Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
1999-04-28
2001-08-14
Bovernick, Rodney (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
C385S142000, C385S144000
Reexamination Certificate
active
06275638
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a single-mode optical fiber used as a transmission line in optical communications, in particular, to a dispersion-shifted fiber suitable for wavelength-division multiplexing (WDM) transmission.
BACKGROUND ART
Conventionally, in optical communication systems employing a single-mode optical fiber as their transmission line, light in a 1.3-&mgr;m wavelength band or 1.55-&mgr;m wavelength band has often been utilized as signal light for communications. Recently, from the viewpoint of lowering the transmission loss in the transmission line, light in the 1.55-&mgr;m wavelength band has been increasingly used. Such a single-mode optical fiber employed in a transmission line for the light in the 1.55-&mgr;m wavelength band (hereinafter referred to as optical fiber for 1.55 &mgr;m) has been designed such that its wavelength dispersion (phenomenon in which pulse waves spread due to the fact that the propagating speed of light varies depending on wavelength) of the light in the 1.55-&mgr;m wavelength band becomes zero (yielding a dispersion-shifted fiber with a zero-dispersion wavelength of 1550 nm). As such a dispersion-shifted fiber, for example, Japanese Patent Application Laid-Open No. 62-52508 proposes a dispersion-shifted fiber having a dual shape core structure in which its core region is constituted by an inner core and an outer core having a refractive index lower than that of the inner core. Also, Japanese Patent Application Laid-Open No. 63-43107 and No. 2-141704 each propose a dispersion-shifted fiber having a depressed cladding/dual shape core structure in which its cladding region is constituted by an inner cladding and an outer cladding having a refractive index greater than that of the inner cladding. Further, Japanese Patent Application Laid-Open No. 8-304655 and No. 9-33744 each propose a dispersion-shifted fiber having a ring core structure.
On the other hand, in recent years, the advent of wavelength-division multiplexing transmission and optical amplifiers has enabled the realization of long-haul transmission. Hence, in order to avoid nonlinear optical effects, there has also been proposed a dispersion-shifted fiber employing the above-mentioned dual shape core structure, depressed cladding/dual shape core structure, or the like, with the zero-dispersion wavelength shifted to a wavelength shorter or longer than the center wavelength of the signal light (Japanese Patent Application Laid-Open No. 7-168046 and U.S. Pat. No. 5,483,612). Here, the nonlinear optical effects refer to phenomena in which, due to nonlinear phenomena such as four-wave mixing (FWM), self-phasemodulation (SPM), cross-phasemodulation (XPM), and the like, signal light pulses are deformed in proportion to the density in light intensity or the like. These effects become factors for restricting transmission speed or the repeater spacing in a relaying transmission system.
In the above-mentioned dispersion-shifted fibers proposed for wavelength-division multiplexing transmission, their zero-dispersion wavelength is set to a value different from the center wavelength of the signal light, thereby restraining the nonlinear optical effects from occurring, or their effective area A
eff
is elongated so as to reduce the density in light intensity, thereby restraining the nonlinear optical effects from occurring.
In particular, in the dispersion-shifted fiber shown in the above-mentioned Japanese Patent Application Laid-Open No. 8-304655 or No. 9-33744 employing a ring core structure, the dispersion slope is made smaller, whereas the effective area A
eff
is made greater, thus realizing a fiber characteristic suitable for wavelength-division multiplexing transmission.
Here, the effective area A
eff
is, as disclosed in Japanese Patent Application Laid-Open No. 8-248251, given by the following expression (1):
A
eff
=2&pgr;(∫
0
∞
E
2
rdr
)
2
/(∫
0
∞
E
4
rdr
) (1)
wherein E is the electric field accompanying the propagated light, and r is the radial distance from a core center.
On the other hand, the dispersion slope is defined by the gradient of the graph indicating the dispersion characteristic in a predetermined wavelength band.
DISCLOSURE OF THE INVENTION
Having studied the conventional dispersion-shifted fibers, the inventors have found the following problems to overcome. Namely, in the above-mentioned dispersion-shifted fiber comprising a structure for effectively restraining the nonlinear optical effects from occurring, there is a problem that a dispersion-shifted fiber whose transmission loss is suppressed to a desired level or lower may not be obtained with a good reproducibility. That is, in the dispersion-shifted fiber of Japanese Patent Application Laid-Open No. 8-304655 or No. 9-33744 employing a ring core structure for suppressing the nonlinear optical effects, the thickness of the outer core (difference between the outer core radius and the inner core radius) is very small, i.e., about 1 to 2 &mgr;m. On the other hand, the average difference between the average relative refractive index difference of the average outer core and the average relative refractive index difference of the inner core is considerably large, i.e., about 1%. For increasing the relative refractive index difference of the outer core, the amount of GeO
2
added to the outer core has been increased in general. Increasing the amount of GeO
2
decreases, inversely, the viscosity of the outer core at a drawing temperature during the making of the optical fiber, however. When viewed along a diametrical direction of the optical fiber being manufactured, the change in viscosity abruptly occurs within the area of the outer core (having a thickness of about 1 to 2 &mgr;m). Such an abrupt change in viscosity in the diametrical direction causes, upon drawing of the optical fiber, an abrupt change in the tensile force applied thereto in the diametrical direction. The abrupt diametrical change in the drawing tension thus applied becomes a cause of structural mismatching or glass defect at the boundary between the inner and outer cores, thereby allowing the resulting optical fiber to increase transmission loss.
In order to overcome the problems such as those mentioned above, it is an object of the present invention to provide a dispersion-shifted fiber for WDM transmission suitable for a long-haul submarine cable or the like, which effectively restrains nonlinear optical effects from occurring, effectively suppresses transmission loss caused by structural mismatching, glass defect, or the average like, and has a structure excellent in reproducibility.
The dispersion-shifted fiber according to the present invention is a transmission medium (for example, silica(SiO
2
) based single-mode (SM) optical fiber), comprising a core region extending along a predetermined axis and a cladding region disposed on an outer periphery of the core region, for propagating signal light in a 1.55-&mgr;m wavelength band (i.e., at least one signal light component having a center wavelength within the wavelength range of 1500 nm to 1600 nm). In this dispersion-shifted fiber, the core region, includes, an inner core which is a glass area, doped with a predetermined amount of fluorine (F), having a first average relative refractive index difference &Dgr;n
1
with respect to a predetermined region (reference region) of the cladding region; and an outer core which is a glass area doped with a predetermined amount of germanium oxide (GeO
2
) and disposed between the inner core and the cladding region, having a second average relative refractive index difference &Dgr;n
2
greater than the first average relative refractive index difference &Dgr;n
1
with respect to the predetermined region of the cladding region. Here, the predetermined region is defined by a single layer in the case that the cladding region is constituted by the single layer, and is also defined by the outermost layer in the case that the cladding region is constituted by a plurality of layers.
In the dispersion
Kato Takatoshi
Sasaoka Eisuke
Urano Akira
Yokoyama Yoshio
Bovernick Rodney
Song Sarah N
Sumitomo Electric Industries Ltd.
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