Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-04-03
2001-05-08
Ullah, Akm E. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Reexamination Certificate
active
06229946
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a single-mode optical fiber used as a transmission line in optical communications or the like; and, in particular, to a dispersion-shifted optical fiber suitable for wavelength-division multiplexing (WDM) transmission.
2. Related Background Art
Conventionally, in optical communication systems employing single-mode optical fibers as their transmission lines, light in the wavelength band of 1.3 &mgr;m or 1.55 &mgr;m has often been utilized as signal light for communications. Recently, among others, the use of light in the wavelength band of 1.55 &mgr;m has been increasing from the viewpoint of reducing transmission loss in transmission lines. Single-mode optical fibers employed in such transmission lines for light in the wavelength band of 1.55 &mgr;m have been designed such that their wavelength dispersion (phenomenon in which pulse waves are broadened due to the fact that the propagation speed of light varies depending on wavelength) with respect to light in the wavelength band of 1.55 &mgr;m is nullified (to yield dispersion-shifted optical fibers having a zero-dispersion wavelength of 1550 nm).
Also, as long-haul transmission has become possible with the advent of WDM transmission or optical amplifiers in recent years, in order to reduce nonlinear phenomena (to suppress distortion of signal light), a dispersion-shifted optical fiber of a dual-shape core structure having an effective area A
eff
of 70 &mgr;m
2
or more, such as that shown in
FIG. 1A
, has been proposed (Japanese Patent Application Laid-Open No. 8-248251). Here, nonlinear optical effects refer to phenomena in which signal light pulses are distorted in proportion to 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, thereby restricting transmission speed or repeater spacing in relay transmission systems.
In such a dispersion-shifted optical fiber, as shown in
FIG. 1B
, the refractive index profile of its core region
10
is designed such that the power distribution PA(r) of propagating light attains its maximum value PA
max
at the center position O
x
(r=0) in the core region
10
. Further, at the outer periphery of the center part of the core region
10
having a higher refractive index, a peripheral area having a lower refractive index is provided. The introduction of peripheral area broadens the envelope of optical power distribution PA(r). Since the envelope is broadened, the effective area A
eff
becomes 70 &mgr;m
2
or more (so as to prevent the optical power from being concentrated at the center of the core), thereby reducing the above-mentioned nonlinear phenomena.
In the conventional dispersion-shifted optical fiber shown in
FIG. 1A
, the center part of the core region
10
with a higher refractive index has an outside diameter of 4.1 &mgr;m, and a relative refractive index difference of 0.94% with respect to its cladding region
20
. The peripheral area of the core region
10
with a low refractive index has an outside diameter of 31.5 &mgr;m, and a relative refractive index difference of 0.20% with respect to the cladding region
20
. When the refractive index profile of the core region
10
is thus designed, its effective area A
eff
becomes 70 &mgr;m
2
or more.
SUMMARY OF THE INVENTION
Having studied the conventional dispersion-shifted optical fiber, however, the inventors have found the following problems. Namely, as shown in
FIG. 1B
, the power distribution PA(r) of light propagating in the conventional dispersion-shifted optical fiber has such a form as the normalized power attains its maximum PA
max
at the center of the core region and decreases toward the outer periphery. Hence, when a dispersion-shifted optical fiber yielding such an optical power distribution employs a refractive index profile which can attain a greater effective area, the enveloe of the optical power distribution become wider, thereby weakening its function of confining light therein. As a result, the optical loss caused by macrobending or microbending (hereinafter simply referred to as bending loss) would increase, thereby making it difficult to transmit light over a long distance without attenuation.
In view of such problems of the prior art, it is an object of the present invention to provide a dispersion-shifted optical fiber comprising a structure which reduces optical loss, such as bending loss in particular, and restrains nonlinear phenomena from occurring.
In order to effectively restrain the above-mentioned nonlinear phenomena from occurring, the dispersion-shifted optical fiber according to the present invention has a core region extending along a predetermined axis, and a cladding region provided on the outer periphery of the core region and having such a refractive index profile that can yield an effective area of 70 &mgr;m
2
or more for one or more light signals whose center wavelengths fall within the range of about 1500 to 1600 nm. In particular, in order to obtain an effective area A
eff
of 70 &mgr;m
2
or more, the refractive index profile of the dispersion-shifted optical fiber according to the present invention is designed such that a peak position where the optical power of the light signals in thier fundamental mode is maximized is radially separated from the center of the core region by a predetermined distance.
Here, as disclosed in Japanese Patent Application Laid-Open No. 8-248251, the above-mentioned effective area A
eff
is given by the following expression (1):
A
eff
=2&pgr;(∫
0
∞
E
2
rdr
)
2
/(∫
0
∞
E
4
rdr
) (1)
where E is the electric field accompanying the propagating light, and r is the radial distance from the center of the core region.
On the other hand, the refractive index profile is represented by the relative refractive index difference &Dgr;n
i
given by the following expression (2):
&Dgr;
n
i
=(
n
i
−n
cd
)/
n
cd
(2)
where n
cd
is the average refractive index of the reference area (SiO
2
) in the cladding region, and n
i
is the average refractive index in each part i constituting the core region. Thus, the relative refractive index difference &Dgr;n
i
is expressed with reference to the average refractive index n
cd
of the reference area in the cladding region. In this specification, relative refractive index difference is expressed in terms of percentage, and an area having a negative relative refractive index difference indicates an area having a refractive index lower than that of the reference area.
When effective area is increased alone in order to restrain nonlinear phenomena from occurring, the envelope of optical power distribution would also be broadened, thereby making it difficult to reduce bending loss. Hence, the refractive index profile of the dispersion-shifted optical fiber according to the present invention is designed such that optical power remarkably decreases at a position separated, by a distance which is five times the center wavelength of the signal light, from the peak position where the optical power of the light signals is maximized.
The optical power distribution broadens as the wavelength of propagating light shifts to the longer wavelength side. Hence, using the signal light wavelength as a parameter, the optical power at a position radially separated from the peak position by a distance which is five times the signal wavelength is employed as a reference for evaluating bending loss. As a result of measurements, the inventors have confirmed that, in the case where the optical power at the evaluation reference position (position radially separated from the peak position by a distance which is five times the signal wavelength) is not greater than {fraction (1/100)} of the optical power at the peak position (maximum value), bending loss can remarkably be reduced (to about {fraction (1/20)}) as compared with the conventional dispersion-shifted optical fiber.
In order to sup
Kato Takatoshi
Sasaoka Eisuke
Pillsbury & Winthrop LLP
Sumitomo Chemical Company Ltd.
Ullah Akm E.
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