JP5461384B2 - Low bending loss multimode fiber - Google Patents

Description

The present invention relates to a multimode fiber that improves a conventional graded index type (hereinafter referred to as GI type) multimode fiber and has a low loss even for a minimum bending diameter. A conventional GI type multimode fiber (hereinafter referred to as GI fiber) has a high refractive index difference between the core and the clad, so it is inherently resistant to bending, but is used for indoor wiring used for FTT H (Fiber to the home). In this case, a low bending loss characteristic is required even for an extremely small bending diameter.

  A conventional GI fiber usually has a core having an α-th power refractive index distribution represented by the following equation (1).

(Where, a is the core radius, n ₁ is the core center refractive index, Δ is the non-refractive index difference, and α is a parameter set so that the band is maximized in the desired wavelength band.)
The conventional GI fiber has a large bending loss with respect to a minimum bending diameter as it is, and cannot satisfy the demand for home wiring.
On the other hand, a method of synthesizing the refractive index distribution of the fiber so as to maximize the band has been proposed (see, for example, Non-Patent Document 1).
The refractive index distribution obtained by this conventional technique is characterized by the fact that there is a region where the refractive index is smaller than the cladding (depressed region) outside the distribution close to the α power refractive index distribution. It is expected that a good fiber will be obtained.

K. Okamoto and T. Okoshi, “Computer-aided synthesis of the optimum refractive index profile for a multimode fiber,” IEEE Trans. Microwave Theory Tech., Vol. MTT-25, pp.213-221, 1976

  However, the refractive index distribution proposed in Non-Patent Document 1 cannot be expressed by a simple formula and is difficult to manufacture because the refractive index distribution must be precisely controlled for fiber manufacture.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a multimode fiber that can be easily manufactured and has excellent bending loss characteristics.

To solve the above problem,
The present invention provides the following formula (1)

(Wherein, a is a core radius, n ₁ is a core center refractive index, Δ is a relative refractive index difference, and α is a parameter set to maximize the band in a desired wavelength band). A multimode fiber having a core having an α power refractive index profile and a clad on the outer side thereof, and a single layer depressed region having a lower refractive index than the clad is provided outside the core,
a = 25 μm, α = 2.04,
Δ = 0.01, r ≦ 25 μm, α power refractive index distribution, r ₁ = 26 μm, r ₂ = 30 μm, −0.004 ≦ Δ ₀ ≦ −0.001 (where r ₁ is the inner boundary of the depressed region) , R ₂ is a radius to the outer boundary of the depressed region, Δ ₀ represents a relative refractive index difference of the depressed region), and has a depressed region having a concave refractive index distribution represented by:
Provided is a low bending loss multimode fiber characterized in that an OFL band of 0.85 μm wavelength band is 1.5 GHz · km or more and a bending loss with respect to bending of φ10 is 5 dB / m or less during steady mode excitation.

  Further, the present invention provides the following formula (1)

（式中、ａはコア半径、ｎ_１はコア中心屈折率、Δは比屈折率差を表し、αは所望の波長帯で帯域が最大になるように設定されるパラメータである。）で表されるα乗屈折率分布をもつコアと、その外側のクラッドとを有するマルチモードファイバであって、コアの外側に、クラッドよりも低屈折率である単層のデプレスド領域が設けられ、
ａ＝３２．５μｍ、α＝２．０４であり、
Δ＝０．０２、ｒ≦２５μｍでα乗屈折率分布をもち、ｒ_１＝３４．５μｍ、ｒ_２＝３６〜３８．５μｍ、−０．００３≦Δ_０≦０（但し、ｒ_１はデプレスド領域内側境界までの半径、ｒ_２はデプレスド領域外側境界までの半径、Δ_０はデプレスド領域の比屈折率差を表す。）で表される凹状屈折率分布をもつデプレスド領域を有することを特徴とする低曲げ損失マルチモードファイバを提供する。

(Wherein, a is a core radius, n ₁ is a core center refractive index, Δ is a relative refractive index difference, and α is a parameter set to maximize the band in a desired wavelength band). A multimode fiber having a core having an α power refractive index profile and a clad on the outer side thereof, and a single layer depressed region having a lower refractive index than the clad is provided outside the core,
a = 32.5 μm, α = 2.04,
Δ = 0.02, r ≦ 25 μm, α power refractive index distribution, r ₁ = 34.5 μm, r ₂ = 36-38.5 μm, −0.003 ≦ Δ ₀ ≦ 0 (where r ₁ is depressed) A radius to the inner boundary of the region, r ₂ is a radius to the outer boundary of the depressed region, and Δ ₀ represents a relative refractive index difference of the depressed region.), And has a depressed region having a concave refractive index distribution. A low bending loss multimode fiber is provided.

ADVANTAGE OF THE INVENTION According to this invention, the low bending loss multimode fiber which has a low bending loss with respect to an extremely small bending diameter can be provided.
The low bending loss multimode fiber of the present invention has a simple structure in which a depressed region having a uniform refractive index distribution with a lower refractive index than the cladding is provided on the outer side of the core. Can be provided at a cost.
Moreover, since the low bending loss multimode fiber of the present invention has a low bending loss even with respect to an extremely small bending diameter, it can be suitably used for home wiring that requires wiring flexibility and workability.
In addition, the VCSEL laser for the 0.85 μm wavelength band is inexpensive, and the low bending loss multimode fiber of the present invention having a high band and a low bending loss in the 0.85 μm wavelength band is obtained from the ease of connection of the GI fiber. It is optimal as a fiber for the user to perform wiring and construct an optical LAN.

It is a graph which shows an example of the refractive index distribution of the low bending loss multimode fiber by this invention. It is a graph which shows the refractive index distribution of the low bending loss multimode fiber produced in Example 1 which concerns on this invention. 4 is a graph showing a relationship between a refractive index and a band of a depressed region of a low bending loss multimode fiber manufactured in Example 1. It is a graph which shows the relationship between the width | variety of a depressed area | region of a low bending loss multimode fiber produced in Example 2 which concerns on this invention, and a zone | band. It is a graph which shows the refractive index distribution of the low bending loss multimode fiber produced in Example 3 which concerns on this invention. It is a graph which shows the relationship between the refractive index and the zone | band of the depressed area | region of the low bending loss multimode fiber produced in Example 4 which concerns on this invention. It is a graph which shows the relationship between the width | variety of a depressed area | region of a low bending loss multimode fiber produced in Example 5 which concerns on this invention, and a zone | band.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram showing an example of a refractive index distribution of a low bending loss multimode fiber according to the present invention. In the figure, reference numeral 1 is a core, 2 is a cladding, 3 is a depressed area, a is a core radius, r is a radius, Δ is a relative refractive index difference with respect to the cladding, r ₁ is a radius to the inner boundary of the depressed area, r ₂ represents the radius to the outer boundary of the depressed region, and Δ ₀ represents the relative refractive index difference of the depressed region.

  This low-bending loss multimode fiber is provided in the core 1 having the α power refractive index distribution represented by the above-described formula (1), the outer cladding 2 thereof, and the outer vicinity of the core 1, which is more than the cladding 2. And a depressed region 3 having a low refractive index and a uniform refractive index distribution. The refractive index profile of the low bending loss multimode fiber shown in FIG. 1 follows the α power refractive index profile in a region where the radius r is smaller than a, and has a ring-shaped depressed region 3 surrounding the core 1 in a region where r ≧ a. . In a preferred example, the core 1 is made of Ge-doped quartz glass, the cladding 2 is made of quartz glass, and the depressed region 3 is made of F-doped quartz glass, but is not limited thereto.

In the illustration of FIG. 1, the depressed region 3 is formed in the region from r ₁ to r _{2 and} has a uniform refractive index distribution lower than that of the cladding 2, that is, a simple concave refractive index distribution.
This low bending loss multimode fiber is a layer made of a low refractive index material such as F-doped quartz glass when a cladding layer is formed on the outer side of the core base material in a conventionally known GI fiber base material manufacturing process. Can be produced by drawing the obtained base material by a conventionally known method.

The low bending loss multimode fiber having the depressed region 3 preferably satisfies any of the following conditions (a) to (f).
(A) In the 0.85 μm wavelength band, the OFL band is 1.5 GHz · km or more, the bending loss with respect to the bending of φ10 is 5 dB / m or less and the relative refractive index difference Δ is 0.011 or less at the time of steady mode excitation. about.
(B) In the 0.85 μm wavelength band, the OFL band is 0.8 GHz · km or more, the bending loss with respect to the bending of φ10 is 3 dB / m or less and the relative refractive index difference Δ is 0.022 or less at the time of steady mode excitation. about.
(C) Δ ≦ 0.011, r ≦ 25 μm, α power refractive index distribution, 25 μm ≦ r ₁ ≦ 28 μm, 26 μm ≦ r ₂ ≦ 32 μm, −0.006 ≦ Δ ₀ ≦ −0.001 A depressed region 3 having a concave refractive index profile.
(D) Δ ≦ 0.022, r ≦ 32.5 μm, α power refractive index distribution, 32.5 μm ≦ r ₁ ≦ 35.5 μm, 33.5 μm ≦ r ₂ ≦ 40.5 μm, −0.005 ≦ It has a depressed region having a concave refractive index profile represented by Δ ₀ ≦ −0.001.
(E) With respect to any wavelength band of 0.85 μm and 1.30 μm, the OFL band is 0.4 GHz · km or more in the 0.85 μm wavelength band, and the OFL band is 0.4 GHz · km or more in the 1.30 mm wavelength band. In the steady mode excitation, the bending loss with respect to the bending of φ10 is 5 dB / m or less, and the relative refractive index difference Δ is 0.011 or less.
(F) In the 0.85 μm wavelength band, the OFL band is 0.3 GHz · km or more, and in the 1.30 mm wavelength band, the OFL band is 0.3 GHz · km or more, for any of the wavelength bands of 0.85 μm and 1.30 μm. In the steady mode excitation, the bending loss with respect to the bending of φ10 is 3 dB / m or less, and the relative refractive index difference Δ is 0.022 or less.

This low bending loss multimode fiber improves the bending loss characteristics without dropping the band by providing a depressed region 3 having a simple refractive index distribution shape outside the core 2 having an α power refractive index distribution. It is possible to have a low bending loss even for a very small bending diameter.
In addition, this low bending loss multimode fiber has a simple structure in which a depressed region having a uniform refractive index distribution with a lower refractive index than that of the cladding is provided outside the core. Can be provided.
Moreover, since this low bending loss multimode fiber has a low bending loss with respect to an extremely small bending diameter, it can be suitably used for home wiring that requires wiring flexibility and workability.
In addition, the VCSEL laser for the 0.85 μm wavelength band is inexpensive, and the low bending loss multimode fiber of the present invention having a high band and a low bending loss in the 0.85 μm wavelength band is obtained from the ease of connection of the GI fiber. It is optimal as a fiber for the user to perform wiring and construct an optical LAN.

In addition, the low bending loss multimode fiber of this invention is not limited to the illustration mentioned above, A various change is possible.
For example, the depressed region 3 is not limited to one layer, and a plurality of depressed regions 3A, 3B, 3C, and 3D having different refractive indexes are provided outside the core 1 as shown in FIG. 5, and these depressed regions 3A, 3B, 3C and 3D may have a multilayer stepwise refractive index distribution.

[Example 1]
As a low bending loss multimode fiber optimized at a wavelength of 0.85 μm, as shown in FIG. 2, it has a conventional α power refractive index profile (α = 2.04, a = 25 μm, Δ = 0.01). A depressed region having a concave refractive index distribution with r ₁ = 26 μm and r ₂ = 30 μm is provided outside the core. When the relative refractive index difference Δ ₀ in the depressed region is changed, the finite element method (K. Okamoto, “Comparison of calculated and measured impulse responses of optical fibers,” Appl. Opt., Vol.18, pp.2199-2206, FIG. 3 shows the change of the bandwidth calculated based on 1979.
As shown in FIG. 3, the structure in which the relative refractive index difference Δ ₀ in the depressed region is set to ₀ to −0.004 has a higher band than the structure without the depressed region. That is, by increasing | Δ ₀ | to about 0.004, it is possible to reduce bending loss without degrading the band.
When using a conventional GI fiber manufacturing core preform (a portion corresponding to r ≦ a) to produce a fiber preform by an external method, a conventional fiber and a depressed region are provided without a depressed region. Fibers were fabricated and the characteristics were compared. The GI fiber without the depressed region has an OFL (Overfilled-launched) band (see standard IEC 60793-1-49) at 2.3 GHz · km and a bending diameter (φ10) of 10 mm (standard). The bending loss of IEC 60793-1-40 was 10 dB / m. However, when a depressed region of r ₁ = 26 μm, r ₂ = 30 μm, Δ ₀ = −0.004 is provided, the OFL band is 2.2 GHz. -At km, the bending loss during steady mode excitation with respect to φ10 was improved to 2.5 dB / m.

[Example 2]
In Example 1, the relative refractive index difference Δ ₀ in the depressed region is optimized to realize a low bending loss GI fiber. However, the same result can be obtained even when Δ ₀ is fixed and the width of the depressed region is changed. can get. FIG. 4 shows changes in the band when r ₁ = 26 μm and Δ ₀ = −0.003 and r ₂ is changed.
Using the same core base material as in Example 1 and providing a depressed region of r ₁ = 26 μm, r ₂ = 28 μm, and Δ ₀ = −0.003 at the time of external attachment, the OFL band is 2.3 GHz · km, φ10 In contrast, the bending loss during steady mode excitation was improved to 3.5 dB / m.

[Example 3]
Examples 1 and 2 are structures having a depressed region having the simplest concave refractive index distribution, but it is not necessary to limit the refractive index distribution of the depressed region to a simple concave shape, and the same effect can be obtained even in a multilayer structure. . 5 has a four-layer structure outside the α power refractive index distribution (α = 2.04, a = 25 μm, Δ = 0.01), r ₁ = 25.5 μm, r ₂ = 26. .8 μm, r ₃ = 28 μm, r ₄ = 29 μm, r ₅ = 30.1 μm, Δ ₀ = −0.001, Δ ₁ = −0.002, Δ ₂ = −0.004, Δ ₃ = −0. When a four-layer depressed region having 003 was provided, the calculated OFL band was 2.68 GHz · km.
When a fiber is manufactured by providing the same depressed region outside the core base material used in Example 1, the OFL band is 2.3 GHz · km, and the bending loss during steady mode excitation with respect to φ10 is 2.8 dB / m. Improved.

[Example 4]
As a low-bending loss multimode fiber with a wavelength of 0.85 μm and Δ of 0.02, outside the core having a conventional α power refractive index profile (α = 2.04, a = 25 μm, Δ = 0.02), A depressed region having a concave refractive index distribution fixed at r ₁ = 34.5 μm and r ₂ = 38.5 μm is provided. FIG. 6 shows a change in the band calculated based on the finite element method when the relative refractive index difference Δ ₀ in the depressed region is changed.
As shown in FIG. 6, the structure in which the relative refractive index difference Δ0 in the depressed region is set to ₀ to −0.0025 has a higher band than the structure without the depressed region. That is, by increasing | Δ ₀ | to about 0.0025, it is possible to reduce the bending loss without degrading the band.
When externally attached using a core base material of a conventional GI fiber, a conventional fiber without a depressed region and a fiber with a depressed region were manufactured, and the characteristics were compared. The GI fiber not provided with the depressed region had an OFL band of 1.5 GHz · km and a bending loss of 4.5 dB / m during steady mode excitation with respect to φ10, but r ₁ = 34.5 μm, r ₂ = 38.5 μm, Δ ₀ = -0.0025, when the depressed region is provided, the OFL band is 1.5 GHz · km, and the bending loss during steady mode excitation is improved to 2.0 dB / m for φ10 .

[Example 5]
In Example 4, the relative refractive index difference Δ ₀ in the depressed region is optimized to realize a low bending loss GI fiber, but the same result is obtained even when Δ ₀ is fixed and the width of the depressed region is changed. can get. FIG. 7 shows changes in the band when r ₁ = 34.5 μm and Δ ₀ = −0.003 and r ₂ is changed.
Using the same core base material as in Example 4 and providing a depressed region of r ₁ = 34.5 μm, r ₂ = 36.0 μm, and Δ ₀ = −0.003 when externally attached, the OFL band is 1.4 GHz · At km, the bending loss during steady mode excitation was improved to 3.2 dB / m for φ10.

  DESCRIPTION OF SYMBOLS 1 ... Core, 2 ... Cladding, 3, 3A, 3B, 3C, 3D ... Depressed area | region.

Claims (2)

The following formula (1)

(Wherein, a is a core radius, n ₁ is a core center refractive index, Δ is a relative refractive index difference, and α is a parameter set to maximize the band in a desired wavelength band). A multimode fiber having a core having an α power refractive index profile and a clad on the outer side thereof, and a single layer depressed region having a lower refractive index than the clad is provided outside the core,
a = 25 μm, α = 2.04,
Δ = 0.01, r ≦ 25 μm, α power refractive index distribution, r ₁ = 26 μm, r ₂ = 30 μm, −0.004 ≦ Δ ₀ ≦ −0.001 (where r ₁ is the inner boundary of the depressed region) , R ₂ is a radius to the outer boundary of the depressed region, Δ ₀ represents a relative refractive index difference of the depressed region), and has a depressed region having a concave refractive index distribution represented by:
A low-bending loss multimode fiber characterized by having an OFL band of 1.5 GHz · km or more in a 0.85 μm wavelength band and a bending loss of 5 dB / m or less with respect to φ10 bending during steady mode excitation. The following formula (1)

(Wherein, a is a core radius, n ₁ is a core center refractive index, Δ is a relative refractive index difference, and α is a parameter set to maximize the band in a desired wavelength band). A multimode fiber having a core having an α power refractive index profile and a clad on the outer side thereof, and a single layer depressed region having a lower refractive index than the clad is provided outside the core,
a = 32.5 μm, α = 2.04,
Δ = 0.02, r ≦ 25 μm, α power refractive index distribution, r ₁ = 34.5 μm, r ₂ = 36-38.5 μm, −0.003 ≦ Δ ₀ ≦ 0 (where r ₁ is depressed) A radius to the inner boundary of the region, r ₂ is a radius to the outer boundary of the depressed region, and Δ ₀ represents a relative refractive index difference of the depressed region.), And has a depressed region having a concave refractive index distribution. Low bending loss multimode fiber.

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