Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1999-11-30
2001-11-27
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S031000, C385S038000, C385S027000, C359S199200
Reexamination Certificate
active
06324317
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength division multiplexing optical transmission system for carrying out repeating transmission of a wavelength division multiplexed signal light via optical amplifiers. More particularly, this invention relates to a wavelength division multiplexing optical transmission system in which a hybrid transmission line formed by combining optical fibers having reciprocal wavelength dispersion is adopted for compensating for wavelength dispersion and wavelength dispersion-slope.
2. Description of the Related Art
In the past, with long-distance optical transmission systems, optical reproduction repeaters have been used for transmission. The optical reproduction repeaters convert an optical signal into an electrical signal and perform re-timing, reshaping, and regenerating. However, the employment of optical amplifiers has come to be a matter of common practice these days. An optical amplification repeater transmission mode using optical amplifiers as linear repeaters is under discussion. The optical reproduction repeaters are replaced with optical amplifier repeaters, whereby the number of parts included in a repeater can be reduced greatly. Consequently, high reliability can be guaranteed and a great reduction in cost is expected.
Moreover, a wavelength division multiplexing (WDM) optical transmission mode has attracted people's attention as one of modes capable of realizing a large-capacity optical transmission system. According to the wavelength division multiplexing optical transmission mode, two or more optical signals having different wavelengths are multiplexed and transmitted over one transmission line.
With a WDM optical amplification repeater transmission mode which is a combination of the optical amplification repeater transmission mode and WDM optical transmission mode, wavelength division multiplexed signal lights can be amplified collectively using optical amplifiers. Moreover, large-capacity and long-distance transmission can be achieved with a simple (economic) configuration.
Conventional WDM optical amplification repeater transmission systems (hereafter abbreviated to WDM optical transmission systems) adopt a method of managing wavelength dispersion generated in a transmission line so as to minimize deterioration of transmission characteristics due to a nonlinear effect generated in the transmission line.
For example, in an article (1) written by N. S. Bergano et al. and entitled “Wavelength Division Multiplexing in Long-haul Transmission Systems” (IEEE Journal of Lightwave Technology, Vol. 14, No. 6, PP.1299-1308, 1996), a transmission line made by combining a dispersion-shifted fiber (DSF) and a single-mode fiber (SMF) is used as shown in FIG.
16
. The dispersion-shifted fiber has a length of approximately 900 km, and is of a zero-dispersion wavelength &lgr;
OD
of 1585 nm, and positive wavelength dispersion-slope. The single-mode fiber has a length of approximately 100 km, and is of a zero-dispersion wavelength &lgr;
OS
of 1310 nm, and positive wavelength dispersion-slope. An average zero-dispersion wavelength &lgr;
OA
of the transmission line is approximately 1558 nm and wavelengths of signal light permitted to be propagated over the transmission line range are from 1556 nm to 1560 nm.
The value of wavelength dispersion generated in the DSF and SMF are approximately −2 ps
m/km and approximately +20 ps
m/km respectively The group velocity of signal light and spontaneously emitted light and the group velocity of each signal light are different between the DSF and SMF The employment of the transmission line made by combining the DSF and SMF makes it possible to shorten the time of interaction by a nonlinear effect. Deterioration in transmission characteristics due to four wave mixing (FWM) and cross phase modulation (XPM) can therefore be minimized. Moreover, since the average zero-dispersion wavelength of the transmission line falls within the wavelengths of signal light, deterioration in transmission characteristics due to self phase modulation (SPM) and wavelength dispersion can also be minimized.
However, a bandwidth used for transmission must be expanded in order to increase a capacity of the WDM optical transmission system. In this case, as far as the foregoing configuration is concerned, due to wavelength dispersion-slope, it is hard to compensate wavelength dispersion so that wavelength dispersion will become zero relative to all wavelengths. Consequently, the waveform of signal light is impaired due to interaction between wavelength dispersion that has not been compensated for but cumulated and the nonlinear effect in an optical fiber.
As a countermeasure, a proposal has been made of a transmission line adopting a dispersion compensation fiber as a second half of a transmission segment thereof. The dispersion compensation fiber compensates for wavelength dispersion and wavelength dispersion-slope generated in a first half of the transmission segment of the transmission line. More particularly, for example, a 1.3 &mgr;m zero-dispersion SMF having positive wavelength dispersion and positive wavelength dispersion-slope is used as the first half of the transmission segment of the transmission line. A dispersion compensation fiber having negative wavelength dispersion and negative wavelength dispersion-slope slope so as to compensate for the wavelength dispersion and wavelength dispersion-slope generated in the 1.3 &mgr;m zero-dispersion fiber is used as the second half of the transmission segment of the transmission line. Thus, the wavelength dispersion-slope is decreased in order to minimize cumulative wavelength dispersion. Eventually, deterioration in transmission characteristics can be alleviated.
According to an article (2) written by M. Murakami et al. and entitled “Quarter terabit (25×10 Gb/s) over 9288 km WDM transmission experiment using nonlinear supported RZ pulse in higher order fiber dispersion managed line” (ECOC'98, PP.79-81, 1998), an average wavelength dispersion-slope may be minimized to 0.0067 ps
m
2
/km. Specifically, a 1.3 &mgr;m zero-dispersion fiber is used as a first half of a transmission segment of a transmission line, and a dispersion compensation fiber is used as a second half thereof. The 1.3 &mgr;m zero-dispersion fiber has a length equivalent to 50% of the transmission segment and positive wavelength dispersion. The dispersion compensation fiber has a length equivalent to 50% of the transmission segment and negative wavelength dispersion.
Moreover, according to an article (3) written by K. Yonenaga et al. and entitled “Dispersion-compensation-free 40-Gbit/s×4-channel WDM transmission experiment using zero-dispersion-flattened transmission line” (OFC'98, PD20, 1998), an average wavelength dispersion-slope may be minimized to −0.0028 ps
m
2
/km. Specifically, a 1.3 &mgr;m zero-dispersion fiber is used as a first half of a transmission segment of a transmission line, and a dispersion compensation fiber is used as a second half thereof. The 1.3 &mgr;m zero-dispersion fiber has a length equivalent to 55% of the transmission segment and positive wavelength dispersion. The dispersion compensation fiber has a length equivalent to 45% of the transmission segment and negative wavelength dispersion.
Furthermore, according to an article (4) written by T. Kashiwada et al. and entitled “Ultra-low chromatic and polarization mode dispersion hybrid fiber links for ultra-high speed transmission systems” (OECC'98, 15C
1-3,
PP.364-365, 1998), an average wavelength dispersion-slope may be minimized to 0.008 ps
m
2
/km. Specifically, a 1.3 &mgr;m zero-dispersion fiber is used as a first half of a transmission segment of a transmission line, and a dispersion compensation fiber is used as a second half thereof. The 1.3 &mgr;m zero-dispersion fiber has a length equivalent to 84% of the transmission segment and positive wavelength dispersion. The dispersion compensation fiber has a length equivalent to 16% o
Naito Takao
Tanaka Toshiki
Fujitsu Limited
Palmer Phan T. H.
Staas & Halsey , LLP
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