JPH04362603A - Low-loss optical fiber - Google Patents

Abstract

PURPOSE:To reduce the tension which remains in an optical fiber core at the time of drawing and to obtain the low-loss optical fiber by providing a core part and a clad part which surrounds the core part and equalizing the softening points of the core part and clad part to each other. CONSTITUTION:Residual stress is generated by drawing the core 1 and clad 2 which differ in softening point temperature with a large tensile force. For the reduction of the residual stress, the core 1 and clad 2 are drawn with a small tensile force or equalized in softening point temperature. It is, however, difficult to draw the core 1 and clad 2 with the small tensile force since transmission loss varies in the length direction of the optical fiber owing to variation in the tension. For the purpose, the core part 1 and clad part 2 are equalized in softening point temperature to eliminate the remaining stress in the core clad which causes an increase in loss after the drawing and also make structure irregularity loss small. Here, GeO2, P2O5, etc., which can reduce only the softening point temperature without exerting any adverse influence upon transmission characteristics are suitable as a dopant.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-loss silica-based optical fiber.

0002

2. Description of the Related Art For future ultra-long-distance, high-capacity optical transmission systems, efforts are being made to reduce the loss of silica-based optical fibers. GeO2 in the core has been put into practical use so far.
There are SiO2-clad single-mode optical fibers doped with SiO2, and F.SiO2-clad single-mode optical fibers using SiO2 in the core.

[0003] The manufacturing method for optical fiber doped with GeO2 in the core has been established, and it has a 0.2 dB performance in the 1.55 μm wavelength band.
A transmission loss of less than /km is available. On the other hand, Si
Optical fibers using O2 as a core have a small Rayleigh scattering coefficient, and are therefore expected to have the lowest loss among silica-based optical fibers. So far, a value close to the theoretical limit of 0.154 dB/km has been obtained at a wavelength of 1.55 μm. FIG. 3(a) shows the softening point temperature distribution of a conventional F.SiO2 clad single mode optical fiber using SiO2 in the core, and FIG. 3(b) shows its refractive index distribution. The core has a softening point temperature of 1580 degrees, and the cladding has a softening point temperature of about 1400 degrees. However, when this optical fiber is solidified from a molten state in the drawing process, the SiO2 glass core, which has a high softening point temperature, solidifies first, and the drawing tension is borne by the small diameter core portion, and the residual stress is frozen. As a result, transmission loss increases as the drawing tension increases. Furthermore, when tension is applied, losses due to structural irregularities at the core-clad interface caused by wire drawing become larger. Figure 4 shows the relationship between wire drawing tension and transmission loss. Here, the effects of residual stress and structural irregularities are added. Increasing the drawing tension in this way increases the transmission loss. When drawing with a tension of 95g, the transmission loss is 1.
It becomes 39dB/km. The average transmission loss of currently manufactured optical fibers is about 0.18 dB/km, which is considerably larger than the theoretical limit.

0004

It has been reported that the transmission loss of an F.SiO2 clad single mode optical fiber using SiO2 in the core varies depending on the drawing conditions as described above.

An object of the present invention is to reduce the tension remaining in the optical fiber core during drawing, and to provide a low-loss optical fiber.

[0006]

[Means for Solving the Problems] In order to achieve the above object, the present invention has a core portion and a cladding portion surrounding the core portion, and the softening point temperature of each of the core portion and the cladding portion is Characterized by equality.

[0007] Here, at least one of the core portion or the cladding portion contains at least one of GeO2, P2O5, F, and Al2O3 as a dopant for adjusting the refractive index and softening point temperature.

[0008]

[Operation] Residual stress is generated by drawing a core and cladding with different softening point temperatures under high tension. In order to reduce residual stress, it is necessary to draw the wire at low tension or to match the softening point temperature of the core cladding. When drawing with low tension, it is difficult to draw the optical fiber because the transmission loss changes in the longitudinal direction of the optical fiber as the tension changes.

[0009] In the present invention, the softening point temperatures of the core part and the cladding part are made to be the same, so that stress in the core cladding, which causes increased loss after drawing, does not remain and structural irregularity loss is also reduced. It is. As dopants for this purpose, GeO2, P2O5, F, Al2 O3 can reduce only the softening point temperature without adversely affecting the transmission characteristics.
etc. are suitable.

As is well known, the refractive index of SiO2 increases by adding GeO2, P2 O5 and Al2O3, and decreases by adding F. These additions lower the softening point temperature of SiO2. Therefore, by adding at least one of these dopants to at least one of the core and the cladding, a desired relative refractive index difference can be given to the core and the cladding, and the softening point temperatures of both can be made equal.

[0011] By matching the softening point temperatures of the core and cladding parts, the tension during drawing that was concentrated in the SiO2 core is also dispersed in the cladding. Therefore, the tension remaining in the core after drawing is small, and the optical fiber transmission loss after drawing is constant regardless of the drawing tension. Also,
Structural irregularity loss at the core-clad interface that occurs during optical fiber drawing can also be reduced by adjusting the softening point temperature of the core-clad.

0012

EXAMPLES The present invention will be explained below with reference to Examples, but the present invention is not limited to these Examples in any way.

Example 1 FIG. 1(a) shows the softening point temperature of an example of the optical fiber of the present invention, and FIG. 1(b) shows its refractive index distribution. In this example, an optical fiber preform was manufactured using the VAD method. P2O5 is added to the core part, and the relative refractive index difference Δ(Δ=(n-n2)/n2)×10 with respect to SiO2
0, n is the refractive index of the core or cladding, n2 is SiO
2) is 0.1%, the cladding part is doped with F, and the relative refractive index difference Δ with respect to SiO2 is -0.2%.
Met. The softening point temperature is approximately 1 for both the core and cladding parts.
It was 420 degrees.

FIG. 2 shows the results of measuring the transmission loss of this optical fiber at a wavelength of 1.55 μm by varying the drawing tension. If the softening point temperatures of the core and cladding are matched, no stress will remain in the core. Therefore, as shown in FIG. 2, the transmission loss of the optical fiber does not depend on the tension during drawing. In addition, when the Rayleigh scattering coefficient was determined from the loss wavelength characteristics, it was 0.65 dB/km・μm4, and SiO2
Rayleigh scattering coefficient of optical fiber using only
．． It was smaller than 75dB/km・μm4. The structural irregularity loss between the core and cladding was determined from the loss wavelength characteristics and was found to be 0.01 dB/km, which was a constant value that did not depend on the tension during drawing.

Example 2 An optical fiber preform was produced using the VAD method. GeO2 was added to the core part, and the relative refractive index difference Δ with respect to SiO2 was set to 0.1%. On the other hand, F was added to the cladding part, and the relative refractive index difference Δ with respect to SiO2 was set to -0.2%. The softening point temperature of both the core part and the clad part was 1420 degrees. This base material is heated in a carbon furnace for approximately 2
When the wire was drawn at a temperature of 1,000 degrees Celsius and the transmission loss was measured, it showed a good value of 0.165 dB/km at a wavelength of 1.55 μm. Changing the tension applied to the optical fiber during drawing did not change the transmission loss.

In the above embodiments, P2 was used as the dopant.
The case where O5, F, and GeO2 are used has been described, but if a small amount of Al2O3 is used as another dopant, it is effective because it has the effect of lowering the softening point temperature. Furthermore, it is also possible to use not only a single dopant, but also a plurality of dopants at the same time. In addition, in the above embodiments, only the case where the core and cladding have the same softening point temperature has been explained, but even if the core softening point temperature is lower than the cladding softening point temperature, if the area ratio of the core and cladding increases, Since most of the tension during drawing remains in the cladding, it has less influence on transmission characteristics than when the core's softening point temperature is higher than the cladding's softening point temperature.

[0017]

[Effects of the Invention] As explained above, according to the present invention,
By matching the softening point temperatures of the core cladding, the stress concentrated in the core can be reduced. the result,
Optical fibers with constant transmission loss can be produced without depending on the tension during drawing.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the softening point temperature distribution and refractive index distribution of the optical fiber of the present invention.

FIG. 2 is a diagram showing transmission loss when the drawing tension of the optical fiber of the present invention is changed.

FIG. 3 is a diagram showing the softening point temperature distribution and refractive index distribution of a conventional optical fiber.

FIG. 4 is a diagram showing transmission loss when the drawing tension of a conventional optical fiber is changed.

[Explanation of symbols]

1 Core 2 Clad

Claims (2)

[Claims] 1. A low-loss optical fiber comprising a core portion and a cladding portion surrounding the core portion, the core portion and the cladding portion having the same softening point temperature. 2. At least one of the core portion and the cladding portion contains GeO2, P2O5, F, and A as a dopant for adjusting the refractive index and softening point temperature.
The low-loss optical fiber according to claim 1, characterized in that it contains at least one type of l2O3.