Optical waveguides – Optical fiber waveguide with cladding – Utilizing nonsolid core or cladding
Reexamination Certificate
2001-02-12
2003-12-30
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing nonsolid core or cladding
C385S123000, C385S124000, C398S152000, C398S164000
Reexamination Certificate
active
06671442
ABSTRACT:
FIELD
This patent specification relates to the field of optical fiber communications. More particularly, it relates to an optical fiber communications system incorporating index-guiding microstructured optical fibers.
BACKGROUND
As the world's need for communication capacity continues to increase, the use of optical signals to transfer large amounts of information has become increasingly favored over other schemes such as those using twisted copper wires, coaxial cables, or microwave links. Optical communication systems use optical signals to carry information at high speeds over an optical path such as an optical fiber. Optical fiber communication systems are generally immune to electromagnetic interference effects, unlike the other schemes listed above. Furthermore, the silica glass fibers used in fiber optic communication systems are lightweight, comparatively low cost, and are able to carry tens, hundreds, and even thousands of gigabits per second across substantial distances.
A conventional optical fiber is essentially an optical waveguide having an inner core and an outer cladding, the cladding having a lower index of refraction than the core. Because of the difference in refractive indices, the optical fiber is capable of confining light that is axially introduced into the core and transmitting that light over a substantial distance. Because they are able to guide light due to total internal reflection principles, conventional optical fibers are sometimes referred to as index-guiding fibers. Conventional optical fibers have a solid cross-section and are made of fused silica, with the core region and the cladding region having different levels of dopants (introduced impurities) to result in the different indices of refraction. The cladding is usually doped to have a refractive index that ranges from 0.1% (single mode fibers) to 2% (multi-mode fibers) less than the refractive index of the core, which itself usually has a nominal refractive index of 1.47.
As known in the art, single-mode fiber is preferred over multi-mode fiber for high-capacity, long-distance optical communications. Single-mode fiber prevents electromagnetic waves from traveling down in the fiber in anything but a single, tightly held mode near its center axis. This is in contrast to multi-mode fiber, in which incident electromagnetic waves may travel down the fiber over several paths of differing distances. Accordingly, single-mode fiber allows for reduced group delay, and thereby allows optical signals to better keep their shape as they travel down the fiber. As described in Dutton,
Understanding Optical Communications
(Prentice Hall 1998), which is incorporated by reference herein, at p. 45, single-mode fibers may be created by (i) making the core region thin enough, (ii) making the refractive index difference between the core and the cladding small enough, or (iii) using a longer wavelength. Conventional single-mode fibers have a core diameter of about 9 &mgr;m and a cladding diameter of about 125 &mgr;m, and are single-mode down to a cutoff wavelength of about 1100 nm, below which they become multi-mode.
FIG. 1
shows a conventional optical fiber communications link
100
comprising a source transmitter 102, a destination receiver 104, and a plurality of regeneration spans
106
therebetween. Optical fiber communications link
100
represents a long-haul, high-capacity optical communications link that would run, for example, between the cities of San Francisco and New York, or between the cities of Los Angeles and Las Vegas. Each regeneration span
106
comprises a plurality of amplifier spans
108
and a regenerator
110
, and has a length L
RG
representing the distance between regenerators
110
. Each amplifier span
108
comprises a fiber span
112
and an amplifier station
114
that houses a dispersion-compensating fiber loop
116
and an optical amplifier
118
, with a length L
A
representing the length of the fiber span
112
. Within each regeneration span
106
the signal from source transmitter
102
remains all-optical, while at each regenerator
110
the optical signal must be regenerated, i.e. converted into digital electrical signals and then reconverted back into a “clean” optical signal for continued transmission down the optical fiber communications link
100
. Generally speaking, each regenerator
110
essentially comprises a combination of an optical receiver similar to the destination receiver
104
and an optical transmitter similar to the source transmitter
102
, along with appropriate electrical circuitry therebetween.
In practical implementations, the fiber spans
112
are placed in buried conduit along rights-of-way owned or leased by communications companies, and run between consecutive amplifier stations
114
and regenerators
110
as shown in FIG.
1
. As known in the art, each regenerator
110
may be coupled to equipment for adding, removing, or routing data to other communications links, usually when the digital data is in electrical form. Likewise, each amplifier station
114
may include equipment for optically adding or dropping channels at different wavelengths for coupling to other optical fiber communications links. Accordingly, the optical fiber communications link
100
of
FIG. 1
generally represents one of several paths between an information source (e.g., in San Francisco) and an information destination (e.g., in New York) within a communications network. However, regardless of the overall communications network in which the optical fiber communications link
100
may be contained, it is important to note that a given information signal originating at the source transmitter
102
, e.g., an information signal f
i
(t) on an optical carrier at wavelength &lgr;
i
, must generally be regenerated several times prior to arrival at destination receiver
104
, and also must generally be optically amplified several times between regenerators.
Amplifier stations
114
and regenerator stations
110
represent substantial costs in installing and maintaining optical fiber communications link
100
. Each amplifier station
114
requires housing in a manhole or other type of communications relay shelter, and requires a consistently maintained environment that includes a reliable electrical power supply for powering the optical amplifiers
118
. Furthermore, each regenerator
110
comprises an extensive amount of high-cost optical, electro-optical, and electronic equipment that requires an even more stable and consistently-maintained environment, such as that provided by a telephone central office or other central telecommunications facility. In addition to the required costs, each amplifier station
114
and regenerator station
110
adds another possible point of failure for the optical communications link
100
, either through a electrical/optical component failure or a power failure. Accordingly, in the design of optical fiber communications link
100
, it is desirable to require as few amplifier stations
114
and regenerator stations
110
as possible between the source transmitter
102
and the destination receiver
104
.
Also shown in
FIG. 1
is a cross-section of a conventional optical fiber
120
used in each fiber span
112
, the optical fiber
120
comprising a solid core region
122
surrounded by a solid cladding region
124
. Conventional optical fibers suffer from several adverse effects that reduce the efficiency of information transfer and the practical distance over which information may be carried by the light. The two primary adverse effects are (a) attenuation, which is a reduction in signal magnitude as it travels down the fiber, and (b) dispersion, which is a loss of signal shape as different component wavelengths travel down the fiber at different speeds. These two adverse effects often overlap in their unfavorable impacts on optical communication system design. In general, however, attenuation effects serve to (i) reduce the range of wavelengths &lgr; that may be used to carry information down a fiber, and (ii) reduce the required spacin
Elrefaie Aly F.
Wang Shih-Yuan
LandOfFree
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