Optical waveguide fiber

Details Optical waveguide fiber Optical waveguide fiber

2000-08-18

2002-12-03

Sanghavi, Hemang (Department: 2874)

Optical waveguides

Optical fiber waveguide with cladding

C385S124000

Reexamination Certificate

active

06490396

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention is directed to a single mode optical waveguide fiber, more particularly to a waveguide fiber in which the total dispersion is maintained at a low value over a selected wavelength range.
Because of the high data rates and the need for long regenerator spacing, the search for high performance optical waveguide fibers designed for long distance, high bit rate telecommunications has intensified. An additional requirement is that the waveguide fiber be compatible with optical amplifiers, which typically show an optimum gain curve in the wavelength range 1530 nm to 1570 nm. Consideration is also given to the potential of expanding the usable wavelength into the L-Band range of about 1570 nm to 1700 nm, more preferably in the range of about 1570 nm to 1625 nm. Another optical waveguide fiber operating wavelength range is the band that extends from about 1250 nm to 1350 nm. Although attenuation in this lower band is greater in comparison to the higher wavelength windows of operation, this lower wavelength band can provide additional information channels that significantly increase overall system capacity.
In cases where waveguide information capacity is increased by means of wavelength division multiplexing (WDM) technology, an additional waveguide fiber property becomes important. For WDM, high bit rate systems, the waveguide should have exceptionally low, but non-zero, total dispersion over the wavelength range of operation, thereby limiting the non-linear dispersion effect of four wave mixing.
Another non-linear effect that can produce unacceptable dispersion in systems having a high power density, i.e., a high power per unit area, is self phase modulation. Self phase modulation may be controlled by designing a waveguide core which has a large effective area, thereby reducing the power density. An alternative approach is to control the sign of the total dispersion of the waveguide so that the total dispersion of the waveguide serves to counteract the dispersion effect of self phase modulation.
A waveguide having a positive dispersion, where positive means shorter wavelength signals travel at higher speed than those of longer wavelength, will produce a dispersion effect opposite that of self phase modulation, thereby substantially eliminating self phase modulation dispersion.
Such a waveguide fiber is disclosed and described in U.S. patent application Ser. No.08/559,954, incorporated herein in its entirety by reference. The present novel profile improves upon the Ser. No.08/559,954 fiber by increasing effective area. In addition the waveguide of this disclosure has a total dispersion over the wavelength window of operation that is everywhere positive and has a lower limit greater than about 2.0 ps
m-km to further reduce the power penalty due to four wave mixing.
Thus there is a need for an optical waveguide fiber which:
is single mode over at least the wavelength range 1530 nm to 1570 nm, and preferably over a range that extends to the lower wavelength 1250 nm;
has a zero dispersion wavelength outside the range 1530 nm to 1570 nm;
has a positive total dispersion over the wavelength range 1530 nm to 1625 nm which is not less than about 2.0 ps
m-km;
has low attenuation, less than about 0.25 dB/km, over the range of about 1570 nm to 1625 nm; and
retains the usual high performance waveguide characteristics such as high strength and acceptable resistance to bend induced loss.
The concept of adding structure to the waveguide fiber core by means of core segments, having distinct profiles to provide flexibility in waveguide fiber design, is described fully in U.S. Pat. No. 4,715,679, Bhagavatula. The segmented core concept can be used to achieve unusual combinations of waveguide fiber properties, such as those described herein.
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. The core refractive index profiles of the invention are described in terms of upper and lower profile boundaries. In addition particular embodiments are described in terms of the relative index &Dgr;(r)% (defined below) value at a number of radius points. The points chosen fully describe the refractive index profile in each case.
The radii descriptive of the index profiles disclosed herein appear in the drawings.
The effective area is,
A
eff
=2&pgr;(∫
E
2
r dr
)
2
/(∫
E
4
r dr
),
where the integration limits are 0 to ∞, and E is the electric field associated with the propagated light. An effective diameter, D
eff
, may be defined as,
A
eff
=&pgr;(
D
eff
/2)
2
.
The initials WDM represent wavelength division multiplexing.
The initials SPM represent self phase modulation, a non-linear optical phenomenon wherein a signal having a power density above a specific power level will travel at a different speed in the waveguide relative to a signal below that power density. SPM causes signal dispersion comparable to that of linear dispersion having a negative sign.
The initials FWM represent four wave mixing, the phenomenon wherein two or more signals in a waveguide interfere to produce signals of different frequencies.
The term, &Dgr;%, represents a relative measure of refractive index defined by the equation,
 &Dgr;%=100×(
n
i
2
−n
c
2
)/2
n
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 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 maximum point of the profile and b
1
is the point at which A(b)% is zero and b is in the range b
i
≦b ≦b
f
, where &Dgr;% 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.
In a computer model of the profile, 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 the 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 when the waveguide fiber is arranged such that no induced bending loss occurs. This waveguide fiber is then woven about the pin array and attenuation again measured. The loss induced by bending 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.
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.
SUMMARY OF THE INVENTION
The low

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