Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2002-07-15
2004-01-27
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S124000, C385S125000, C385S126000, C385S127000
Reexamination Certificate
active
06684016
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber for a wavelength division multiplexing (WDM) optical transmission system, and more particularly to an optical fiber capable of suppressing influences, caused by a non-linearity of optical fibers, to a maximum in order to obtain a maximum transmission capacity per optical fiber. Also, the present invention relates to an optical fiber which can operate efficiently even when a reduced channel spacing is used for an increase in transmission capacity, while being usable even in a wavelength band of 1,450 to 1,530 nm called an “S-band” expected to be used in WDM optical transmission systems in future.
2. Description of the Related Art
Optical fibers can transmit a large quantity of data within a short period of time while involving a reduced transmission loss. The use of such optical fibers has been greatly increased in accordance with recent development of communications. In particular, optical transmission techniques have been remarkably 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. In pace with such development of optical transmission techniques, techniques associated with optical fibers has been greatly advanced.
However, known optical fibers involve a chromatic dispersion, that is, a phenomenon that a signal is spread due to a difference in group velocity inverse to a variation in phase constant for different wavelength components of the signal, that is, different mode frequencies. 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 band 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.
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 becomes more severe at a reduced channel spacing or a decreased dispersion of the optical fiber.
U.S. Pat. No. 5,327,516 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, the NZ-DSF disclosed in U.S. Pat. No. 5,327,516 insufficiently suppresses the four-wave mixing phenomenon occurring in current WDM systems using a channel spacing reduced from 200 GHz to 50 GHz via 100 GHz. For this reason, it is difficult for this NZ-DSF to be applied to a WDM long-distance optical transmission system using a narrow channel spacing of about 50 GHz.
FIG. 1
schematically illustrates an example of a WDM optical transmission system using NZ-DSFs.
The optical fiber system of
FIG. 1
has 8 channels with a channel spacing of 50 GHz. This optical fiber system, which is denoted by the reference numeral
10
, receives optical power of 0 dBm per channel from a light source. NZ-DSFs
14
are distributed over a total distance of 480 km. A dispersion compensation optical fiber (DCF)
15
is also arranged in every span, along with an optical amplifier
13
. The detailed specification of the optical transfer system
10
illustrated in
FIG. 1
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
Optical Amplifier Distribution Span Length
80 km
Optical Fiber Loss
0.2 dB/km
The optical transmission system of
FIG. 1
mainly includes eight transmitters (Tx)
11
respectively adapted to provide lights of different wavelengths, a multiplexer for multiplexing the lights of different wavelengths transmitted from the transmitting terminals
11
, a plurality of optical amplifiers
13
each adapted to amplify a multiplexed light outputted from the multiplexer, a plurality of DCFs
15
each adapted to compensate for an amplified light outputted from an associated one of the optical amplifiers
13
arranged just upstream from the DCF
15
, a demultiplexer for demultiplexing the light finally outputted after passing through the optical amplifiers
13
and DCFs
15
, and a receiver (Rx)
12
for receiving the demultiplexed light from the demultiplexer. A plurality of NZ-DSFs
14
are distributed between the transmitters
11
and receiver
12
. The optical amplifiers
13
are arranged so that each of them is spaced apart from an associated one of the NZ-DSFs
14
by a desired distance.
Each of the NZ-DSFs
14
used in the optical transmission system of
FIG. 1
exhibits an average dispersion of 3.0 ps
m-km. The average dispersion is a value obtained by dividing a dispersion value accumulated during the transmission of an optical signal by a transmission distance. Each NZ-DSF
14
exhibits an accumulated dispersion value of about 240 ps
m at a point of 80 km. This accumulated dispersion value of each NZ-DSF
14
is compensated for by an associated one of the DCFs
15
each having a dispersion value of −240 ps
m.
FIG. 2
a
is an eye diagram of an optical signal transmitted in the optical transmission system illustrated in FIG.
1
.
As apparent from
FIG. 2
a,
the eye of the optical signal is unclear, and partially opene
Jung Yun-chul
Kim Dae-Weon
Kim Dong-young
Lee Dong-wook
Lee Kwon-moo
Berkowitz Marvin C.
Doan Jennifer
LG Cable Ltd.
Nath & Associates PLLC
Novick Harold L.
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