Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1999-02-19
2001-03-20
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S123000, C385S027000, C359S341430
Reexamination Certificate
active
06205268
ABSTRACT:
TECHNICAL FIELD
This invention relates to singlemode optical fiber systems, and more particularly to wavelength-division multiplexing (WDM) on such systems.
BACKGROUND OF THE INVENTION
Dispersion is a phenomenon whereby different optical wavelengths travel at different speeds through a dispersive media such as glass. And since a modulated carrier signal comprises many wavelengths, the optical signal that emerges from the distant end of a glass fiber is a smeared version of the signal that was launched into the near end. In the case of linear dispersion, this is solved by periodically providing compensation along an optical fiber route, and fewer compensation stages are better.
Conventional singlemode fiber systems primarily operate in the wavelength region between 1285 and 1335 nanometers (nm) and have a zero-dispersion wavelength at about 1310 nm. However, the optical fiber used in such systems is poorly suited for transmitting multiple closely spaced carrier wavelengths because of nonlinear interactions and mixing between the channels. The limiting form of such nonlinear phenomena—4-photon mixing (4PM)—is described in the literature (see e.g., article by D. Marcuse, A. Chraplyvy and R. Tkach entitled: “Effect of Fiber Nonlinearity on Long-Distance Transmission,”
Journal of Lightwave Technology
, vol. 9, No. 1, January 1991, pp. 121-128). Briefly, 4PM appears as a fluctuating gain or loss due to constructive and destructive interference between different signal channels. The magnitude of 4PM is power dependent and may be reduced by decreasing launch power.
Multi-channel optical systems provide the most efficient use of an optical fiber and include wavelength-division multiplexers, which operate to combine an number of closely spaced channels (wavelength regions) onto a single optical path in one direction of transmission, and to separate them from the optical path in the other direction of transmission. And while conventional singlemode fiber systems do provide WDM operation in the 1.55 &mgr;m wavelength region, there is too much linear dispersion (e.g., about 17 ps
m-km) that needs to be compensated. For example, compensation is required every 50 to 100 kilometers, which is an impractical short distance.
Contemplated uses of optical fiber include the transmission of all type of digital and analog information, both separately and together. Particular uses include data (such as Internet traffic) as well as broadcast television (TV) signals, which typically utilize amplitude modulated, vestigial-sideband (AM-VSB) modulation. Analog signals are inherently noise sensitive, and noise is readily observable in TV pictures. In particular, when multiple wavelengths such as WDM signals are transmitted on a single fiber, stimulated Raman scattering (SRS) causes energy to be transferred from the WDM signals into another wavelength region that is as much as 120 nm longer. At the present time, there are no systems that provide WDM and analog TV signals over the same optical fiber.
It has been observed in the article entitled: Fabrication Of Completely OH-Free V.A.D. Fiber in
Electronics Letters
, Aug. 28, 1980 Vol. 16 No. 19, that a completely OH-free optical fiber, with no loss peaks due to OH ions at any wavelengths in the loss spectrum from ultraviolet to infrared, has been desired for some time; and that such a fiber will play an important role as a transmission medium for WDM systems. However, this article provides no information regarding the dispersion characteristics of such a fiber, and it provides no information regarding the allocation of optical channels within the usable loss spectrum of optical fiber.
For these and other reasons, an optical transmission system that is compatible with apparatus that was designed for conventional singlemode fiber systems, which permits WDM operation without 4PM interference among WDM signals, and which avoids SRS interference between WDM and analog TV signals, would be of great interest. This application discloses such a system.
TERMINOLOGY
Conventional Fiber—Singlemode glass fiber that is characterized by a dispersion null at about 1310 nm, minimum loss at about 1550 nm, and a region of high loss at about 1385 nm that is attributable to the absorption of optical energy by hydroxyl (OH) ions.
Dispersion—When used alone, this term refers to chromatic dispersion—a linear effect due to wavelength-dependent velocity within the carrier spectrum.
Span—Reference is made here to a length of optical fiber having no regenerators. This length, which likely includes optical amplifiers, is the distance between stations at which the signal has been converted to/from electronic form (commonly the distance between nearest signal regenerators). This span may define an entire system, or may be combined with one or more additional spans.
Average System Wavelength—a specific wavelength determined by the arithmetic average of the carrier frequencies of a group of WDM channels.
Wavelength Region—a shorthand expression for a particular range of wavelengths. The 1.3 micron (&mgr;m) wavelength region is defined herein to include all wavelengths between 1285 and 1335 nm; the 1.4 &mgr;m wavelength region is defined herein to include all wavelengths between 1335 and 1435 nm; and the 1.55 &mgr;m wavelength region is defined herein to include all wavelengths between 1500 and 1600 nm.
WDM—Wavelength-Division Multiplexing. Situation whereby multiple communication channels, each having a different central wavelength, are combined onto a single transmission path such as an optical fiber.
SUMMARY OF THE INVENTION
We have discovered that the above-described deficiencies of prior art systems can be overcome in an optical transmission system that is arranged to transmit multiple WDM channels in the 1.4 &mgr;m wavelength region. Such a system includes optical fiber having a suitably low loss at 1385 nm, a zero dispersion wavelength (&lgr;
0
) at about 1310 nm, and linear dispersion between about 1.5 and 8.0 ps
m-km within the 1.4 &mgr;m wavelength region. This small-but-critical amount of dispersion significantly reduces the effect of four-photon mixing. In its broadest terms, the present invention reflects a number of observations including: (i) four-photon mixing is a relevant mechanism that must be considered in the design of WDM systems; (ii) stimulated Raman scattering from WDM channels has an adverse effect on analog signal transmission at 1550 nm, and (iii) desirably, new WDM systems should be reverse compatible with apparatus used in conventional singlemode fiber systems.
It is an advantage of the present invention that by positioning WDM channels in the 1.4 &mgr;m wavelength region, the wavelength regions around 1310 nm and 1550 nm are available for use by equipment that has traditionally operated in these regions.
It is another advantage of the present invention that by positioning WDM channels in the 1.4 &mgr;m wavelength region, where dispersion is less than about 8 ps
m-km, dispersion compensation can be avoided in most metropolitan optical transmissions systems where distances are shorter than about 200 kilometers.
It is yet another advantage of the present invention that by positioning WDM channels in the 1.4 &mgr;m wavelength region, a substantial buffer (about 120 nm) exists to protect analog signals, such as broadcast television, operating at 1550 nm from SRS noise.
In illustrative embodiments of the invention, Raman amplifiers are used for amplification of the WDM channels in the 1.4 &mgr;m region, whereas Erbium amplifiers are used for amplification of channels operating in the 1.55 &mgr;m region.
REFERENCES:
patent: 3933454 (1976-01-01), DeLuca
patent: 4578101 (1986-03-01), Clark et al.
patent: 4820322 (1989-04-01), Baumgart et al.
patent: 5000771 (1991-03-01), Fleming, Jr. et al.
patent: 5002350 (1991-03-01), Dragone
patent: 5136671 (1992-08-01), Dragone
patent: 5224183 (1993-06-01), Dugan
patent: 5327516 (1994-07-01), Chraplyvy et al.
patent: 5412744 (1995-05-01), Dragone
patent: 5587830 (1996-12-01), Chraplyvy et al.
patent: 5623508 (1997-04-01), Grub
Chraplyvy Andrew Roman
Eichenbaum Bernard Raymond
Emery Gary Patrick
Haber Janice Bilecky
Kalish David
Lucent Technologies - Inc.
Morra Michael A.
Palmer Phan T. H.
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