Optical waveguides – Optical fiber waveguide with cladding – Utilizing nonsolid core or cladding
Reexamination Certificate
2001-02-12
2003-09-02
Palmer, Phan T. H. (Department: 2882)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing nonsolid core or cladding
C385S123000, C385S124000, C385S126000
Reexamination Certificate
active
06614974
ABSTRACT:
FIELD
This patent specification relates to the field of optical fiber communications. More particularly, it relates to an optical fiber having a greater range of wavelengths and core diameters for which the optical fiber exhibits single-mode operation.
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.
FIG. 1
illustrates a cross-section of a conventional optical fiber
100
comprising a solid core region
102
surrounded by a solid cladding region
104
. As described in Dutton,
Understanding Optical Communications,
Prentice-Hall (1998), which is incorporated by reference herein, at p. 45, optical fibers may be made single-mode 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. More particularly, as described in Hecht,
Understanding Fiber Optics,
Prentice-Hall (1999), which is incorporated by reference herein, at pp. 68-71, for a given propagation wavelength &lgr;, a maximum core diameter D
max
for single-mode operation is given by Eq. (1) below, where n
1
is the refractive index of the core material, n
2
is the refractive index of the cladding material can be represented by:
D
max
=
2.4
λ
π
n
1
2
-
n
2
2
{
1
}
Also as described in Hecht, supra, for a given core diameter D, a cutoff wavelength &lgr;
c
below which propagation becomes multi-mode can be given by Eq. (2):
λ
c
=
π
D
n
1
2
-
n
2
2
2.4
{
2
}
More generally, a condition for which single-mode propagation will occur can be stated in terms of the ratio of the core diameter D to the wavelength &lgr; according to Eq. (3):
D
λ
≥
2.4
π
n
1
2
-
n
2
2
{
3
}
From a practical implementation perspective, it is desirable to make the diameter of the core region as large as possible while still maintaining single-mode operation in the wavelengths of operation. A larger core diameter allows for light to be more easily introduced into the fiber from light sources, thereby reducing the costs of both light sources and optical coupling equipment. A larger core diameter also allows for looser tolerances (i.e., reduced costs) in fiber splicing operations, and allows for other practical advantages. As indicated by Eq. (3) above, the maximum allowable core diameter increases as the refractive indices of the core material and cladding material get closer together. Of course, as these refractive indices get closer together, a corollary result is that the optical fiber may be made single-mode across a wider range of wavelengths for a fixed core diameter.
A problem, however, arises with conventional optical fibers in that current optical fiber manufacturing methods are restricted in their ability to precisely control the indices of refraction of the core material (n
1
) and the cladding material (n
2
). Because of this restricted ability, in commercially practical fiber the closeness of n
1
and n
2
is usually limited by design to no less than 0.1%. This, in turn, restricts the designed size of the core diameter for a given wavelength, and/or restricts the wavelengths of single-mode operation of a fiber for a given core diameter. For example, one common optical fiber manufacturing method referred to as flame hydrolysis uses a burner to fire a combination of metal halide particles and SiO
2
(called a “soot”) onto a rotating graphite or ceramic mandrel to make the optical fiber perform. See Keiser,
Optical Fiber Communications,
2
nd
ed., McGraw-Hill (1991), which is incorporated by reference herein, at pp. 63-68. The index of refraction is controlled by controlling the constituents of the metal halide vapor stream during the deposition process. The process is “open loop” without a feedback mechanism to precisely control the ultimate index of refraction of the optical material. Moreover, the metal halide vapor stream is limited in its controllability and in its ability to control the ultimate index of refraction of the optical material.
Thus, the above flame hydrolysis technique and similar prior art methods used to vary the relative refractive indices of the core and cladding material, which are generally referred to as “chemical” techniques herein, are generally limited in their ability to control these indices to closer than 0.1% from each other. Also, these techniques may introduce a substantial amount of unwanted impurities into the optical fiber, increasing Rayleigh scattering and reducing the quality and effectiveness of the optical fiber. Furthermore, a substantial degree of unwanted local or global variations in the doping may occur in the chemical deposition process and, because the optical fiber preform cannot be reheated to high temperatures without losing its desired refractive index profile, these variations remain in the final optical fiber and lessen its quality and effectiveness.
One wavelength band of great significance is the 1500-1610 nm band of operation of Erbium-Doped Fiber Amplifiers (EDFAs) used in most high-capacity, long-distance Dense Wavelength Division Multiplexing (DWDM) optical communications systems. Applying a refractive index difference of 0.1% between the core and the cladding and a wavelength of 1500 nm in Eq. (1), the maximum diameter of a conventional solid-core fiber would be about 17.8 &mgr;m for single-mode operation; applying a wavelength of 1100 nm in Eq. (1), the maximum diameter of a conventional solid-core fiber would be about 13.1 &mgr;m for single-mode operation. More commonly, a larger refractive index difference of 0.2% between the core and the cladding is used, for which the maximum diameter of a conventional solid-core fiber would be about 13.1 &mgr;m for a wavelength of 1500 nm and 9.6 &mgr;m for a wavelength of
Elrefaie Aly F.
Wang Shih-Yuan
Cooper & Dunham, LLP.
Gazillion Bits Inc.
Palmer Phan T. H.
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