JPS61167906A - Optical fiber - Google Patents

Abstract

PURPOSE:To realize zero dispersion without spoiling the low-loss characteristics in a 1.5mum wavelength band by using doped glass, making the refractive index distribution of a core stepwise, and setting a specific condition among the difference in specific refractive index between the core and a clad, the refractive index of the core, and in-use wavelength. CONSTITUTION:An optical fiber includes the core of quartz glass and doped glass which has a lower refractive index than pure quartz and the refractive index distribution of the core is stepwise; and the standardized frequency V shown by an equation is 0.8-1.1 in a 1.6mum wavelength band and DELTA>0.004, where 2a is the diameter of the core, DELTA the difference in specific refractive index between the core and clad, n1 the refractive index of the core, and lambdathe in-use wavelength. The wavelength dispersion is eliminated in the 1.5mum band where the loss of a quartz optical fiber is minimum, and the loss is reduced by >=0.1dB/Km as compared with a conventional 1.5mum band zero- dispersion optical fiber of this kind and reduced to the same loss of 0.12dB/Km with an optical fiber with a low specific refractive index DELTA. Consequently, the band is widened greatly and the loss is extremely small.

Description

Detailed Description of the Invention [Industrial Field of Application] The present invention is a silica-based optical fiber that is used in optical communications. The present invention relates to an optical fiber that can eliminate chromatic dispersion, which is a loss and a cause of transmission band deterioration.

〔overview〕

In the present invention, in a 1.5 μ band silica optical fiber, by setting the parameters to specific conditions, material dispersion and waveguide dispersion are set to cancel each other out, thereby obtaining an extremely low loss optical fiber. It is something.

[Conventional technology]

The transmission band of single mode fiber is limited by chromatic dispersion. The chromatic dispersion is given by the sum of the material dispersion that depends on the fiber material, the relative refractive index difference Δ, and the waveguide dispersion that depends on the core diameter 2a. However, when the refractive index of the core is n1 and the refractive index of the cladding is n2, it is defined as follows. The material dispersion of silica-based fibers is positive in the wavelength region of 1.3 μm or more, while the four-wave path dispersion is negative in the so-called single mode waveguide region (V<2.4) in the case of step fibers. ,
It has been revealed that the chromatic dispersion given by the sum of these can be made zero in a long wavelength region of 1.3 μm or more. However, ■ is given by when the wavelength used is λ. Specifically, the zero color dispersion in the 1.5 μm band is V
This is achieved by setting Δ>0.004 in the vicinity of ~1.

Literature: 1), Tsuchiya and N, Imot.

Dispersion free sjngle-mo
fiber in 1, 5μm wave length
th region ”Electronics Le
tters 19th July vol1, 15 No.
, 5P, 476 (1979) The 1.5 μm wavelength band is the wavelength band where the optical loss of silica-based frame synchronization is the minimum, but Δ#0.003, 2a'; 10. The chromatic dispersion of a typical single mode optical fiber with lJm is 16-20 ps/nm-k
Because of its large diameter of m, it was not suitable for optical communications that require an ultra-wideband. The above-mentioned 1.5 μm band zero-dispersion single-mode optical fiber was devised to compensate for this drawback, but G
In conventional optical fibers made of a quartz core and a pure silica cladding, a method was used to increase the relative refractive index difference Δ by increasing the core dopant to increase the refractive index of the core. As a result, Ley scattering loss, which is the main cause of optical loss in optical fibers, increases as Δ increases, making it impossible to realize a low-loss 1.55μ band zero-dispersion fiber. Rayleigh scattering loss is caused by fluctuations in the refractive index that are smaller than the wavelength order, and compared to pure quartz, Ge
is increased by adding as a dopant.

FIG. 3 shows the Rayleigh scattering coefficient versus the relative refractive index Δ or the molar ratio of Ge. Here, the Lely failure coefficient is the wavelength (μ
m) is raised to the fourth power, the loss value dB/ is defined as +*. The Rayleigh scattering loss at a wavelength of 1.55 μm is 0.12 dB/lo++ for pure quartz, while it is the same for Ge-doped quartz with Δ=0.003 <0.
It becomes 18dB/km.

From Figure 3, the Ley scattering loss of the Ge-doped core optical fiber with Δ=0.006 is 0.234 dB/km, which is 0.06 dB compared to the normal single-mode Ge-doped core optical fiber with Δ=0.003. /km increases.

[Problem that the invention seeks to solve]

This loss increase is 0.2 dB/km (ffi tM loss optical fiber is a non-negligible value), and it has been desired to develop a dopant material that has a high value of Δ and does not increase scattering loss.

The present invention is intended to solve the above-mentioned drawbacks, and aims to realize a zero-dispersion fiber without impairing the low loss property at a wavelength of 1.5μ steel strip.

[Means for solving problems]

The present invention provides an optical fiber including a core made of quartz glass and a cladding made of doped glass with a refractive index lower than that of pure silica, in which the refractive index distribution of the core is step-shaped, the core diameter is 2a, and the distance between the core and the cladding is When the relative refractive index difference between them is Δ, the refractive index of the core is fl1, and the wavelength used is λ, the normalized frequency V given by λ is 0.
It is characterized by taking a value between 8 and 1.1 and setting Δ > 0.004 or more.

[For production]

By setting such parameters, chromatic dispersion and waveguide dispersion cancel each other out to realize zero dispersion.

〔Example〕

FIG. 1 is a diagram showing the radial refractive index distribution of an optical fiber according to an embodiment of the present invention. C is an incoming signal, and the core 1 portion and the cladding 2 portion are adjusted so that their refractive indexes change as step-like as possible, and the above-mentioned conditions are applied to the refractive index difference Δ.

FIG. 2 is a diagram showing the radial refractive index distribution of an optical fiber according to an embodiment of the present invention, and in this example, a quartz jacket 3 is provided on the outside.

As is clear from FIG. 3, since pure quartz has a smaller Ley difference than doped quartz, the Ley difference can be reduced by using pure quartz as the core material. In this case, a dopant is required in the cladding to lower the refractive index. Fluorine (F) or boron (B) can be considered as a dopant for this purpose, but since 8203 doped quartz causes absorption loss in the long wavelength range of 1 μm or more, fluorine, which does not have this, is suitable as a dopant. .

Fluorine can form an equivalent refractive index difference with a smaller doping amount than germanium (Ge), and the amount is about 173. Furthermore, due to the difference in molecular structure, rifno-doped quartz has essentially less refractive index fluctuation, which causes Rayleigh scattering, than germanium-doped quartz. For the above two reasons, the Rayleigh scattering of fluorine-doped quartz is almost equal to that of pure quartz, and even when the relative refractive index difference Δ is increased, the Rayleigh scattering does not increase. Incidentally, the Rayleigh scattering loss at a wavelength of 1.55 μm is 0.12 dB/km.

Fluorine-doped optical fibers are manufactured using the VAD method, which is widely used as an optical fiber base material manufacturing method, using the following steps. First, a pure quartz base material, which serves as the core, is manufactured using the VAD method and vitrified. Next, P-SzOz is deposited around this base material again by the VAD method.The example shown in Figure 1 is a case where this is directly vitrified, and the example shown in Figure 2 is a case where a quartz jacket 3 is placed on top of this for dimension adjustment. This is the case when it is covered with glass and then vitrified.

FIG. 4 shows the relationship between material dispersion, waveguide dispersion, and core radius a of a stepped single-mode file at a wavelength of 1.55 μm for various relative index differences Δ.

As the relative refractive index difference Δ increases, the waveguide dispersion increases, and Δ>
It can be seen that at 0.004, it is possible to make the sum of redispersion zero. Figure 5 shows a and Δ that can achieve zero dispersion at a wavelength of 1.4.
Shown for ~1,6 μm. The broken line in the figure represents the cutoff of the first higher-order mode, and the area below the broken line is the single mode waveguide region.

Therefore, values for a and Δ can be selected for each zero-dispersion wavelength in this single mode waveguide area.

From FIG. 5, in order to achieve zero dispersion when λ>1.5 μl, it is necessary to set Δ>0.004, and the value of ■ in this case is based on the definition of V above, and the wavelength used λ By substituting the relative refractive index Δ obtained from FIG. 5 and the core radius a, 0.8 to 1.1 is obtained.

〔Effect of the invention〕

As explained above, this optical fiber can make the chromatic dispersion zero in the wavelength band of 1.5 μm, where the loss of silica-based optical fiber is minimum, and is superior to the conventional one of this type.
．． 0.1 d loss compared to 5 μm band zero dispersion optical fiber
The loss can be reduced to 0.12 dB/km, which is equivalent to an optical fiber with a low relative refractive index Δ. Therefore, the optical fiber according to the present invention has an ultra-wide band and an ultra-low loss, and is very promising as a transmission line for long-distance non-repeater ultra-high capacity optical communications in the future.

[Brief explanation of drawings]

FIGS. 1 and 2 are diagrams showing refractive index distributions of optical fibers according to embodiments of the present invention. Figure 3 shows the Lely failure coefficient and the relative refractive index difference Δ or Ge
A diagram showing the relationship with dopants. FIG. 4 is a diagram showing the relationship between dispersion and core radius. FIG. 5 is a diagram showing the relationship between the core radius and the relative refractive index difference Δ with respect to the zero dispersion wavelength, and the kayak-off condition. 1... Core, 2... Clad, 3... Quartz jacket.

Claims (1)

[Claims] (1) In an optical fiber that includes a core made of silica glass and a cladding made of doped glass with a refractive index lower than that of pure silica, the refractive index distribution of the core is step-shaped, with a core diameter of 2a and a distance between the core and the cladding. When the relative refractive index difference of 0.
An optical fiber having a value between 8 and 1.1, and set to Δ>0.004 or more. However, when the refractive index of the cladding is n_2, the relative refractive index difference Δ is defined as Δ=(n_1−n_2)/n_2.