JPH01147412A - Single mode optical fiber - Google Patents

Abstract

PURPOSE:To decrease the stress load on a core at the time of drawing and to lessen the increase in transmission loss by bending by lowering the softening point of a clad to the softening point lower than the softening point of the core and jacket and specifying the thickness of the jacket. CONSTITUTION:The clad 2 is formed to the softening point lower than the softening point of the core 1 and the jacket 3 and the thickness of the jacket 3 is specified to 2.5-10mum. Since the thickness of the jacket 3 is specified to 2.5-10mum, the increase in the transmission loss by bending is suppressed and since the jacket 3 is provided, the stress acting upon the core 1 at the time of drawing is decreased and the transmission loss is lessened even if the jacket is thin. The single mode optical fiber which has a low transmission loss value and lessens the increase in the transmission loss by bending is thereby obtd. at a high yield.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to improvements in single mode optical fibers.

[Prior art]

In recent years, optical fibers have been applied to many fields such as public communications, factory control systems, and electrical equipment (compact disc players, etc.).

By the way, optical fibers used for public communications have low loss.
Characteristics such as high reliability are required, but in particular single mode optical fibers used for long distance information transmission such as submarine cables have ultra-low loss in order to increase relay distance and reduce costs. are in demand. Specifically, ultra-low loss in the 1.55 μm band is desired.

Therefore, in order to obtain a single-mode optical fiber with ultra-low loss, the core is made only of pure silica glass without using dopants such as germanium or phosphorus, which are commonly used in ordinary optical fibers. A structure in which the cladding is doped with fluorine (shown in FIG. 4) has been proposed to prevent Rayleigh scattering.

However, in this structure, there is a difference in softening point of 200°C to 300'C (the cladding 2 is lower) between the core l made of pure silica glass and the cladding 2 doped with fluorine. Due to its presence, a large stress is applied to the core 1 when the optical fiber is drawn from the optical fiber base material, resulting in a problem that it is not possible to manufacture an ultra-low loss optical fiber with a high yield.

In order to reduce this stress load, it has been proposed to provide a jacket 3 made of pure silica glass on the outside of the cladding 2, as shown in FIG.

However, although the provision of the jacket 3 can reduce the stress applied to the core 1 during drawing, on the other hand, the leaky mode caused by the provision of the jacket 3 tends to cause an increase in loss due to bending. However, it could not be put to practical use.

[Object of the Invention] In view of the above-mentioned problems, an object of the present invention is to provide a single-mode optical fiber that reduces the stress load on the core during drawing and has less increase in transmission loss due to bending.

[Structure of the invention]

To achieve the above-mentioned object, the present invention provides a single-mode core, which has a core, a craft provided around the core and having a refractive index smaller than that of the core, and a jacket provided around the cladding and having a larger refractive index than the cladding. In the optical fiber, the softening point of the cladding is lower than that of the core and the jacket, and the thickness of the jacket is 2.
．． It is characterized by having a diameter of 5 μt to 10 μm.

[Embodiments of the invention]

The present inventor arrived at the present invention by prototyping and evaluating three types of single mode optical fibers as shown in Table 1 below. In addition, all of these samples are VAI
It was manufactured using the I method.

Here, the refractive index distribution of each sample is shown in the drawing below each sample number.

Table-1 Refractive index distribution Transmission loss Transmission loss due to bending Ca
55μm) increase (atl, 55μ latent) No. 1
0.187 dB/lv 0.03 dB/m
(Figure 4) 0.220 0.020.20
1 0.03 AV O,203AV O,03 No,2 (Figure 5) 0.185 1°200,
162 0.65 0, 177 1.90 AV 0.175 AV 1.25No,
3 (Figure 1) 0.176 0.030,
163 0.03 0, 180 0.02 AV 0.173 AV 0.03 The relative refractive index difference Δ- between the core 1 and cladding 2 of each sample is all 0.38%, and the mode field diameter is 9. 7μen,
The cutoff wavelength was 1.3 μm, and the increase in transmission loss due to bending was measured by winding the sample around a mandrel with a diameter of 1 cm.

As shown in Table 1 above, the optical fiber of sample No. 1 has a small increase in transmission loss due to bending, but the transmission loss itself is large, while the optical fiber of sample No. 2 has a small transmission loss, but the transmission loss due to bending is small. Large increase in loss,
On the other hand, in sample No. 3, both the value of transmission loss and the amount of increase in transmission loss due to bending are smaller than those of samples No. 41 and No. 2, which are representative examples of conventional single mode optical fibers.

From this result, the present inventor inferred as follows.

Sample No. 1, which is a so-called mantid clad type having the refractive index distribution shown in FIG. Since the difference in the softening points of the optical fibers is large, a large stress is applied to the core 1 when this optical fiber is drawn from the optical fiber base material. Specifically, sample No. 1 has a tension of 1.
When drawn at 0g, it is 1.6 xlO” g/μl11
Two stresses were applied to core 1. It is thought that this stress changes the glass structure, scattering light, and increasing transmission loss. However, since it is a matte clad type, there is no no-key mode, and the transmission is by letting it go (the increase in setting fire is small).

Sample No. 2, which has the refractive index distribution shown in FIG. The stress applied to the core 1 during wire drawing is relaxed. For example, when this sample is drawn with a tension of 10 g, the result is 1.7 Xl0-' g/μ
m'', and it was confirmed that the value was two fingers smaller than that of No. 1 above.

We believe that this is the reason why transmission loss can be reduced. However, since light leaks into the jacket 3 made of pure silica glass due to the leaky mode, there is a drawback that the amount of increase in transmission loss due to bending becomes large.

However, in sample No. 3, which corresponds to an embodiment of the present invention shown in FIG. 1, the thickness of the jacket 3 is 5 μm, compared to 17.5 μm in No. It is estimated that the increase in transmission loss due to bending could be suppressed since the curve was set to about 173. Moreover, although it is said to be thin, the provision of the jacket 3 reduces the stress applied to the core 1 during drawing, and the transmission loss is also small. Specifically, in this sample No. 3, the optical fiber base material was drawn with a tension of 10 g. However, the stress applied to core 1 is 5. I Xl0−”g/μm2, i.e.
It has been found that even though the jacket 3 is made thinner, the stress applied to the core 1 during drawing can be considerably reduced.

This relationship is shown in FIG.

As shown in Figure 3, the stress value applied to the core 1 and the increase in transmission loss due to bending have a contradictory relationship with respect to the thickness of the jacket 3, but it is possible to reduce the stress applied to the core 1 by about one order of magnitude. If possible, transmission loss can be effectively lowered,
Moreover, since there is no practical problem as long as the increase in transmission loss due to bending is about one order of magnitude smaller than that of sample No. 2, it was confirmed that the thickness of the jacket 3 should be set to 2.5 μm to 10 μm.

FIG. 2 shows another embodiment of the single mode optical fiber of the present invention shown in FIG. 1. In this optical fiber, the core 1 is also doped with fluorine, and everything except the jacket 3 is made of fluorine-doped quartz glass. It is composed of As a result of experiments, substantially the same effect as shown in FIG. 1 could be obtained with this type of device.

Further, if the jacket 3 has a higher softening point than the cladding 2, it is expected that the same effect as described above can be obtained.

Therefore, when the jacket 3 was constructed of 5402-TiO□-based glass or 5iOz Zr0z-based glass, the stress applied to the core 1 during wire drawing was reduced to 10-''g/μm g
The thickness of the jacket 3 could be lowered to an order of magnitude, and almost the same effect as when the jacket 3 was made of pure silica glass was obtained.

From the above results, the stress applied to the core during drawing is 10-"
A similar effect can be expected with a multi-mode optical fiber having a similar structure if it can be made on the order of g/μm2. Furthermore, in addition to reducing transmission loss, it is also possible to reduce the concentration of structural defects remaining in the core, so it is expected that radiation resistance characteristics will be improved.

[Effects of the Invention] As described above, according to the present invention, a single mode optical fiber having a small transmission loss value and a small increase in transmission loss due to bending can be obtained with a high yield.

[Brief explanation of the drawing]

FIGS. 1 and 2 show refractive index distributions of one embodiment and other embodiments of the single mode optical fiber of the present invention. Also the third
The figure is a graph showing changes in the stress applied to the core depending on the thickness of the jacket and changes in the increase in transmission loss due to bending. Figures 4 and 5 show the refractive index distribution of a conventional single mode optical fiber. . 1~Core 2~Clad 3~Jacket Patent Applicant
Furukawa Electric Co., Ltd. Figure 2 Jacket thickness (Pm) Figure 3

Claims (3)

[Claims] (1) In an optical fiber having a core, a cladding provided around the core and having a smaller refractive index than the core, and a jacket provided around the cladding and having a larger refractive index than the cladding, the softening point of the cladding is lower than that of the core and the jacket, and the thickness of the jacket is 2.5 μm to 10 μm. (2) The single mode optical fiber according to claim 1, wherein at least one of the core and the cladding contains fluorine as a dopant. (3) The jacket is made of pure silica glass, SiO_2-
The single mode optical fiber according to claim 1, characterized in that it is made of either TiO_2-based glass or SiO_2-ZrO_2-based glass.