Optical communications – Transmitter and receiver system – Including compensation
Reexamination Certificate
2000-01-31
2004-01-20
Pascal, Leslie (Department: 2633)
Optical communications
Transmitter and receiver system
Including compensation
C398S068000, C398S081000, C398S097000, C398S159000, C398S160000
Reexamination Certificate
active
06681082
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength division multiplexing optical transmission system for transmitting wavelength division multiplexed (WDM) signal light, as well as to an optical amplifier and a dispersion compensator both applicable to such a system; and more particularly to a wavelength division multiplexing optical transmission system, optical amplifier and dispersion compensator adapted to utilize a hybrid transmission path comprising combined optical fibers having wavelength dispersion characteristics conflicting with each other, so as to effectively compensate wavelength dispersions of respective wavelength bands, in case of transmitting broadband wavelength division multiplexed signal light containing a plurality of wavelength bands.
2. Related Art
Conventionally, there has been performed transmission of an optical signal, by utilizing an optical regenerating repeater which converts an optical signal into an electric signal to thereby perform retiming, reshaping and regenerating. At present, however, practical use of an optical amplifier has been advanced, so that there is being investigated an optical amplifying-and-repeating transmission method which adopts an optical amplifier as a linear repeater. By substituting an optical regenerating repeater with an optical amplifying repeater, it is expected that the number of parts within the repeater is remarkably reduced to thereby ensure reliability and permit cost reduction. Further, as one method for realizing a large capacity of an optical transmission system, attention has been directed to a wavelength division multiplexing (WDM) optical transmission method which multiplexes two or more optical signals having wavelengths different from each other to transmit within a transmission path.
In a WDM optical amplifying-and-repeating transmission method obtained by combining the aforementioned optical amplifying-and-repeating transmission method and WDM optical transmission method, it is possible to collectively amplify WDM signal lights making use of an optical amplifier, to thereby permit realization of large capacity and long distance transmission with a simple (economic) constitution.
In the conventional WDM optical amplifying-and-repeating transmission system (hereinafter abbreviated to “WDM optical transmission system”), there is used a method for managing a wavelength dispersion of a transmission path, so as to reduce transmission characteristic degradation due to non-linear effect of a transmission path.
For example, in an article “Wavelength Division Multiplexing in Long-Haul Transmission Systems, IEEE Journal of Lightwave Technology. vol. 14, No. 6, pp. 1299-1308, 1996” of N. S. Bergano et al., there is used a transmission path obtained by combining: a dispersion-shifted fiber (DSF) of a length of about 900 km having a zero-dispersion wavelength of 1585 nm and a positive wavelength dispersion slope; with a single mode fiber (SMF) of a length of about 100km having a zero-dispersion wavelength of 1310 nm and a positive wavelength dispersion slope. This transmission path has an averaged zero-dispersion wavelength of about 1558 nm, and accommodates wavelengths of signal lights ranging from 1556 nm to 1560 nm.
Wavelength dispersions of DSF and SMF are approximately −2 ps
m/km and +20 ps
m/km, respectively, in which a group velocity of signal light and spontaneous emission light and a group velocity of mutual signal lights are different from each other. Thus, by adopting a transmission path obtained by combining DSF and SMF, it becomes possible to shorten an interaction period of time of non-linear effect, and to reduce degradation of a transmission characteristic such as due to four wave mixing (FWM) and a cross phase modulation (XPM). Further, since the transmission path has the averaged zero-dispersion wavelength within a signal light wavelength, there is also reduced a degradation of transmission characteristic due to self phase modulation (SPM) and wavelength dispersion.
However, when it is required to expand a transmission band to thereby increase a capacity of a WDM optical transmission system, it will be difficult to compensate such that wavelength dispersions become zero for all signal light wavelengths in the above described constitution, because of an affection of wavelength dispersion slope. As such, there are necessarily caused degradations of signal light waveforms due to interaction between: wavelength dispersion which is not compensated but accumulated; and non-linear effect within an optical fiber.
As a countermeasure for such a situation, there has been proposed a transmission path having a latter stage of a transmission section thereof applied with a dispersion compensation fiber for compensating wavelength dispersions and dispersion slopes thereof caused in a former stage of the transmission section. Concretely, it has been proposed to reduce dispersion slope to thereby reduce accumulated wavelength dispersions so as to reduce a degradation of a transmission characteristic, by adopting: for a former stage of a transmission section, a 1.3 &mgr;m zero-dispersion SMF having a positive wavelength dispersion and a positive dispersion slope; and for a latter stage of the transmission section, a dispersion compensation fiber having a negative wavelength dispersion and a negative dispersion slope, which compensates the wavelength dispersion and dispersion slope of the 1.3 &mgr;m zero-dispersion fiber.
In an article “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” of M. Murakami et al., it has been permitted to reduce an averaged wavelength dispersion slope up to 0.0067 ps
m
2
/km, by adopting: for a former section of a transmission section, a 1.3 &mgr;m zero-dispersion fiber of a 50% length of the transmission section having a positive wavelength dispersion; and for a latter stage of the transmission section, a dispersion compensation fiber of a 50% length of the transmission section having a negative wavelength dispersion.
There has been recently further proposed an optical transmission technique adopting WDM signal light including a plurality of wavelength bands such as 1550 nm band and 1580 nm, so as to increase a transmission capacity of a WDM optical transmission system.
According to, for example, an article “Ultra-wide band, long distance WDM transmission demonstration: 1 Tb/s (50×20 Gb/s), 600km transmission using 1550 and 1580 nm wavelength bands, PD11, OFC '98, 1998” of S. Aisawa, it has been permitted to increase a transmission capacity by adopting WDM signal lights of two wavelength bands, 1550 nm band and 1580 nm band, to thereby allow 50 waves of signal lights to be multiplexed. Here, since the wavelength dispersions and the dispersion slopes accumulated within SMF transmission path are different from each other for the respective wavelength bands, respectively, there are inserted dispersion compensators for compensating wavelength dispersions and dispersion slopes in the respective wavelength bands, into optical amplifiers linked in multiple stages, respectively. Each of the optical amplifiers has a constitution to demultiplex the input WDM signal lights in each of wavelength bands, and to send the demultiplexed WDM signal lights of each of the wavelength bands to each of corresponding dispersion compensators so as to compensate wavelength dispersion and dispersion slope caused within the SMF transmission path for each of wavelength bands. In this way, there is reduced a degradation of transmission characteristic of WDM signal lights in both wavelength bands of 1550 nm band and 1580 nm band.
Further, there has been proposed a technique for transmitting WDM signal light of a plurality of wavelength bands, making use of a hybrid transmission path obtained by combining a plurality of optical fibers having wavelength dispersion characteristics different from each other suc
Naito Takao
Tanaka Toshiki
Pascal Leslie
Staas & Halsey , LLP
Tran Dzung
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