JP2005114766A - Graded-index multimode fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a graded-index (GI) multimode fiber in which a maximum transmission band width at a certain wavelength is obtained regardless of the wavelength of signal light beams within the fiber. <P>SOLUTION: In the GI multimode fiber which is provided with a core made by quartz glass and a clad provided at the outer periphery, phosphorus and fluorine are added to the core. The GI multimode fiber has a refractive index profile which satisfies an equation (1). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

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

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

 Currently, GI multimode fiber has almost reached its performance limit, and in order to increase the transmission bandwidth of GI multimode fiber beyond this, wavelength division multiplexing (WDM) transmission must be used. .

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 glass and a clad provided on the outer periphery of the core, wherein the core comprises phosphorus and fluorine. A graded index type multimode fiber is provided.

  The graded index multimode fiber having the above-described configuration preferably has a refractive index profile that satisfies the following formula (1).

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. Represents the distribution order.

In the graded index multimode fiber having the above-described configuration, when the relative refractive index difference with respect to the cladding of phosphorus is Δ _P and the relative refractive index difference with respect to the cladding of fluorine is Δ _F , the maximum relative refractive index difference Δ of the core with respect to the cladding is It is preferable to be represented by the following formula (2).

In graded index multimode fiber having the above-described configuration, the maximum relative refractive index difference delta 0.005 or more, 0.025 or less, the relative refractive index difference with respect to the cladding of phosphorus delta _P is 0 or more, the maximum relative refractive index hereinafter difference delta, the relative refractive index difference with respect to the cladding of the fluorine delta _F is 0 or more, it is preferable that the equal to or less than the maximum relative refractive index difference delta.

  In the above-described graded index multimode fiber, the maximum relative refractive index difference Δ is preferably 0.005 or more and 0.025 or less, and the core radius a is 10 μm or more and 35 μm or less.

  In the graded index type multimode fiber having the above-described configuration, the maximum relative refractive index difference Δ is 0.009 or more, the numerical aperture is 0.185 or more, and the transmission bandwidth is 0.8 to 1.4 μm. It is preferable to exceed 2 GHz · km.

  In the graded index type multimode fiber having the above-described configuration, the maximum relative refractive index difference Δ is 0.019 or more, the numerical aperture is 0.26 or more, and the transmission bandwidth is at a wavelength of 0.8 μm to 1.4 μm. It is preferable to exceed 1.5 GHz · km.

  Since the graded index type multimode fiber of the present invention has a structure in which phosphorus and fluorine are contained in the core, the transmission bandwidth becomes large in a very wide wavelength region. Therefore, the graded index multimode fiber of the present invention is an optical fiber suitable for wavelength division multiplexing transmission.

Hereinafter, a graded index type multimode fiber embodying the present invention will be described.
The GI multimode fiber of the present invention includes a core made of silica glass containing phosphorus (P) and fluorine (F), and a clad provided concentrically with the core around the core. It is an optical fiber.

The GI multimode fiber of the present invention is an optical fiber having a refractive index profile that satisfies the following formula (1), and the core uses the WKB method (Wentzel-) in order to maximize the transmission bandwidth in the used wavelength region. See Kramers-Brillouin method, R. Olshansky and DB Keck, “Pulse broadcasting in graded-index optical fibers”, Appl. Opt., Vol. The optimal value α _opt of the refractive index distribution order α in the following formula (1) is substantially monotonously decreasing as the wavelength becomes longer, and the optimum value α _opt becomes longer as the wavelength becomes longer. In general, it contains monotonically increasing fluorine.

In the above formula (1), 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, Δ is the maximum relative refractive index difference of the core with respect to the cladding, a Represents the core radius, and α represents the refractive index profile order.

  The refractive index profile represented by the above formula (1) has a maximum refractive index at the center of the core, and has a shape such that the refractive index gradually decreases as the radius increases. 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.

On the other hand, 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. When there is only one kind of dopant, the optimum value α _opt is roughly divided into a substance A that decreases as the wavelength increases, and a substance B that increases as the wavelength increases.

FIG. 1 shows the wavelength of the optimum value α _opt of the refractive index distribution order α in the above equation (1) representing the refractive index profile in 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.

  In addition, in the calculations related to FIG. 1 and all the following calculations, the material dispersion coefficients of pure quartz and germanium-added quartz are described in Document A (Noriyoshi Shibata, Takao Edahiro, “Refractive index dispersion characteristics of glass for optical fibers”, Vol.OQE80-114, pp.85-90, 1980), and the material dispersion coefficient of fluorine-added quartz is described in Document B (JW Fleming, “Material dispersion in lightguide glasses”, Electron Lett. , Vol.14, pp.326-328, 1978).

From FIG. 1, it can be seen that, in the GI multimode fiber including fluorine in the core, the optimum value α _opt becomes a minimum near the wavelength of 0.7 μm, but generally increases monotonously as the wavelength becomes longer. On the other hand, in the GI multimode fiber containing germanium or phosphorus in the core, it can be seen that the optimum value α _opt monotonously decreases as the wavelength becomes longer.

Also, from FIG. 1, when these GI multimode fibers are all optimized, for example, at a wavelength of 0.85 μm, the value of the refractive index distribution order α at a wavelength other than this wavelength deviates from the optimum value α _opt. Thus, it can be seen that a large transmission bandwidth cannot be obtained at wavelengths other than 0.85 μm.

In addition, from FIG. 1, the GI multimode fiber containing germanium in the core shows the largest variation in the optimum value α _opt with respect to the change in wavelength, and thus the wavelength dependence of the transmission bandwidth is most prominent. I understand that.
On the other hand, in the GI multimode fiber containing phosphorus or fluorine in the core, it can be seen that the variation of the optimum value α _opt with respect to the change in wavelength is small. Therefore, these GI multimode fibers also have a small wavelength dependency of the transmission bandwidth.

  Therefore, by adding phosphorus and fluorine to the core, the wavelength dependence of the transmission bandwidth is suppressed, and the transmission bandwidth of the GI multimode fiber is increased in a wide wavelength region.

  Further, by adding phosphorus to quartz glass, the viscosity of the quartz glass at a high temperature can be lowered. In the GI multimode fiber of the present invention, since a large amount of phosphorus is added to the center of the core, the viscosity at the center of the core is reduced when the fiber preform is manufactured. Therefore, PCVD (Plasma Chemical Vapor Deposition Method: Plasma Chemistry) When a fiber preform is manufactured by a vapor phase deposition method) or an MCVD (Modified Chemical Vapor Deposition Method), a collapse process can be easily performed.

Furthermore, when the fiber preform is manufactured, the viscosity at the center of the core is reduced, so that the occurrence of a dip (center dip) at the center of the core can be suppressed. Therefore, it becomes easy to control the refractive index profile of the fiber.
The center dip is caused by the removal of the dopant in the collapse process, and has an undesirable property for the fiber.

  Further, by adding phosphorus to the core, the Rayleigh scattering coefficient in the fiber is reduced, so that it is possible to realize a fiber with a reduced loss particularly on the short wavelength side.

Further, the graded index type multimode fiber of the present invention has a maximum relative refractive index difference Δ of the core relative to the cladding, where Δ _P is the relative refractive index difference with respect to the cladding of phosphorus and Δ _F is the relative refractive index difference with respect to the cladding of fluorine. Is an optical fiber represented by the following formula (2).

The GI multimode fiber of the present invention, the maximum relative refractive index difference delta is 0.005 ≦ Δ ≦ 0.025 in the above equation (2), and, relative refractive index difference delta _P is 0 ≦ Δ _P ≦ Δ, the ratio it is preferred refractive index difference delta _F is 0 ≦ Δ _F ≦ Δ.
When the maximum relative refractive index difference Δ is less than 0.005, the NA (numerical aperture) of the fiber becomes small, and the coupling with the light source becomes difficult. On the other hand, when the maximum relative refractive index difference Δ exceeds 0.025, the number of modes increases and the transmission bandwidth decreases.

In the GI multimode fiber of the present invention, the maximum relative refractive index difference Δ in the equation (2) is 0.005 ≦ Δ ≦ 0.025, and the core radius a in the equation (1) is 10 μm ≦ a ≦. The thickness is preferably 35 μm, and more preferably 20 μm ≦ a ≦ 30 μm.
When the core radius a is less than 10 μm, it becomes difficult to couple the fibers or between the fiber and the light source. On the other hand, when the core radius a exceeds 35 μm, the number of modes increases too much, the dispersion between modes increases, and the transmission bandwidth decreases.

The GI multimode fiber of the present invention has a maximum relative refractive index difference Δ ≧ 0.009, numerical aperture ≧ 0.185, and a transmission bandwidth exceeding 2 GHz · km at a wavelength of 0.8 μm to 1.4 μm. is there.
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.

Furthermore, the GI multimode fiber of the present invention has a maximum relative refractive index difference Δ ≧ 0.019, a numerical aperture ≧ 0.26, and a transmission bandwidth exceeding 1.5 GHz · km at a wavelength of 0.8 μm to 1.4 μm. Is.
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 in a wavelength region of a wavelength of 0.8 μm to 1.4 μm.

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 a plasma chemical vapor deposition method (PCVD) or a modified chemical vapor deposition method (MCVD). Then, fluorine is added to the quartz glass serving as the core, and the addition concentration is accurately controlled to produce a fiber preform made of quartz glass having a desired refractive index profile. A high temperature is applied to this fiber preform, and the GI multimode fiber is manufactured 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 phosphorus and fluorine and a clad made of pure silica glass concentrically provided around the core was manufactured.
Further, the maximum relative refractive index difference Δ of the core center with respect to the cladding of the GI multimode fiber was set to 0.01, and the core radius a was set to 25 μm. Note that Δ = Δ _P (specific refractive index difference with respect to phosphorus cladding) + Δ _F (specific refractive index difference with respect to fluorine cladding), and the ratio between Δ _P and Δ _F was changed.

Δ = Δ _P + Δ _F = 0.01 was fixed, the ratio between Δ _P and Δ _F was changed, and the wavelength dependence of the optimum value α _opt of the refractive index distribution order α was examined. The results are shown in FIG.

From the results shown in FIG. 2, the ratio of the delta _P and delta _F, i.e., by changing the ratio of the addition amount of germanium and fluorine, the wavelength characteristic of the optimum value alpha _opt is changed, the wavelength characteristics of the GI multimode fiber It was confirmed that it could be improved. In particular, when Δ _P = 0.004 and Δ _F = 0.006, it was confirmed that the optimum value α _opt is almost flat.

  Therefore, it was found that the wavelength characteristics of the GI multimode fiber can be improved by producing a GI multimode fiber containing germanium and fluorine in the core. Furthermore, it was found that the transmission bandwidth of the GI multimode fiber can be increased in a very wide wavelength region by optimizing the ratio of the addition amount of germanium and fluorine.

(Example 2)
A GI multimode fiber having a core made of quartz glass containing phosphorus and fluorine 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, the maximum relative refractive index difference Δ at the core center with respect to the cladding of the GI multimode fiber was 0.01, and the core radius a was 25 μm.
Further, Δ = Δ _P + Δ _F = 0.01 was fixed, and the ratio of Δ _P and Δ _F was changed.
The wavelength dependence of the OFL (Over-filled Launch) transmission bandwidth of the obtained GI multimode fiber was examined. The results are shown in FIG.

From the results of FIG. 3, it was confirmed that the GI multimode fiber containing phosphorus and fluorine in the core has a larger transmission bandwidth in a wider wavelength region than the GI multimode fiber containing only phosphorus or fluorine in the core. Further, it was confirmed that those having Δ _P = 0.005 and Δ _F = 0.005 exhibit the best characteristics.

FIG. 4 is a graph showing the relative refractive index difference distribution when Δ _P = 0.005 and Δ _F = 0.005.

(Example 3)
A GI multimode fiber having a core made of quartz glass containing phosphorus and fluorine 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 1.30 μm, the maximum relative refractive index difference Δ at the core center with respect to the cladding of the GI multimode fiber was 0.01, and the core radius a was 25 μm.
Further, Δ = Δ _P + Δ _F = 0.01 was fixed, and the ratio of Δ _P and Δ _F was changed.
The wavelength dependence of the OFL transmission bandwidth of the obtained GI multimode fiber was investigated. The results are shown in FIG.

From the results of FIG. 5, it was confirmed that the GI multimode fiber containing phosphorus and fluorine in the core has a larger transmission bandwidth in a wider wavelength region than the GI multimode fiber containing only phosphorus or fluorine in the core. Further, it was confirmed that those having Δ _P = 0.004 and Δ _F = 0.006 exhibit the best characteristics.

FIG. 6 is a graph showing the relative refractive index difference distribution when Δ _P = 0.004 and Δ _F = 0.006.

Example 4
A GI multimode fiber having a core made of quartz glass containing phosphorus and fluorine and a clad made of pure silica glass concentrically provided around the core was manufactured.
Further, the maximum relative refractive index difference Δ of the core center with respect to the cladding of this GI multimode fiber was set to 0.02, and the core radius a was set to 31.25 μm. Incidentally, a Δ = Δ _P + Δ _F, is varied the ratio of the delta _P and delta _F.

Δ = Δ _P + Δ _F = 0.02 was fixed, the ratio between Δ _P and Δ _F was changed, and the wavelength dependence of the optimum value α _opt of the refractive index distribution order α was examined. The results are shown in FIG.

From the results of FIG. 7, the ratio of the delta _P and delta _F, i.e., by changing the ratio of the addition amount of germanium and fluorine, the wavelength characteristic of the optimum value alpha _opt is changed, the wavelength characteristics of the GI multimode fiber It was confirmed that it could be improved. In particular, when Δ _P = 0.008 and Δ _F = 0.012, it was confirmed that the optimum value α _opt is almost flat.

(Example 5)
A GI multimode fiber having a core made of quartz glass containing phosphorus and fluorine 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, the maximum relative refractive index difference Δ at the core center with respect to the cladding of the GI multimode fiber was 0.02, and the core radius a was 31.25 μm.
Further, Δ = Δ _P + Δ _F = 0.02 was fixed, and the ratio between Δ _P and Δ _F was changed.
The wavelength dependence of the OFL transmission bandwidth of the obtained GI multimode fiber was investigated. The results are shown in FIG.

From the results of FIG. 8, it was confirmed that the GI multimode fiber containing phosphorus and fluorine in the core has a larger transmission bandwidth in a wider wavelength region than the GI multimode fiber containing only phosphorus or fluorine in the core. Moreover, it was confirmed that what set (DELTA) _P = 0.010 and (DELTA) _F = 0.010 showed the best characteristic.

FIG. 9 is a graph showing a relative refractive index difference distribution when Δ _P = 0.010 and Δ _F = 0.010.

(Example 6)
A GI multimode fiber having a core made of quartz glass containing phosphorus and fluorine 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 1.30 μm, the maximum relative refractive index difference Δ at the core center with respect to the cladding of this GI multimode fiber was 0.02, and the core radius a was 31.25 μm.
Further, Δ = Δ _P + Δ _F = 0.02 was fixed, and the ratio between Δ _P and Δ _F was changed.
The wavelength dependence of the OFL transmission bandwidth of the obtained GI multimode fiber was investigated. The results are shown in FIG.

From the results of FIG. 10, it was confirmed that the GI multimode fiber containing phosphorus and fluorine in the core has a larger transmission bandwidth in a wider wavelength region than the GI multimode fiber containing only phosphorus or fluorine in the core. Further, it was confirmed that Δ _P = 0.006 and Δ _F = 0.014 showed the best characteristics.

FIG. 11 is a graph showing a relative refractive index difference distribution when Δ _P = 0.006 and Δ _F = 0.014.

  The GI multimode fiber of the present invention is also applicable to the construction of a wavelength division multiplexing system based on the use of multifiber.

In the GI multimode fiber in which the dopant contained in the core is only germanium, phosphorus, and fluorine, respectively, the optimum value α _opt of the refractive index distribution order α in the above formula (1) representing the refractive index profile of the GI multimode fiber It is a graph which shows wavelength dependence. In the GI multimode fiber containing phosphorus and fluorine in the core, it is a graph showing the wavelength dependence of the optimum value α _opt of the refractive index distribution order α in the above formula (1) representing the refractive index profile. It is a graph which shows the wavelength dependence of the transmission bandwidth of GI multimode fiber which contains phosphorus and fluorine in a core. It is a graph which shows relative refractive index difference distribution when (DELTA) _P = 0.005 and (DELTA) _F = 0.005. It is a graph which shows the wavelength dependence of the transmission bandwidth of GI multimode fiber which contains phosphorus and fluorine in a core. It is a graph which shows relative refractive index difference distribution when (DELTA) _P = 0.004 and (DELTA) _F = 0.006. In the GI multimode fiber containing phosphorus and fluorine in the core, it is a graph showing the wavelength dependence of the optimum value α _opt of the refractive index distribution order α in the above formula (1) representing the refractive index profile. It is a graph which shows the wavelength dependence of the transmission bandwidth of GI multimode fiber which contains phosphorus and fluorine in a core. It is a graph which shows relative refractive index difference distribution when (DELTA) _P = 0.010 and (DELTA) _F = 0.010. It is a graph which shows the wavelength dependence of the transmission bandwidth of GI multimode fiber which contains phosphorus and fluorine in a core. It is a graph which shows relative refractive index difference distribution when (DELTA) _P = 0.006 and (DELTA) _F = 0.014.

Claims (7)

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 contains phosphorus and fluorine. The graded index multimode fiber according to claim 1, wherein the graded index multimode fiber has a refractive index profile that satisfies the following formula (1).

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. Represents the distribution order. When the relative refractive index difference with respect to the cladding of phosphorus is Δ _P and the relative refractive index difference with respect to the cladding of fluorine is Δ _F , the maximum relative refractive index difference Δ of the core with respect to the cladding is expressed by the following equation (2). The graded index type multimode fiber according to claim 1, wherein the graded index type multimode fiber is used.

The maximum relative refractive index difference delta 0.005 or more, 0.025 or less, the relative refractive index difference with respect to the cladding of phosphorus delta _P is 0 or more, the maximum relative refractive index difference delta less, a relative refractive index relative to the cladding of the fluorine the difference delta _F is 0 or more, graded index multimode fiber according to any one of claims 1 to 3, wherein the equal to or less than the maximum relative refractive index difference delta.   5. The graded index type according to claim 1, wherein the maximum relative refractive index difference Δ is 0.005 or more and 0.025 or less, and the core radius a is 10 μm or more and 35 μm or less. Multimode fiber.   6. The wavelength of 0.8 μm to 1.4 μm, wherein the maximum relative refractive index difference Δ is 0.009 or more, the numerical aperture is 0.185 or more, and the transmission bandwidth exceeds 2 GHz · km. The graded index type multimode fiber according to any one of the above.   2. The maximum relative refractive index difference Δ is 0.019 or more, the numerical aperture is 0.26 or more, and the transmission bandwidth exceeds 1.5 GHz · km at a wavelength of 0.8 μm to 1.4 μm. The graded index type multimode fiber according to any one of 5 to 5.

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