JPH0672967B2 - Zero-dispersion single-mode optical fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] At present, communication using an optical fiber is almost in a practical stage, and many communication systems using, for example, light having a wavelength of 1.3 μm are in operation.

The optical fiber used is quartz glass, but the minimum value of its transmission loss is in the wavelength band of 1.55 μm. When a light source in this wavelength band is used, the non-repeatered transmission distance is nearly doubled as compared with the case of 1.3 μm wavelength, and therefore it is attracting attention as a next-generation system.

This next-generation system must be capable of long-distance transmission and at the same time capable of transmitting a large amount of information.

Therefore, a single mode optical fiber having a wide transmission bandwidth is used, but the single mode optical fiber has chromatic dispersion that limits the transmission bandwidth.

Therefore, the development of a single mode optical fiber with zero wavelength dispersion in the wavelength band of 1.55 μm is underway.

The present invention also relates to such a zero wavelength dispersion single mode optical fiber.

[Principle of zero dispersion] Strictly speaking, chromatic dispersion consists of material dispersion and structural dispersion. However, since it cannot be completely separated and discussed, it is usually discussed as chromatic dispersion.

As an example, if the chromatic dispersion of the step type optical fiber is shown by an equation, Becomes

Where k is the phase constant of light in vacuum, c is the speed of light in vacuum, β is the propagation constant of light in the optical fiber, N ₁ is the amount called the group refractive index, v is a quantity called a normalized frequency, and v ² = k ² ▲ n ² ₁ ▼ a ² 2 △ (4) n ₁ is the refractive index of the core, a is the radius of the core, △ is the relative refractive index between the core and the clad The rate difference, b is an amount called the normalized propagation constant, n ₂ is the refractive index of the clad.

The first term in the equation (1) corresponds to the material dispersion, and the second term corresponds to the structural dispersion.

FIG. 7 shows an example of the calculation result of the equation (1).

The material dispersion has a negative value at a wavelength of 1.3 μm or more, whereas the structural dispersion always has a positive value, so at a specific wavelength, the sum of the two is zero. .

Therefore, by appropriately adjusting the relative refractive index difference Δ and the core diameter 2a, it becomes possible to cancel the chromatic dispersion near the wavelength of 1.55 μm.

Even if the refractive index distribution is other than the step type, if the profile of the refractive index distribution is determined, it is possible to obtain a diagram showing the relationship between the wavelength dispersion and the relative refractive index difference Δ and the core diameter 2a. It is possible to design a zero-dispersion single-mode optical fiber by considering the field diameter (MFD) and the cutoff wavelength of the second mode, but until now, sufficient studies have been made for all profiles. It does not mean that

[Prior Art] A 1.55 μm zero-dispersion optical fiber that has been studied so far is shown below.

(1) High refractive index difference step type (Fig. 8 (a)) Fig. 9 shows the calculation results of what the chromatic dispersion becomes at 1.55 µm when the relative refractive index difference Δ and the core radius a are changed. Show.

From this figure, the relative refractive index difference Δ is 0.7% or more and the core radius a is
It can be seen that the wavelength dispersion at 1.55 μm can be made zero by setting around 2.3 μm.

In this case, when the core radius is a <2 μm and the relative refractive index difference is Δ <0.6%, the effect of confining light in the core is weak (MFD is too large) and it cannot be used as a practical optical fiber. Even if the chromatic dispersion value is small, it is not considered.

(2) Triangular profile type (Fig. 8 (b)) Fig. 10 shows how the chromatic dispersion at 1.55 µm becomes when the relative refractive index difference Δ and the core radius a are changed.

From this figure, the relative refractive index difference Δ is 0.8% or more and the core radius a is
It can be seen that the wavelength dispersion at 1.55 μm can be made zero by setting around 3.3 μm.

Also in this case, when the core radius is a <2.5 μm and the relative refractive index difference Δ <0.8%, the effect of confining light in the core is weak, and a stable optical fiber cannot be obtained.

(3) Trapezoidal profile type (Fig. 8 (c)) It is said that the high refractive index portion at the center of the above triangular profile type is removed to make it easier to make, and the characteristics are said to be almost the same as the triangular profile type. There is.

(4) Double clad type (FIG. 8 (d)) This is an optical fiber that has attempted to reduce wavelength dispersion in a wide wavelength range including 1.55 μm. In this case, core radius a
Figure 11 shows how the wavelength at which chromatic dispersion becomes zero when is changed.

When the relative refractive index difference Δ is set to 0.85% and the core radius a is set to 3.05 μm, the wavelength dispersion at 1.55 μm can be made zero.

[Problems to be Solved by the Invention] (1) In the case of high refractive index difference step type (Fig. 8 (a)) 1) As can be seen from Fig. 9, in the case of relative refractive index difference Δ = 0.8% The core radius a within which the dispersion value of 1.55 μm is within ± 3 ps / km / nm is 2.2 to 2.4 μm, and the allowable width is only ± 5%.

That is, even if the core radius a deviates slightly from the target value, the chromatic dispersion value changes greatly. Therefore, extremely high controllability of the core diameter is required, and manufacturing becomes difficult.

In actual manufacturing, the only parameter that can be adjusted before drawing the preform is the core diameter. If the cladding diameter of the optical fiber (optical fiber outer diameter) is out of the set value in order to set the core diameter to the target value, the preform is ground into concentric cylinders, or a quartz glass tube prepared separately is used. The clad diameter can be adjusted by covering it.

However, as described above, when the permissible width of the core radius after drawing is about ± 5%, extremely high precision control is required.

2) To obtain a high relative refractive index difference Δ, a large amount of GeO ₂ is usually doped throughout the core. Therefore, Rayleigh scattering is likely to be high, and scattering due to the effect of refractive index fluctuations at the boundary between the core and the cladding is likely to occur.

(2) Triangular profile type (Fig. 8 (b)) Although the problem in the high index difference step type described above is greatly improved, the refractive index at the core center is considerably high, It is hard to say that it is easy to manufacture.

In the trapezoidal profile type (FIG. 8 (c)), that point is improved, but as described later, the problem of manufacturing difficulty by the VAD method remains.

(3) Double clad type (FIG. 8 (d)) As is known from FIG. 11, when the core radius a changes slightly, the chromatic dispersion value and the wavelength of zero dispersion largely change. Therefore, it is difficult to control the core diameter during manufacturing.

(4) Relationship with manufacturing method All of the optical fibers shown in Fig. 8 above were all prototyped by the MCVD method, and the VAD method, which is developing in Japan, has not been tried so much (however, (There are some reports on the step type in FIG.

When the MCVD method is used, the profile shape as shown in the figure is approximated by multiple layers, so that a considerably complicated refractive index distribution can be dealt with.

However, in principle, the VAD method cannot handle a profile that is too complicated.

However, as the history of low loss of optical fiber, especially silica optical fiber since 1980 shows, the VAD method is always superior to other manufacturing methods in terms of loss. In addition, the fact that the fiber length obtained from one preform is long, considering that this type of fiber is likely to be used in a submarine optical fiber cable, the VAD method is 1.55μ.
Applicability to m-zero-dispersion fiber should be called a request of the times.

Figure 12 shows an example of the VAD method.

Burner 20 for core formation and burner 30 for clad formation
From the above, the raw material gases 22 and 32, which are to become glass, respectively, are introduced into the flame, and the generated glass fine powder is collected as the sintered body 40. 50 is an exhaust pipe.

The refractive index distribution of the core is controlled by adjusting the combustion conditions and the flow rate conditions of the core forming burner 20, the temperature distribution of the bottom surface of the glass fine powder sintered body 40, etc. Since it is not necessarily larger than the powder sintered body 40, it is not good at making a stepwise change in the refractive index.

[Object of the Invention] As described above, the present invention provides: 1) Controllability of chromatic dispersion that varies depending on a profile, that is, a change in chromatic dispersion when a target value of core diameter or relative refractive index difference Δ deviates. , 2) Profile controllability from the viewpoint of manufacturing method, that is,
This was done in consideration of the ease of fabrication by the VAD method, etc. It is intended to provide an optical fiber with small wavelength dispersion in the 1.55 μm wavelength band with good manufacturability.

[Means for Solving Problems] The present invention is good at the VAD method as shown in FIG. 1 without making the profile of the refractive index distribution in the radial direction of the core angular as in the conventional case. It is characterized by a Gaussian distribution.

[Explanation] (1) Gaussian distribution: Its typical shape is as shown in FIG. If you show it with a mathematical formula, Becomes

In addition, the core radius a is the relative refractive index difference Δ seen from the clad.
It is defined by the radius that can be reduced to 1 / e ² .

Also, in actual manufacturing, it is impossible to make an optical fiber whose refractive index changes to infinity, so the refractive index change ends at a finite radius, specifically 2a or more, and a flat It is sufficient to enter the refractive index portion (a part of the clad).

(2) Regarding chromatic dispersion at 1.55 μm: FIG. 2 shows the calculation result of what the chromatic dispersion becomes when the refractive index distribution is expressed by the equation (6).

The change in chromatic dispersion with respect to the change in core diameter is as small as that of the above-mentioned triangular profile type.

For example, when the relative refractive index difference Δ = 0.8%, the wavelength is 1.55 μm.
The core radius a within which the dispersion value of is within ± 3ps / km / nm is 1.8
~ 2.3 μm, ± 12% allowed.

Also, the change in the dispersion value of 1.55 μm with respect to the change in the relative refractive index difference Δ is slight.

Therefore, it becomes easy to control the core diameter during manufacturing.

(3) MFD: Fig. 3 shows the MFD of the propagation mode of Gaussian distribution. If the MFD is too large, the required high-purity glass clad becomes thick and is easily damaged by bending, which is not preferable.

From the figure, for example, the relative refractive index difference Δ for setting the MFD to 10 μm or less needs to be 0.8% or more when the core radius a = 2 μm, and 0.66% or more when the core radius a = 2.5 μm.

(4) Cutoff wavelength: Fig. 4 shows the cutoff wavelength of the LP ₁ mode, which is the second mode.

The relative refractive index difference Δ is 0.8% or more, and the core radius a is 3.5 μm.
In the above case, the single mode is not achieved at the wavelength of 1.55 μm or less which is the target of the present invention.

Therefore, it is necessary to design the appropriate range of the relative refractive index difference Δ and the core radius a in consideration of the cutoff wavelength and the MFD.

(5) Regarding single-mode optical fiber having Gaussian refractive index distribution: In a multimode optical fiber, a graded refractive index distribution is adopted in order to eliminate mode dispersion.

However, in a single mode optical fiber without mode dispersion, the basic profile of the refractive index profile is stepwise.

In the conventional 1.3 μm wavelength band, the relative refractive index difference Δ is 0.4
%, The chromatic dispersion of the step-type single-mode optical fiber with a core diameter of about 10 μm is almost zero dispersion (at least ±
Within 3ps / km / nm).

The development of zero dispersion at a wavelength of 1.55 μm was also started by making the step type relative refractive index difference Δ very high as shown in FIG. 8 (a). It was about 1980 in Britain, Japan, and the United States.

After that, we proceeded to the triangular profile shape of FIG.

However, the profiles of these refractive index distributions were all step-shaped and their modifications, and had a clear profile of the boundary between the core and the cladding. In addition, it was one of the strengths of the MCVD method.

Until now, single-mode optical fibers having a Gaussian profile refractive index profile have never been tried.

[Example] A glass fine powder sintered body 40 was produced using a system similar to that shown in FIG. However, in order to obtain a sufficient clad diameter, four clad forming burners 30 were used. The structure of the burners 20 and 30 for forming the core and the clad is a four-layer concentric multi-tube.

Source gas, SiCl _4, as a dopant to the core using GeCl ₄ for SiO ₂ occurs.

The combustion gas is hydrogen and the auxiliary twisting gas is oxygen. The flow rate is
The SiCl ₄ for the core was set to about 100 cc / min, the dopant GeCl ₄ for the core was set to 20 cc / min, and the SiCl ₄ for the clad was set to about 200 to 400 cc / min for each burner.

The obtained glass fine powder sintered body 40 was made into a transparent glass in a He atmosphere in an electric furnace having a maximum heating temperature of about 1450 ° C. while adding a small amount of SOBr ₂ .

The refractive index distribution of the obtained preform was a roughly Gaussian distribution, as shown in FIG.

This preform was heat-stretched to have an outer diameter of 10 mm and then covered with a separately prepared transparent quartz glass tube. As a result, the final preform had an outer diameter of 38 mm and a core diameter of 1.25 mm.

It was spun in an electric furnace having a maximum heating temperature of about 1250 ° C. to obtain an optical fiber having an outer diameter of 125 μm and a core diameter of about 4.1 μm.

When the transmission loss of the optical fiber was measured, it was 0.22 dB / km at the wavelength of 1.55 μm, which is the current focus.

When the wavelength dispersion characteristics were measured using a Raman fiber pumped by a YAG laser, the characteristics shown in FIG. 6 were obtained. The wavelength giving zero dispersion was 1.56 μm.

In view of the loss of this optical fiber, non-relay of 100 km or more and high-speed transmission of 1 Gbit / s or more can be realized in the 1.55 μm band.

[Advantages of the Invention] Since the profile of the refractive index distribution in the radial direction of the core is a Gaussian distribution, (1) As described above, the change in chromatic dispersion with respect to the change in the core diameter or the relative refractive index difference Δ is small.

Therefore, it becomes easy to control the core diameter when manufacturing a zero-dispersion single-mode optical fiber at 1.55 μm.

(2) Due to the smoothness of the shape of the profile of the refractive index distribution, it is suitable for a manufacturing method in which fine glass powder, which is likely to cause diffusion, volatilization, and migration of dopant during preform production, is formed as an intermediate product. Easy to make with advanced VAD method.

[Brief description of drawings]

1 to 6 relate to the optical fiber of the present invention, FIG. 1 is an explanatory view of a typical profile of the refractive index distribution, FIG. 2 is a wavelength dispersion characteristic diagram, and FIG. 3 is an MFD characteristic diagram. FIG. 4 is a characteristic diagram of the cutoff wavelength of the secondary mode, FIG. 5 is a refractive index distribution diagram of the final base material in the example, and FIG. 6 is a wavelength dispersion characteristic diagram of the optical fiber of the example. FIG. 7 is an explanatory view of the principle by which chromatic dispersion can be made zero, and (a) to (d) of FIG. 8 are explanatory views of different examples of the conventional 1.55 μm zero-dispersion single mode optical fiber. FIGS. 9, 10 and 11 are wavelength dispersion characteristic diagrams of different examples of the conventional 1.55 μm zero-dispersion single mode optical fiber, and FIG. 12 is an explanatory diagram of the VAD method. 20: Burner for core formation 30: Burner for clad formation 40: Sintered fine glass powder

 ─────────────────────────────────────────────────── --- Continuation of the front page (72) Inventor Cho Fukuda 1440 Rokusaki, Sakura City, Chiba Prefecture Sakura Factory, Fujikura Electric Wire Co., Ltd. (56) References JP 57-39401 (JP, B2) JP 56-2923 (JP, B2)

Claims (1)

[Claims] 1. A single-mode optical fiber having a chromatic dispersion of zero in the wavelength band of 1.55 μm, characterized in that the profile of the refractive index distribution in the radial direction of the core is a Gaussian distribution. One mode light fiber.