Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-10-30
2003-03-11
Nasri, Javaid (Department: 2839)
Optical waveguides
Optical fiber waveguide with cladding
Reexamination Certificate
active
06532331
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a single-mode optical fiber usable as a long-haul transmission line for optical communications and the like; and, in particular, to a dispersion-shifted fiber suitable for large-capacity optical communications-such as wavelength division multiplexing (WDM) transmission and the like.
2. Related Background Art
Conventionally, light in a 1.3-&mgr;m wavelength band or 1.55-&mgr;m wavelength band has often been utilized as light signals for communications in optical communications systems employing single-mode optical fibers as their transmission lines. Recently, however, the use of light in 1.55-&mgr;m wavelength band has been increasing from the viewpoint of lowering transmission loss in transmission lines. Single-mode optical fibers employed in such a transmission line for light in the 1.55-&mgr;m wavelength band (hereinafter referred to as 1.55-&mgr;m single-mode optical fibers) have been designed such that their chromatic dispersion (phenomenon in which pulse waves widen because of the fact that the propagation speed of light varies depending on wavelength) with respect to light in the 1.55-&mgr;m wavelength band becomes zero (so as to yield dispersion-shifted fibers having a zero-dispersion wavelength of 1.55 &mgr;m).
As such a dispersion-shifted fiber, Japanese Patent Application Laid-Open No. HEI 8-304655 (U.S. Pat. No. 5,613,027) and U.S. Pat. No. 5,659,649, for example, propose a dispersion-shifted fiber having a refractive index profile of a ring-like core structure, whose core region is constituted by an inner core and an outer core having a higher refractive index than the inner core. Also, Japanese Patent Application Laid-Open No. HEI 8-248251 (European Patent Publication No. 0 724 171 A2) and Japanese Patent Application Laid-Open No. HEI 9-33744 propose a dispersion-shifted fiber having a refractive index profile of a dual ring core structure, whose core region is constituted by a first core, a second core having a higher refractive index than the first core, a third core having a lower refractive index than the second core, and a fourth core having a higher refractive index than the third core.
On the other hand, Japanese Patent Application Laid-Open- No. SHO 63-43107 and Japanese Patent Application Laid-Open No. HEI 2-141704 propose a depressed cladding structure whose cladding region is constituted by an inner cladding and an outer cladding having a higher refractive index than the inner cladding.
In recent years, the advent of wavelength division multiplexing (WDM) transmission and optical amplifiers has further enabled long-haul transmission, and various improvements have been made as to optical fibers such as those mentioned above in order to avoid nonlinear phenomena. Here, nonlinear optical effects refer to phenomena in which light signal pulses distort in proportion to the density of light intensity or the like due to nonlinear phenomena such as four-wave mixing (FWM), self-phase modulation (SPM), cross-phase modulation (XPM), and the like, and become a factor restricting the transmission speed and the repeater spacing in repeating transmission systems.
SUMMARY OF THE INVENTION
In general, the amount of occurrence of nonlinear phenomena has been known to be proportional to the amount of change in refractive index given by the following expression (1):
(
N
2
/A
eff
)×
P
(1)
where N
2
is the nonlinear refractive index (unit: m
2
/W) , A
eff
is the effective area (unit: &mgr;m
2
), and P is the optical power.
Here, the nonlinear refractive index N
2
is defined as follows. Namely, the refractive index <N> of a medium under strong light varies depending on the optical power. Therefore, the lowest-order effect on this refractive index <N> is given by the following expression (2):
<
N>=<N
0
>+<N
2
>·I
(2)
where <N
0
> is the refractive index with respect to linear polarization, <N
2
> is the nonlinear refractive index with respect to nonlinear polarization, and I is the light intensity. Under strong light, the refractive index <N> of the medium is given by the sum of its normal value <N
0
> and the increase proportional to the light intensity. In particular, the constant of proportion <N
2
> (unit: m
2
/W) in the second term is known as nonlinear refractive index.
On the other hand, as shown in Japanese Patent Application Laid-Open No. HEI 8-248251 (EP 0 724 171 A2), the effective area A
eff
is given by the following expression (3):
A
eff
=
2
π
(
∫
0
∞
E
2
r
ⅆ
r
)
2
/
(
∫
0
∞
E
4
r
ⅆ
r
)
(
3
)
where E is the electric field accompanying the propagating light, and r is the radial distance from the core center (the center axis of the optical fiber).
Each of the above-mentioned conventional optical fibers has been designed so as to enhance the effective area A
eff
in order to suppress the occurrence of nonlinear phenomena. However, the inventors have studied the conventional optical fibers and, as a result, have found problems as follows. Namely, there is inevitably a limit to the enhancement of effective area A
eff
since it increases the transmission loss upon bending the optical fiber at a predetermined radius (hereinafter referred to as macrobending loss) and the transmission loss due to an external pressure (side pressure) applied to the side face of the optical fiber (hereinafter referred to as microbending loss). Though the microbending loss of optical fibers having a refractive index profile of a ring-like core structure is lower than that of optical fibers having other refractive index profiles such as those of dual ring core and multilayer core structures in general, there is still an upper limit to the above-mentioned enhancement. Thus, the increase in macrobending loss and microbending loss along with the enhancement of effective area A
eff
is an essential problem of optical fibers, which is inescapable.
In order to overcome problems such as those mentioned above, it is an object of the present invention to provide an optical fiber comprising a structure which effectively suppresses the occurrence of nonlinear phenomena without increasing transmission loss such as macrobending loss.
The optical fiber according to the present invention is suitable for a single-mode optical fiber which is mainly composed of silica glass and which has a core region extending along a predetermined axis and a cladding region provided on the outer periphery of the core region. Also, in view of its application to the wavelength division multiplexing transmission and the like in recent years, the optical fiber according to the present invention has, with respect to light having a wavelength of 1550 nm, a dispersion with an absolute value of 1.0 to 4.5 ps
m/km and an effective area of 70 &mgr;m
2
or more, and has a cutoff wavelength of 1.3 &mgr;m (1300 nm) or more at a fiber length of 2 m.
Specifically, as shown in
FIGS. 1A and 1B
, the optical fiber according to the present invention comprises, at least, a center region
101
extending along a center axis and having a predetermined refractive index; a first annular region
102
provided on the outer periphery of the center region
101
and having a higher refractive index than the center region
101
; and a second annular region
103
provided on the outer periphery of the first annular region
102
and having a lower refractive index than the first annular region
102
.
In particular, in order to lower the nonlinear refractive index N
2
so as to suppress the occurrence of nonlinear phenomena (see the above-mentioned expression (1)), contents of fluorine, which is a refractive index reducing dopant, in the second annular region
103
is adjusted such that the refractive index thereof radially decreases from the center of the optical fiber
100
. Also, in the optical fiber according to the present invention, the radi
Kato Takatoshi
Onishi Masashi
Sasaoka Eisuke
Nasri Javaid
Sumitomo Electric Industries Ltd.
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