Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-12-31
2002-04-02
Negash, Kinfe-Michael (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06366376
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the an optical communication system which combines a plurality of signal lights into a wavelength division multiplexed signal light for transmission through an optical fiber transmission line. More particularly, the present invention relates to an arrangement of frequencies of the signal lights to eliminate the effects of four-wave mixing (FWM).
2. Description of the Related Art
Optical communication systems can provide long-distance, large capacity transmission. Therefore, such optical communication systems will likely be used for future multimedia networks which require these characteristics.
Various techniques are being studied for increasing the capacity of optical communication systems. Such techniques include the use of time-division multiplexing (TDM), optical time-division multiplexing (OTDM) and wavelength-division multiplexing (WDM). Of these techniques, WDM is considered the most advantageous, since the transmission speed for each optical carrier signal light can be set at a lower value for the same transmission capacity, thereby resulting in greater tolerance to wavelength dispersion and nonlinear optical effects of an optical fiber transmission line.
Moreover, the development of erbium-doped fiber amplifiers (EDFA) having a wide “gain region” (that is, a wavelength region where a desired gain is obtained) has allowed optical communication systems to sufficiently amplify wavelength division multiplexed signals, thereby increasing the use of WDM. It is hopeful that WDM will allow optical communication systems to provide a flexible lightwave network in which cross connects, branching, insertion, and multiplexing of different kinds of services can be performed at optical levels by utilizing the wide gain region of an EDFA.
However, waveform degradation caused by wavelength dispersion of an optical fiber becomes significant in optical communication systems where the transmission speed for each optical carrier signal light exceeds several gigabits per second. Moreover, with long-distance transmission, input power to an optical fiber transmission line must be increased to obtain a required signal-to-noise ratio (SNR) . Unfortunately, this increase causes an increase of self-phase modulation (SPM), a nonlinear optical effect in optical fibers.
SPM causes many problems. For example, wavelength chirping of signal light by SPM causes waveform degradation through interaction with the group velocity dispersion (GVD) of an optical fiber, thereby providing an SPM-GVD effect. To eliminate or suppress the SPM-GVD effect, a dispersion-shifted fiber (DSF) at 1.55, &mgr;m can be used as an optical fiber transmission line to minimize the dispersion value at signal light wavelength. However, in WDM transmission using the low-dispersion region of the DSF, the occurrence of a nonlinear optical effect in optical fiber, known as four-wave mixing (FWM) between signal lights, will become noticeable. The effect of FWM manifests itself in the form of crosstalk and attenuation. More specifically, crosstalk is caused by the selection and reception of FWM light together with the signal wavelength. Attenuation of the signal light is caused by energy transfer to the FWM light. Crosstalk and attenuation degrade SNR, and, in the worst case, make transmission impossible.
The occurrence efficiency of FWM increases as the transmission channel dispersion decreases, signal light power increases, wavelength separation reduces, or the number of wavelengths increases. Since FWM occurs at a lower power level than other nonlinear optical effects, the effects of FWM tend to easily occur. In reality, the effects of FWM increase because signal light power must be increased by using a low-dispersion region of a DSF transmission line and transmissions must be performed using closer channel spacing due to the limited signal band resulting from the wavelength-dependence of optical components and the gain region of optical amplifiers. Accordingly, sufficient consideration must be given to the effects of FWM in designing a WDM system.
Various methods have been proposed to suppress FWM. Such methods include separating the signal band substantially away from the zero dispersion wavelength &lgr;
o
of an optical fiber transmission line (H. Miyata et al., “Study on the Effects of FWM in WDM Transmission Considering Fiber Dispersion Variations”, Technical Report, The Institute of Electronics, Information and Communication Engineers, SSE93-143, OCS93-73 (1994-03); N. S. Bergano et al., “100 Gb/s WDM Transmission of Twenty 5 Gb/s NRZ Data Channels Over Trcosoceanic Distances Using a Gain Flattened Amplifier Chain”, Proc. 21st Enr. Conf. on Opt. Comm. (ECOC '95—Brussels)). However, with this method, the signal band may undesireably shift from the gain region of optical amplifiers. Also, the zero dispersion wavelength &lgr;
o
must be managed with high accuracy. Further, dispersion compensation becomes necessary for long-distance transmission because fiber dispersion increases.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a WDM optical communication system that can substantially eliminate or suppress the effects of FWM.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
Objects of the present invention are achieved by providing an optical transmitting device which combines a plurality of signal lights, each having a different, corresponding, frequency, into a wavelength division multiplexed signal light. For at least three signal lights of the plurality of signal lights, the difference in frequencies of any pair-combination of the at least three signal lights is different from the difference in frequencies between any other pair-combination of the at least three signal lights.
Moreover, objects of the present invention are achieved by providing the at least three signal lights to include n signal lights having respectively corresponding frequencies f
1
through f
n
arranged in order from f
1
to f
n
along a frequency spectrum, where frequencies f
1
through f
n−1
have respectively corresponding integer spacing coefficients m
1
through m
n−1
, and frequencies f
i
and f
i+1
are separated by m
i
·&Dgr;f
S−F
, where i=1 to (n−1) and &Dgr;f
S−F
is a unit of spacing between frequencies. m
1
through m
n−1
should all be different from each other. In addition, m
1
through m
n−1
and the sum of any consecutive (n−2) spacing coefficients m
1
through m
n−1
should all be different from each other. m
1
through m
n−1
should be selected to minimize the sum of m
1
through m
n−1
, and thereby reduce the required bandwidth.
Objects of the present invention are also achieved by providing an optical transmitting device for combining first, second and third signal lights, each having a different, corresponding, frequency, into a wavelength division multiplexed signal light. The difference in frequencies of any pair-combination of the first, second and third signal lights is different from the difference in frequencies between any other pair-combination of the first, second and third signal lights.
Moreover, objects of the present invention are achieved by providing the first, second and third signal lights so that the first, second and third signal lights have frequencies f
1
, f
2
and f
3
, respectively, arranged in order from f
1
to f
3
along a frequency spectrum, where frequencies f
1
and f
2
have respectively corresponding integer spacing coefficients m
1
and m
2
, frequency f
2
is separated by m
1
·&Dgr;f
S−F
from frequency f
1
, and frequency f
3
is separated by m
2
·&Dgr;f
S−F
from frequency f
2
. &Dgr;f
S−F
is a unit of spacing between frequencies. m
1
and m
2
are different from each other.
In addition, objects of the present i
Chikama Terumi
Miyata Hideyuki
Onaka Hiroshi
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