Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-03-08
2003-07-15
Lee, John D. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S024000, C385S027000
Reexamination Certificate
active
06594428
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a dispersion compensating transmission line and system, and more specifically to a dispersion compensating optical transmission line and system capable of long haul transmission of wavelength-division-multiplexed (WDM) signal light.
BACKGROUND OF THE INVENTION
In a long haul optical amplification transmission system, dispersion compensation fibers are disposed at appropriate intervals in order to control an accumulated chromatic dispersion within a predetermined value (cf. Japanese Patent Laid-open publication No. Heisei 6-11620 or U.S. Pat. No. 5,361,319).
In wavelength-division-multiplexing optical transmission widely noticed as a method for increasing a transmission capacity, since a chromatic dispersion of an optical transmission fiber differs according to respective wavelengths (dispersion slope), an accumulated chromatic dispersion of each wavelength becomes also different. Firstly, proposed was a configuration that compensated differences of accumulative chromatic dispersion values among the wavelengths at a receiving or transmitting side. However, the dispersion amount practically compensated at the transmitting or receiving side is quite limited. Furthermore, an allowable difference of the dispersion values tends to decrease as a transmission rate per wavelength increases.
Then secondly proposed was an optical transmission system wherein each optical repeating span is composed of a single mode optical fiber having a zero dispersion wavelength at a 1.3 &mgr;m band and a slope compensating dispersion compensation fiber for compensating both of its chromatic dispersion and dispersion slope; namely a difference of accumulative chromatic dispersions among wavelengths (cf. op. cit. Japanese Patent Laid-open publication No. Heisei 6-11620 or U.S. Pat. No. 5,361,319 and D. Le Guen et al., “Narrow Band 640 Gbit/s Soliton WDM transmission over 1200 km of Standard Fibre with 100 km 21 dB Amplifier Spans”, ECOC '98, September 1998, Postdeadline papers, pp. 61-63).
FIGS.
2
(A) and
2
(B) show an optical transmission line for compensating an accumulated chromatic dispersion according to a cycle of several repeating spans and a distance variation of the accumulated chromatic dispersion. A dispersion slope compensating dispersion compensation fiber for compensating the dispersion slope at the same time is employed in the embodiment. FIG.
2
(A) shows the configuration of the transmission line and FIG.
2
(B) shows the distance variation of the accumulated chromatic dispersion corresponding to the transmission line shown in FIG.
2
(A). Reference numeral
10
denotes an optical transmitter for outputting signal light and reference numeral
12
denotes an optical transmission fiber comprising a single mode optical fiber (a dispersion shifted fiber) having a zero dispersion wavelength at a 1.5 &mgr;m band. Reference numerals
14
and
16
respectively denote an optical repeating amplifier and a slope compensating dispersion compensation fiber (SCDCF) for reducing accumulated chromatic dispersion values of respective wavelengths &lgr;l-&lgr;n into a predetermined value. The dispersion compensation fibers
16
are disposed at a plurality of the optical repeating intervals. The slope compensating dispersion compensation fiber
16
comprises, for instance, a fiber in which polarities of its chromatic dispersion value and dispersion slope are both reverse to those of the optical transmission fiber
12
.
Davg indicates a desired chromatic dispersion value of the whole system. A desired value of dispersion compensation by each dispersion compensation fiber
16
is derived by multiplying the desired value Davg by a transmission distance z from a starting point. Dlocal shows a chromatic dispersion value before the dispersion compensation by the dispersion compensation fiber
16
, namely the chromatic dispersion value of the optical transmission fiber
12
. The accumulated chromatic dispersion increases at the coefficient Dlocal according to the transmission distance. The dispersion compensation fiber
16
reduces, namely compensates the accumulated chromatic dispersion of each wavelength into a value obtained by multiplying Davg by the transmission distance z. Dlocal generally varies according to a wavelength.
In long haul transmission, since nonlinear effect exists to no small extent, an average chromatic dispersion value Davg of a whole system is generally set low other than zero in order to balance the nonlinear effect with the chromatic dispersion value.
FIGS.
3
(A) and
3
(B) show a conventional transmission line for compensating an accumulated chromatic dispersion at an optical amplification repeating cycle, and a distance variation of the accumulated chromatic dispersion respectively. FIG.
3
(A) shows the transmission line and FIG.
3
(B) shows the distance variation of the accumulated chromatic dispersion on the transmission line shown in FIG.
3
(A). Reference numerals
20
and
22
respectively denote an optical transmitter for outputting signal light and an optical amplification repeater, reference numeral
24
denotes an optical transmission fiber composed of a single mode optical fiber having a zero dispersion wavelength at a 1.3 &mgr;m band, and reference numeral
26
denotes a slope compensating dispersion compensation fiber (SCDCF). The optical transmission fiber
24
and slope compensating dispersion compensation fiber (SCDCF)
26
are inserted in each optical repeating span formed by the optical amplification repeater
22
. That is, the dispersion compensating cycle is equal to the optical amplification repeating cycle.
A nonlinearity of an optical fiber is generally expressed as n
2
/A
eff
. The reference symbols n
2
and A
eff
denote a nonlinear constant and an effective core area respectively. The nonlinearity n
2
/A
eff
of a SCDCF is larger than that of a standard single mode optical fiber. In a conventional system that the dispersion compensation fibers
26
are inserted at frequent intervals, the nonlinear effect, which affects the transmission characteristics, becomes larger. In order to perform the long haul transmission while balancing the nonlinear effect with the chromatic dispersion value, the chromatic dispersion value Davg after the dispersion compensation by the dispersion compensation fiber
26
, namely the chromatic dispersion value of the whole system should be set relatively high.
As already mentioned above, because the nonlinear effect exists to no small extent in long haul transmission such as transoceanic transmission, the average chromatic dispersion value Davg of the whole system preferably should be a low value other than zero for balancing the nonlinear effect with the chromatic dispersion value.
In the conventional system shown in FIGS.
2
(A) and
2
(B), the dispersion-shifted fiber is employed as the optical transmission fiber. The chromatic dispersion value of the dispersion-shifted fiber is low at the 1.5 &mgr;m band and therefore the influence due to the nonlinearity becomes relatively too large. To put it concretely, in the WDM transmission, owing to the lowness of the local chromatic dispersion value at the interval before the dispersion compensation by the dispersion compensation fiber
16
, each interaction length among the respective wavelengths becomes too long causing the large influence of cross phase modulation (XPM), which makes the stable long haul transmission impossible.
On the other hand, when a single mode fiber, which chromatic dispersion value is high at the 1.5 &mgr;m band, is employed as the optical transmission fiber
12
, each interaction length among the respective wavelengths of the WDM signal light is shortened and thus the influence of the XPM is also suppressed. However, in order to control the accumulated chromatic dispersion value (the absolute value) within a predetermined value, the dispersion compensation fibers
14
should be inserted at shorter intervals. In other words, the dispersion compensating cycle should be shorter and consequently this configurati
Edagawa Noboru
Suzuki Masatoshi
Tanaka Keiji
Tsuritani Takehiro
Christie Parker & Hale LLP
KDD Corporation
Lee John D.
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