Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-09-14
2002-11-05
Tweel, John (Department: 2632)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C385S028000, C385S123000
Reexamination Certificate
active
06476951
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to fiber-optic communications systems, such as telecommunications systems and data communications systems.
2. Description of the Related Art
In today's marketplace, as the demand for high-rate communications systems continues to increase, commercial communications system designers continue to attempt to find cost savings and technological improvements in components, performance efficiencies, and transmission rates. To address these system goals, communications system designers have implemented designs and solutions using optical fiber as a transmission medium that exhibits high bandwidth and low transmission loss. Traditional fiber-optic communications systems exhibit superior data rates and demonstrate rapid recovery of capitalized costs. As used herein, the term “optical fiber” has its accustomed meaning of a fiber (e.g., a thin rod shape) containing one or more core regions within which light travels, and, in certain fabrications, a cladding layer outside of the outermost core region.
For decades, optical fiber has been the preferred transmission medium in high-capacity communications networks and long-distance communications systems. More recently, optical fiber has also begun to be used in short-distance communications applications, such as local area networks, integrated applications, and intra-system applications. In general, long-distance applications have communications links greater than about 1 km, while typical short-distance applications have communications links shorter than about 1 km, and often as short as tens of meters.
Typically, optical implementations of short-distance communications systems employ glass optical fiber that generally satisfies the desired physical and performance characteristics previously discussed. In particular, existing glass optical fiber exhibits low transmission losses, generally less than 1 dB/km. As used herein, the term “glass optical fiber” refers to any suitable silica-based fiber with or without optical doping impurities.
For example, a technically superior solution for making short-distance links is known to be based on single-mode glass optical fiber. This type of fiber offers exceptionally high bandwidth, since it supports only one propagating electromagnetic mode. Typically, the optical core of a single mode fiber is only several microns in diameter. Consequently, connecting to the optical core of the fibers requires precision optical and mechanical couplings that significantly increase the material and labor cost of the links.
Attempts to reduce the high costs associated with single-mode glass optical fiber systems include the approach of implementing links based on multimode glass optical fiber. Multimode glass optical fiber possesses a larger core diameter than single-mode fiber, generally in the range of approximately 50-62.5 microns. Thus, the mechanical and optical couplings required for multimode fiber connections are generally less precise, and therefore less expensive to manufacture and install, than those used for single mode fibers. However, the resulting bandwidth of multimode fiber is degraded by dispersion of the various electromagnetic modes which propagate in the fiber. While fiber manufacturers attempt to fabricate multimode fibers with an index profile that minimizes this intermodal dispersion, manufacturing limitations often result in non-ideal index profiles and associated degradation of bandwidth.
To further reduce the cost of optical links, the prior art also includes the use of multimode plastic optical fiber. Since plastic is less brittle than glass, plastic fibers can have significantly larger cores than multimode glass fiber, thereby allowing even further relaxation in the mechanical and optical tolerances of couplings at fiber endpoints. These relaxed tolerances further lower the cost of the optical transceivers and couplings in the links. Also, since methods exist to terminate plastic fiber endfaces with very little skill and effort, the cost required to install plastic fiber links may also be lower than comparable links using glass fibers. Although multimode plastic fibers offer simplicity and lower cost, serious limitations exist in the technologies for fabricating these fibers. As a result, plastic fibers are usually produced with either a step refractive index profile or a graded refractive index profile that differs significantly from that required for maximum bandwidth.
One way to overcome the bandwidth limitations of glass and plastic multimode fibers is to introduce nonuniformities during manufacture, such that power diffuses between one or more pairs of the electromagnetic modes of the fiber. The presence of such nonuniformities is referred to herein as “mode coupling.” Mathematically, the diffusion of power between a pair of modes (labeled i and j) may be described with the differential equation dP
j
(x)/dx=C
ij
P
i
(x), where x is the spatial coordinate parallel to the fiber axis, P
j
(x) is the amount of power in the j
th
mode at point x, P
i
(x) is the amount of power in the i
th
mode at point x, and C
ij
is the coupling constant between modes i and j. A fiber of length L will be considered herein to be “mode coupled” if any of the coupling constants C
ij
are sufficiently large to produce a significant change in modal power distribution as an optical signal traverses the length of the fiber. A fiber that does not meet this condition is referred to herein as a “standard” multimode fiber. Since the coupling constants discussed above are often unknown in practice, a simpler, but less precise, definition of mode coupling strength will be introduced below. Note that a mode coupled fiber with a given set of coupling constants will cease to be mode coupled if it is cut to a sufficiently short length. By the same token, fiber that operates as a standard multimode fiber at one length will be mode coupled at a sufficiently long length.
In mode-coupled fibers, photons injected into the fiber sample many of the various electromagnetic modes while transiting the fiber. As a result, they arrive at the output end of the fiber with a narrower distribution of arrival times than they would in the absence of such mode coupling. The net result of this mode coupling is to reduce the effective intermodal dispersion of the multimode fiber, thereby increasing fiber bandwidth. This phenomenon has been well documented in both glass and plastic multimode optical fibers, and can be conveniently used to parameterize the strength of the mode coupling. Herein, if a multimode fiber has an index profile such that its monochromatic bandwidth would be B
o
in the absence of mode coupling, and if mode coupling acts to increase the observed monochromatic bandwidth to a value B
c
, then the mode coupling will be said to be of strength F=B
c
/B
o
.
While mode coupling improves the bandwidth of multimode fibers, it also increases their attenuation compared to a comparable standard multimode fibers. Many types of mode coupling non-uniformities result in an additional loss that increases quadratically with mode coupling strength. Mathematically, the excess loss &agr;
c
due to mode coupling is &agr;
c
=F
2*
0.5 dB/km.
While a significant body of prior art teaches methods for creating mode coupling in optical fibers, this knowledge has found little practical application, due to the increased loss that accompanies mode coupling. Historically, optical fibers were developed for use in long-distance links, where the large length scales involved made minimal fiber attenuation imperative. Since even a modest bandwidth improvement, say F=2, necessitated 2 dB/km excess loss, such fibers were not employed. In later years, when multimode optical fibers began to be significantly used for short-distance links, systems were designed with an assumption that the fiber medium would exhibit the very low losses achieved for long-distance transmission. Thus, existing link designs allow very little budget for attenuation in t
Fitel USA Corp.
Mendelsohn Steve
Tweel John
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