JP2004020836A - Optical fiber and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new optical fiber causing no concentration of anisotropic stress to a core region due to melting residual stress and having a superior PMD (Polarization Mode Dispersion) characteristic. <P>SOLUTION: In the optical fiber provided with a clad layer 3 in the circumference of the core region 2, an assembly region of air bubbles 5 is formed in the clad layer 3. This eliminates the concentration of the anisotropic stress to the core region 2 due to the melting residual stress as seen in the conventional photonic crystal optical fiber, so that the superior PMD characteristic can be displayed to easily attain fine refractive index distribution control of the inner clad layer 3. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical fiber used in the field of optical communication, which enables further large-capacity communication, and a method for manufacturing the same.
[0002]
[Prior art]
Optical fibers that enable high-capacity and high-speed communication are indispensable for constructing optical communication networks. However, in recent and future optical communication networks, optical fibers have become faster and more information is required. Accordingly, an optical fiber having a larger capacity has been demanded, and an optical fiber called a so-called photonic crystal optical fiber has attracted attention as a new optical fiber satisfying this demand.
[0003]
The photonic crystal fiber is an optical fiber using a photonic crystal (PC) having a uniform two-dimensional periodic structure in the longitudinal direction of the fiber as a cladding covering the core, and a region corresponding to the cladding. A photonic band gap (PBG: Photonic Band Gap) is provided in the core, and a light wave is confined in the core by Bragg reflection.
[0004]
In the photonic crystal optical fiber proposed so far, for example, like a holey fiber (HF), a continuous hole is provided in the cladding without interruption in the longitudinal direction of the fiber, and the effective refraction in that region is provided. This is achieved by reducing the rate. The holey fiber has characteristics that cannot be achieved with ordinary optical fibers, such as an ultra-wide band single mode transmission region, a large effective core area, a high refractive index difference (High- △), and a large structural dispersion, depending on the design of the holes in the cladding. Is feasible.
[0005]
For example, as shown in FIG. 3, such a photonic crystal optical fiber is obtained by cutting a small-diameter quartz rod a having an outer diameter of about 500 μm and a small-diameter quartz tube b having an inner diameter of about 300 μm to about 300 mm in length, The small diameter quartz rod a is bundled so as to be surrounded by several hundred small diameter quartz tubes b, b, and the bundle c is inserted into a quartz jacket tube d having an inner diameter of about 10 to 15 mm and an outer diameter of about 25 mm. After forming the preform e, the preform e is fused and integrated with the bundle c of the small diameter quartz rods a and the small diameter quartz tubes b, b, and the quartz jacket tube d by a normal optical fiber drawing process. While drawing, a predetermined fiber diameter of 100 to 150 μm is drawn.
[0006]
In such a photonic crystal optical fiber, as shown in FIG. 3, an axial center portion composed of the small-diameter quartz rod a serves as a core region for transmitting light, and a small-diameter quartz surrounding the core region. The portion consisting of the tubes b, b,... Becomes an inner cladding layer having a large number of holes, and the surrounding solid portion consisting of the quartz jacket tube 5 becomes an outer cladding layer. The light wave is efficiently propagated by being reflected by the cladding layer and confined in the core region.
[0007]
[Problems to be solved by the invention]
By the way, in the conventional photonic crystal optical fiber obtained in this way, when the small-diameter quartz rod a and the small-diameter quartz tubes b, b. Residual fusion stress remains in the glass, giving anisotropic stress to the core region, deteriorating PMD (Polarization Mode Dispersion) characteristics, etc., and making it difficult to delicately control the refractive index distribution of the inner cladding layer. is there.
[0008]
Therefore, the present invention has been devised in order to effectively solve such a problem, and an object of the present invention is to prevent anisotropic stress from concentrating on a core region due to fusion residual stress and to provide excellent PMD characteristics. The present invention provides a novel optical fiber having the same and a method for manufacturing the same.
[0009]
[Means for Solving the Problems]
According to a first aspect of the present invention, there is provided an optical fiber having a cladding layer around a core region, wherein an aggregate region of bubbles is formed in the cladding layer. Specifically, the aggregate region of the bubbles is formed in a layer shape concentric with the core region, as described in claim 2, and the clad layer is formed of an inner clad and an inner clad as defined in claim 3. And an outer cladding provided around the periphery of the airbag, and the aggregate region of the bubbles is formed in the inner cladding.
[0010]
This eliminates the concentration of the anisotropic stress in the core region due to the fusion residual stress unlike the conventional fiber, so that excellent PMD characteristics can be exhibited and delicate refractive index distribution control of the inner cladding layer can be easily achieved. .
[0011]
In addition, as described in claim 4, when the bubbles are formed to have a diameter of 4 μm or less, the above-mentioned effects can be remarkably exhibited.
[0012]
Further, as described in claim 5, the distribution density of the bubbles is changed in the radial direction so that the equivalent refractive index of the aggregate region of the bubbles has a distribution in the radial direction. If the diameter of the bubble is changed in the radial direction and the equivalent refractive index of the aggregate region of the bubble has a radial distribution, the structural dispersion is controlled, and a large positive dispersion fiber and a negative dispersion / negative dispersion are controlled. A fiber with a slope is obtained.
[0013]
On the other hand, the present invention provides a method for manufacturing an optical fiber in which a clad layer is formed around a core region, wherein a glass soot layer is formed around a core glass preform that becomes a core region, The glass soot layer is heated and vitrified in a gas atmosphere containing an inert gas whose diffusion coefficient is smaller than that of helium, thereby forming a glass layer having bubbles of the inert gas.
[0014]
Thereby, the above-described optical fiber of the present invention can be obtained.
[0015]
Specifically, as described in claim 8, the gas atmosphere is formed only of an inert gas whose diffusion coefficient in glass is smaller than helium. If the gas is formed of a mixed gas of helium gas and an inert gas having a diffusion coefficient smaller than that of helium in the glass, the above effect can be remarkably exhibited. As the inert gas having a diffusion coefficient smaller than that of helium in the glass, a nitrogen gas or an argon gas is preferable.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, a preferred embodiment of the present invention will be described with reference to the accompanying drawings.
[0017]
FIG. 1 shows an embodiment of an optical fiber 1 according to the present invention.
[0018]
As shown in the figure, the optical fiber 1 has an inner cladding layer 3 around a core region 2 located at an axial center, and an outer cladding layer 4 having an outer diameter of about 125 μm around the inner cladding layer 3. Are integrally provided.
[0019]
The inner clad layer 3 is formed with a region in which a large number of independent bubbles 5 having a diameter of 4 μm or less are densely gathered, and exists in a layer so as to surround the core region 2.
[0020]
In the optical fiber 1 of the present invention having such a structure, the small-diameter quartz rod a and the small-diameter quartz tubes b are fused and integrated like a conventional photonic crystal optical fiber. Unlike the conventional one, since the anisotropic stress is not concentrated on the core region due to the fusion residual stress of the small-diameter quartz tube b, excellent PMD characteristics are exhibited, and delicate refractive index distribution control of the inner cladding layer 3 is performed. Can be easily achieved.
[0021]
That is, the optical fiber 1 uses an inert gas having a small diffusion coefficient, for example, a nitrogen gas or an argon gas as a sintering gas (atmosphere gas) in a sintering process of the inner cladding layer 3 in a preform manufacturing process. After the sintering gas is left in the sintered glass layer to form a cellular layer around the core region, the preform is drawn to an outer diameter of about 125 μm. For this reason, the conventional fusion residual stress does not occur, and a situation such as concentration of anisotropic stress in the core region does not occur. The bubbles 5 have a substantially spherical shape during the preform manufacturing process, but are drawn into a long hole extending in the longitudinal direction by subsequent drawing. Further, by setting the equivalent relative refractive index difference between the core region 2 and the inner cladding layer 3 to 1%, a fiber having a large absolute value of the dispersion value can be obtained.
[0022]
Here, the diameter of the core region 2 is not particularly limited, but is set to a size of 2 to 10 μm for performing single mode transmission. The thickness of the inner cladding layer 3 varies depending on the refractive index profile, dispersion characteristics, and the like of the optical fiber to be obtained, but is generally set in the range of 5 to 30 μm. Further, it is an absolute condition that the diameter of the bubble 5 is equal to or less than the thickness of the inner cladding layer 3. However, in consideration of forming a bubble density distribution in the inner cladding layer 3, the diameter is 4 μm or less, preferably 1 μm or less. It is desirable that the thickness be 3 μm. The optical fiber 1 according to the present invention is not limited to these dimensional conditions at all.
[0023]
FIG. 3 shows another embodiment of the present invention, in which the refractive index difference distribution is controlled by changing the density of bubbles 5 constituting the inner cladding layer 3 in the radial direction. That is, the inner cladding layer 3 is divided into three layers on the concentric circle centering on the core region 2, the bubbles 5 are present in the region on the inner cladding layer 3 side at a high density (high-density region), and the outer region is formed. The low density (low density region) is obtained, and the outer region is further densified (medium density region). As a result, a multi-step equivalent relative refractive index difference Δn can be generated in the inner cladding layer 3 as shown in the figure, controlling the structural dispersion, and having a large positive dispersion fiber and a negative dispersion / negative dispersion slope. Fiber is obtained. Even if the distribution density of the bubbles in the inner cladding layer is changed in the radial direction, or the outer diameter of the bubbles 5 forming the inner cladding layer 3 is changed in the radial direction, the equivalent refractive index is similarly distributed. It becomes possible.
[0024]
Here, the outer diameter of the bubble can be controlled by the following two methods.
One method is to change the bulk density of the quartz glass soot layer deposited around the core base material. When the soot is vitrified by heating (sintering), the higher the bulk density of the soot, the smaller the gap through which the gas escapes, so the proportion of large bubbles remaining increases, and the lower the bulk density of the soot, the smaller bubbles remain. The percentage increases.
[0025]
The other is a method of changing the ratio of helium gas and inert gas in the gas atmosphere in the furnace during sintering. The higher the proportion of helium gas, the smaller the bubbles tend to be, and the lower the proportion of helium gas, the larger the bubbles tend to be.
[0026]
Note that the larger the bubble, the higher the density of the bubble, and the smaller the bubble, the lower the density of the bubble. Therefore, by controlling the bulk density of the quartz glass soot layer and the ratio of the helium gas and the inert gas in the furnace gas atmosphere during sintering, both the bubble diameter and the bubble density can be controlled. Can be.
[0027]
Further, by controlling the size and position of the bubble and adjusting the distance between the bubbles to one half of the wavelength of the signal light to be propagated, a photonic band gap structure can be realized in the optical fiber of the present invention. .
[0028]
In the above-described embodiment, the case where the core region is made of quartz glass has been described. However, the present invention is not limited to this. Pure quartz glass, a well-known impurity for increasing the refractive index (eg, Ge) , Ti) or a quartz glass to which a rare earth element such as Er is added.
[0029]
Further, the core region may be hollow. In this case, a hollow quartz tube may be used in place of the above-described core base material, that is, the small-diameter quartz tube a. The quartz tube can be made of either pure quartz glass, quartz glass to which a known impurity (for example, Ge or Ti) for increasing the refractive index is added, or quartz glass to which a rare earth element such as Er is added. It is.
[0030]
【Example】
Next, examples of the present invention will be described. In test 1, a bubble aggregate region consisting of one high-density bubble region was provided in the inner cladding layer, and in test 2, three types of high-density bubble region, low-density bubble region, and medium-density bubble region were formed in the inner cladding layer. This is provided with a bubble aggregate region composed of a layer.
[0031]
<Test 1>
First, a transparent glass base material serving as a core region was prepared by a VAD method and stretched to an outer diameter of 25 mm.
[0032]
A soot layer serving as a high-density bubble region was deposited on the outer periphery of the core glass base material by a CVD method to obtain an external base material having an outer diameter of 60 mm. This base material was heat-treated at 1600 ° C. in an electric furnace in a 100% nitrogen gas atmosphere. The obtained glass preform had an outer diameter of 45 mm, and was in a translucent state because many bubbles of nitrogen gas remained in the closed cell aggregate region on the outer peripheral portion. Note that the bubble density in the high-density bubble region was about 0.5.
[0033]
Here, the diffusion coefficient of nitrogen gas is much smaller than that of helium gas usually used in the heat treatment step for vitrification, and soot tends to remain in the glass when soot is vitrified. By using a 100% nitrogen gas atmosphere, a glass base material containing bubbles of nitrogen gas at high density can be obtained.
[0034]
Next, the obtained glass base material with an inner clad layer having a high-density bubble region is stretched to an outer diameter of 25 mm, a soot layer serving as an outer clad layer is deposited by a CVD method, and an outer base material having an outer diameter of 120 mm is formed. Obtained. This base material was subjected to a heat treatment in an electric furnace in a 100% helium gas atmosphere at a temperature of 1600 ° C. to obtain a glass base material having an outer diameter of 60 mm and a transparent outer cladding layer at the outer peripheral portion.
[0035]
Next, the glass base material was drawn into an optical fiber having an outer diameter of 125 μm by an ordinary drawing method. The obtained optical fiber had a core diameter of 7 μm, a layer thickness of the inner cladding layer of 10 μm, and an outer diameter of bubbles of 1 to 3 μm.
[0036]
<Test 2>
First, a transparent glass base material serving as a core region was prepared by a VAD method and stretched to an outer diameter of 25 mm.
[0037]
A soot layer serving as a high-density bubble region was deposited on the outer periphery of the core glass base material by a CVD method to obtain an external base material having an outer diameter of 60 mm. This base material was heat-treated at 1600 ° C. in an electric furnace in a 100% nitrogen gas atmosphere. Here, the reason for using a 100% nitrogen atmosphere gas is as described in Test 1. The glass base material thus obtained had an outer diameter of 45 mm, and was in a translucent state because many nitrogen bubbles remained in the high-density bubble region on the outer periphery. The high-density bubble region had a bubble density of about 0.5.
[0038]
Next, the glass base material was stretched to an outer diameter of 25 mm, and a soot layer serving as a low-density bubble region was deposited by a CVD method to obtain an external base material having an outer diameter of 60 mm. This base material was heat-treated at a temperature of 1600 ° C. in an electric furnace in an atmosphere of 20% nitrogen gas and 80% helium gas inside the furnace. Here, the reason that a mixed gas of nitrogen gas and helium gas is used as the atmosphere gas is that the transparency of the glass after the heat treatment becomes higher by mixing the helium gas than in the 100% nitrogen gas atmosphere. This is because a glass base material having a low bubble density can be obtained. The glass base material thus obtained has an outer diameter of 45 mm, and is slightly transparent to nitrogen bubbles in the low-density bubble region on the outer periphery thereof, as compared with the high-density bubble region. However, it was not completely transparent. Note that the bubble density in the low-density bubble region was about 0.3.
[0039]
Next, the obtained glass base material was stretched to an outer diameter of 25 mm, and a soot layer serving as a medium density bubble region was deposited by a CVD method to obtain an external base material having an outer diameter of 60 mm. This base material was heat-treated at 1600 ° C. in an electric furnace in an atmosphere of 50% nitrogen gas and 50% helium gas in the furnace. The glass base material thus obtained had an outer diameter of 45 mm, and the transparency of the medium density bubble region on the outer peripheral portion was intermediate between the high density bubble region and the low density bubble region. The bubble density in the low-density bubble region was about 0.4.
[0040]
Next, the obtained glass base material with an inner cladding layer including the high-density bubble region, the low-density bubble region, and the medium-density bubble region is stretched to an outer diameter of 25 mm, and a soot layer serving as an outer cladding layer is deposited by a CVD method. Then, an external base material having an outer diameter of 120 mm was obtained. This base material was heated in an electric furnace in a 100% helium gas atmosphere at a temperature of 1600 ° C. to obtain a glass base material having an outer diameter of 60 mm and a transparent outer cladding layer at the outer peripheral portion. .
[0041]
Next, the glass base material was drawn into an optical fiber having an outer diameter of 125 μm by an ordinary drawing method. The obtained optical fiber has a core diameter of 4 μm, a layer thickness of the inner cladding layer of 12 μm (high-density bubble region 4 μm, low-density bubble region 5 μm, medium-density bubble region 3 μm), and the outer diameter of the bubbles in each region is 1-2 μm.
[0042]
【The invention's effect】
In short, according to the present invention, since the clad layer having the aggregate of bubbles is provided around the core region, the concentration of the anisotropic stress in the core region due to the fusion residual stress as in the conventional photonic crystal optical fiber. , And excellent PMD characteristics can be exhibited. As a result, it has low PMD characteristics and exhibits characteristics that cannot be achieved with ordinary optical fibers, such as large effective core area, high refractive index difference, large anomalous dispersion (positive dispersion), negative dispersion and negative dispersion slope fiber. It has excellent effects such as being able to perform.
[Brief description of the drawings]
FIG. 1 is an enlarged sectional view showing an embodiment of an optical fiber according to the present invention.
FIG. 2 is an enlarged sectional view showing another embodiment of the optical fiber according to the present invention.
FIG. 3 is an enlarged perspective view showing a structure of a preform for obtaining a conventional photonic crystal optical fiber.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Optical fiber 2 Core area 3 Inner cladding layer 4 Outer cladding layer 5 Bubbles

Claims (10)

An optical fiber having a cladding layer around a core region, wherein an aggregate region of bubbles is formed in the cladding layer. 2. The optical fiber according to claim 1, wherein the aggregate region of the bubbles is formed in a layer shape concentric with the core region. 3. The light according to claim 1, wherein the cladding layer includes an inner cladding and an outer cladding provided around the inner cladding, and the aggregate region of the bubbles is formed in the inner cladding. fiber. The optical fiber according to any one of claims 1 to 3, wherein the bubble has a diameter of 4 µm or less. The optical fiber according to any one of claims 1 to 4, wherein the distribution density of the bubbles is changed in the radial direction so that the equivalent refractive index of the aggregate region of the bubbles has a distribution in the radial direction. The optical fiber according to any one of claims 1 to 5, wherein the diameter of the bubble is changed in the radial direction so that the equivalent refractive index of the aggregate region of the bubble has a radial distribution. In a method for manufacturing an optical fiber in which a cladding layer is formed around a core region, a glass soot layer is formed around a core glass preform that becomes a core region, and an inert gas whose diffusion coefficient in glass is smaller than helium is A method for producing an optical fiber, characterized in that the glass soot layer is heated and vitrified in a gas atmosphere containing the gas so as to form a glass layer having bubbles of an inert gas. The method for manufacturing an optical fiber according to claim 7, wherein the gas atmosphere is formed only of an inert gas whose diffusion coefficient in glass is smaller than that of helium. The method according to claim 7, wherein the gas atmosphere is formed of a mixed gas of helium gas and an inert gas whose diffusion coefficient in glass is smaller than helium. 10. The method of manufacturing an optical fiber according to claim 7, wherein the inert gas whose diffusion coefficient in the glass is smaller than that of helium is nitrogen gas or argon gas.

Images