Low attenuation optical waveguide

Details Low attenuation optical waveguide Low attenuation optical waveguide

2001-07-03

2002-06-11

Palmer, Phan T.H. (Department: 2874)

Optical waveguides

Optical fiber waveguide with cladding

With graded index core or cladding

C385S127000, C385S126000, C385S123000

Reexamination Certificate

active

06404965

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to an optical waveguide fiber optimized for low attenuation. In particular, waveguide fiber attenuation is minimized for any core refractive index profile by proper selection of the core refractive index profile variables.
The dependence of waveguide properties upon the configuration of the refractive index profile has been described in the pioneering patent, U.S. Pat. No. 4,715,679, Bhagavatula. In that patent, core refractive index profiles are disclosed which provide for a variety of waveguide fiber properties, especially those having a zero dispersion wavelength shifted into the 1550 nm operating window and those which have a relatively constant dispersion over an extended wavelength range such as 1250 nm to 1600 nm.
In response to demands for specialized waveguide fibers, particularly with regard to high performance waveguides, investigation of waveguide core refractive index profiles has intensified. For example in U.S. Pat. No. 5,483,612, Gallagher et al., (the '612 patent) there is disclosed a core profile design which provides low polarization mode dispersion, low attenuation, a shifted dispersion zero, and low dispersion slope. Other core refractive index profiles have been designed to meet the requirements of applications which include the use of higher power signals or optical amplifiers.
A problem which may arise when a core profile is altered in order to arrive at a desired property is that the property is realized at the expense of another essential property. For example, a certain core refractive index profile design may provide increased effective area, thus reducing non-linear distortion of the signal. However, in this large effective area waveguide fiber, the bend resistance may be seriously compromised. Thus, core profile design is an exacting task, in which model studies usually precede the manufacturing stage of product development.
The interaction of the profile variables is such that one skilled in the art usually cannot, except perhaps in a very general way, predict the impact of a refractive index profile change upon such waveguide properties as, bend resistance, attenuation, zero dispersion wavelength, and total dispersion and total dispersion slope over a selected wavelength range. Therefore, studies of waveguide refractive index profiles usually include a computer simulation of the particular profile or family of profiles. Manufacturing testing is then carried out for those refractive index profiles which exhibited the desired properties.
In a continuation of the work disclosed in the '612 patent, a family of profiles was found which produced a high performance fiber having a zero dispersion wavelength above a pre-selected band of wavelengths and excellent bend resistance. A description of this work has been filed recently as a provisional application, Ser. No. 60/050550.
As further model studies and manufacturing tests were completed, it became clear that:
a particular family of profiles could be found to provide a selected set of operating parameters; and, most surprisingly,
the profiles of the particular family could be further adjusted to optimize attenuation without materially changing the operating parameters.
Definitions
The radii of the regions of the core are defined in terms of the index of refraction. A particular region has a first and a last refractive index point. The radius from the waveguide centerline to the location of this first refractive index point is the inner radius of the core region or segment. Likewise, the radius from the waveguide centerline to the location of the last refractive index point is the outer radius of the core segment. Other definitions of core geometry may be conveniently used.
Unless specifically noted otherwise in the text, the parameters of the index profiles discussed here are defined as follows:
radius of the central core region is measured from the axial centerline of the waveguide to the intersection with the x axis of the extrapolated central index profile;
radius of the second annular region is measured from the axial centerline of the waveguide to the center of the baseline of the second annulus; and,
the width of the second annular region is the distance between parallel lines drawn from the half refractive index points of the index profile to the waveguide radius.
The dimensions of the first annular region are determined by difference between the central region and second annular region dimensions.
Core refractive index profile is the term which describes the refractive index magnitude defined at every point along a selected radius or radius segment of an optical waveguide fiber.
A compound core refractive index profile describes a profile in which at least two distinct segments are demarcated.
The relative index percent (&Dgr;%) is:
&Dgr;%=[(n
1
2
−n
c
2
)/2n
1
2
]×100, where n
1
is a core index and n
c
is the minimum clad index. Unless otherwise stated, n
1
is the maximum refractive index in the core region characterized by a % &Dgr;.
The term alpha profile refers to a refractive index profile which follows the equation,
n(r)=n
0
(1−&Dgr;[r/a]
&agr;
) where r is radius, &Dgr; is defined above, a is the last point in the profile, r is chosen to be zero at the first point of the profile, and I is a real number. For example, a triangular profile has &agr;=1, a parabolic profile has &agr;=2. When &agr; is greater than about 6, the profile is essentially a step. Other index profiles include a step index, a trapezoidal index and a rounded step index, in which the rounding may be due to dopant diffusion in regions of rapid refractive index change.
The profile volume is defined as 2∫
r1
r2
(&Dgr;% r dr). The inner profile volume extends from the waveguide centerline, r=0, to the crossover radius. The outer profile volume extends from the cross over radius to the last point of the core. The units of the profile volume are % &mgr;m
2
because refractive index is dimensionless. To avoid confusion, the profile volumes will be connoted a number followed by the word units.
The crossover radius is found from the dependence of power distribution in the signal as signal wavelength changes. Over the inner volume, signal power decreases as wavelength increases. Over the outer volume, signal power increases as wavelength increases.
The bend resistance of a waveguide fiber is expressed as induced attenuation under prescribed test conditions. A bend test referenced herein is the pin array bend test which is used to compare relative resistance of waveguide fiber 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. During testing, sufficient tension is applied to make the waveguide fiber conform to a portion of the pin surface.
The bend test used in the model calculations was a single turn of waveguide fiber around a 30 mm diameter mandrel.
The effective group refractive index (n
geff
) is the ratio of the velocity of light to the group velocity. The mathematical expression for n
geff
in terms of electromagnetic field, refractive index, wavelength and propagation constant, derives from Maxwell's equations, or, more particularly, from the scalar wave equation.
The propagation constant &bgr;, also called the effective refractive index is an electromagnetic field parameter related to field propagation velocity and is found by solving the scalar wave equation for a selected waveguide. Because &bgr; depends upon waveguide geometry, one may expect that bending the waveguide will change &bgr;. An example of a scalar wave equation descriptiv

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