Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-06-29
2003-04-01
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06542275
ABSTRACT:
The present invention is directed toward a wavelength division multiplexed communication system having a dispersion compensation element that provides spectrally uniform nonlinearity behavior.
BACKGROUND OF THE INVENTION
Optical signals transmitted in a fiber optic communication system typically constitute a series of pulses of digital information. Although the pulses are usually at a single nominal wavelength, each pulse is actually composed different spectral components. These spectral components propagate through the transmission fiber at different speeds with higher frequency components traveling slower than lower frequency components. This effect, known as “chromatic dispersion”, can result in spectral components of one pulse arriving at a receiver at substantially the same time as a succeeding pulse, thereby causing degraded receiver sensitivity. Chromatic dispersion becomes increasingly pronounced at higher bit rates, e.g. those associated with synchronous optical network (SONET) OC-192 transmission speeds.
Dispersion compensated fiber, commercially available from Corning, for example, can be used to offset chromatic dispersion. It is known, however, that dispersion compensated fiber has a nonlinearity coefficient &ggr;, which is related to a nonlinearity property of the refractive index of the fiber, n
2
, and the mode field diameter, otherwise referred to as the fiber effective area A
eff
(see Agrawal, “Nonlinear Fiber Optics”, Academic Press, Inc., 1995, pp. 37-43). In particular, &ggr; can be expressed as follows:
γ
=
2
π
n
2
λ
A
eff
In general, n
2
depends on the fiber composition and dopants, e.g., fluoride doped fibers have a different n
2
value than germanium doped fibers. A
eff
, however, is related to fiber geometry, doping profile and waveguiding characteristics. For single wavelength transmission applications, n
2
and A
eff
have fixed values associated with the transmission wavelength &lgr;.
Recently, wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of existing fiber optic networks. In a WDM system, plural optical signal channels are carried over a single optical fiber with each channel being assigned a particular wavelength. The wavelengths are typically within a narrow range about 1550 nm, the absorption minimum of silica fiber. At high data speeds, such as OC-192 rates, the dispersion associated with each channel must be compensated. Moreover, the nonlinearity coefficient &ggr; for each channel should be substantially the same, i.e., &ggr; should be spectrally uniform. Otherwise, some channels may have more errors than others, thereby degrading system performance.
SUMMARY OF THE INVENTION
The nonlinearity coefficient &ggr; of dispersion compensated fiber has been assumed to be spectrally uniform over a relatively broad range of optical wavelengths. It has been found, however, that &ggr; can vary substantially over a relatively narrow wavelength range of wavelengths. In particular, for wavelengths within a narrow range about 1550 nm, n
2
/A
eff
values of dispersion compensated fiber as high as 1.7×10
−9
(1/W) (at about 1543 nm) and as low as low as 1.2×10
−9
(1/W) (at about 1557 nm) have been measured. Accordingly, in WDM systems in which optical signals carry high-speed data at these wavelengths, selected channels can have a 40% higher &ggr; than other channels. As a result, the higher &ggr; channels may exhibit different system performance as characterized by different error rates. Accordingly, system design is made difficult.
Accordingly, the present invention provides a WDM optical communication apparatus comprising a dispersion compensating element having at least one dispersion compensating fiber. The dispersion compensating element is configured to mitigate the effects of a spectrally nonuniform nonlinearity coefficient behavior of the fiber, thereby giving an effectively uniform nonlinearity behavior for the plural optical signals in a WDM system.
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Agrawal, “Nonlinear Fiber Optics” 2n Edition, pp. 16, 17, and 36-43, Academic Press 1995.
Yamauchi, “Fiber Nonlinearity: Measurements and Countermeasures”, OFC'95.
Marhic, M.E., et al.; “Optimizing the Location of Dispersion Compensators in Periodically Amplified Fiber Links in the Presence of Third-Order NonLinear Effects”; IEEE Photonics Technology Letters, vol. 8, No. 1, pp. 145-147, (1996).
Ciena Corporation
Pascal Leslie
Soltz David L.
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