Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2002-06-17
2004-11-02
Bruce, David V. (Department: 2882)
Optical waveguides
Optical fiber waveguide with cladding
C385S028000, C398S081000
Reexamination Certificate
active
06813425
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a fiber optic cable used in a wavelength division multiplexing (WEM) optical transmission system, and more particularly to a fiber optic viable capable of suppressing influences, caused by a non-linearity of optical fibers, to a maximum while controlling dispersion characteristics to an appropriate level in order to obtain a maximum transmission capacity per optical fiber. Also, the present invention relates to WDM optical transmission system using the fiber optic cable, and more particularly to a WrM optical transmission system which can operate efficiently even when it uses a reduced channel spacing for an increase in transmission capacity.
BACKGROUND ART
Optical transmission techniques using optical fibers have rapidly been developed in that they can transmit a large quantity of data within a short period of time while involving a reduced transmission loss. In particular, such optical transmission techniques have further been advanced by virtue of development of a new optical fiber capable of transmitting signals for a long distance.while involving a reduced signal loss, and development of a superior light source such as a semiconductor laser.
However, known optical fibers involve a chromatic dispersion, that is, a phenomenon in which a signal is spread due to a difference in group velocity among the wavelength components of the signal. Due to such a chromatic dispersion, a signal overlap occurs at the receiving terminal, thereby resulting in a fatal problem such as an impossibility of demodulation. For this reason, attempts to minimize such a chromatic dispersion (hereinafter, simple referred to as a “dispersion”) have been made. By virtue of such attempts, it has been found that a zero dispersion is achieved at an operating wavelength of 1,310 nm.
Meanwhile, it has been found, on the basis of the relation between the total loss and the wavelength in an optical fiber, that a minimum signal loss is exhibited at a wavelength of 1,550 nm even though an increased dispersion occurs, as compared to that occurring at 1,310 nm. In this connection, the operating wavelength of 1,550 nm could be used by virtue of the development of a new optical amplifier capable of amplifying the wavelength range of 1,530 nm to 1,565 nm. As a result, a non-repeating long distance transmission has been possible. This has resulted in the advent of a dispersion-shifted fiber (DSF) adapted to shift the zero dispersion from the wavelength of 1,310 nm, at which the zero dispersion is achieved in conventional cases, to the wavelength of 1,550 nm in order to obtain a minimum dispersion and a minimum signal loss.
In addition to such a development of optical fibers, a WDM system has been developed, which serves to multiplex a plurality of optical signals having different wavelengths so as to simultaneously transmit those optical signals through a single optical fiber. Using such a WDM system, it is possible to more rapidly transmit an increased amount of data. An optical communication system using a WDM scheme at a wavelength of 1,550 nm has already been commercially available.
Where the above mentioned DSF is used in such a WDM optical transmission. system, however, a signal distortion may occur even though a desired zero dispersion may be achieved. This is because the zero dispersion in the optical fiber may result in a non-linearity of the optical fiber, for example, a four-wave mixing in which lights of different wavelengths may be mixed together, even though a reduced signal distortion is obtained.
In particular, the most practical method usable in the WDM optical transmission system for a further increase in transmission capacity is to increase the number of channels used. In order to increase the number of channels used, however, it is necessary to use a reduced channel spacing because optical amplifiers use a limited amplification band. Such a reduced channel spacing may result in a more severe problem associated with the non-linearity of the optical fiber such as the four-wave mixing.
The non-linearity of an optical fiber is reduced at an increased channel spacing or an increased dispersion of the optical fiber. However, where the dispersion of the optical fiber increases, a degradation in transmission quality occurs inevitably due to a distortion of optical signals resulting from the increased dispersion.
Therefore, it is necessary to control the dispersion of the optical fiber in order to obtain a maximum transmission capacity of the WDM optical transmission system. In other words, an excessively high dispersion results in an increased signal distortion whereas an excessively low dispersion approximating to the zero dispersion results in a non-linearity of optical signals such as the four-wave mixing phenomenon, thereby generating a signal degradation. In this regard, it has been strongly required to develop an optical fiber capable of solving both the problem resulting from the dispersion and the problem resulting from the non-linearity.
U.S. Pat. No. 5,327,516 issued on Jul. 5, 1994 discloses an optical fiber for a WDM system which exhibits a dispersion ranging from 1.5 ps
m-km to 4 ps
m-km at a wavelength of 1,550 nm in order to achieve a suppression in non-linearity. The optical fiber disclosed in this patent is called a “non-zero dispersion-shifted fiber (hereinafter, referred to as an “NZ-DSF”) in that it is configured to obtain a non-zero dispersion. Such an optical fiber is commercially available from Lucent Technologies In., U.S.A.
The NZ-DSF is significant in that it can suppress the four-wave mixing phenomenon by virtue of its dispersion value ranging from 1.5 ps
m-km to 4 ps
m-km. However, where a long-distance transmission is carried out using such an NZ-DSF or an increased number of channels, it is difficult to compensate for a surplus dispersion increased by a dispersion slope on the increased number of channels even though the dispersion accumulated on one channel may be compensated for by use of a dispersion compensation module (DCM) with a high negative dispersion value.
Furthermore, the NZ-DSF exhibits a relatively low dispersion value while having a relatively small effective area of 55 &mgr;m
2
(in the case of a single-mode optical fiber, it has an effective area of about 80 &mgr;m
2
. Since the effective area of the optical fiber is the actual area of an optical signal within the optical fiber, the optical signal has a reduced density for the same optical power as the optical fiber has an increased effective area. At a reduced density of the optical signal, the optical fiber exhibits a relatively reduced non-linearity. In this regard, where a very narrow channel spacing is used, it is difficult to sufficiently suppress the four-wave mixing phenomenon in the NZ-DSF with a relatively small effective area.
In particular, current WDM optical transfer systems show a tendency to use a channel spacing gradually reduced from 200 GHz to 100 GHz, and to 50 GHz. Such a tendency is due to the necessity of an increase in transmission capacity. However, where a very narrow channel spacing of 50 GHz is used, it is difficult for the NZ-DSF to be applied to WDM long-distance optical transmission systems.
FIG. 2
a
schematically illustrates an example of a WDM optical fiber system using an NZ-DSF. The illustrated optical fiber system, which is denoted by the reference numeral
20
, has a channel spacing of 50 GHz and 8 channels. This optical fiber system
20
receives optical power of 0 dBm per channel from a light source. NZ-DSFs
24
are distributed over a total distance of 480 km. A dispersion-shifted optical fiber (DCF)
25
is also arranged in every span, along with an optical amplifier
23
. The detailed specification of the optical transfer system
20
illustrated in
FIG. 2
a
is described in the following Table 1.
TABLE 1
System Specification
Value
Data Transmission Rate
10
Gb/s
Channel Spacing
50
GHz
Optical Power
0
dBm per channel
Number of Channels
8
Total Fiber Optic Cable Length
480
km
Distribution Span Length of
80
km
Optical
Chung Yun Chur
Kim Dong Young
Barber Therese
Bruce David V.
LG Cable Ltd. and Korea Advanced Institute of Science and Techno
Santangelo Law Offices P.C.
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