Large effective area waveguide fiber

Details Large effective area waveguide fiber Large effective area waveguide fiber

2000-07-05

2002-12-10

Sanghavi, Hemang (Department: 2874)

Optical waveguides

Optical fiber waveguide with cladding

Utilizing multiple core or cladding

C385S124000, C385S123000, C385S127000

Reexamination Certificate

active

06493495

ABSTRACT:

BACKGROUND
The invention relates to a single mode optical waveguide fiber having a large effective area, A
eff
, for light transmission. The large effective area reduces non-linear optical effects, including self phase modulation, four wave mixing, cross phase modulation, and non-linear scattering processes, which can cause degradation of signals in high power systems. In general, a mathematical description of these non-linear effects includes the ratio, P/A
eff
, where P is optical power. For example, a non-linear optical effect usually follows an equation containing a term, exp [PxL
eff
/A
eff
], where L
eff
is effective length. Thus, an increase in A
eff
produces a decrease in the non-linear contribution to the degradation of a light signal.
The requirement in the telecommunication industry for greater information capacity over long distances, without regenerators, has led to a reevaluation of single mode fiber index profile design.
The focus of this reevaluation has been to provide optical waveguides which:
reduce non-linear effects such as those noted above;
are optimized for the lower attenuation operating wavelength range around 1550 nm;
are compatible with optical amplifiers; and,
retain the desirable properties of optical waveguides such as high strength, fatigue resistance, and bend resistance.
A waveguide fiber, having at least two distinct refractive index segments was found to have sufficient flexibility to meet and exceed the criteria for a high performance waveguide fiber system. The genera of segmented core designs are disclosed in detail in U. S. Pat. 4,715,679, Bhagavatula. Species of the profiles disclosed in the '679 patent, having properties especially suited for particular high performance telecommunications systems, are disclosed in applications Ser. Nos. 08/323,795 and 08/287,262.
The present invention is yet another core index profile species which reduces non-linear effects and which is particularly suited to transmission of high power signals over long distances without regeneration. The definition of high power and long distance is meaningful only in the context of a particular telecommunication system wherein a bit rate, a bit error rate, a multiplexing scheme, and perhaps optical amplifiers are specified. There are additional factors, known to those skilled in the art, which have impact upon the meaning of high power and long distance. However, for most purposes, high power is an optical power greater than about 10 mw. For example, a long distance is one in which the distance between electronic regenerators can be in excess of 100 km.
Considering the Kerr non-linearities, i.e., self phase modulation, cross phase modulation and four wave mixing, the benefit of large A
eff
can be shown from the equation for refractive index. The refractive index of silica based optical waveguide fiber is known to be non-linear with respect to the light electric field. The refractive index may be written as,
n=n
0
+n
2
P/A
eff
,
where n
0
is the linear refractive index, n
2
is the non-linear index coefficient, P is light power transmitted along the waveguide and A
eff
is the effective area of the waveguide fiber. Because n
2
is a constant of the material, increase in A
eff
is essentially the only means for reducing the non-linear contribution to the refractive index, thereby reducing the impact of Kerr type non-linearities.
Thus there is a need for an optical waveguide fiber designed to have a large effective area. The window of operation of greatest interest at this time is that near 1550 nm.
Definitions
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
An alpha profile is
n=n
0
(1−&Dgr;(
r/a
)
&agr;
),
 where n
0
is the refractive index at the first point of the alpha index profile, &Dgr; is defined below, r is radius, and a is the radius measured from the first to the last point of the alpha index profile, and r is chosen to be zero at the first point of the alpha index profile.
The width of an index profile segment is the distance between two vertical lines drawn from the respective beginning and ending points of the index profile to the horizontal axis of the chart of refractive index vs. radius.
The % index delta is
% &Dgr;=[(
n
1
2
−n
c
2
)/2
n
1
2
]×100,
 where n
1
is a core index and n
c
is the clad index. Unless otherwise stated, n
1
is the maximum refractive index in the core region characterized by a % &Dgr;.
A tapered step index profile, is a step index profile which has been modified by dopant diffusion during the waveguide fiber manufacturing process. The dopant diffusion causes the substantially right angles at the top and bottom of the step to become rounded and the sides of the step to be tapered. The amount of diffusion depends upon several variables including the details of the process steps and upon the initial height and width of the step index profile.
The exact amount of taper is not a critical determinant of the waveguide fiber properties herein discussed. However, a general description of degree of taper may be given.
A sharply tapered step is one in which the width at half the % &Dgr; is in the range of about 30 to 50% of the base width and the width at 0.9 of the % &Dgr; is in the range of about 15 to 25% of the base width.
A moderately tapered step is one in which the width at half the % &Dgr; is in the range of about 60 to 80% of the base width and the width at 0.9 of the % &Dgr; is in the range of about 35 to 50% of the base width.
The index profiles discussed herein, in general, are in the ranges of sharply or moderately tapered profiles. However, the invention is not limited to profile segments having a particular degree of taper.
SUMMARY OF THE INVENTION
This invention meets the need for a waveguide fiber having an index profile tailored for high performance operation in the 1550 nm window while maintaining a relatively large effective transmission area. It is noteworthy that a large effective area is achieved while maintaining good bend resistance.
A first aspect of the invention is a single mode waveguide fiber having an operating range from about 1500 nm to 1600 nm. A waveguide designed for operation in this wavelength range may be called a dispersion shifted waveguide. That is, the zero of total dispersion lies in range of about 1500 nm to 1600 nm.
The waveguide has a core glass region comprising at least two segments surrounded by a clad glass layer of refractive index n
c
. The index profiles of the segments comprising the core region are tailored to provide an effective area of at least 70 microns
2
.
In an embodiment of the first aspect, the core region comprise three segments. The central segment is a tapered step index profile having a maximum % &Dgr; and a width, measured at the base of the step. The exact amount of taper and the shape of the top of the index profile, whether triangular or uneven, is in general not of critical importance. Unless expressly stated otherwise, all widths are measured at the base of a particular core segment. This central segment includes an index depression on the waveguide centerline, i.e., the line of symmetry along the long waveguide fiber axis. This depression approximates the shape of an inverted cone. The central depression is due to the well known dopant loss by diffusion. It is also well known that process differences can increase or decrease the size of this central depression. However, with proper process the central depression can be held relatively constant from waveguide to waveguide. In general, this central depression is not cylindrically symmetric.
A first annular segment, adjacent the central segment, has a substantially constant % &Dgr; and a width. A second annular segment, adjacent the first annular segment, has a tapered step index profile and a wi

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