Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2001-01-22
2003-09-23
Bovernick, Rodney (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
C385S123000, C385S029000
Reexamination Certificate
active
06625362
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dispersion compensating optical fiber for compensating for wavelength dispersion and a dispersion slope of an optical transmission path in an optical communication system, and particularly to a dispersion compensating optical fiber for achieving an increased slope rate.
2. Related Art
In an optical communication system, there is a case where an optical signal of 1.55 &mgr;m wavelength band is transmitted over a long distance, through an optical transmission path such as constituted of a single mode fiber (SMF) which exhibits zero-dispersion near 1.3 &mgr;m wavelength. In such a case, the optical transmission path has wavelength dispersion on the order of about 16 ps
m/km for the optical signal of 1.55 &mgr;m band, resulting in a problem that the transmission of optical signal over a long distance at a high speed leads to accumulation of wavelength dispersion to thereby distort the waveform of the optical signal. For example, if an optical signal is transmitted over a distance of 100 km, a positive wavelength dispersion of about 1,600 ps
m is accumulated, thereby requiring compensation therefor. Further, when a wavelength division multiplexing (WDM) transmission is performed using the aforementioned optical transmission path, it is also required to compensate for a dispersion slope of the optical transmission path. Concretely, it is known that a 1.3 &mgr;m zero-dispersion SMF has a positive dispersion slope on the order of about 0.05 ps
m
2
/km.
Conventionally, to compensate for the wavelength dispersion of an optical transmission path having such a positive wavelength dispersion and a positive dispersion slope, there has been utilized a dispersion compensating optical fiber (DCF) having a negative wavelength dispersion and a negative dispersion slope. As conventional dispersion compensating optical fibers, various structures have been concretely proposed.
For example, there has been disclosed a basic structure having a “W” shaped refractive index profile as shown in
FIG. 8
in the conventional dispersion compensating optical fibers such as described in Japanese Unexamined Patent Publication Nos. 7-261048, 8-136758, 8-313750, 10253847, 9-318833 and 11-95056. Concretely, the conventional dispersion compensating optical fiber has the basic structure wherein a low refractive index portion
52
is formed on the periphery of a core portion
51
positioned at the center of the optical fiber, and a cladding portion
53
is formed on the periphery of the low refractive index portion
52
. The core portion
51
has a high refractive index of which specific refractive index difference to pure silica is a positive value, and the low refractive index portion
52
has a low refractive index of which specific refractive index difference refractive index is a negative value to pure silica, while the cladding portion
53
is formed of pure silica (i.e., its specific refractive index difference is zero). By forming the basic structure in such a “W” shaped refractive index profile, a dispersion compensating optical fiber having a negative wavelength dispersion and a negative dispersion slope is realized.
Further, as shown in
FIG. 9
, there has been also disclosed a structure wherein, in the cladding portion
53
, a part adjacent to the low refractive index portion
52
has a slightly high refractive index in the “W” shaped refractive index profile, in conventional dispersion compensating optical fibers such as described in Japanese Unexamined Patent Publication Nos. 2000-47048, 6-222235, 7-270636, 11-507445 and 10300965. By forming such a structure, there is realized a dispersion compensating optical fiber capable of such as reducing transmission loss.
Meantime, in many cases, there is adopted a dispersion shifted fiber (DSF) as an optical transmission path in an optical communication system for transmitting an optical signal of 1.55 &mgr;m band over a long distance. This DSF is an optical fiber obtained by shifting the zero-dispersion wavelength of the 1.3 &mgr;m zero-dispersion SMF to the 1.55 &mgr;m band and being specialized in optical transmission in 1.55 &mgr;m band where the transmission loss of silica optical fiber becomes a minimum. However, when performing a WDM transmission in 1.55 &mgr;m band through an optical transmission path adopting a DSF, the smaller wavelength dispersion in 1.55 &mgr;m band rather leads to a defective susceptibility to a nonlinear optical effect. As such, there has been proposed a system utilizing a non-zero dispersion shifted fiber (NZ-DSF) wherein the zero-dispersion wavelength of DSF has been intentionally shifted to the outside of a signal light wavelength band. Since such an NZ-DSF has a finite wavelength dispersion and a finite dispersion slope in the signal light wavelength band of 1.55 &mgr;m, it is required to compensate for the wavelength dispersion and dispersion slope in a long distance high-speed transmission of a WDM optical signal.
To compensate corresponding to the wavelength dispersion and dispersion slope of the NZ-DSF, there is required a dispersion compensating optical fiber having a large “slope rate” defined by a value obtained by dividing the dispersion slope by the value of the wavelength dispersion compared to the 1.3 &mgr;m zero-dispersion SMF. However, it is not easy to achieve an increase in slope rate in the dispersion compensating optical fiber having the aforementioned conventional structure. As one concrete way to increase the slope rate, it is considered to reduce the refractive index of the low refractive index portion
52
. According to the technique to date, however, it is difficult to reduce the refractive index down to a desired level, due to such as a manufactural problem. Even when a dispersion compensating optical fiber having a greater slope rate corresponding to the NZ-DSF can realized, such a dispersion compensating optical fiber should be so expensive.
Further, in the conventional dispersion compensating optical fiber having the structure as shown in
FIG. 9
, there are caused light in higher mode propagated through the part with higher refractive index of the cladding portion
53
. If this light in higher mode is sent out to the transmission path, then there is caused a problem in that the signal waveform of the transmitted light would be deteriorated.
SUMMARY OF THE INVENTION
The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to provide a dispersion compensating optical fiber having a negative wavelength dispersion and a negative dispersion slope and exhibiting a large slope rate. It is a further object of the present invention to provide a dispersion compensating optical fiber for eliminating higher mode optical components to thereby transmit fundamental mode optical signals only.
To achieve the aforementioned object, one aspect of the dispersion compensating optical fiber according to the present invention includes: a core portion positioned at the center of the optical fiber; a low refractive index portion provided at the periphery of the core portion and having a refractive index lower than that of the core portion; and an intermediate refractive index portion provided at the periphery of the low refractive index portion and having a refractive index higher than that of the low refractive index portion and lower than that of the core portion, and has a negative wavelength dispersion and a negative dispersion slope, wherein the intermediate refractive index portion is formed by doping to pure silica a dopant for increasing a refractive index such that a first specific refractive index difference (&Dgr;
3
) thereof relative to pure silica has a positive value, the low refractive index portion is formed by doping to pure silica a dopant for reducing a refractive index such that a value obtained by subtracting the first specific refractive index difference (&Dgr;
3
) of the intermediate refractive index portion from a
Inagaki Shinya
Takeyama Tomoaki
Bovernick Rodney
Fujitsu Limited
Staas & Halsey , LLP
Stahl Mike
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