Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-05-04
2001-10-09
Lee, John D. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S127000
Reexamination Certificate
active
06301419
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber applicable to large-capacity, high-speed WDM (Wavelength Division Multiplexing) optical transmission systems; and an optical transmission line including the same.
2. Related Background Art
A WDM optical transmission system enables large-capacity, high-speed optical communications since a plurality of signal lights (hereinafter referred to as WDM signals) in a
1.5-
&mgr;m wavelength band (1500 nm to 1600 nm) propagate there through by way of a network of optical fiber transmission lines. In general, this optical transmission system comprises an optical amplifier for optically amplifying the WDM signals together, and the like in addition to optical fiber transmission lines which are a transmission medium. In such WDM communications, various techniques are under study in order to enable further larger capacity and higher speed.
How to reduce the dispersion and dispersion slope has been an important subject for study with respect to optical transmission lines. It is because of the fact that, though being monochromatic, each signal light propagating through an optical transmission line has a predetermined bandwidth, whereby the signal light sent out from a transmitting station may deform its waveform when reaching a receiving station by way of the optical transmission line, thus deteriorating its reception.
For securing the quality of an optical transmission line, it is desirable that the dispersion of the optical transmission line in its signal light wavelength band be as small as possible. For realizing larger-capacity communications, on the other hand, it is necessary for the optical transmission line to suppress the dispersion in a wavelength band as wide as possible, and it is desirable that the dispersion slope of the optical transmission line be as small as possible. Therefore, dispersion-flattened optical fibers in which both the dispersion and dispersion slope are substantially zero in the
1.5-
&mgr;m wavelength band have conventionally been studied for use as an optical transmission line. Here, the dispersion slope refers to the gradient of a graph indicating the wavelength dependence of dispersion.
SUMMARY OF THE INVENTION
As a result of studies concerning the above-mentioned conventional techniques, the inventors have found the following problems.
Namely, as compared with typical single-mode optical fibers having a zero-dispersion wavelength near a wavelength of 1.3 &mgr;m, the wavelength band dispersion-flattened optical fibers tend to have a greater optical energy per unit cross-sectional area since their effective area is smaller, though yielding a smaller dispersion in the
1.5-
&mgr;m wavelength band. It means that nonlinear optical phenomena (four-wave mixing in particular) are relatively easily occur in the dispersion-flattened optical fibers. On the other hand, in an optical transmission system employing a dispersion-flattened optical fiber as an optical transmission line, it is necessary to reduce the power of signal light sent out from its transmitting station or repeater station, whereby the repeater spacing is inevitably shortened. It means that the number of stations to be installed would increase, whereby the optical transmission system to be realized becomes more expensive.
The optical transmission system employing a single-mode optical fiber as its optical transmission line and comprising a dispersion-compensating module for compensating for the dispersion of the optical transmission line is designed such that both the dispersion and dispersion slope are substantially zero in the
1.5-
&mgr;m wavelength band. Also, as the dispersion-compensating module, one having a negative dispersion in the 1.5-&mgr;m wavelength band is employed. Since the dispersion-compensating module is desired to have a smaller size, the dispersion-compensating fiber to be employed in the dispersion-compensating module is required to be designed to yield large absolute values of both dispersion and dispersion slope so that the dispersion of the whole optical transmission line can be compensated for by a short length of the dispersion-compensating fiber. Therefore, the effective area of the dispersion-compensating optical fiber is very small. Since the dispersion-compensating fiber is wound like a coil having a diameter on the order of 50 to 100 mm, how to reduce its bending loss is an important technical issue in such a dispersion-compensating fiber. Here, since the dispersion-compensating fiber employed in the dispersion-compensating module has a large dispersion value, it cannot be utilized as a main line though it constitutes a part of the optical transmission line.
For overcoming the problems such as those mentioned above, it is an object of the present invention to provide an optical transmission line which enables WDM communications in a wide signal light wavelength band (1.5-&mgr;m wavelength band) centered at a wavelength of 1.55 &mgr;m and effectively restrains nonlinear optical phenomena from occurring, and an optical fiber constituting a part of the optical transmission line. In the following, the optical fiber according to the present invention will be referred to as a “dispersion-equalizing optical fiber”, the use of which reduces dispersion and dispersion slope in the optical transmission line as a whole.
The dispersion-equalizing optical fiber according to the present invention is employed in a part of an optical transmission line which is a transmission medium for WDM communications between stations such as between a transmitting station and a receiving station, between repeater stations, between the transmitting station and a repeater station, between a repeater station and the receiving station, or the like. This dispersion-equalizing optical fiber is an optical fiber for reducing deviations in dispersion among wavelengths in its signal light wavelength band; and comprises a core region extending along a predetermined axis and a cladding region disposed on the outer periphery of the core region. The above-mentioned cladding region may comprise a depressed cladding structure constituted, at least, by an inner cladding, disposed on the outer periphery of the core region, having a refractive index lower than that of the core region; and an outer cladding, disposed on the outer periphery of the inner cladding, having a refractive index higher than that of the inner cladding.
In particular, the dispersion-equalizing optical fiber according to the present invention has a dispersion D (unit: ps
m/km) and a dispersion slope S (unit: ps
m
2
/km) satisfying the following conditions:
−83
≦D≦−
18
0.0050
×D≦S≦
0.0025
×D
with respect to light having a wavelength of 1.55 &mgr;m.
Further, as characteristics with respect to light having a wavelength of 1.55 &mgr;m, this dispersion-equalizing optical fiber has an effective area of 15 &mgr;m
2
or more, preferably 17 &mgr;m
2
or more, further preferably 19 &mgr;m
2
or more. Here, as disclosed 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 (1):
A
eff
=
2
π
(
∫
0
∞
E
2
r
ⅆ
r
)
2
/
(
∫
0
∞
E
4
r
ⅆ
r
)
(
1
)
where E is the electric field accompanying the propagating light, and r is the radial distance from the core center.
Since the dispersion D and dispersion slope S at the wavelength of 1.55 &mgr;m satisfy the above-mentioned conditions in this dispersion-equalizing optical fiber, when the ratio between the length of the dispersion-equalizing optical fiber and the length of a single-mode optical fiber having a zero-dispersion wavelength in the 1.3-&mgr;m wavelength band is appropriately adjusted, the respective absolute values of dispersion and dispersion slope in the whole optical transmission line constituted by the dispersion-equalizing optical fiber and single-mode optical fiber ca
Sasaoka Eisuke
Tanaka Shigeru
Tsukitani Masao
Lee John D.
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
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