JPS6389810A - Optical fiber tap - Google Patents

Abstract

PURPOSE:To obtain branched light with excellent transmission characteristics while suppressing the mode conversion loss of transmission light by optically coupling a main line with a tap fiber without reducing the radius of curvature of a main line optical fiber too small. CONSTITUTION:The main line optical fiber 1 consists of a multi-mode optical fiber having a core 3 and a clad 5 and a part of light in a waveguide mode is converted into clad mode light at a mode conversion area 8 in a bent part 1a. Since a gap between the area 8 and the one end part 2a of the tap fiber 2 is filled with a light translucent material 7 having >=1 diffractive index, the clad mode light in the area 8 can be lead into the material 7 and made incident on the fiber 2. Since only a part of the waveguide mode light is converted into the clad mode light in the area 8 without converting the clad mode light up to radiation mode light and it is unnecessary to reduce the radius R of curvature of the bend part 1a too small, the mode conversion loss of the transmission light can be suppressed.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention provides an optical method for optically coupling a first optical fiber and a second optical fiber without destroying the first optical fiber. It concerns fiber taps.

[Summary of the invention]

In the optical fiber tap as described above, the present invention forms a mode conversion region in the first optical fiber that converts a part of light in the waveguide mode into light in the Talarado mode, and also provides mode conversion of the first optical fiber. By filling a translucent material with a refractive index greater than 1 between the region and one end of the second optical fiber, it is possible to suppress the mode conversion loss of the transmitted light and produce branched light with good transmission characteristics. It is made so that it can be obtained.

[Conventional technology]

In optical communication systems, light wave transmission systems, etc., optical fiber taps are used to guide light from a main optical fiber to a tapped optical fiber, or vice versa. ing.

There are various types of optical fiber taps, but optical fiber taps that optically couple the trunk optical fiber and the tapped optical fiber without destroying the trunk optical fiber and are for multimode optical fibers include, for example: Unexamined Japanese Patent Publication 1986
There is an optical fiber tap as shown in Japanese Patent No. 14607.

This non-destructive optical fiber tap bends the main optical fiber, converts part of the waveguide mode light into radiation mode light in the mode conversion area of the bend, and taps this radiation mode light. It guides the light to the optical fiber and also guides the light from the tap optical fiber to the trunk optical fiber.

[Problem that the invention seeks to solve]

By the way, in order to convert part of the waveguide mode light into radiation mode light as in the above-mentioned conventional example, all that is required is to remove the transmitted light in the core of the trunk optical fiber and leak it into the land. In order to leak light from inside the craft to the outside of the main optical fiber, it is necessary to considerably reduce the radius of curvature of the bent portion of the main optical fiber.

However, if the radius of curvature of the bent portion of the trunk optical fiber is made too small, the mode conversion loss of the transmitted light in the trunk optical fiber becomes large.

[Means for solving problems]

The optical fiber tap according to the present invention is made of a multimode optical fiber having a core 3 and a cladding 5, and has a bent portion 1a that forms a mode conversion region 8 that converts a part of light in the guided mode into light in the Talarado mode. a first optical fiber 1 , and a second optical fiber 2 , whose one end portions 2 a and 12 a are disposed near the mode converting region 8 so as to receive light emitted from the mode converting region 8 . 12
and a translucent material 7 filled between the mode conversion region 8 and the one end portions 2a, 12a and having a refractive index greater than 1.

[Effect]

In the optical fiber tap according to the present invention, only a part of the guided mode light is converted into cladding mode light in the mode conversion region 8 of the first optical fiber 1; Conversion region 8 and second optical fiber 2.1
Since the light-transmitting material 7 having a refractive index larger than 1 is filled between the end portion 2a112a of the second end portion 2a112a, the light in the cladding mode in the mode conversion region 8 is guided into the light-transmitting material 7 and further It can be made to enter the optical fiber 2.

Moreover, in the mode conversion region 8 of the first optical fiber 1,
It is only necessary that a part of the light in the waveguide mode is converted to light in the cladding mode, and there is no need to convert it to light in the radiation mode, so there is no need to make the radius of curvature R of the bent portion 1a very small.

〔Example〕

Hereinafter, first and second embodiments of the present invention will be described with reference to FIGS. 1 to 11.

FIG. 1 shows a first embodiment. In this first embodiment, the main optical fiber 1 is held in a bent state with a radius of curvature R, and due to this bending, the main optical fiber 1 has a bent part 1a and straight parts 1b and 1c continuous on both sides of the bent part 1a. is formed.

The tap optical fiber 2 also has one end 2a coaxial with one straight section 1b of the main optical fiber 1 and a bent section 1a.
It is held so that it is in contact with the In addition, trunk optical fiber 1,
Each of the tap optical fibers 2 is a multimode optical fiber having a core 3.4 with a refractive index of n and a cladding 5.6 with a refractive index of n2.

Between the cladding 5 of the bent portion 1a of the trunk optical fiber 1 and the end surface of the one end portion 2a of the tap optical fiber 2, there is a refractive index nc.
is filled with a translucent material 7 having a larger value than 1.

The light-transmitting material 7 is made of an ultraviolet curing resin, a thermosetting resin, or the like, but if these resins diffuse to areas other than the bonding portion before curing, light loss increases. Therefore, these resins preferably have a high viscosity, specifically, the viscosity before curing is preferably 1000 cP or more.The transparent material 7 is preferably a gel-like material with good workability or a highly elastic rubber. It may be composed of a shaped molded product or the like.

Further, in order to reduce loss due to shape mismatch of the optical waveguide, the transparent material 7 is attached to one end 2 of the tapped optical fiber 2.
At the interface with the end surface of the one end portion 2a, the cross-sectional shape is substantially the same as that of the end surface of the one end portion 2a.

In the first embodiment, when light is propagated through the trunk optical fiber 1 from the straight section 1b to the bent section 1a, the field distribution held by both the straight section 1b and the bent section 1a changes at the boundary between the straight section 1b and the bent section 1a. The mismatch in causes mode conversion losses (
(Partly written by Akira Yogami: Optical Waveguide, Asakura Shoten). Therefore, the vicinity of the boundary between the straight portion 1b and the bent portion 1a serves as the mode conversion region 8.

The radius of curvature R described above is selected so that in this mode conversion region 8, a part of the light in the guided mode is only converted into light in the cladding mode, but not into light in the radiation mode. Therefore, this radius of curvature R is larger than the radius of curvature when converting to radiation mode light.

The light converted to the cladding mode in the mode conversion region 8 is
The light propagates through the cladding 5, enters the interface 9 between the cladding 5 and the transparent material 7, is refracted at the interface 9, and travels into the transparent material 7.

The light that travels into the translucent material 7 in this manner is emitted so as to have a peak in the axial direction of the straight portion 1b.

This is a phenomenon that has been confirmed in measurements in the microwave region (E.G., Neumann & H.D., Rud.
olph, “Ra-dtation from Ben
ds in Dielectrjc Rod Tran
s-mission Lines”, IEEE Tr.
ans, Mtcrowave Theory and
Techniques, vol, MTT-23
+ Nol + pp142-14911975).

Figure 2 uses an LED with a wavelength of 660 nm as a light source,
Far-field image in the steady mode distribution of the trunk optical fiber 1, that is, the trunk, when a plastic optical fiber whose core 3 and cladding 5 have refractive indexes n1 and n2 of 1.492 and 1.405, respectively, is used as the trunk optical fiber 1. It shows the measured value of the far-field pattern of the optical fiber 1 in an unbent state. As is clear from FIG. 2, the light intensity of this far-field pattern has a full width at half maximum of 33''.

3A and 3B, the radius of curvature is R-10 mm in the first embodiment shown in FIG. 1, and the refractive index nc is used as the translucent material 7.
= 1.432, and the light having the mode distribution as shown in Fig. 2 is transmitted from the straight part 1b to the bent part 1a.
It shows the mode distribution of light emitted from the straight portion IC and the one end portion 2a when the light is propagated toward the straight portion IC and the one end portion 2a.

As is clear from FIG. 3A, the light emitted from the straight portion IC has a full width at half maximum of 40°, and mode conversion loss is small.

Further, as is clear from FIG. 3B, the light emitted from the one end portion 2a has a full width at half maximum of 21°, and has good transmission characteristics.

FIG. 3C shows the mode distribution of light emitted from the straight portion 1b when light having the mode distribution as shown in FIG. 2 is propagated from one end portion 2a toward the bent portion 1a. ing.

As is clear from FIG. 3C, higher-order mode excitation occurs in this case, and the full width at half maximum is 58°.

FIG. 4 shows the influence of the attachment angle θ of the one end portion 2a with respect to the axial direction of the straight portion 1b on the branching of light. As is clear from the measured values shown in FIG. 3B, the light 10 emitted from the bent portion 1a has a peak in the axial direction of the straight portion 1b. Therefore, the above-mentioned mounting angle θ may be selected so that the light 10 emitted in the axial direction of the straight portion 1b can be made to enter the tapped optical fiber 2.

Incidentally, the maximum angle θwax at which light can enter the tap optical fiber 2 from the transparent material 7 is expressed as follows. Therefore, the mounting angle θ should be selected so that the following relationship is satisfied: -θ□8≦θ≦θ□.

Incidentally, also when the light 11 emitted from the tap optical fiber 2 is made to enter the main optical fiber 1 as shown in FIG. 4B, the formula (2) naturally holds true due to the principle of light ray reversal.

FIG. 5 shows a second embodiment. In this second embodiment, the trunk optical fiber 1 is bent by 180' so that the straight portion 1b and the straight portion 1c of the trunk optical fiber 1 are approximately parallel, and the straight portion 1b and the IC are approximately parallel to each other. Two tapped optical fibers 2.12 are held in a ring shape.

In the second embodiment, a part of the light propagating through the trunk optical fiber 1 from the straight section 1b toward the bent section 1a is branched to the tap optical fiber 2, and
The light propagated through the main optical fiber 1 enters the trunk optical fiber 1.

Therefore, trunk optical fiber 1 and tap optical fiber 2.12
The second embodiment forms a bidirectional optical fiber tap.

FIG. 6 shows the dependence of the branching characteristics of the second embodiment on the refractive index nc of the transparent material 7. Here, the branched light is light that propagates through the straight portion lb and then propagates through the one end portion 2a, and the branching ratio is a value defined by . However, P is the intensity of light propagating through the straight portion lb and propagating through the bottom portion 1c, and P2 is the intensity of the branched light.

FIG. 7 shows the dependence of the coupling characteristics of the second embodiment on the refractive index nc of the transparent material 7. Here, the coupled light is light that propagates through the one end portion 12a and then propagates through the straight portion 1c, and is a value defined by the coupling loss. However, Po is the intensity of light propagating through the straight portion lb of the trunk optical fiber 1 in an unbent state and propagating through the straight portion IC, and Pl! is the intensity of the combined light.

FIG. 8 shows the dependence of the excess loss and insertion loss of the second example on the refractive index nc of the transparent material 7. Here, the excess loss and insertion loss are values defined by .

As is clear from the above FIGS. 6 to 8, in the range of nc≦n, all values show good characteristics. vice versa,
In the range of nc>n, the branching ratio decreases, and the full width at half maximum, coupling loss, and excess loss of the branched light increases.

Particularly, paying attention to the branching characteristics shown in FIG. 6, the closer nc is to n2, the better the amount of transmitted light and the transmission mode characteristics become. This is because the smaller the value of the full width at half maximum, the more low-order mode components there are and the more favorable the transmission mode distribution is.

Moreover, when paying attention to the coupling characteristics in FIG. 7, the coupling loss is the minimum at nc#n2. On the other hand, although the full width at half maximum of the coupled light itself has little dependence on nc, the lower-order mode components actually increase as nc increases. This is also clear from FIG.

That is, Fig. 9 shows the measured value of the far-field image of the combined light when nc = fi, = 1.492, but nc = 1.43.
It can be seen that the value in FIG. 9 has more low-order mode components than the value in FIG. 3C for case 2.

That is, when the main focus is on high coupling efficiency, a value close to n8 may be selected as the ncO value, and when the main focus is on low-order mode coupling, a large value of nc may be selected.

Furthermore, paying attention to FIG. 8, the smaller the nc, the smaller the excess loss and insertion loss. In particular, the insertion loss is ne≦
n! In the range , the amount that can be taken out of the thalassode mode component held at the bent portion 1a of the trunk optical fiber 1 is small, so that the amount becomes small. Therefore, from FIGS. 6 and 8, regarding the branching characteristic, n is close to the value of n3.
It is preferable to select c.

Figure 10 ASB is nc=nt and fi c+1111
fi.

The results of ray tracing based on the meridional ray neighborhood are shown in each case. From Figure 10B, new
It can be seen that in the case of , the coupling efficiency decreases due to total reflection at the interface 9.

Therefore, in the case as shown in FIG. 10B, by using a low NA optical fiber as the tap optical fiber 2, the components of the light incident on the interface 9 can be limited and the coupling efficiency can be improved. Note that this can also be achieved by using an optical fiber having a smaller diameter than the trunk optical fiber 1 as the tap optical fiber 2.

FIG. 11 shows the relationship between the full width at half maximum of the light incident on the straight portion 1b toward the bent portion 1a and the branching ratio, and the full width at half maximum of the light emitted from the straight portion 1c and one end portion 2a in the second embodiment, that is, It shows the dependence of the branching characteristics on the full width at half maximum of the incident light.

As is clear from FIG. 11, as the full width at half maximum of the incident light increases, even if the radius of curvature R of the bent portion 1a remains the same, the amount of light converted to the cladding mode becomes relatively large, and the branching ratio increases. At the same time, the full width at half maximum of the light emitted from the tapped optical fiber 2 is also large.

However, the full width at half maximum of the light emitted from the straight portion 1c has a small dependence on the full width at half maximum of the incident light.

This is because the higher-order mode components of the incident light are branched to the tapped optical fiber 2, and mixing between modes is promoted at the bent portion 1a.

In other words, the second embodiment has a function not only as an optical fiber tap but also as a mode scrambler.

Therefore, when the optical fiber tap of the second embodiment is incorporated into an optical communication system etc., even if such optical fiber taps are connected in multiple stages, light with a stable mode distribution is always transmitted to the next stage optical fiber tap. can be provided.

[Effects of the Invention] In the optical fiber tap according to the present invention, it is possible to optically couple the first optical fiber and the second optical fiber without reducing the radius of curvature of the bent portion of the first optical fiber too much. Therefore, branched light having good transmission characteristics can be obtained while suppressing mode conversion loss of transmitted light.

[Brief explanation of the drawing]

FIG. 1 is a plan view of the main part of the first embodiment of the present invention, FIG. 2 is a graph showing a far-field image of light emitted from the end face of the optical fiber in steady mode, and FIG. 3 is the first embodiment. FIG. 4 is a plan view showing the influence of the installation angle of the tapped optical fiber in the first embodiment, and FIG. 5 is a graph showing the main points of the second embodiment. FIG. 6 is a plan view of the part, and FIG.
A graph showing the dependence of the branching characteristics of the example. FIG. 7 is a graph showing the dependence of the coupling characteristics of the second example on the refractive index nc of the transparent material. FIG. 8 is a graph showing the dependence of the coupling characteristic of the second example on the refractive index nc of the transparent material A graph showing the dependence of excess loss and insertion loss of the second embodiment on n, and FIG. 9 is a far-field image of coupled light in the second embodiment when the refractive index of the transparent material is 1.492. FIG. 10 is a plan view of the second embodiment showing the results of ray tracing, and FIG. 11 is a graph showing the dependence of the branching characteristic of the second embodiment on the full width at half maximum of the incident light. In addition, in the symbols used in the drawings, 1...-------1...-- Trunk optical fiber 1a--...Bending portions 2, 12-- ---Tap optical fibers 2a, 12a--One end 3
・−−−−−1−・・・・−・−・・・・Core 5・−・
−・−・−−−−−−・Clad 7・−・−・・・・・
-...Translucent material 8-...------1--...
- Mode conversion area.

Claims (1)

[Claims] 1. A multimode optical fiber having a core and a cladding, and having a bent portion forming a mode conversion region that converts a part of light in the guided mode into light in the cladding mode. a second optical fiber having one end disposed near the mode conversion region so as to receive light emitted from the mode conversion region; the mode conversion region and the one end; and a translucent material having a refractive index greater than 1 filled in between the optical fiber taps. 2. The bent portion is near the one end portion such that the light emitted from the one end portion of the second optical fiber into the transparent material enters the first optical fiber at the bent portion. An optical fiber tap according to claim 1, which is arranged in a. 3. Claim 1, wherein the bent portion forms a plurality of the mode conversion regions, and a plurality of the second optical fibers are arranged corresponding to the plurality of mode conversion regions.
The optical fiber tap according to item 1 or 2. 4. The optical fiber tap according to any one of claims 1 to 3, wherein the light-transmitting material is made of any one of a curable resin, a gel-like material, or a rubber-like molded product. . 5. Claims 1 to 4, wherein the translucent material has substantially the same cross-sectional shape as the end face of the second optical fiber at the interface with the second optical fiber. The optical fiber tap according to any one of the items. 6. The refractive index n_c of the light-transmitting material is n_c≧n_1 with respect to the refractive index n_1 of the core of the first optical fiber.
An optical fiber tap according to any one of claims 1 to 5. 7. The refractive index n_c of the light-transmitting material satisfies n_2≦n_c<n_1 with respect to the refractive index n_1 of the core and the refractive index n_2 of the cladding of the first optical fiber. The optical fiber tap according to any one of item 5. 8. The optical fiber tap according to any one of claims 1 to 7, wherein the second optical fiber has a lower NA than the first optical fiber. 9. The optical fiber tap according to any one of claims 1 to 7, wherein the diameter of the second optical fiber is smaller than the diameter of the first optical fiber. 10. The bent portion of the first optical fiber has a uniform radius of curvature throughout the bent portion, according to any one of claims 1 to 9. fiber optic tap.