphery at the optical coupling portion. The optical fiber is coupled to the fiber, and is approximately equally branched into 1 + N one central fiber and N outer peripheral fibers. Further, the optical output from the central fiber may be substantially zero, and the optical fiber may be branched into N outer peripheral fibers substantially equally. Furthermore, the insertion loss at the output end of the optical signal was good, close to the ideal loss value, and the uniformity was small.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a state of optical coupling and a propagation mode in a multi-branch coupler.

FIG. 2 is a diagram illustrating a cross section of an optical coupling portion of a multi-branch coupler.

FIG. 3A is a perspective view of a twisted optical fiber bundle, and FIG. 3B is a perspective view of a multi-branch coupler formed by heating, fusing and stretching the bundle to form a biconical taper; C) ~
(G) is a sectional view at the position (B) corresponding to the arrow.

4A is a cross-sectional view of a close-packed configuration of an optical fiber bundle, FIG. 4B is a cross-sectional view of a non-close-packed configuration having a gap between peripheral fibers, and FIG. 4C is a cross-sectional view of FIG. FIG. 4D is a cross-sectional view in a process in which the shape of the cross section of the outer peripheral fiber is flattened by heating in the cross-sectional arrangement of FIG. ,
It is a favorable sectional view obtained after heating, fusion, and extension.

FIGS. 5A to 5D are cross-sectional views of an optical fiber bundle before heating, fusion, and stretching, and FIGS. 5E to 5G are optical coupling portions after heating, fusion, and stretching. FIG.

FIG. 6 shows the uniformity and the diameter D of the central fiber.
FIG.

FIG. 7 is a diagram showing a region (132) where good uniformity is obtained in the relationship between the diameter D of the central fiber and the diameter d of the outer peripheral fiber.

FIG. 8 is a diagram plotting insertion loss of an optical signal with respect to each output port.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tsuneo Mori 2-13-1, Isobe, Annaka-shi, Gunma Shin-Etsu Kagaku Kogyo Co., Ltd. Precision Functional Materials Research Laboratory (72) Inventor Jun Abe Isobe, Annaka-shi, Gunma 2-13-1 Shin-Etsu Kagaku Kogyo Co., Ltd. Precision Functional Materials Laboratory (72) Inventor Shinji Makikawa 2-13-1 Isobe Annaka-shi, Gunma Prefecture Shin-Etsu Kagaku Kogyo Co., Ltd.

Claims (14)

[Claims] 1. An optical fiber bundle in which N (N> 5) peripheral fibers are arranged at substantially equal intervals around a central fiber having a clad and a core, and the optical fiber bundle has a cross section of an optical fiber. (A) the diameter D of the central fiber
The relationship between the diameter _{d 0} of the diameter _{D 0} and the outer peripheral fibers of the central fiber of close-packed structure _{d 0 / D 0 = sin (} π / N) / (1-sin (π / N)) (1 (B) The diameter d of the outer peripheral fiber is larger than D ₀ defined in the above and is in the range of 1.7 × 10 ⁻³ <(D−D ₀ ) / D ₀ <2.0 × 10 ⁻² (2). The above (a) and (b), which are smaller than d ₀ defined by the expression (1) and in the range of 1.7 × 10 ⁻³ <(d ₀ −d) / d ₀ <2.0 × 10 ⁻² (3) (B) is satisfied, the optical fiber bundle is fixed in at least two places in the longitudinal direction by pressing with clamps, and the portion between the clamps is heated to fuse the optical fibers, and at the same time, the fixing clamps are separated from each other. A method for manufacturing an optical fiber type multi-branch coupler, wherein an optical coupling portion having a biconical tapered shape is formed in a fusion portion. 2. The optical fiber mold according to claim 1, wherein the optical fiber bundle is formed by twisting the optical fiber bundle and then fusing the optical fiber bundle when heating and fusing the portion between the clamps of the optical fiber bundle. A method for producing a branched coupler. 3. The light according to claim 1, wherein a cross-sectional shape perpendicular to the longitudinal direction of the optical fiber bundle before heating, fusion, and stretching is a non-close-packed structure having a gap of an appropriate size. A method for producing a fiber type multi-branch coupler. 4. The optical coupling section having a biconical tapered shape, wherein the central fiber and the outer peripheral fiber have no unfused portion, and all optical fibers are integrally fused. 4. The method for producing an optical fiber type multi-branch coupler according to 3. 5. An optical fiber type multi-branch coupler obtained by the manufacturing method according to claim 1. Description: 6. An optical signal incident on the central fiber from one end of the optical fiber bundle is coupled to the outer peripheral fiber at an optical coupling section, and one of the central fiber and one of the N outer fibers is coupled.
The optical fiber type multi-branch coupler according to claim 5, wherein the optical fiber multi-branch coupler is approximately equally branched into + N branches. 7. An optical signal incident on the central fiber from one end of the bundle of optical fibers is coupled to the outer peripheral fiber at an optical coupling section, the optical output from the central fiber is made substantially zero, and the optical fiber is approximately equally branched into N outer peripheral fibers. The optical fiber type multi-branch coupler according to claim 5, wherein 8. The center fiber and the outer peripheral fiber have substantially the same refractive index between the cores and between the claddings, and the refractive index distribution in the core portion of the central fiber and in the vicinity thereof is close to that in the core portion of the outer peripheral fiber and in the vicinity thereof. 8. The optical fiber type multi-branch coupler according to claim 5, wherein the refractive index distribution is similar to the distribution shape. 9. The optical fiber type multi-branch coupler according to claim 5, wherein a propagation constant, a cutoff wavelength, and a mode field diameter of the center fiber and the outer fiber are substantially the same. 10. The optical fiber type multi-branch coupler according to claim 5, wherein the central fiber and the outer peripheral fiber are single mode optical fibers. 11. An optical fiber having a cladding surface layer doped with titania (TiO ₂ ).
2. The multi-branch optical fiber coupler according to item 1. 12. At one end on the light input side, N outer fibers are cut, and the cut surface is subjected to non-reflection termination processing. Then, the non-reflection termination processing portion is housed in a coupler housing to be integrated with the coupler. The multi-branch optical fiber coupler according to any one of claims 5 to 11, wherein: 13. After cutting the center fiber at one end on the light output side and subjecting the cut surface to non-reflection termination processing,
13. The multi-branch optical fiber coupler according to claim 5, wherein the non-reflection terminating portion is housed in a coupler housing and integrated with the coupler. 14. At the optical signal input / output end of the central fiber, after the central fiber is fusion-spliced to the same optical fiber as the outer peripheral fiber, the fusion spliced part is housed in a coupler housing and integrated with the multi-branch coupler. The multi-branch optical fiber coupler according to any one of claims 5 to 13.