Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2002-05-15
2004-10-19
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
Reexamination Certificate
active
06807350
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to optical fibers and more specifically to optical fibers with a controlled refractive index profile.
BACKGROUND OF THE INVENTION
Beam splitters and combiners are widely used today in optical communications systems. Optical splitters divide a single light beam into a plurality of light beams. Conversely, combiners take a plurality of light beams and combine them into a fewer number of beams (e.g. eight to one). Combiners and splitters generally introduce two losses into the overall system performance, “insertion loss” and “excess loss.” Insertion loss is the loss in a transmission line that results from inserting (i.e., introducing) the component in the transmission line (e.g., a 3 dB 2×2 coupler inserted into a line will introduce a 50% or 3 dB loss in the line). Excess loss is the internal loss attributable to the specific component itself.
Excess loss arises, for example, from fiber fusion and/or tapering of the fiber, and is generally related to the number of splits or combinations of fibers made in the component. (Fusion refers to localized heating and softening of two or more fibers to attach them together; tapering relates to reduction in the size of the fiber geometry, e.g., as a result of fusion or polishing.) For example, an 8-to-1 splitter will generally have a higher excess loss than a 4-to-1 splitter designed for the same operation band, with the same fibers. Fibers can be joined by means of cladding-fused and cladding-polished couplers.
With reference to
FIG. 1
, a typical single mode optical fiber
100
used to create either a cladding-polished or cladding-fused coupler includes a core
110
and a cladding
120
. In a cladding-fused coupler, shown in
FIG. 2
, the claddings of two single mode fibers
100
a
,
100
b
are fused together, thereby fixing cores
110
a
,
110
b
in close proximity to each other at the region of joinder. Cross-talk between cores
110
a
,
100
b
results in transfer of a portion of the signal propagating through one of the cores to the other core. In cladding-polished couplers, the core
110
of each fiber is polished flat and mechanically aligned to the other core
110
, thereby inducing cross-talk between the cores.
The coupling efficiency of both cladding-fused and cladding-polished couplers is reduced because a portion of the signal propagating in the core is transferred to cladding
120
at the point of coupling, thereby increasing the excess loss of the device incorporating the coupled fibers.
Accordingly, there exists a need for an optical fiber with reduced or minimal excess loss resulting from coupling.
SUMMARY OF THE INVENTION
The present invention is directed to improving the coupling efficiency of both cladding-fused and cladding-polished optical couplers thereby reducing excess loss.
One aspect of the invention is directed to an optical fiber including a core having a diameter d
c
and a refractive index n
c
, a primary cladding concentrically surrounding the core and having an outer diameter d
pc
, and a refractive index n
pc
that is less than the core refractive index n
c
and a secondary cladding concentrically surrounding the primary cladding. The secondary cladding has an outer diameter d
sc
and a refractive index n
sc
that is less than the primary cladding refractive index n
pc
. The primary cladding outer diameter d
pc
is a fraction of the secondary cladding outer diameter d
sc
. The difference between the primary cladding refractive index n
pc
and the secondary cladding refractive index n
sc
is greater than or equal to a constant and less than or equal to the core refractive index n
c
less the constant.
In other embodiments, the primary cladding is updoped or the secondary cladding is downdoped. The primary cladding outer diameter is a fraction of the secondary cladding outer diameter. The fraction may range from >0% to 75%; is desirably 10% to 50%; and is most preferably around 20%. The above-mentioned constant relating the refractive indices of the primary and secondary claddings is preferably equal to or greater than 0.0002.
In another embodiment, the cladding and/or the core includes a material inducing photosensitivity, such that exposure to radiation creates a refractive-index modulation within the cladding and/or the core.
In another aspect, the invention is directed to a method of coupling a pair of optical fibers. The method includes the steps of providing a pair of fibers and fusing the fibers along a portion of their longitudinal extents to couple them.
The primary claddings may or may not be fused. Thus, in some embodiments, the primary claddings are fused, while in other embodiments, the primary claddings are not fused but the secondary claddings are fused proximate to the primary claddings.
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Patent Cooperation Treaty, International Search Report, International Application No. PCT/US02/15409, mailed on Nov. 21, 2002, 8 pages.
Awaleh Musa
Fournier Paula
MacDonald John
Soufiane Abdelouahed
Rahll Jerry T
Testa Hurwitz & Thibeault LLP
Ullah Akm Enayet
Verrillon Inc.
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