Optical fiber and colored optical fiber

Abstract

A low attenuation optical fiber having a core doped with Ge is offered. The optical fiber consists of a glass part and a covering part formed around the glass part. The glass part is made of silica glass and includes: a Ge-doped center core region; an optical cladding layer formed around the center core region; and an optical cladding layer formed around the cladding layer. The relationship of 1>32 holds, where 1, 2, and 3 are the relative refractive index differences of the center core region, the cladding layer, and the cladding layer 30, respectively with respect to pure silica glass. The average outer diameter of the glass part is in the range of 1250.5 m in the longitudinal direction, and 3 is in the range of 0.1 m to 0.5 m, where is the standard deviation of the outer diameter in the longitudinal direction.

Claims

1. An optical fiber comprising: a glass part made of silica glass and having a first outer diameter; and a covering part around the glass part, the glass part including: a center core region containing Ge and having a relative refractive index difference 1 with respect to pure silica glass; an optical cladding layer provided around the center core region, containing fluorine, and having a relative refractive index difference 2 with respect to the pure silica glass; and a jacket layer provided around the optical cladding layer and having a relative refractive index difference 3 with respect to the pure silica glass, wherein the relative refractive index differences have a relationship of 1>32, an average of the first outer diameter along the longitudinal direction being in the range of 1250.5 m, and 3 being in the range of 0.1 m to 0.5 M, where is a standard deviation of the first outer diameter along the longitudinal direction.

2. An optical fiber according to claim 1, wherein the relative refractive index difference 1 is in the range of 0.15% to 0.35% and the relative refractive index difference 2 is in the range of 0.15% to 0.00%.

3. An optical fiber according to claim 1, wherein the relative refractive index difference 3 is in the range of 0.05% to 0.05%.

4. An optical fiber according to claim 1, wherein the glass part has a fictive temperature of 1620 C. or less.

5. A colored optical fiber including an optical fiber as set forth in claim 1, wherein the covering part consists of two protective covering layers and an identification colored layer is provided around the covering part, the colored layer having a second outer diameter of 180 m or more and 210 m or less.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a cross-sectional view of the glass part of an optical fiber relating to an embodiment of the present invention.

(2) FIG. 2 is a cross-sectional view of the colored optical fiber relating to an embodiment of the present invention.

(3) FIG. 3 is a schematic diagram showing a refractive index profile of the glass part of the optical fiber of FIG. 1.

(4) FIG. 4 is a graph showing the relationship between the attenuation at 1.31 m and the variation in fluctuation of the outer diameter of a glass part.

(5) FIG. 5 is a graph showing the relation between the attenuation at 1.55 m and the variation in fluctuation of the outer diameter of a glass part.

DETAILED DESCRIPTION OF THE INVENTION

(6) Hereinafter, examples of an optical fiber and a colored optical fiber according to embodiments of the present invention will be described, referring to the accompanying drawings. It should be noted that the present invention is not limited to these examples but is shown by the claims and equivalents to the claims, including all modifications within the scope of claims.

(7) In an optical fiber which contains Ge in the core, the core exhibits compressive stress. Therefore, even if the residual stress of the core region is made smaller by adjusting viscosity differences between the core and the cladding so as to reduce the concentration of stress to the core as in the case of an optical fiber described in JP2002-148466A, it is not enough for reduction of Rayleigh scattering loss, and the effect of reducing the attenuation is small.

(8) FIG. 1 is a sectional view of glass part 1 of an optical fiber relating to an embodiment of the present invention. This optical fiber includes a glass part 1 and a covering part. The glass part 1, which is made of silica glass, includes a center core region 10, an optical cladding layer 20 formed around the perimeter of the center core region 10, and a jacket layer 30 formed around the perimeter of the optical cladding layer 20. A predetermined quantity of germanium as a dopant for controlling the refractive index is added to the center core region 10. The covering part is omitted in FIG. 1.

(9) FIG. 2 is a sectional view schematically showing the cross-sectional structure of a colored optical fiber relating to an embodiment of the present invention. This colored optical fiber is formed such that a colored layer 60 for identification is provided around the perimeter of the optical fiber shown in FIG. 1. The covering part of the optical fiber consists of two protective covering layers 40 and 50.

(10) FIG. 3 is a schematic diagram typically showing the refractive index profile of the glass part 1. The relative refractive index difference shown on the ordinate is based on the refractive index of pure SiO.sub.2, and defined by expressing the refractive-index difference in terms of percentage (%) with respect to pure SiO.sub.2. This optical fiber exhibits a relationship of 1>32, where 1 is the relative refractive index difference of the center core region 10, 2 is the relative refractive index difference of the optical cladding layer 20, and 3 is the relative refractive index difference of the jacket layer 30.

(11) FIG. 4 is a graph showing the relationship between the attenuation at 1.31 m and the variation in the fluctuation of the outer diameter of the glass part 1. Here, when the attenuation () (dB/km) of an optical fiber at a wavelength is approximated by the formula:
()=A/.sup.4+B+C(),
the first term, A/.sup.4 (dB/km), of the formula shows the Rayleigh scattering loss, which is caused by density fluctuation in the optical fiber. The coefficient A of the first term (henceforth, value A) is called Rayleigh scattering loss coefficient (dB/(km.Math.m.sup.4)), and the attenuation of the optical fiber is reduced as a whole by lowering the value A. The second term B of this formula, which shows a loss due to the structural disorder of the optical fiber, is a component independent of the wavelength . The third term C() of this formula shows other losses, such as OH absorption and infrared absorption on the long wavelength side.

(12) In FIG. 4, the ordinate shows ratios of fibers (percentage in length) of low attenuation fibers in which the attenuation .sub.1.31 was 0.32 dB/km or less at the wavelength of 1.31 m (1310 nm), that is, the index for defining the low attenuation fiber at the wavelength obtained from the measurement on 10,000 km optical fibers. Here, a horizontal dashed line is drawn at the level of 90% for indicating an acceptability criterion of not less than 90% as mass-production of optical fibers.

(13) On the other hand, the abscissa of FIG. 4 shows the value of 36, where expresses the standard deviation of the outer diameter of the glass part 1 in the case where the outer diameter of the glass part 1 is measured at intervals of 1 m with respect to the fiber length of 10 km. Here, the range in which 3 is from 0.1 m to 0.5 m is shown by two vertical dashed lines and the arrow between them.

(14) The lower limit of 0.1 m of the above-mentioned 3 is set as the minimum within the range of 3 in which .sub.1.310.32 dB/km is 90% or more. The upper limit 0.5 m in the above-mentioned range is set as the maximum within the range where .sub.1.310.32 dB/km is 90% or more and the requirement of the international communication standard (ITU-T), that is, the outer diameter of the glass part 1 must be within the range of 1251 m, is satisfied substantially (99% or more), when the average value is set to 1250.5 m in an actual production.

(15) At a drawing process in the manufacture of an optical fiber, the structural disorder (density fluctuation) of glass will decrease if the optical fiber is slowly cooled from a high temperature state so that the glass constituting the fiber may be put in a state closer to a low-temperature thermal equilibrium by promoting the structural relaxation. However, in such case, the fluctuation in the outer diameter of the glass part 1 will be larger. As described above, the Rayleigh scattering loss is caused by density fluctuation in an optical fiber, and the smaller the value A, the larger the fluctuation in outer diameter of the glass part 1 will be.

(16) As mentioned above, in the optical fiber in which Ge is added to the core, if 3 ( is the standard deviation in the longitudinal direction of the outer diameter of the glass part 1) is in the range of 0.1 m to 0.5 m, it is possible to obtain a low attenuation optical fiber in which the attenuation at the wavelength of 1310 nm is 0.32 dB/km or less and the outer diameter of the glass part 1 satisfies the requirement of the international standard.

(17) In an optical fiber in which Ge is added to the core, it is preferable that 1 be in the range of 0.15% to 0.35% and 2 be in the range of 0.15% to 0.00% in the case where the jacket layer 30 is pure silica glass or no dopants other than Cl are added. The lower limit of 2, that is 0.15%, is set on the basis of the limit that can be achieved by doping the cladding layer 20 with fluoride, using CF.sub.4 (carbon tetrafluoride) as a material.

(18) In the optical fiber in which Ge is added to the core, it is preferable that 3 be in the range of 0.05% to 0.05% in the case where the jacket layer 30 is made of silica glass which does not contain any dopants other than fluoride. Furthermore, in the optical fiber in which Ge is added to the core, it is preferable that the fictive temperature be 1620 C. or less. Here, the fictive temperature is a temperature of a super-cooled liquid the structure of which corresponds to the structure frozen in the glass and it can be an index for indicating the extent of structural relaxation in the glass. That is, it expresses the uniformity of glass structure, and the higher the fictive temperature, the larger the density fluctuation is. By promoting the structure relaxation, the Rayleigh scattering loss within the optical fiber decreases and the attenuation can be reduced.

(19) FIG. 5 is a graph showing the relationship between the attenuation at 1.55 m and the variation of fluctuation with respect to the outer diameter of the glass part 1. The ordinate and abscissas of FIG. 5 are the same as those of FIG. 4. As described above, the reduction in the value A affects the total wavelength. Therefore, if 3 is 0.1 or more and 0.5 or less, it is possible to obtain an optical fiber in which the attenuation .sub.1.55 at the wavelength of 1.55 m is 0.184 dB/km or less (i.e., an index for a low attenuation fiber at this wavelength), and in which the outer diameter of the glass part 1 satisfies the requirement of the international standard.

(20) It is preferable that the outer diameter of the colored layer be 180 m or more and 210 m or less in a colored optical fiber which has two ultraviolet-curing protective covering layers 40 and 50 coated around the perimeter of the glass part 1 including the core doped with Ge and which has a colored layer 60 for identification. In a colored optical fiber having an outer diameter of about 200 m by providing a thin coating, the micro bending loss is high as compared with the conventional colored optical fiber including a colored layer and having an outer diameter of about 250 m. However, as compared with the conventional colored optical fibers, the optical fiber according to the embodiment of the present invention has a lower Rayleigh scattering loss and an equivalent attenuation, so that it is capable of actual use. Therefore, it is possible to put the colored optical fibers of the present invention in a cable more than the conventional colored optical fibers.

(21) As described above, according to the embodiments of the present invention, regarding an optical fiber made of silica glass and including: a center core region doped with Ge; an optical cladding layer formed around the perimeter of the center core region; and a jacket layer formed around the perimeter of the optical cladding layer, it is possible to provide a low attenuation optical fiber and colored optical fiber, in which the Rayleigh scattering loss is lessened by controlling variation in fluctuation of the outer diameter of the glass part 1 within a given range while the relative refractive index differences of the center core region, the optical cladding layer, and the jacket layer are respectively maintained within the predetermined relationships with respect to pure silica glass.