Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2001-05-30
2003-12-30
Ullah, Akm E. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
C359S199200
Reexamination Certificate
active
06671445
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an optical waveguide fiber designed to compensate total dispersion, and particularly to an optical waveguide fiber designed to compensate total dispersion substantially equally over a range of wavelengths.
2. Technical Background
Dispersion compensation techniques in telecommunications systems or links have been used successfully. A technique useful in links already installed is one in which total dispersion (also called chromatic dispersion) is compensated by an appropriately designed waveguide fiber formed into a module that can be inserted into the link at an access point such as an end of the link. A drawback of this technique is that the compensation module adds loss to the system without adding useful system length. For situations in which the system loss budget has a small margin, the addition of a compensation module can cause unacceptably low signal to noise ratio.
Another dispersion compensation scheme involves the use of both positive and negative dispersion fibers in the cables of the link. Each cable can contain both positive and negative total dispersion waveguide fibers, or the link can be formed using cables having only positive dispersion together with cables having only negative dispersion. This compensation scheme avoids the drawback associated with the compensation module but necessarily complicates the installation and maintenance of the system. That is, the dispersion sign of a particular cable or of the fibers in the cable must be identified during installation. Also, an inventory of replacement cables would be increased over that required for standard systems because dispersion sign is an additional variable that must be taken into account in maintaining an effective inventory.
More recently, an alternative dispersion compensation technique has been developed in conjunction with a particular optical waveguide fiber having a total dispersion and a total dispersion slope which effectively mirrors that of the transmission fiber. That is, the ratio of total dispersion to total dispersion slope, &kgr;, has the same value for the transmission fiber and for the compensating fiber. This fiber type is disclosed and discussed in U.S. provisional application S.No. 60/217,967, incorporated herein by reference in its entirety.
For the telecommunications system in which mirror fiber is used, the compensation is said to be perfect in that the end to end accumulated dispersion of a span including a transmission fiber and a compensating fiber is zero across the wavelength range of operation. The result of such a configuration is that signals in the fiber traverse significant span lengths in which the total dispersion is zero or near zero.
However, in certain applications it may be desirable to use the 1:1 length ratio of transmission to dispersion compensating optical waveguide fiber, as in the case of certain mirror fiber, but still maintain a non-zero local dispersion to avoid dispersion penalties due to four wave mixing and cross phase modulation. In this case, one would need a compensating waveguide fiber that mirrored the total dispersion slope but not the total dispersion of the transmission fiber.
In addition, perhaps because of consideration of the effective area or attenuation of the compensating fiber, one may wish to use a length ratio other than 1:1, for example a ratio of 1.5:1, or 2:1, where the longer length is typically taken to be the that of the transmission fiber.
There is therefore a need for dispersion compensating optical waveguide fibers designed to meet a variety of compensation formats that derive from the variety of system performance requirements together with a desired transmission to compensating fiber length ratio.
Definitions
The following definitions are in accord with common usage in the art.
The refractive index profile is the relationship between refractive index or relative refractive index and waveguide fiber radius.
A segmented core is one that is divided into at least a first and a second waveguide fiber core portion or segment. Each portion or segment is located along a particular radial length, is substantially symmetric about the waveguide fiber centerline, and has an associated refractive index profile.
The radii of the segments of the core are defined in terms of the respective refractive indexes at respective beginning and end points of the segments.
The definitions of the radii used herein are set forth in the figures and the discussion thereof.
Total dispersion, sometimes called chromatic dispersion, of a waveguide fiber is the sum of the material dispersion, the waveguide dispersion, and the inter-modal dispersion. In the case of single mode waveguide fibers the inter-modal dispersion is zero.
The sign convention generally applied to the total dispersion is as follows.
Total dispersion is said to be positive if shorter wavelength signals travel faster than longer wavelength signals in the waveguide. Conversely, in a negative total dispersion waveguide, signals of longer wavelength travel faster.
The effective area is
A
eff
=2&pgr;(∫
E
2
rdr
)
2
/(∫
E
4
rdr
),
where the integration limits are 0 to ∞, and E is the electric field associated with light propagated in the waveguide.
The relative refractive index percent, &Dgr;%=100×(n
i
2
−n
c
2
)/2n
i
2
, where n
i
is the maximum refractive index in region i, unless otherwise specified, and n
c
is the average refractive index of the cladding region. In those cases in which the refractive index of a segment is less than the average refractive index of the cladding region, the relative index percent is negative and is calculated at the point at which the relative index is most negative unless otherwise specified.
The term &agr;-profile refers to a refractive index profile, expressed in terms of &Dgr;(b)%, where b is radius, which follows the equation,
&Dgr;(
b
)%=&Dgr;(
b
o
)(1−[
b−b
o
/(
b
1
−b
o
)]
&agr;
),
where b
o
is the point at which &Dgr;(b)% is maximum, b
1
is the point at which &Dgr;(b)% is zero, and b is in the range b
i
≦b≦b
f
, where delta is defined above, b
i
is the initial point of the &agr;-profile, b
f
is the final point of the &agr;-profile, and &agr; is an exponent which is a real number.
A waveguide fiber telecommunications link, or simply a link, is made up of a transmitter of light signals, a receiver of light signals, and a length of waveguide fiber having respective ends optically coupled to the transmitter and receiver to propagate light signals therebetween. The length of waveguide fiber can be made up of a plurality of shorter lengths that are spliced or connected together in end to end series arrangement. A link can include additional optical components such as optical amplifiers, optical attenuators, optical switches, optical filters, or multiplexing or demultiplexing devices. One may denote a group of inter-connected links as a telecommunications system.
The pin array bend test is used to compare relative resistance of waveguide fibers to bending. To perform this test, attenuation is measured for a waveguide fiber with essentially no induced bending loss. The waveguide fiber is then woven about the pin array and attenuation again measured. The loss induced by bending, typically expressed in units of dB, is the difference between the two attenuation measurements. The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center to center. The pin diameter is 0.67 mm. The waveguide fiber is caused to pass on opposite sides of adjacent pins. During testing, the waveguide fiber is placed under a tension just sufficient to make the waveguide conform to a portion of the periphery of the pins. The test pertains to macro-bend resistance of the waveguide fiber.
Another bend test referenced herein is the lateral load test. In this test a prescribed length of waveguide fiber is
Bickham Scott R.
Cain Michael B.
Kumar Shiva
Mishra Snigdharaj K.
Srikant V.
Chevenak William J.
Corning Incorporated
Ullah Akm E.
Wayland Randall S.
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