JP2002156543A - Optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent an optical fiber from the increase of a transmission loss caused by residual stress generating in the boundary between a core and a clad and a core part adjacent to the clad or the like after being formed into the optical fiber after wire drawing and a deviation of characteristics caused by a change in a distribution of a refractive index in wire drawing. SOLUTION: In the optical fiber which is composed by incorporating the core and the clad, a place where viscosity at a high temperature is maximized exists in a region wherein a distance from the center of the optical fiber is >=1.3 times the mode field diameter in the specific wavelength within range of wavelength enabling a single mode operation and the outer surface of the optical fiber is not contained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical fiber mainly used for an optical communication system.

[0002]

2. Description of the Related Art In recent years, due to a rapid increase in communication demand, studies on expanding the transmission capacity in an optical fiber transmission line have been actively conducted. Accordingly, the capacity of the wavelength division multiplexing (WDM) optical transmission system has been increased, that is, the number of channels has been increased and the transmission band has been expanded.

In order to increase the capacity of a WDM optical transmission line,
As an optical fiber mainly constituting an optical transmission line, an optical fiber capable of inputting high-output signal light, an optical fiber having a small dispersion gradient, and the like are indispensable.

[0004] Further, an optical fiber (hereinafter, referred to as DCF) for compensating the dispersion of the optical fiber constituting the optical transmission line, and an optical fiber (hereinafter, referred to as DSCF) for compensating the dispersion slope of the optical fiber constituting the optical transmission line. ) Etc. are increasing.

FIGS. 3 to 6 show examples of the refractive index distribution used for each of the above-mentioned optical fibers.

FIG. 3 is an explanatory diagram showing an example of a refractive index distribution of a conventional optical fiber. Here, 31 is the first core, 3
2 is a second core, and 33 is a clad.

FIG. 4 is an explanatory view showing another example of the refractive index distribution of a conventional optical fiber. Here, 41 is a first core, 42 is a second core, 43 is a third core, and 44 is a clad.

FIG. 5 is an explanatory view showing still another example of the refractive index distribution of the conventional optical fiber. Where 5
1 is a first core, 52 is a second core, and 53 is a clad.

FIG. 6 is an explanatory view showing still another example of the refractive index distribution of a conventional optical fiber. Where 6
1 is a first core, 62 is a second core, 63 is a third core, 64
Is a cladding.

[0010]

Incidentally, each of the above optical fibers contains germanium as a dopant for increasing the refractive index and contains fluorine as a dopant for decreasing the refractive index. It is common. In general, in conventional optical fibers, the amount of dopant is adjusted to adjust the refractive index distribution.

However, the cladding of each of the above-mentioned optical fibers is often substantially made of silica (SiO ₂ ), and when the optical fiber is drawn at a high temperature, the difference in viscosity between the core and the cladding is high. After the wire becomes an optical fiber after drawing, a large residual stress is generated at the interface between the core and the clad or at the core portion adjacent to the clad.

The residual stress causes an increase in transmission loss and a shift in characteristics due to a change in the refractive index distribution during drawing. Therefore, it is necessary to minimize the residual stress in the optical fiber.

[0013]

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a method described below in which a refractive index distribution is relatively complicated and a change in characteristics at the time of drawing is prevented. It is to provide a small optical fiber.

According to a first aspect of the present invention, there is provided an optical fiber including a core and a clad, wherein a portion having a maximum viscosity at a high temperature has a single mode distance from the center of the optical fiber. It is characterized by being present in a region which is at least 1.3 times the mode field diameter at a specific wavelength within the operable wavelength range and does not include the outer surface of the optical fiber.

According to a second aspect of the present invention, in the first aspect, the wavelength range in which the single mode operation is possible is on a longer wavelength side than a wavelength of 1250 nm.

According to a third aspect of the present invention, in the first aspect, the specific wavelength in the wavelength range in which the single mode operation is possible is a wavelength of 1450 nm to 1650 nm.
In the range.

Here, since the mode field diameter of an optical fiber generally tends to increase as the wavelength becomes longer, a specific wavelength in a wavelength range in which a single mode operation can be performed is set to a wavelength band actually used for optical transmission. If the wavelength is the longest wavelength side (hereinafter referred to as a used wavelength band), it is possible to obtain an optical fiber in which the influence of residual stress is small in the entire used wavelength band.

If the location where the viscosity at the high temperature becomes maximum includes a region less than 1.3 times the mode field diameter at a specific wavelength in the wavelength range in which the single mode operation is possible, the characteristic due to the residual stress is reduced. It is not desirable because it becomes difficult to prevent the change. On the other hand, if the location where the viscosity at high temperature becomes maximum includes the outer surface of the optical fiber, tensile stress remains on the outer surface of the optical fiber, so if the outer surface of the optical fiber is damaged, it will break. It is not desirable because it is easy to perform.

Therefore, the location where the viscosity at the time of high temperature becomes maximum is 1.3 times the mode field diameter at a specific wavelength in the wavelength range in which the single mode operation is possible, and does not include the outer surface of the optical fiber. This makes it possible to obtain an optical fiber that is less affected by residual stress.

Further, by setting the wavelength range in which the single mode operation can be performed to a longer wavelength side than the wavelength of 1250 nm, an optical signal in a wavelength band around 1300 nm or a wavelength band around 1550 nm currently used in optical communication can be used. W
DM optical transmission becomes possible.

Further, by setting the specific wavelength in the wavelength range in which the single mode operation can be performed to be in the range of 1450 nm to 1650 nm, it is possible to more reliably obtain an optical fiber in which the influence of residual stress is small in the entire used wavelength band. .

That is, the optical fiber of the above solution is
Stress remaining in the core portion of the optical fiber is reduced, and the optical fiber has excellent characteristics.

[0023]

Embodiments of the present invention will be described with reference to the drawings. The first embodiment of the present invention is an optical fiber having a viscosity distribution at a high temperature as shown in FIG. Here, in FIG. 1, 11 is a first core, 12 is a second core, 13 is a first clad, 14 is a second clad, and 15 is a third clad. Here, high temperature means a temperature at the time of heating at the time of drawing, and specifically, 800
It means the temperature range above ℃.

By increasing the viscosity of the second cladding 14, stress during drawing is mainly applied to the second cladding 14, and residual stress is mainly applied to the interface adjacent to the second cladding 14 and the second cladding 14. , The stress remaining in the first core 11 and the second core 12 not adjacent to the second cladding 14 becomes relatively small.

Here, it is desirable that the inner diameter of the second cladding 14 is larger than 1.3 times the mode field diameter.
More preferably, it is larger than 1.5 times the mode field diameter. Further, it is desirable that the wavelength defining the mode field diameter is set to the wavelength on the longest wavelength side of the used wavelength band.

The smaller the cross-sectional area of the second cladding 14, the lower the heating temperature at the time of drawing can be. It is not desirable that the cross-sectional area of the two claddings 14 be smaller than twice the cross-sectional area of the core.
On the other hand, the larger the cross-sectional area of the second clad 14 is, the larger the area receiving the tension at the time of drawing becomes. However, it is necessary to increase the drawing heating temperature, so that the cross-sectional area of the second clad 14 is 100 times the cross-sectional area of the core portion. It is not desirable to make it larger.

That is, the sectional area of the second cladding 14 is
The cross-sectional area of the core portion is desirably 2 times to 100 times, and the cross-sectional area of the core portion is desirably 5 times to 50 times.

In this embodiment, the dispersion value is about -80 ps / nm.
/ Km DCF and actually produced a prototype. Here, the first core 11 is doped with germanium, and the second core 1 is doped with germanium.
2 is doped with fluorine, and the first to third claddings 13 to 15 are doped with chlorine, but the amount of chlorine doped in the second cladding 14 is the smallest. The inner diameter of the second cladding 14 was 1.5 times the mode field, and the cross-sectional area was about 50 times the cross-sectional area of the core.

As a result, the transmission loss at a wavelength of 1550 nm was reduced by about 0.002 dB / km on average, and the standard deviation indicating the variation in other characteristics was reduced to about half of the conventional value, thereby improving the yield.

Next, FIG. 2 shows a second embodiment of the present invention. The second embodiment of the present invention is an optical fiber having a viscosity distribution at a high temperature as shown in FIG. Here, FIG.
, 21 is a first core, 22 is a second core, 23 is a third core, 24 is a first clad, 25 is a second clad, 2
Reference numeral 6 denotes a third clad.

Here, the conditions of the inner diameter and the sectional area of the second clad 25 are the same as those of the second clad 14 of the first embodiment.

In this embodiment, the dispersion value is about -100 ps / n.
An m / km DCF was applied to make a prototype. here,
The first core 21 and the third core 23 are doped with germanium, and the second core 22 is doped with fluorine.
The cladding 24 to the third cladding 26 are doped with chlorine, but the amount of chlorine doped in the second cladding 25 is the smallest. The inner diameter of the second clad 25 was 1.5 times the mode field, and the sectional area was about 10 times the sectional area of the core.

As a result, the transmission loss at the wavelength of 1550 nm was reduced by about 0.005 dB / km on average, and the standard deviation indicating the dispersion of other characteristics was reduced to about half of the conventional value, thereby improving the yield.

In the above embodiments, chlorine is added as a dopant to the first to third claddings, but the dopant may be, for example, fluorine, germanium, aluminum, boron, phosphorus, etc. These dopants may be co-added.

In each of the above embodiments, the type of the dopant added to the first to third claddings is the same, but different dopants may be added respectively.

In each of the above embodiments, the dopant concentration of the second cladding is lower than that of the first cladding and the third cladding, but the second cladding is substantially free of the dopant. You may.

That is, what is important here is that the viscosity of the second clad is higher than the viscosity of the first clad and the viscosity of the third clad.

[0038]

According to the present invention, the stress remaining in the optical fiber, especially in the core after the drawing of the optical fiber is reduced, and an optical fiber having excellent characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating an example of a viscosity distribution of an optical fiber according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram illustrating an example of a viscosity distribution of an optical fiber according to a second embodiment of the present invention.

FIG. 3 is an explanatory diagram showing an example of a conventional refractive index distribution structure of an optical fiber.

FIG. 4 is an explanatory view showing another example of the refractive index distribution structure of the conventional optical fiber.

FIG. 5 is an explanatory view showing still another example of a conventional refractive index distribution structure of an optical fiber.

FIG. 6 is an explanatory view showing still another example of a conventional refractive index distribution structure of an optical fiber.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 1st core 12 2nd core 13 1st clad 14 2nd clad 15 3rd clad 21 1st core 22 2nd core 23 3rd core 24 1st clad 25 2nd clad 26 3rd clad

Claims (3)

[Claims] In an optical fiber including a core and a clad, a portion at which a viscosity at a high temperature becomes maximum is a distance from a center of the optical fiber to a specific wavelength range in which a single mode operation is possible. An optical fiber, wherein the optical fiber is present in a region not less than 1.3 times a mode field diameter at a wavelength and not including an outer surface of the optical fiber. 2. The optical fiber according to claim 1, wherein the wavelength range in which the single mode operation can be performed is on a longer wavelength side than a wavelength of 1250 nm. 3. The optical fiber according to claim 1, wherein the specific wavelength in the wavelength range in which the single mode operation can be performed is in a wavelength range of 1450 nm to 1650 nm.