JP2005107514A - Grated index type multimode fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a grated index type multimode fiber that obtains the maximum transmission band width at the wavelength of signal light propagated in a fiber without depending upon the wavelength. <P>SOLUTION: The grated index type multimode fiber having a core made of quartz-based glass and a clad provided at the outer periphery of the core has one of germanium or phosphorus added to the center part of the core and fluorine added to the outer peripheral part. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a graded index type multimode fiber.

  Of the multimode fibers, a graded index type multimode fiber (hereinafter abbreviated as “GI multimode fiber”) has a high numerical aperture and is widely used as a transmission line for an optical LAN. . Up to now, the accuracy in controlling the refractive index profile of the GI multimode fiber has been improved with the demand for higher speed optical LAN.

  Currently, the GI multimode fiber has almost reached the performance limit, and in order to further increase the transmission bandwidth of the GI multimode fiber, it is necessary to perform wavelength division multiplexing (WDM).

However, in a conventional GI multimode fiber containing germanium in the core, the optimum refractive index profile greatly depends on the wavelength of signal light propagating in the fiber (see, for example, Non-Patent Document 1). Therefore, a fiber having a refractive index profile optimized at a specific wavelength has a problem that the transmission bandwidth becomes very small at different wavelengths and cannot be applied to wavelength division multiplexing.
Takayoshi Ohkoshi, Katsunari Okamoto, Kazuo Hoba, “Optical Fiber”, Chapter 7, pp. 182-184, Ohmsha, 1984

  The present invention has been made in view of the above circumstances, and provides a graded index type multimode fiber capable of obtaining the maximum transmission bandwidth at the wavelength without depending on the wavelength of the signal light propagating in the fiber. For the purpose.

  In order to solve the above-mentioned problems, the present invention provides a graded index type multimode fiber comprising a core made of silica-based glass and a clad provided on the outer periphery of the core, wherein the core has a central portion. Provides a GI multimode fiber containing either germanium or phosphorus and fluorine on the outer periphery thereof.

  The GI multimode fiber having the above configuration preferably has a refractive index profile that satisfies the following expressions (1) to (4).

Where n (r) is the refractive index at a distance r from the core center of the optical fiber, n ₁ is the refractive index at the core center, Δ is the maximum relative refractive index difference of the core with respect to the cladding, a is the core radius, and α is the refractive index. Distribution order, α ₁ is the refractive index distribution order of germanium or phosphorus contained in the central part of the core, α ₂ is the refractive index distribution order of fluorine contained in the outer peripheral part of the core, and a ₀ is the central part and outer peripheral part of the core , N ₀ represents the refractive index of pure quartz.

In the GI multimode fiber having the above-described configuration, the refractive index distribution orders α ₁ and α ₂ in the above formulas (2) to (4) each independently have an optimum value that maximizes the transmission bandwidth in the used wavelength region. Is preferred.

  In the GI multimode fiber configured as described above, germanium is contained in the center of the core, fluorine is contained in the outer periphery of the core, the core has a diameter of 50 μm, and the cladding has a diameter of 125 μm. It is preferable that the transmission bandwidth at 8 to 1.1 μm exceeds 3 GHz · km.

  In the GI multimode fiber configured as described above, germanium is contained in the center of the core, fluorine is contained in the outer periphery of the core, the core has a diameter of 62.5 μm, and the cladding has a diameter of 125 μm. It is preferable that the transmission bandwidth at 0.85 to 1.1 μm exceeds 2 GHz · km.

  In the GI multimode fiber configured as described above, phosphorus is contained in the center of the core, fluorine is contained in the outer periphery of the core, the core has a diameter of 50 μm, the cladding has a diameter of 125 μm, and a wavelength of 0. It is preferable that the transmission bandwidth in 8 to 1.3 μm exceeds 3 GHz · km.

  In the GI multimode fiber having the above-described configuration, phosphorus is contained in the central portion of the core, fluorine is contained in the outer peripheral portion of the core, the core diameter is 62.5 μm, the cladding diameter is 125 μm, and the wavelength It is preferable that the transmission bandwidth at 0.8 to 1.2 μm exceeds 2 GHz · km.

  In the GI multimode fiber having the above configuration, phosphorus is contained in the central portion of the core, fluorine is contained in the outer peripheral portion of the core, and transmission loss at a wavelength of 0.8 to 1.3 μm is 2 dB / km or less. Is preferred.

  The GI multimode fiber of the present invention has a configuration in which either germanium or phosphorus is contained in the center of the core and fluorine is contained in the outer periphery of the core, so that the transmission bandwidth is increased in a wide wavelength region. It becomes an optical fiber suitable for wavelength division multiplexing transmission.

  In addition, since the GI multimode fiber of the present invention is a fiber formed by adding different dopants to two different regions of the core, the fiber is different from a fiber in which two different types of dopants are added to the same region of the core. Since it is easy to adjust the dopant concentration distribution, the refractive index distribution of the core, etc., the manufacturing is easy.

The present invention will be described in detail below.
The GI multimode fiber of the present invention is provided in the center, and includes a core made of quartz glass containing germanium (Ge) or phosphorus (P) at the center and fluorine (F) at the outer periphery. And a clad provided concentrically with the core around the core.
The center of the core is the inner part of the core region that concentrically surrounds the center of the core and includes either germanium or phosphorus, and is a region whose distance from the center of the core is 0 to about 70% of the core radius. That is. The outer peripheral portion of the core is the remaining core region surrounding the central portion concentrically.

  The GI multimode fiber of the present invention is an optical fiber having a refractive index profile that satisfies the following formulas (1) to (4).

In the above formulas (1) to (4), n (r) is the refractive index at the distance r from the core center of the optical fiber, n ₁ is the refractive index at the core center, and Δ is the maximum relative refractive index of the core with respect to the cladding. The difference in rate, a is the core radius, α is the refractive index distribution order, α ₁ is the refractive index distribution order of germanium or phosphorus contained in the center of the core, and α ₂ is the refractive index distribution order of fluorine contained in the outer periphery of the core. , A ₀ represents the boundary between the central portion and the outer peripheral portion of the core, and n ₀ represents the refractive index of pure quartz.

Further, the refractive index distribution order α (α ₁ , α ₂ ) is controlled so that the transmission bandwidth at a desired wavelength is maximized, and the optimum value α _opt is a dopant added to quartz glass (for example, It depends on the type of germanium, phosphorus and fluorine.

  The refractive index profile of the GI multimode fiber of the present invention represented by the above formulas (1) to (4) has the maximum refractive index at the center of the core, and the refractive index gradually decreases as the radius increases. It is a shape like this. Therefore, the signal light propagating in the GI multimode fiber in the low-order mode propagates at a low speed although the propagation path is short. On the other hand, the signal light propagating in the higher-order mode has a long propagation path, but has a small refractive index near the boundary between the core and the clad and propagates at a high speed.

Therefore, by appropriately adjusting the α value that determines the shape, the arrival times at the output ends of the signal light propagated in the GI multimode fiber in each propagation mode can be made uniform. At this time, the mode dispersion is theoretically minimized, and the maximum transmission bandwidth at the wavelength of the signal light can be realized. The optimum value α _opt of α varies depending on the wavelength used. The change varies depending on the type and concentration of the dopant added to the core.

In the present invention, the refractive index distribution order α _{1 of} germanium or phosphorus contained in the central portion of the core and the refractive index distribution order α ₂ of fluorine contained in the outer peripheral portion of the core are used independently regardless of the size. It can be controlled to have an optimum value that maximizes the transmission bandwidth in the wavelength region. Thereby, as shown in FIG. 1, the refractive index distribution order α ₁ and the refractive index distribution order α ₂ are independently independent of the magnitude of the refractive index distribution order α ₁ , and the refractive index distribution order α ₁ is independently a refractive index profile at the center of the core. The refractive index profile order α ₂ can determine the refractive index profile of the outer periphery of the core.

Also, included in the center of the core (r _<a 0 )Nifukumarerudopanto(gerumaniumumatawarin)niyorukuraddonitaisurukoanochushinbunosaidaihikussetsuritsusa_deruta _in To,koanogaishubu_(r> _{a 0)} The maximum relative refractive index difference _Δout of the outer periphery of the core with respect to the clad by the dopant (fluorine) is expressed by the following equations (5) and (6), respectively.

However, there is a relationship represented by the following formula (7) between n ₁ and n ₂ .

  Therefore, the maximum relative refractive index difference Δ with respect to the cladding of the entire core is expressed by the following formula (8).

In the refractive index profile that satisfies the relationships represented by the above formulas (1) to (8), the maximum transmission bandwidth is such that the refractive index distribution order α ₁ and the refractive index distribution order α ₂ are α-powered only by the respective dopants. It is obtained when the optimum value in the case of configuring the refractive index profile (the refractive index profile represented by the above formulas (1) to (4)) is obtained.

Here, FIG. 2 shows a GI multimode fiber in which the dopant contained in the core is only germanium, phosphorus, and fluorine, respectively, and the core has a maximum relative refractive index difference Δ = 0.01. It is a graph which shows the wavelength dependence of optimal value (alpha) _opt of refractive index distribution order (alpha) in said Formula (1)-(4) showing a refractive index profile.

Germanium and phosphorus are dopants that increase the refractive index, and fluorine is a dopant that decreases the refractive index.
From FIG. 2, the GI multimode fiber containing germanium in the core shows the most significant variation in the wavelength dependence of the transmission bandwidth because the optimum value α _opt shows the largest fluctuation with respect to the change in wavelength.

From FIG. 2, the GI multimode fiber containing phosphorus or fluorine has a smaller variation in the optimum value α _opt with respect to the change in wavelength than that containing germanium. Therefore, these GI multimode fibers also have a small wavelength dependency of the transmission bandwidth.

  Therefore, in the GI multimode fiber of the present invention, the wavelength region having a large transmission bandwidth can be further increased by adding germanium and fluorine or phosphorus and fluorine in combination to the core.

  However, it is difficult in manufacturing to add a plurality of types of dopants to the core. Further, when a GI multimode fiber is manufactured using MCVD (Modified Chemical Vapor Deposition Method), the maximum relative refractive index difference Δ cannot be increased only by adding fluorine.

  Therefore, in the present invention, the graded index type multimode fiber has a structure in which either germanium or phosphorus is added to the center of the core and fluorine is added to the outer peripheral portion, so that only one kind of dopant is added. A value of the maximum relative refractive index difference Δ that cannot be achieved when added is obtained.

In addition, as shown in FIG. 2, the optimum value α _opt of germanium or phosphorus generally decreases monotonically as the wavelength increases, and the optimum value α _{opt of} fluorine increases approximately monotonously as the wavelength increases. Therefore, by adding these dopants at an appropriate ratio, a GI multimode fiber having a wide transmission bandwidth in a wide wavelength region can be realized.

Here, for example, a GI multimode fiber in which germanium is added to the central portion of the core and fluorine is added to the outer peripheral portion will be examined.
In this GI multimode fiber, the refractive index distribution order α _{1 of} germanium contained in the central portion of the core and the refractive index distribution order α _{2 of} fluorine contained in the outer peripheral portion were optimized at a wavelength of 0.85 μm, respectively. To do. Since this GI multimode fiber is optimized at a wavelength of 0.85 μm, the transmission bandwidth is maximized at this wavelength.

When the wavelength is longer than 0.85 μm, the effect of the higher-order mode proceeding slower than the lower-order mode due to the refractive index distribution order α ₁ being larger than the optimum value, and the refractive-index distribution order α ₂ is more than the optimum value. Since the effects of the higher order mode proceeding faster than the lower order mode are compensated for each other, a wide transmission bandwidth is maintained even at wavelengths exceeding 0.85 μm.

In the GI multimode fiber of the present invention, germanium is contained in the center of the core, fluorine is contained in the outer periphery of the core, the core has a diameter of 50 μm, and the cladding has a diameter of 125 μm. The transmission bandwidth at a wavelength of 0.8 to 1.1 μm exceeds 3 GHz · km.
Here, the transmission bandwidth is represented by the product of the transmission rate that can be transmitted and the distance of the optical fiber, and indicates the transmission capacity of the optical fiber.
Therefore, the GI multimode fiber of the present invention is an optical fiber that has a high transmission rate and enables wavelength division multiplex transmission at wavelengths of 0.8 to 1.1 μm.

In the GI multimode fiber of the present invention, germanium is contained in the center of the core, fluorine is contained in the outer periphery of the core, and the core has a diameter of 62.5 μm and the cladding has a diameter of 125 μm. The transmission bandwidth at a wavelength of 0.85 to 1.1 μm exceeds 2 GHz · km.
Therefore, the GI multimode fiber of the present invention is an optical fiber that has a high transmission rate and enables wavelength division multiplexing transmission at wavelengths of 0.85 to 1.1 μm.

The GI multimode fiber of the present invention has a wavelength of 0 when phosphorus is contained in the center of the core, fluorine is contained in the outer periphery of the core, the core has a diameter of 50 μm, and the cladding has a diameter of 125 μm. The transmission bandwidth in .8 to 1.3 μm exceeds 3 GHz · km.
Therefore, the GI multimode fiber of the present invention is an optical fiber that has a high transmission speed and enables wavelength division multiplex transmission at a wavelength of 0.8 to 1.3 μm.

The GI multimode fiber of the present invention has a configuration in which phosphorus is contained in the center of the core, fluorine is contained in the outer periphery of the core, the core has a diameter of 62.5 μm, and the cladding has a diameter of 125 μm. The transmission bandwidth at a wavelength of 0.8 to 1.2 μm exceeds 2 GHz · km.
Therefore, the GI multimode fiber of the present invention is an optical fiber that has a high transmission rate and enables wavelength division multiplex transmission at a wavelength of 0.8 to 1.2 μm.

Furthermore, the GI multimode fiber of the present invention has a transmission loss of 2 dB / km at a wavelength of 0.8 to 1.3 μm when phosphorus is contained in the center of the core and fluorine is contained in the outer periphery of the core. It is as follows.
Therefore, the GI multimode fiber of the present invention is an optical fiber having a low transmission loss at a wavelength of 0.8 to 1.3 μm and suitable for wavelength division multiplex transmission.

Next, a method for manufacturing a GI multimode fiber according to the present invention will be described.
The manufacturing method of the GI multimode fiber according to the present invention is based on PCVD (Plasma Chemical Vapor Deposition Method) or MCVD (Modified Chemical Vapor Deposition Method: Internal Chemical Vapor Deposition). Kinds of dopants are sequentially added to the outer peripheral part and the central part of the core, and the additive concentration is accurately controlled to produce a base material having a desired refractive index profile. A high temperature is applied to the base material, and the GI multimode fiber is produced by drawing it thinly.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention is not limited to a following example.

(Example 1)
A GI multimode fiber having a core made of quartz glass containing germanium in the center and fluorine in the outer periphery and a clad made of pure silica glass concentrically provided around the core was manufactured. .
The GI multimode fiber was optimized at a wavelength of 0.85 μm, and the relative refractive index difference Δ at the core center with respect to the cladding was set to 0.01. Furthermore, the core diameter was 50 μm (core radius a = 25 μm), and the cladding diameter was 125 μm.
In addition, the Δ = Δ _Ge + Δ _F, changing the ratio of Δ _Ge = Δ _in the Δ _F = Δ _out.
The wavelength dependence of the transmission bandwidth of the obtained GI multimode fiber was investigated. The results are shown in FIG.

From the results of FIG. 3, the GI multimode fiber containing germanium in the center of the core and fluorine in the outer periphery has a wider transmission bandwidth in a wider wavelength region than the GI multimode fiber containing only germanium in the core. confirmed. Further, the wavelength characteristic of the optimum value α _opt of the refractive index distribution order α of fluorine is much more stable than that of germanium, so that an optical fiber having a wider transmission bandwidth can be obtained by increasing the amount of fluorine added. It was confirmed.

Further, in FIGS. 4-8, showing the relative refractive index difference distribution of the core when varying the ratio of the delta _Ge and delta _F. 4 is a relative refractive index difference distribution when Δ _Ge / Δ _F = 0.001 / 0.009, and FIG. 5 is a relative refractive index difference distribution when Δ _Ge / Δ _F = 0.002 / 0.008. 6 is a relative refractive index difference distribution when Δ _Ge / Δ _F = 0.003 / 0.007, and FIG. 7 is a relative refractive index difference distribution when Δ _Ge / Δ _F = 0.004 / 0.006. FIG. 8 shows a relative refractive index difference distribution when Δ _Ge / Δ _F = 0.005 / 0.005.

(Example 2)
A GI multimode fiber having a core made of quartz glass containing germanium in the center and fluorine in the outer periphery and a clad made of pure silica glass concentrically provided around the core was manufactured. .
This GI multimode fiber was optimized at a wavelength of 0.85 μm, and the relative refractive index difference Δ at the core center with respect to the cladding was set to 0.02. Further, the core diameter was 62.5 μm (core radius a = 31.25 μm), and the cladding diameter was 125 μm.
In addition, the Δ = Δ _Ge + Δ _F, changing the ratio of Δ _Ge = Δ _in the Δ _F = Δ _out.
The wavelength dependence of the transmission bandwidth of the obtained GI multimode fiber was investigated. The results are shown in FIG.

From the results shown in FIG. 9, the GI multimode fiber containing germanium in the center of the core and fluorine in the outer periphery has a wider transmission bandwidth in a wider wavelength region than the GI multimode fiber containing only germanium in the core. confirmed. Further, it was confirmed that Δ _Ge = 0.002 and 0.004 show the best characteristics.

Further, in FIGS. 10 to 14, showing the relative refractive index difference distribution of the core when varying the ratio of the delta _Ge and delta _F. FIG. 10 shows a relative refractive index difference distribution when Δ _Ge / Δ _F = 0.002 / 0.018, and FIG. 11 shows a relative refractive index difference distribution when Δ _Ge / Δ _F = 0.004 / 0.016. FIG. 12 shows a relative refractive index difference distribution when Δ _Ge / Δ _F = 0.006 / 0.014, and FIG. 13 shows a relative refractive index difference distribution when Δ _Ge / Δ _F = 0.008 / 0.012. FIG. 14 shows a relative refractive index difference distribution when Δ _Ge / Δ _F = 0.010 / 0.010.

(Example 3)
A GI multimode fiber having a core made of quartz glass containing phosphorus in the center and fluorine in the outer periphery and a clad made of pure silica glass concentrically provided around the core was manufactured. .
The GI multimode fiber was optimized at a wavelength of 0.85 μm, and the relative refractive index difference Δ at the core center with respect to the cladding was set to 0.01. Furthermore, the core diameter was 50 μm (core radius a = 25 μm), and the cladding diameter was 125 μm.
Further, a Δ = Δ _P + Δ _F, is varied the ratio of Δ _P = Δ _in the Δ _F = Δ _out.
The wavelength dependence of the transmission bandwidth of the obtained GI multimode fiber was investigated. The results are shown in FIG.

From the results shown in FIG. 15, the GI multimode fiber containing phosphorus in the center of the core and fluorine in the outer peripheral portion has a wider transmission bandwidth in a wider wavelength region than the GI multimode fiber containing only phosphorus in the core. confirmed. Further, it was confirmed that Δ _P = 0.001 and 0.002 show the best characteristics.

  Also, adding a small amount of phosphorus to the core has the effect of reducing Rayleigh scattering, which is particularly effective at short wavelengths. Furthermore, since the center part of the core is made of quartz glass containing phosphorus, the melt viscosity is low. Therefore, when the fiber preform is manufactured by PCVD or MCVD, the precursor of the preform of the preform is easily collapsed, so that the fiber preform can be easily manufactured.

Example 4
A GI multimode fiber having a core made of quartz glass containing phosphorus in the center and fluorine in the outer periphery and a clad made of pure silica glass concentrically provided around the core was manufactured. .
This GI multimode fiber was optimized at a wavelength of 0.85 μm, and the relative refractive index difference Δ at the core center with respect to the cladding was set to 0.02. Further, the core diameter was 62.5 μm (core radius a = 31.25 μm), and the cladding diameter was 125 μm.
Further, a Δ = Δ _P + Δ _F, is varied the ratio of Δ _P = Δ _in the Δ _F = Δ _out.
The wavelength dependence of the transmission bandwidth of the obtained GI multimode fiber was investigated. The results are shown in FIG.

  From the results of FIG. 16, the GI multimode fiber containing phosphorus in the center of the core and fluorine in the outer periphery has a wider transmission bandwidth in a wider wavelength region than the GI multimode fiber containing only phosphorus in the core. confirmed.

  The GI multimode fiber of the present invention is applicable even at an optimum wavelength of 0.85 μm or less.

It is a graph which shows the fiber refractive index profile which adds a different dopant to two area | regions where a core differs, respectively. The wavelength of the optimum value α _opt of the refractive index distribution order α in the above formulas (1) to (4) representing the refractive index profile for a GI multimode fiber in which the dopant contained in the core is only germanium, phosphorus, and fluorine, respectively. It is a graph which shows dependence. It is a graph which shows the wavelength dependence of the transmission bandwidth of GI multimode fiber. It is a graph which shows the relative refractive index difference distribution of a core when changing the ratio of (DELTA) _Ge and (DELTA) _F. It is a graph which shows the relative refractive index difference distribution of a core when changing the ratio of (DELTA) _Ge and (DELTA) _F. It is a graph which shows the relative refractive index difference distribution of a core when changing the ratio of (DELTA) _Ge and (DELTA) _F. It is a graph which shows the relative refractive index difference distribution of a core when changing the ratio of (DELTA) _Ge and (DELTA) _F. It is a graph which shows the relative refractive index difference distribution of a core when changing the ratio of (DELTA) _Ge and (DELTA) _F. It is a graph which shows the wavelength dependence of the transmission bandwidth of GI multimode fiber. It is a graph which shows the relative refractive index difference distribution of a core when changing the ratio of (DELTA) _Ge and (DELTA) _F. It is a graph which shows the relative refractive index difference distribution of a core when changing the ratio of (DELTA) _Ge and (DELTA) _F. It is a graph which shows the relative refractive index difference distribution of a core when changing the ratio of (DELTA) _Ge and (DELTA) _F. It is a graph which shows the relative refractive index difference distribution of a core when changing the ratio of (DELTA) _Ge and (DELTA) _F. It is a graph which shows the relative refractive index difference distribution of a core when changing the ratio of (DELTA) _Ge and (DELTA) _F. It is a graph which shows the wavelength dependence of the transmission bandwidth of GI multimode fiber. It is a graph which shows the wavelength dependence of the transmission bandwidth of GI multimode fiber.

Claims (8)

A graded index type multimode fiber comprising a core made of quartz glass and a clad provided on the outer periphery of the core,
The graded index multimode fiber, wherein the core includes one of germanium and phosphorus at the center thereof and fluorine at the outer periphery thereof. The graded index multimode fiber according to claim 1, wherein the graded index multimode fiber has a refractive index profile that satisfies the following expressions (1) to (4):

[Where n (r) is the refractive index at a distance r from the core center of the optical fiber, n ₁ is the refractive index at the core center, Δ is the maximum relative refractive index difference of the core with respect to the cladding, a is the core radius, and α is the refractive index. Index distribution order, α ₁ is the refractive index distribution order of germanium or phosphorus contained in the central part of the core, α ₂ is the refractive index distribution order of fluorine contained in the outer peripheral part of the core, and a ₀ is the central part and outer peripheral part of the core And n ₀ represents the refractive index of pure quartz. ] The refractive index distribution orders α ₁ and α ₂ in the above formulas (2) to (4) each independently have an optimum value that maximizes the transmission bandwidth in the used wavelength region. Graded index multimode fiber.   Germanium is contained in the central part of the core, fluorine is contained in the outer peripheral part of the core, the core diameter is 50 μm, the cladding diameter is 125 μm, and the transmission bandwidth at a wavelength of 0.8 to 1.1 μm The graded index multimode fiber according to any one of claims 1 to 3, characterized in that is over 3 GHz · km.   Germanium is contained in the central part of the core, fluorine is contained in the outer peripheral part of the core, the core diameter is 62.5 μm, the cladding diameter is 125 μm, and transmission at a wavelength of 0.85 to 1.1 μm The graded index type multimode fiber according to any one of claims 1 to 3, wherein the bandwidth exceeds 2 GHz · km.   The core contains phosphorus, the core contains fluorine, the core has a diameter of 50 μm, the cladding has a diameter of 125 μm, and a transmission bandwidth at a wavelength of 0.8 to 1.3 μm. The graded index multimode fiber according to any one of claims 1 to 3, characterized in that is over 3 GHz · km.   Phosphorus is contained in the central part of the core, fluorine is contained in the outer peripheral part of the core, the core diameter is 62.5 μm, the cladding diameter is 125 μm, and transmission at a wavelength of 0.8 to 1.2 μm The graded index type multimode fiber according to any one of claims 1 to 3, wherein the bandwidth exceeds 2 GHz · km. 4. The center of the core includes phosphorus, the outer periphery of the core includes fluorine, and the transmission loss at a wavelength of 0.8 to 1.3 [mu] m is 2 dB / km or less. The graded index multimode fiber according to any one of the above.

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