Dispersion shifted optical fiber

Details Dispersion shifted optical fiber Dispersion shifted optical fiber

1998-03-19

2001-01-16

Lee, John D. (Department: 2874)

Optical waveguides

Optical fiber waveguide with cladding

Utilizing multiple core or cladding

C126S147000, C126S147000, C126S144000

Reexamination Certificate

active

06175680

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an optical fiber for long-haul transmissions, and in particular relates to a dispersion shifted optical fiber, in which optical signals having a wavelength band of 1.5 &mgr;m are used, suitable for long-haul and high bit rate transmissions.
BACKGROUND OF THE INVENTION
It is well known that SiO
2
-based single mode optical fibers having zero dispersion at a wavelength of 1.3 &mgr;m have a remarkably large chromatic dispersion of 15 ps/km·nm or more at a wavelength band of 1.5 &mgr;m, and are thus not suitable for high bit rate and long-haul optical transmissions at a wavelength of 1.5 &mgr;m.
As a means of overcoming this limitation, dispersion shifted optical fiber, in which a zero dispersion wavelength is shifted to 1.5 &mgr;m band having a dual-shaped core profile shown in
FIG. 5
, has been proposed as an optical fiber which enables high bit rate and long-haul optical transmissions.
OBJECT AND SUMMARY OF THE INVENTION
Usually, in a dispersion shifted optical fiber having a dual-shaped core (DSC) profile like this, a great deal of Ge is doped on the first core
11
at the center region of the fibre, and the cladding
13
is caused to have a large relative refractive index difference of 0.8 to 1.0%, wherein zero dispersion wavelength is shifted to a 1.5 &mgr;m band.
It is typical for a great deal of Ge (germanium) to be doped on the first core
11
, a lesser amount of Ge to doped on the second core
12
than is doped on the first core
11
, and a pure silica clad is provided. However, in such dispersion shifted optical fiber, optical scattering may occur resulting from a great deal of doped GeO
2
. Furthermore, the optical scattering is liable to cause an increase of transmission loss.
That is, a DSC type dispersion shifted optical fiber, in which a great deal of Ge is doped on the core, has a tendency of increasing the transmission loss.
Therefore, a DSC type dispersion shifted optical fiber is proposed, wherein F (fluorine), which has a comparatively small influence giving a transmission loss to the second core
12
and is able to lower the refractive index so that the relative refractive index difference can be increased even though Ge is doped on the first core
11
, and a great deal of F (fluorine) is doped on the clad
13
through the second core
12
.
However, in a dispersion shifted optical fiber of this type, the hardest second core
12
is under tension due to the amount of a dopant when carrying out fiber drawing, and this tension will likely increase the transmission loss.
That is, when carrying out fiber drawing, the first core
11
and clad
13
remain soft although the portion of the second core
12
, at which the amount of dopant is small, is in a hard state. Therefore, the first core
11
and clad
13
, which were soft, will be solidified after the second core
12
is solidified. Thus, a time lag occurs in the stage of solidification of the respective portions.
Due to minute sways which are produced in an optical fiber from a state where the optical fiber is still soft to a state where the same is solidified during fiber drawing, and changes of temperature distribution on the section of the optical fiber resulting from disorder of gas flows around the optical fiber when the same is solidified, unequal distortion may remain in the sectional direction on the second core
12
which has large tension. per sectional area. The distortion remaining in the second core
12
will cause an increase of polarization mode dispersion (PMD).
In order to solve the abovementioned problem, it is therefore an object of the invention to provide a dispersion shifted optical fiber which is able to suppress an increase of transmission loss generated when drawing fibers is carried out, and to suppress an increase of PMD, and has zero dispersion in a wavelength band of 1.5 &mgr;m.
The present invention is provided with the following means in order to solve the abovementioned shortcomings and problems.
An optical fiber according the invention is provided with the first core (refractive index n
1
), the second core (refractive index n
2
), the first clad (refractive index n
3
) and the second clad (refractive index n
4
) in the order from the center to outside, wherein the abovementioned refractive indexes are in a relationship of n
1
>n
4
>n
2
>n
3
, Ge which is at a ratio of 0.4 to 1.0% for the value of relative refractive index difference with respect to the second clad is doped on the first core, F which is at a ratio of 0.1 to 0.5% for the value of relative refractive index difference with respect to the second clad is doped on the second core, and the second clad is SiO
2
, which has a thickness from 1 &mgr;m or more to 40 &mgr;m or less.
Since an optical fiber according to the invention is such that the amount of doping of GeO
2
at the center of the first core is made less than ever, the optical scattering resulting from GeO
2
is lessened to cause an increase in transmission.
Furthermore, since the second clad is constructed of pure SiO
2
glass although F is doped on the second core and the first clad, the hardest second clad is able to sustain a majority of tension when fiber drawing is carried out, wherein an increase of PMD can be prevented since the second core will not receive any substantial unequal distortion in the cross-sectional direction, which may influence propagation of light, resulting from disturbance.


REFERENCES:
patent: 4822399 (1989-04-01), Kanamori et al.
patent: 63-217311 (1988-09-01), None
patent: 64-24040 (1989-01-01), None
patent: 64-24041 (1989-01-01), None
patent: 5-155639 (1993-06-01), None
patent: 9-218318 (1997-08-01), None

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