Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2001-10-10
2004-03-02
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S124000, C398S087000
Reexamination Certificate
active
06701050
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates to methods of improving transmissions in optical fibers and, more particularly, to methods and apparatuses for decreasing pulse degradation and bit sequence deterioration resulting from random chromatic dispersion in optical fibers.
2. The Relevant Technology
Optical fiber communication systems, especially high-bit rate communication systems, are under active development worldwide. Numerous techniques and apparatuses are being reported for achieving high-bit rates for local area, metropolitan, and long haul optical communications. However, in transmitting information through the optical fibers at high-bit rates the optical signal degrades due to effects of chromatic dispersion (phase change of the optical signal induced by chromatic dispersion) and fiber Kerr nonlinearity (self phase modulation induced by Kerr nonlinearity).
Stable optical solitons result from exact compensation of self phase modulation and phase change due to chromatic dispersion. Soliton pulse propagation has been proposed as a method for transmitting of the information bits in optical fiber telecommunication systems in the presence of chromatic dispersion and Kerr nonlinearity. For example, U.S. Pat. No. 5,558,921, which is hereby incorporated by this reference, discloses a soliton-based single frequency optical fiber communication system with uniformly distributed fiber chromatic dispersion. Further enhancement of soliton-based optical fiber systems by implementing of multi-frequency channel technique (wavelength division multiplexing—WDM), however, is limited due to severe nonlinear inter-channel interaction.
U.S. Pat. No. 6,011,638, which is hereby incorporated by this reference, discloses a method that can effectively eliminate inter-channel interaction in soliton based telecommunication systems with lumped in-line optical amplifiers through proper dispersion management. In one embodiment of this method, the fiber chromatic dispersion decreases exponentially along a fiber as the energy of optical signal decreases. In another embodiment of this method, the exponentially decreasing profile is approximated in a step-wise manner using fiber spans with different values of chromatic dispersion uniformly distributed over each span.
A method for significantly improving the performance of optical communication system was proposed in the paper “Optical-pulse equalization of low-dispersion transmission in single-mode fibers in the 1.3-1.7-&mgr;m spectral region” by C. Lin, H. Kogelnik and L. G. Cohen, Opt. Lett. 5, 476 (1980). This method reduces signal deterioration due to chromatic dispersion by means of periodical inserting additional fiber spans with the opposite sign of the dispersion (dispersion management) that are required to keep the overall dispersion within one period close to zero. U.S. Pat. No. 5,471,333, which is hereby incorporated by this reference, discloses a method that eliminates pulse deterioration due to Kerr self-phase modulation by using the effects of the chromatic dispersion within one period of the dispersion map and provides stable oscillating pulses (dispersion managed solitons) was disclosed by. Dispersion managed solitons are stable, compatible with the WDM technique, and well suited for high bit rate telecommunications. Consequently, dispersion managed solitons have been proposed as a method for transmitting the information bits in optical fiber telecommunication systems with dispersion management. For example, U.S. Pat. No. 6,243,181, which is incorporated by this reference, discloses a method of reducing the inter-channel interaction in the soliton based optical communication systems with dispersion management.
The value of chromatic dispersion in the fiber spans of soliton based telecommunication systems is considered to be deterministic (predictable). However, the dispersion is known to randomly vary along the fiber, for example, in dispersion shifted fibers. Random variation of the zero dispersion wavelength was indicated in “Four-wave mixing in an optical fiber in the zero-dispersion wavelength region” by K. Inoue, J. Lightwave Technol. 10, pp. 1553-1561 (1992). Variation of the zero dispersion point was obtained by cutting a 10 km length of dispersion shifted fiber into four 2.5 km segments and subsequent measurement of the average zero dispersion wavelength. The randomness of fiber chromatic dispersion was demonstrated using a nondestructive accurate method of dispersion measurement along a fiber, which method has been disclosed in U.S. Pat. No. 5,956,131, which is hereby incorporated by this reference.
Historically unrecognized effects of random chromatic dispersion have the potential for uncontrolled growth of additional pulse deterioration due to the presence of randomness in fiber chromatic dispersion. Such uncontrolled growth of the pulse deterioration imposes penalties in the transmission system by two different mechanisms. The first mechanism is optical pulse broadening that eventually deteriorates information bit pattern. The second mechanism is shedding of nonlocal continuous radiation from localized optical pulses. Since this continuous radiation is nonlocal, optical pulses experience interaction through continuous radiation, resulting in increased separation between pulses. These penalties rapidly increase with shortening of the pulse width, i.e., when the bit-rate is increased.
Refining fiber optic production technology is expensive and currently does not offer an absolute cure to the adverse effects of random chromatic dispersion. Given the ever-increasing demand for higher transmission rates, minimizing pulse degradation in optical fibers resulting from random chromatic dispersion is important to increasing the bandwidth of optical fiber.
Accordingly, a need exists for methods and optical fibers for decreasing pulse degradation resulting from random chromatic dispersion in optical fibers. It would be a further advancement in the state of the art fiber optics technology to provide such a method and apparatus in a cost-effective manner. It would also be an advancement in the art to provide a method of minimizing pulse degradation resulting from random chromatic dispersion in the existing and newly manufactured cables.
BRIEF SUMMARY OF THE INVENTION
The present invention provides methods and apparatuses for decreasing pulse degradation resulting from random chromatic dispersion in optical fibers. More specifically, the present invention provides methods for periodically pinning (approximating) an actual (random) accumulated chromatic dispersion to a predicted (nominal) accumulated dispersion through relatively simple modifications of fiber-optic manufacturing methods or retrofitting of existing fibers. Through use of these methods and apparatuses, increased optical transmission speeds may be enabled.
If the pinning occurs with sufficient frequency (at a distance less than or equal to a correlation scale, Z
&xgr;
), pulse degradation resulting from random chromatic dispersion is minimized. The correlation scale may be defined by the following equation:
Z
&xgr;=&tgr;
4
/D,
where &tgr; is a pulse (bit of information) width at which signal is launched into an optical fiber and D indicates dispersion noise strength. Dispersion noise strength is a measure of the variation of actual (random) dispersion relative to predicted (nominal) dispersion.
Pinning may occur periodically (less than or equal to the correlation scale) or quasi-periodically along the length of the optical fiber. Quasi-periodic pinning may involve pinning at irregular intervals. For example, with quasi-periodic pinning, a distance between each consecutive instance of pinning may be between approximately zero and approximately two to three times the correlation scale. In another embodiment, such a distance may be between one half of the correlation scale and one and one half times the correlation scale.
Pinning involves points (“pinning points”) along an optical fiber where actual accumulated dispersion approx
Chertkov Michael
Gabitov Ildar
Madson & Metcalf
Sanghavi Hemang
The Regents of the University of California
Wong Eric
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