Optical waveguides – Optical fiber waveguide with cladding – With graded index core or cladding
Reexamination Certificate
1999-06-07
2001-09-18
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
With graded index core or cladding
C385S123000, C385S126000, C385S127000
Reexamination Certificate
active
06292612
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a multi-mode optical fiber having an improved refractive index profile and to a communications system comprising the inventive fiber.
BACKGROUND OF THE INVENTION
Optical fibers are well known in the art and useful for many applications, including transmission laser devices and amplifiers. Basically, an optical fiber comprises an inner core fabricated from a dielectric material having a certain index of refraction and a cladding surrounding the core. The cladding is comprised of a material having a lower index of refraction than the core. As long as the refractive index of the core exceeds that of the cladding, a light beam propagated along the core exhibits total internal reflection, and it is guided along the length of the core. In most practical applications, the refractive indices of the core and cladding differ from each other by less than a few percent.
Designs for optical fibers vary depending upon the application, the desired mode of transmission of the light beam, or the materials used in fabrication. Fibers can be fabricated to propagate light with a single mode or multiple modes; an optical fiber which supports more than one guided mode is referred to as a multi-mode fiber. Multi-mode fibers typically have a larger core diameter than single-mode fibers to enable a larger number of modes to pass through the fiber. Additional design constraints are posed by multi-mode applications. Such constraints may include, for example, the choice of materials used to fabricate the core and cladding and the refractive index profile of the fiber (e.g., the profile reflects the radial variation in refractive index from the center of the fiber to the outer circumference of the cladding). Various types of refractive index profiles are known, e.g., step index, graded index, depressed clad, or W-type variety.
For high bandwidth, the group velocities of the various modes in multi-mode fibers should be as close to equal as possible. The differential group velocities can be controlled by grading the refractive index of the material comprising the core, which means specifying a functional form of the index as a function of the fiber radius. In a conventional multi-mode fiber, the design goal has been to achieve an &agr;-profile, which is defined as:
n
(
r
)
-
n
clad
n
(
r
)
=
Δ
(
1
-
(
r
/
r
core
)
α
)
,
where r is the radius of the fiber, r
core
is the radius of the core, n
clad
is the refractive index of the cladding, and &agr; and &Dgr; are free parameters. The optimal choice of parameters depends on the properties of the materials comprising the fiber and the intended application.
There are inherent limitations, however, in the &agr;-class profiles, and manufacturing variables make it difficult, in practice, to achieve the theoretically optimal &agr;-profile. For example, an inherent limitation with &agr;-class profiles is that high order modes are not properly compensated (high order modes are those belonging to principle mode groups of high order; principle mode groups are groups of modes which propagate with nearly equal phase velocity, and the high order mode groups are those nearest to cutoff). Additionally, manufacturing variations can occur anywhere in the profile. With the two most frequently-used fabrication techniques, OVD and MCVD, anomalies are particularly problematic near the center of the fiber, i.e., anomalies occur near the center with greater frequency and magnitude than at other regions of the fiber. A common side effect of the MCVD process is a pronounced index depression, or dip, in the center of the fiber, which results in poorly compensated low order modes (i.e. those with small principle mode number). In other words, when a center dip is present the modes which have fields confined near to the central axis of the fiber have substantially different group velocities than the majority of the modes. Poorly compensated low order modes can dramatically affect fiber performance for certain applications, e.g. under launch from a semiconductor laser.
Efforts have been made to develop fiber index profiles to equalize high order modes in a multi-mode fiber and to compensate for the center dip. See Okamoto et al., “Computer-Aided Synthesis of the Optimum Refractive-Index Profile for a Multi-Mode Fiber,” IEEE T
RANS.
M
ICROWAVE
T
HEORY AND
T
ECHNIQUES,
Vol. MTT-25, No. 3 (March 1977), at p. 213 (incorporated herein) (hereinafter “Okamoto”). In Okamoto, a computer-aided synthesis is applied to develop an optimal profile, which is reported to be a smoothed W-shaped profile (e.g.,
FIG. 1
thereof). Essentially, this profile involves an extension of the alpha shape below the cladding (e.g., outside the core/cladding boundary region), with a negative cladding jump and then a further numerical refinement of the shape of the profile. See also Okamoto et al., “Analysis of Wave Propagation in Optical Fibers Having Core with &agr;-Power Refractive-Index Distribution and Uniform Cladding,” IEEE T
RANS.
M
ICROWAVE
T
HEORY AND
T
ECHNIQUES,
Vol. MTT-24, No. 7 (July 1976), at p. 416 (incorporated herein), discussing use of numerical analysis to propose a similar profile. While such profiles may be advantageous in leading to high bandwidths, they are difficult to manufacture, and it is believed they may lead to leaky modes. In Geshiro et al., “Truncated Parabolic-Index Fiber with Minimum Mode Dispersion,” IEEE T
RANS.
M
ICROWAVE
T
HEORY AND
T
ECHNIQUES,
Vol. MTT-26, No. 2 (February 1978), at p. 115 (incorporated herein), a parabolic index profile is combined with a cladding jump, which leads to higher bandwidths than with a parabolic profile with no cladding jump. In a parabolic index profile, the core has a refractive index profile that has a parabolic distribution and is surrounded by a cladding having a constant refractive index.
As may be appreciated, those concerned with the development of optical communications systems continually search for new components and designs including new fiber designs. As optical communications systems become more advanced, there is growing interest in multi-mode fibers and increased fiber performance. The instant invention provides a multi-mode fiber having a refractive index profile that is relatively easy to manufacture and yet improves the behavior of high-order modes propagated by the fiber and compensates for the presence of a center dip, thereby improving the behavior of low order modes. Further advantages may appear more fully upon considering the description given below.
SUMMARY OF THE INVENTION
Applicants have discovered that a multi-mode fiber having increased performance may be made by modifying a conventional &agr;-profile by (i) modifying the profile near the core-cladding boundary, to improve the behavior of high-order modes relative to that of an &agr;-profile, and (ii) adding an index ridge near the center of the fiber to compensate for the presence of a center dip, thereby improving the behavior of the low-order modes. Gains in performance also may be achieved by modifying the profile with a ripple adjacent the core-cladding boundary.
REFERENCES:
patent: 4560247 (1985-12-01), Aldebert
patent: 5013131 (1991-05-01), Frotheringham
patent: 5729645 (1998-03-01), Garito et al.
patent: 5940567 (1999-08-01), Garito
patent: 5956448 (1999-09-01), Smolka et al.
patent: 6078715 (2000-06-01), Fujii et al.
Geshiro et al., “Truncated Parabolic-Index Fiber with Minimum Mode Dispersion”,IEEE Transactions on Microwave Theory and Techniques., vol. MTT 26, No. 2, Feb. 1978 pp 115-119.
Okamoto et al., “Analysis of Wave Propagation in Optical Fibers Having Core with &agr;-Power Refractive-Index Distribution and Uniform Cladding”,IEEE Transactions on Microwave Thery and Techniques, vol. MTT-24, No. 7, Jul. 1976 pp 416-421.
Okamato et al., “Computer-Aided Synthesis of the Optimum Refractive-Index Profile for a Multimode Fiber”,IEEE Transactions on Microwave Theory and Techniques., vol. MTT 25, No. 3, Mar. 1977 pp 213-221.
Golowich Steven Eugene
Reed William Alfred
Ritger Albert John
Lowenstein & Sandler PC
Lucent Technologies - Inc.
Sanghavi Hemang
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