Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Utility Patent
1999-07-08
2001-01-02
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
C385S123000, C385S124000
Utility Patent
active
06169837
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a dispersion-flattened optical fiber suitable for a communication system using an optical fiber network.
BACKGROUND ART
Communication systems constituted by optical fiber networks are capable of long-haul and high-capacity communications, and their capacity has further been increased by time-division multiplexing transmission and wavelength-division multiplexing soliton transmission (see, for example, N. Edagawa, et al., “Long Distance Soliton WDW transmission using a dispersion-flattened fiber,” OFC '97, PD19). Such an optical communication system is constituted by a high-performance receiver and transmitter for transmitting and receiving signal light, an optical amplifier for optically amplifying the signal light, an optical fiber for transmitting the signal light, and the like. Among them, the optical amplifier, which is essential for obtaining a high S/N, has an optically amplifiable wavelength band ranging from 1530 nm to 1560 nm, thereby the wavelength of usable signal light is substantially limited within this wavelength bandwidth of 30 nm.
DISCLOSURE OF THE INVENTION
In an optical communication system using an optical amplifier, the strength of signal light in its optical fiber is intensified, thereby nonlinear optical phenomena such as four-wave mixing and self-phase modulation may occur within the optical fiber. Among them, the four-wave mixing can be restrained from occurring when the absolute value of dispersion at the wavelength of signal light is set to about 1 ps
m/km to 5 ps
m/km. On the other hand, while the mutual action between self-phase modulation and wavelength dispersion (hereinafter simply referred to as dispersion) is not problematic in the case of soliton transmission of single-wavelength signal light, it may yield the following problems in the case where a plurality of wavelength-multiplexed signal light components are transmitted. Namely, dispersion-shifted optical fibers whose zero-dispersion wavelength has been shifted to a wavelength of about 1550 nm usually have a wavelength dispersion slope (hereinafter simply referred to as dispersion slope) of about 0.07 ps
m
2
/km, thereby yielding different dispersion values depending on the wavelength of each signal light component. As a consequence, it may become out of balance with self-phase modulation, thus causing signal light pulses to collapse. Therefore, in such a case, a dispersion-flattened optical fiber having a very small dispersion slope is needed.
When the nonlinear refractive index of an optical fiber is N2, its effective cross-sectional area is A
eff
, the power of signal light is P, and the effective length of the optical fiber is L
eff
, the amount of occurrence of nonlinear phenomena in the optical fiber is given by the following expression (1):
N2.P.L
eff
/A
eff
(1)
In order to attain a high S/N by enhancing the signal light power P without increasing the amount of occurrence of nonlinear optical phenomena, a large effective cross-sectional area A
eff
is necessary. In the case of single-wavelength time-division multiplexing transmission, in order to restrain nonlinear optical phenomena from occurring, it is necessary that not only the effective cross-sectional area A
eff
be enhanced but also the absolute value of dispersion be suppressed to 1 ps
m/km or less, and the dispersion slope be made very small. Further, in order to keep the loss upon cabling optical fibers from increasing, it is required that bending loss be small. To this end, it is necessary for cutoff wavelength to be set to an appropriate value.
The above-mentioned nonlinear refractive index N2 is defined as follows. Namely, the refractive index N of a medium under strong light differs depending on the power of the light. Consequently, the lowest-order effect with respect to the refractive index N is represented by the following expression (2):
N=N0+N2.E
2
(2)
wherein
N0 is the refractive index with respect to linear polarization;
N2 is the nonlinear refractive index with respect to the third-order nonlinear polarization; and
E is the photoelectric field amplitude.
Under strong light, the refractive index N of the medium is given by the sum of the normal value N0 and the increment proportional to the square of the photoelectric field amplitude E. In particular, the constant of proportionality N2 (unit: m
2
/V
2
) in the second term is known as nonlinear refractive index.
As disclosed in Japanese Patent Application Laid-Open No.8-248251, the above-mentioned cross-sectional 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
)
wherein E is the electric field accompanying the propagated light, and r is the radial distance from a core center.
The dispersion slope is defined by the gradient of a graph showing the dispersion characteristic in a predetermined wavelength band.
As a result of studies concerning conventional dispersion-flattened optical fibers, the inventors have found the following problems. Namely, in the conventional dispersion-flattened optical fibers, though the dispersion slope is small, the effective cross-sectional area is only about 30 &mgr;m
2
to 40 &mgr;m
2
, thereby optical power density is high in their core region, thus making it easy for nonlinear optical phenomena such as four-wave mixing to occur intensively. As a consequence, the conventional dispersion-flattened optical fibers have not been suitable for wavelength-division multiplexing optical communication systems using optical amplifiers.
For example, the dispersion-flattened optical fiber disclosed in M. Ohashi, et al., “Dispersion-modified Single-Mode Fiber by VAD Method with Low Dispersion in the 1.5 &mgr;m Wavelength Region,” ECOC '88, pp. 445-448 comprises a core region composed of a center core (first core), a second core surrounding the center core, and a third core surrounding the second core; and a cladding surrounding the core region; while having a triple cladding type refractive index profile in which Ge element is added to the center core (first core) such that the relative refractive index difference of the center core with respect to the cladding is enhanced to 0.87%, F element is added to the second core such that the relative refractive index difference of the second core with respect to the cladding is lowered to −0.41%, and Ge element is added to the third core such that the relative refractive index difference of the third core with respect to the cladding is enhanced to 0.23%.
Though this conventional dispersion-flattened optical fiber attains a dispersion slope of 0.023 ps
m
2
/km at a wavelength of 1550 nm, its effective cross-sectional area A
eff
is only about 37 &mgr;m
2
.
Also, the dispersion-flattened fiber disclosed in Y. Kubo, et al., “Dispersion Flattened Single-Mode Fiber for 10,000 km Transmission System,” ECOC '90, pp. 505-508 comprises a core region composed of a center core and a second core surrounding the center core, and a cladding surrounding the core region; while having a W-shaped refractive index profile in which Ge element is added to the center core such that the relative refractive index difference of the center core with respect to the cladding is enhanced to 0.9%, and F element is added to the second core such that the relative refractive index difference of the second core with respect to the cladding is lowered to −0.4%.
Though this conventional dispersion-flattened optical fiber also attains a dispersion slope of 0.023 ps
m
2
/km at a wavelength of 1550 nm, its effective cross-sectional area A
eff
is 30 &mgr;m
2
or less.
On the other hand, though typical dispersion-shifted optical fibers have a relatively large effective cross-sectional area A
eff
of about 50 &mgr;m
2
, their dispersion slope is about 0.07 ps
m
2
/km, thus being greatly influenced by dispersion. Therefore, they are not suitable for long-distance optical
Hatayama Hitoshi
Kato Takatoshi
Nishimura Masayuki
Sasaoka Eisuke
Pillsbury Madison & Sutro LLP
Sanghavi Hemang
Sumitomo Electric Industries Ltd.
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