Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-07-27
2002-08-06
Ullah, Akm E. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Reexamination Certificate
active
06430346
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a long haul optical waveguide fiber, and particularly to such an optical waveguide fiber that has negative dispersion. The waveguide fiber of the invention can be cabled and used to form all or a portion of an optical telecommunications link.
2. Technical Background
Communications systems operating at bit rates above about a giga-hertz or which include wavelength division multiplexing are facilitated through use of high performance waveguides. In such high performance systems launched power can range from 0.1 mW to 10 mW and higher. In the higher power systems, the desired properties of the waveguide fiber include larger effective area. New system strategies are being sought to decrease cost even while system performance is being enhanced.
A promising strategy is one that involves matching system components in such a way that a particular property of one component compensates a drawback in another component. Preferably, the component matching strategy is one in which a given component is designed to allow another component to operate more efficiently or effectively. Such compensation schemes have been effective, for example, in reducing dispersion penalty by adding a dispersion compensating module to within a communications link, thereby providing for a desired signal to noise ratio or signal pulse shape after the signal pulse has traversed the optical waveguide fiber of the link. Another example of effective compensation is the use of large effective area waveguide fiber in communications systems in which non-linear effects are a major source of signal degradation.
One area which can provide an increase in performance and a decrease in cost is that of matching a signal source to a fiber. A cost effective signal source, having relatively high power output and good longevity is the distributed feedback laser (DFB) which is directly modulated. However a directly modulated DFB laser is always positively chirped. That is, the leading edge of the pulse is shifted to longer wavelengths (red shifted) and the trailing edge is blue shifted. When such a pulse propagates in a positive dispersion fiber, the positive chirp results in pulse broadening. Efforts have been made to reduce the effect of positive chirp by biasing the semi-conductor laser above threshold. See
Fiber Optic Communications Systems
, G. P. Agrawal, p. 223.
DEFINITIONS
The following definitions are in accord with common usage in the art.
The refractive index profile is the relationship between 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 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-mode 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.
A chirped laser is one that produces an output pulse wherein the wavelengths within the pulse wavelength are shifted backward or forward in time. That is, the output pulse is red or blue shifted. A laser having a positive chirp is one in which the leading edge of the output pulse is red shifted and the trailing edge blue shifted.
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. An effective diameter, D
eff
, may be defined as,
A
eff
=&pgr;(
D
eff
/2)
2
.
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.
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 a is an exponent which is a real number. The initial and final points of the &agr;-profile are selected and entered into the computer model. As used herein, if an &agr;-profile is preceded by a step index profile or any other profile shape, the beginning point of the &agr;-profile is the intersection of the &agr;-profile and the step or other profile.
In the model, in order to bring about a smooth joining of the &agr;-profile with the profile of the adjacent profile segment, the equation is rewritten as;
&Dgr;(
b
)%=&Dgr;(
b
a
)+[&Dgr;(
b
o
)−&Dgr;(
b
a
)]{(1
−[¦b−b
o
/(
b
1
−b
o
)]
&agr;
},
where b
a
is the first point of an adjacent segment.
The pin array bend test is used to compare relative resistance of waveguide fibers to bending. To perform this test, attenuation loss 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 is the difference between the two measured attenuations. 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.
Another bend test referenced herein is the lateral load test. In this test a prescribed length of waveguide fiber is placed between two flat plates. A #70 wire mesh is attached to one of the plates. (The market code #70 mesh is descriptive of screen made of wire having a diameter of 0.178 mm. The screen openings are squares of side length 0.185 mm.) A known length of waveguide fiber is sandwiched between the plates and a reference attenuation is measured while the plates are pressed together with a force of 30 newtons. A 70 newton force is then applied to the plates and the increase in attenuation in dB/m is measured. This increase in attenuation is the lateral load attenuation of the waveguide.
Adiabatic chirp is proportional to the output power of the signal.
Transient chirp is proportional to the derivative of the output power of the signal and so is present only in the time periods when the signal power is in transition between a 0 and a 1 (or a 1 to a 0).
Gain compression factor, also known as the nonlinear gain parameter, refers to a semiconductor laser and is a proportionality constant that relates semiconductor laser material optical gain of the active region of the laser to the number of photons in the active region. In the relationship, G=f(&egr;P), G is the gain of the laser, &egr; is the gain compression factor, P is number of photons in the active region (which is directly related to the laser output power) and f is a function. See
Fiber Optic Communications S
Conradi Jan
Kumar Shiva
Rosenblum Steven S.
Chervenak William J.
Corning Incorporated
Ullah Akm E.
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