Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
1998-12-01
2001-10-30
Palmer, Phan T. H. (Department: 2877)
Optical waveguides
Optical fiber waveguide with cladding
C385S124000, C385S126000, C359S199200
Reexamination Certificate
active
06311002
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to optical fiber communication systems and more particularly to an optical fiber communication system that employs dispersion mapping to reduce penalties arising from nonlinear interactions.
BACKGROUND OF THE INVENTION
Transmission performance of long-haul optical communication systems such as transoceanic systems is limited by a number of phenomena, including amplified spontaneous emission (ASE) noise accumulation, dispersion, and a nonlinear component to the refractive index of the fiber. If the signal travels at the zero dispersion wavelength it will not suffer any temporal distortions. However, at the zero dispersion wavelength the signal and the ASE noise generated by the optical amplifiers travel at similar velocities (good phase matching) and thus have the opportunity to interact over long distances, via the Kerr effect. The result is the transfer of power out of the signal and into unwanted wavelengths.
Conversely if the signal propagates at a wavelength for which the dispersion is large then there is a large phase mismatch (group velocity difference) between the signal and noise, which greatly reduces the efficiency of four wave mixing. However, large values of dispersion result in increased inter-symbol interference due to the temporal spreading of the signal.
One of the most deleterious nonlinear interactions is known as modulation instability, which occurs for signal wavelengths that are less than about one nm greater than the zero dispersion wavelength. As a result of modulation instability, any fluctuations in the signal intensity caused by noise or dispersive effects that change the signal envelope shape will be enhanced by the Kerr effect through self phase modulation.
In known transmission systems dispersive and nonlinear distortions are simultaneously minimized by a dispersion management technique. One dispersion management technique is known as dispersion mapping, in which the zero dispersion wavelengths of the constituent fibers are chosen so that they are appropriately far from the system's operating wavelengths. Constituent fibers with different zero dispersion wavelengths are then arranged in some periodic fashion so that the path average dispersion for the whole transmission line is appropriately small. For example, the transmission line may be divided into two or more sections approximately equal length. In one section, the optical fiber has a zero dispersion wavelength less than the operating wavelengths. The following section has optical fiber with a zero dispersion wavelength greater than the operating wavelengths. The overall transmission line is thus constructed in a periodic manner from a concatenation of fiber sections having different zero dispersion wavelengths. By constructing the transmission line out of alternating lengths of positive and negative dispersion fiber, the path average dispersion can be adjusted so that it causes minimal temporal distortion. Moreover, by selecting the local dispersions of the constituent fibers to be large in magnitude, nonlinear interactions can be suppressed. Typically in dispersion managed systems the amount of positive and negative dispersion that is provided are equal to one another so that there is no net accumulated dispersion over the entire transmission line.
One factor complicating the dispersion map for a wavelength-division multiplexed transmission system is that optical fiber generally has a nonzero dispersion slope. That is, different wavelengths experience different dispersion values in a given fiber. As a result, in a WDM transmission system employing a plurality of wavelengths, the dispersion map can return the accumulated dispersion to zero for only one wavelength. All the other wavelengths will accumulate some net dispersion, which can be compensated on a channel by channel basis at the transmitter or receiver. As with a single channel system employing dispersion management, the net accumulated dispersion of the individual wavelengths in a WDM system is typically returned to zero.
Optimal or near optimal performance can be achieved by precise tailoring of the dispersion map to reduce nonlinear impairments for a given wavelength. In particular, it would be desirable to provide a dispersion map in which nonlinear penalties arising from self-phase modulation and the nonlinear interaction between the signal and ASE noise are reduced.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method and apparatus is provided for compensating for dispersion in a wavelength division multiplexed (WDM) optical communication system. The system includes a transmitting and receiving terminal for transmitting and receiving, respectively, an optical signal having a plurality of channels, and an optical fiber transmission path coupling the first and second terminals. The fiber transmission path has a dispersion substantially equal to zero for a selected channel, positive dispersion for a first set of channels, and negative dispersion for a second set of channels. The method begins by providing positive dispersion compensation to the second set of channels at one of the terminals. Negative dispersion compensation is provided to the first set of channels, also at one of the terminals.
In accordance with one aspect of the invention, the step of providing positive dispersion compensation to the second set of channels is performed by the transmitting terminal.
In accordance with another aspect of the invention, the step of providing negative dispersion compensation to the first set of channels is performed by the receiving terminal.
The present invention advantageously allows the accumulated dispersion of each channel of a WDM signal to be individually compensated so that each channel's performance can be optimized. Optimization is achieved by appropriately adjusting the portion of the total dispersion compensation that is provided at the transmitting and, receiving terminals.
REFERENCES:
patent: 5559920 (1996-09-01), Chraplyvy et al.
patent: 5696614 (1997-12-01), Ishikawa et al.
patent: 6021235 (2000-02-01), Yamamoto et al.
Evangelides Steven G.
Golovchenko Ekaterina A.
Mazurczyk Vincent J.
Pilipetskii Alexei N.
Merlino Amanda
Palmer Phan T. H.
TyCom (US) Inc.
LandOfFree
Method and apparatus for reducing nonlinear penalties by... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for reducing nonlinear penalties by..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for reducing nonlinear penalties by... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2605525