JP3088856B2 - Mode field diameter conversion fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber capable of converting a mode field diameter at a desired portion, a mode field diameter converting fiber using the optical fiber, and a mode field diameter converting method.

[0002]

2. Description of the Related Art FIG. 12 is a diagram showing a conventional mode field diameter conversion method. By thermally diffusing the dopant used to form the core of the optical fiber, the mode field diameter is locally increased at the end of the optical fiber. FIG.
2 (a) shows a change in the concentration distribution of the dopant, and FIG.
FIG. 12B shows a change in the refractive index distribution, and FIG. 12C shows a change in the intensity of the mode field. If the mode field diameter at the end of the optical fiber is increased by such a method, an optical fiber having a small mode field diameter can be connected to an optical fiber having a large mode field diameter with low loss.

[0003]

However, the above-described method has a problem that an optical fiber having a large mode field diameter cannot be connected to an optical waveguide device having a small mode field diameter with low loss. Further, in order to increase the mode field diameter, heating for a long time was required.

Accordingly, an object of the present invention is to provide a mode field diameter conversion fiber in which the mode field diameter of a desired portion is converted in a shorter time as compared with the related art.

[0005]

In order to solve the above-mentioned problems, an optical fiber according to the present invention has a function of increasing the refractive index of a fiber material and has a first heat diffusion coefficient with respect to the fiber material. A core formed by adding a dopant and a second dopant having an effect of lowering the refractive index of the fiber material and having a second thermal diffusion coefficient greater than the first thermal diffusion coefficient for the fiber material. Is to be provided. One form of the dopant distribution is characterized in that both the first dopant and the second dopant have a substantially uniform core concentration distribution. Another form of dopant distribution is the second
Makes the concentration distribution in the core substantially uniform.
The dopant concentration distribution in the core is substantially higher at the center and lower at the periphery to lower the refractive index at the periphery of the core than at the cladding, and to reduce the core diameter to the mode field diameter in the fundamental mode of guided light. It is characterized in that it is formed smaller than the value to be minimized. At this time, the concentration distribution of the first dopant is, for example, substantially parabolic or substantially stepped.

[0006]

Further, in the mode field diameter conversion fiber according to the present invention, a predetermined portion of the optical fiber is heated, and the mode field diameter is changed at the predetermined portion. In the method for converting the mode field diameter according to the present invention,
A predetermined portion of the optical fiber is heated, and the mode field diameter is changed at the predetermined portion.

[0008]

When a predetermined portion of the optical fiber is heated, the first and second dopants are thermally diffused from the core of the predetermined portion. In this case, since the second thermal diffusion coefficient is larger than the first thermal diffusion coefficient, the second dopant diffuses to a region farther away from the center of the core than the first dopant. As a result, the difference between the refractive index of the region close to the center of the core and the refractive index of the region far from the center of the core is relatively increased in both the one dopant distribution mode and the other dopant distribution mode. For this reason, the mode field diameter is reduced in the predetermined portion subjected to the heat treatment. Further, in the case of another dopant distribution mode, the mode field diameter is also reduced by the effect of the substantial increase in the core diameter, which becomes higher in refractive index than the cladding due to the diffusion of the second dopant.

[0009]

As a result, it is possible to obtain a mode field diameter conversion fiber in which the mode field diameter is changed at a desired predetermined portion. Such a mode field diameter conversion fiber functions not only as an optical transmission line but also as an optical element that converts the mode field diameter with low loss.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be briefly described below with reference to the drawings.

(First Embodiment) FIG. 1 is a diagram schematically showing the structure of an optical fiber according to a first embodiment, showing changes in various distributions before and after a heat treatment for changing the mode field diameter. Is shown. FIG. 1A shows a change in the concentration distribution of the first dopant added to the core, and FIG.
Shows a change in the concentration distribution of the second dopant added to the core, and FIG. 1C shows a change in the refractive index distribution near the core. The graph of FIG. 2 shows the relationship between the mode field diameter, the core diameter, and the difference in the refractive index between the core and the cladding when the refractive index distribution in the fiber is a step type.

The optical fiber before heating in FIG. 1 is a single mode fiber, and is formed by a known VAD method in the process shown in FIG. First, a quartz soot preform to which germanium (Ge) as a first dopant was added was synthesized (see FIG. 3A).
Before the transparentization, fluorine (F) as a second dopant is added (see FIG. 3B). Next, the soot preform for the core is made transparent, stretched to an appropriate outer diameter, and then the soot for the cladding is synthesized around the core so that the soot is used as an axis (FIG. 3).
(C)). Thereafter, the cladding soot is made transparent to obtain a preform for forming an optical fiber. This preform is drawn under appropriate conditions (see FIG. 3D).
Thereby, an optical fiber for mode field diameter conversion in which Ge and F are added to the core can be obtained.

The method and principle of forming the above-mentioned optical fiber into a mode field diameter conversion fiber will be described with reference to FIGS. 1 (a) to 1 (c) and FIG. In the following, discussion will be made on the assumption that fluorine is diffused sufficiently quickly by heating as compared to germanium, and that the concentration distribution of fluorine decreases substantially uniformly after heating, and that the concentration distribution of germanium hardly changes. The core diameter of the optical fiber before the heat treatment is 4 μm.

As shown in FIG. 1A, in the optical fiber before thermal diffusion, Ge is added at a substantially uniform concentration in the core region. Further, as shown in FIG. 1B, F is also added to this core region at a substantially uniform concentration. Here, it is assumed that the contribution Δ (Ge) of Ge to the refractive index difference is 0.5%, and the contribution Δ (F) of F is to −0.3%. Consider a case in which a desired portion of an optical fiber having such a structure is subjected to a heat treatment so that Ge and F are thermally diffused. Ge hardly diffuses as shown on the right side of FIG. 1A, but F diffuses widely as shown on the right side of FIG. 1B, and the contribution of the F to the refractive index difference Δ (F) = in the core portion = It changes to -0.2%.

With reference to FIG. 1C, a change in the refractive index near the core before and after thermal diffusion will be considered. The optical fiber before thermal diffusion shows a substantially uniform refractive index distribution as shown by a solid line in the core region. At this time, the refractive index difference Δ (n) between the core part and the clad part
= 0.2%. The dotted line indicates the refractive index distribution caused by Ge or F. When Ge and F are thermally diffused by subjecting a desired portion of the optical fiber to heat treatment, as shown by a solid line on the right side of the drawing, the core diameter hardly changes, and the refractive index difference between the core portion and the cladding portion is not changed. Δ (n) = 0.3%. The change in the mode field diameter corresponding to the above change will be considered with reference to FIG. Before heating, the refractive index difference Δ
(n) = 0.2%, core diameter = 4 μm, corresponding to point A in the graph of FIG. 2, and the mode field diameter at this time was 32 μm. After thermal diffusion by heating, the refractive index difference Δ (n) = 0.
3%, the core diameter = 4 μm, the transition to the point A ′ in the graph of FIG. 2 occurs, and the mode field diameter decreases to 14 μm.

(Second Embodiment) FIG. 4 is a diagram schematically showing the structure of an optical fiber according to a second embodiment. FIG. 4 shows changes in various distributions before and after heat treatment for changing the mode field diameter. Is shown. FIG. 4A shows a change in the concentration distribution of the first dopant added to the core, and FIG.
Shows a change in the concentration distribution of the second dopant added to the core, and FIG. 4C shows a change in the refractive index distribution near the core. The graph of FIG. 5 shows the relationship between the mode field diameter, the core diameter, and the refractive index difference between the core part and the clad part when the refractive index distribution in the fiber is a graded type.

The optical fiber before heating in FIG. 4 is a single mode fiber, and is formed by the known VAD method and the rod-in-tube method in the process shown in FIG. First, a soot preform for quartz made of quartz to which germanium (Ge) as a first dopant is added is synthesized (see FIG. 6A), and fluorine (F) as a second dopant is prepared before the transparent soot preform. (See FIG. 6B). Next, the core soot preform is made transparent, stretched and inserted into a cylindrical cladding preform to form a fiber preform (see FIG. 6 (c)). Thereafter, the fiber preform is drawn under appropriate conditions (see FIG. 6D). Thus, an optical fiber for mode field diameter conversion in which Ge and F are added to the core can be obtained.

Referring to FIGS. 4A to 4C and FIG. 5, a method and principle for forming the above-mentioned optical fiber into a mode field diameter conversion fiber will be described. In the following, discussion will be made on the assumption that fluorine is diffused sufficiently quickly by heating as compared to germanium, and that the concentration distribution of fluorine decreases substantially uniformly after heating, and that the concentration distribution of germanium hardly changes. The core diameter of the optical fiber before the heat treatment is set to 10 μm.

As shown in FIG. 4A, in the optical fiber before the thermal diffusion, Ge is added in the core region in a substantially parabolic concentration distribution which is one of the graded distributions. As shown in FIG. 4B, F is added to this core region at a substantially uniform concentration. Here, the contribution Δ (Ge) of Ge to the refractive index difference is 0.4%, and the contribution Δ (F) of F to the refractive index difference.
= -0.2%. Consider a case where Ge and F are thermally diffused by performing a heat treatment on a desired portion of the optical fiber having such a structure. Ge is hardly diffused as shown on the right side of FIG. As shown on the right side of FIG. 4 (b), the light diffuses widely and contributes to the difference in the refractive index of F in the core portion Δ
(F) =-0.12%.

With reference to FIG. 4C, a change in the refractive index near the core before and after thermal diffusion will be considered. The optical fiber before thermal diffusion exhibits a substantially parabolic refractive index distribution as indicated by a solid line in the core region. In the periphery of the core, the contribution of the decrease in the refractive index by F is greater than the contribution of the increase in the refractive index by Ge, and the substantial core diameter at which the refractive index is larger than that of the clad portion is the core diameter at the time of fabrication in the process of FIG. Smaller than the diameter. The dotted line indicates the refractive index distribution caused by Ge or F. A desired portion of the optical fiber is subjected to a heat treatment to
When F and F are thermally diffused, as shown by the solid line on the right side of the drawing, the refractive index of the core portion increases and the substantial core diameter increases. The change in the mode field diameter corresponding to the above change will be considered with reference to FIG. Before heating,
Ge refractive index contribution Δ (Ge) = 0.4%, F refractive index difference contribution Δ
(F) = − 0.2%, substantial core diameter = 10 μm or less.
From the coordinate position of point B in the graph, the mode field diameter at this time was at least about 40 μm. After thermal diffusion by heating, the refractive index difference contribution of Ge Δ (Ge) = 0.4%, the refractive index difference contribution of F Δ (F) = − 0.12%, the substantial core diameter = about 10 μm
Then, the state transits to the point B 'in the graph of FIG. 5, and the mode field diameter decreases to about 11 μm. In this embodiment, the effect of the substantial increase in the core diameter in addition to the increase in the refractive index difference between the core portion and the clad portion of the first embodiment also contributes to the reduction of the mode field diameter due to the heating. The mode field diameter can be reduced.

(Third Embodiment) FIG. 7 is a diagram schematically showing the structure of an optical fiber according to a third embodiment, showing how various distributions change before and after heat treatment for changing the mode field diameter. Is shown. FIG. 7A shows a change in the concentration distribution of the first dopant added to the core, and FIG.
Shows the change in the concentration distribution of the second dopant added to the core, and FIG. 7C shows the change in the refractive index distribution near the core.

The optical fiber before heating in FIG. 7 is a single mode fiber, and is formed by a known VAD method and a rod-in-tube method in the process shown in FIG. First, a soot preform for quartz made of quartz to which germanium (Ge) as a first dopant is added is synthesized (see FIG. 8A), and fluorine (F) as a second dopant is synthesized before its transparency. (See FIG. 8B). Next, the core soot preform is made transparent, stretched, and inserted into a cylindrical cladding preform to form a fiber preform (see FIG. 8C). Thereafter, the fiber preform is drawn under appropriate conditions (see FIG. 8D). Thus, an optical fiber for mode field diameter conversion in which Ge and F are added to the core can be obtained.

With reference to FIGS. 7A to 7C, a method and a principle of forming the above-mentioned optical fiber into a mode field diameter conversion fiber will be described. In the following, discussion will be made on the assumption that fluorine is diffused sufficiently quickly by heating as compared to germanium, and that the concentration distribution of fluorine decreases substantially uniformly after heating, and that the concentration distribution of germanium hardly changes.

As shown in FIG. 7A, in the optical fiber before thermal diffusion, Ge is added in the core region in a substantially stepwise (two-step) concentration distribution. Also, as shown in FIG.
F is added to this core region at a substantially uniform concentration.
Consider a case where Ge and F are thermally diffused by performing a heat treatment on a desired portion of the optical fiber having such a structure. Ge is hardly diffused as shown on the right side of FIG. The light diffuses widely as shown on the right side of FIG.

With reference to FIG. 7C, a change in the refractive index near the core before and after thermal diffusion will be considered. The optical fiber before thermal diffusion exhibits a substantially step-like refractive index distribution as shown by a solid line in the core region. In the core peripheral portion, the contribution of the decrease in the refractive index by F is greater than the contribution of the increase in the refractive index by Ge.
Is smaller than the core diameter at the time of preparation. The dotted line indicates the refractive index distribution caused by Ge or F. A desired portion of the optical fiber is subjected to a heat treatment to
When F and F are thermally diffused, as shown by the solid line on the right side of the drawing, the refractive index of the core portion increases and the substantial core diameter increases. That is, in this embodiment, as in the second embodiment, the factor for reducing the mode field diameter due to heating is due to the substantial increase in the core diameter in addition to the increase in the refractive index difference between the core portion and the cladding portion in the first embodiment. Since the effect also contributes, it is possible to efficiently reduce the mode field diameter.

(Fourth Embodiment) FIG. 9 is a diagram schematically showing the structure of an optical fiber according to a first embodiment, showing changes in various distributions before and after a heat treatment for changing a mode diameter and a field diameter. It shows the situation. FIG. 9A shows a change in the concentration distribution of the first dopant added to the core.
FIG. 9B shows a change in the concentration distribution of the second dopant added to the core, and FIG. 9C shows a change in the refractive index distribution near the core. The graph of FIG. 10 shows the relationship between the mode field diameter, the core diameter, and the difference in the refractive index between the core and the cladding when the refractive index distribution in the fiber is a step type.

The optical fiber before heating in FIG. 9 is a single mode fiber, and is formed by a known rod-in-tube method in the process shown in FIG. First,
A preform for a cylindrical columnar core made of quartz to which germanium (Ge) as a first dopant is added and a preform for a cylindrical cladding made of quartz to which fluorine (F) as a second dopant is added (FIG. 11 (a)). Next, the preform for core is inserted into the hollow portion of the preform for cladding to obtain a preform for forming an optical fiber (see FIG. 11B). This preform is integrated under appropriate conditions (see FIG. 11 (c)). Thus, an optical fiber for mode field diameter conversion in which Ge is added to the core and F is added to the cladding can be obtained.

The method and principle of forming the above optical fiber into a mode field diameter conversion fiber will be described with reference to FIGS. 9 (a) to 9 (c) and FIG. In the following, discussion will be made on the assumption that fluorine is diffused sufficiently quickly by heating as compared to germanium, and that the concentration distribution of fluorine decreases substantially uniformly after heating, and that the concentration distribution of germanium hardly changes. The core diameter of the optical fiber before the heat treatment is 4 μm.

As shown in FIG. 9A, the optical fiber before thermal diffusion is doped with Ge at a substantially uniform concentration in the core region. Further, as shown in FIG. 9B, F is also added to this core region at a substantially uniform concentration. Here, it is assumed that Ge contributes to the refractive index difference Δ (Ge) = 0.2% and F contributes to the refractive index difference Δ (F) = 0.1%. Consider the case where a desired portion of the optical fiber having such a structure is subjected to a heat treatment to thermally diffuse Ge and F. Ge hardly diffuses as shown on the right side of FIG. As shown on the right side of FIG. 9B, the light diffuses widely, and changes to the contribution Δ (F) = 0% of the refractive index difference of F in the core portion.

With reference to FIG. 9C, a change in the refractive index near the core before and after thermal diffusion will be considered. The optical fiber before thermal diffusion shows a substantially uniform refractive index distribution as shown by a solid line in the core region. At this time, the refractive index difference Δ (n) between the core part and the clad part
= 0.3%. The dotted line indicates the refractive index distribution caused by Ge or F. When Ge and F are thermally diffused by subjecting a desired portion of the optical fiber to heat treatment, as shown by a solid line on the right side of the drawing, the core diameter hardly changes, and the refractive index difference between the core portion and the cladding portion is not changed. Δ (n) = 0.2%. The change in the mode field diameter corresponding to the above change will be considered with reference to FIG. Before heating, the refractive index difference Δ
(n) = 0.3%, core diameter = 4 μm, which corresponds to point C in the graph of FIG. 10, and the mode field diameter at this time is 14 μm
Met. After thermal diffusion by heating, the refractive index difference Δ (n) =
0.2%, core diameter = 4 μm, and C ′ in the graph of FIG.
The point changes to a point, and the mode field diameter increases to 32 μm.
Actually, the first dopant Ge also diffuses slightly. This diffusion of Ge also contributes to a decrease in the difference in the refractive index, so that the decrease in the difference in the refractive index quickly increases the mode field diameter.

The mode field diameter conversion fiber obtained as described above can be applied to various uses that require a mode field diameter to be reduced. For example, a fiber having a large mode field diameter can be connected with low loss to an optical component such as an optical waveguide device or a fiber having a small mode field diameter. In addition to reducing or increasing the mode field diameter at the end of the fiber, the mode field diameter can be reduced or increased at an arbitrary portion between the ends of the fiber.

Although the present invention has been described with reference to the embodiment, various modifications are possible. For example, the mode field diameter conversion fiber as described above can be formed by various manufacturing methods such as an MCVD method, an OVD method, and a double crucible method. Further, the first and second dopants are not limited to Ge and F, and various dopants can be used. Further, the refractive index difference between the core and the clad can be set to a desired value depending on the conditions for setting the temperature for thermal diffusion. In addition, it is considered that a similar effect can be realized not only with a single mode fiber but also with a multimode fiber.

[0034]

As described above, according to the optical fiber of the present invention, the core is doped with the first dopant that increases the light refractive index, and the core and the clad are lowered in the light refractive index so that the core and the clad have higher heat than the first dopant. Since the second dopant having a large diffusion coefficient is added with a distribution, by heating a predetermined portion, the difference between the refractive index of the region near the center of the core and the refractive index of the region far from the center of the core is relatively large. And the mode field diameter decreases in a short time at a predetermined portion subjected to the heat treatment. Also, in the case of an optical fiber in which the mode field diameter is reduced by heating, the core diameter is set to be smaller than the diameter at which the mode field diameter in the fundamental mode is minimized, and the dopant distribution is set so that the core diameter is substantially increased by heating. By doing so, the mode field diameter can be efficiently reduced. As a result, a mode field diameter conversion fiber in which the mode field diameter is changed in a desired portion can be obtained. Such a mode-field diameter conversion fiber has an advantage that an optical fiber having a large or small mode field diameter can be connected to an optical component having a small or large mode field diameter with low loss.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a method for converting a mode field diameter of a fiber according to a first embodiment.

FIG. 2 is a graph showing a relationship between a core diameter and a refractive index distribution and a mode field diameter according to the first embodiment.

FIG. 3 is a view showing a manufacturing process of the fiber of the first embodiment.

FIG. 4 is a diagram illustrating a method for converting a mode field diameter of a fiber according to a second embodiment.

FIG. 5 is a graph showing a relationship between a core diameter and a refractive index distribution and a mode field diameter according to the second embodiment.

FIG. 6 is a process chart of manufacturing a fiber according to a second embodiment.

FIG. 7 is a diagram illustrating a method for converting the mode field diameter of a fiber according to a third embodiment.

FIG. 8 is a process chart of manufacturing a fiber according to a third embodiment.

FIG. 9 is a diagram illustrating a method for converting a mode field diameter of a fiber according to a fourth embodiment.

FIG. 10 is a graph showing a relationship between a core diameter and a refractive index distribution and a mode field diameter according to the second embodiment.

FIG. 11 is a manufacturing process diagram of the fiber of the second embodiment.

FIG. 12 is a diagram showing a conventional mode field diameter conversion method.

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Masayuki Nishimura 1-chome, Taya-cho, Sakae-ku, Yokohama-shi, Kanagawa Prefecture Sumitomo Electric Industries, Ltd. Yokohama Works (72) Inventor Shigeru Tomita 1-1-6 Uchisaiwai-cho, Chiyoda-ku, Tokyo Japan (56) References JP-A-4-65326 (JP, A) JP-A-4-128704 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02B 6 / 10-6/14

Claims (11)

(57) [Claims] A first dopant having a function of increasing the refractive index of the fiber material and having a first thermal diffusion coefficient with respect to the fiber material; and a first dopant having a function of lowering the refractive index of the fiber material with respect to the fiber material. An optical fiber, comprising: a core formed by adding a second dopant having a second thermal diffusion coefficient larger than the first thermal diffusion coefficient. 2. The optical fiber according to claim 1, wherein the concentration distribution of the first dopant in the core is substantially uniform. 3. The optical fiber according to claim 1, wherein the concentration distribution of the first dopant in the core is substantially high in a central portion and low in a peripheral portion. 4. The optical fiber according to claim 3, wherein the concentration distribution of the first dopant in the core is substantially parabolic. 5. The optical fiber according to claim 3, wherein the concentration distribution of the first dopant in the core is substantially stepped. 6. The optical fiber according to claim 1, wherein the concentration distribution of the second dopant in the core is substantially uniform. 7. The optical fiber according to claim 1, wherein a diameter of the core is smaller than a value at which a mode field diameter of a transmitted light in a fundamental mode is minimized. 8. The method of claim 1, wherein the first dopant is germanium and the second dopant is fluorine.
An optical fiber as described. 9. A mode field diameter conversion fiber according to claim 1, wherein a predetermined portion of the optical fiber according to claim 1 is heated to reduce a mode field diameter at said predetermined portion. 10. The method of claim 1, wherein the first dopant is germanium,
10. The mode field diameter conversion fiber according to claim 9, wherein the second dopant is fluorine. 11. A method for converting a mode field diameter of an optical fiber according to claim 1, wherein a predetermined portion of the optical fiber is heated to reduce a mode field diameter at the predetermined portion.