Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-04-27
2002-03-26
Lee, John D. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S015000
Reexamination Certificate
active
06363195
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a dispersion compensated multimode waveguide link which includes a compensating multimode optical waveguide fiber which is optically coupled to the link to increase bandwidth at one or more preselected wavelengths.
Multimode optical waveguide fiber has long been preferred for use in shorter link length systems, such as local area networks, in which the link length is typically less than 5 km and the data transmission rate is of the order of hundreds of Mbits/sec. The large core diameter of multimode waveguides, typically 50 &mgr;m, 62.5 &mgr;m, 100 &mgr;m or larger, allows for low loss splicing and connecting loses. In addition, multimode waveguides provide for operation at two wavelength windows, centered around 850 nm and 1300 nm, and have sufficient bandwidth at both wavelengths to meet local area network data rate requirements.
Because the waveguide attenuation at the 1300 nm window of operation is lower, the multimode waveguide manufacturing process may be adjusted to provide higher bandwidth at the higher wavelength window. This adjustment provides a higher wavelength window capable of carrying higher data rates over longer distances, in comparison to the lower wavelength window. In this way full use is made of the lower attenuation at 1300 nm. Thus, for example, multimode fibers having a bandwidth of 160 MHz-km at 850 nm and 500 MHz-km at 1300 nm (160/500 fiber) has been specified for many local network or other short length applications.
Applications in which bandwidth is optimized at lower operating windows is required in certain systems. In these cases, the wavelength of peak bandwidth is moved to a lower wavelength such as 780 nm or 850 nm.
However, as laser sources at the lower window have become more powerful, narrower in line width, and relatively free of chirp, a need has arisen for higher bandwidth in the wavelength region centered on 850 nm. In addition, for certain local area net applications the demand for increased bit rate continues. Thus a practical need has arisen for higher bandwidth in the 850 nm window while maintaining sufficient bandwidth in the 1300 nm window.
Because many networks have been installed using the two window bandwidth values of 160 MHz-km and 500 MHz-km at the respective 850 nm and 1300 nm windows, a search has been undertaken to find an economically feasible way to adjust or compensate the two window bandwidths in installed multimode waveguide fiber links.
Definitions
Refractive Index profile is a statement of the value of the refractive index of a material along a line having a first and a last point. In the case of an optical waveguide fiber, a value of refractive index profile is defined at each point along a waveguide radius.
A general expression for an index profile is,
n
2
(
r
)=
n
1
2
[1−2
&Dgr;f
(
r/a
)],
in which.
n(r) is the refractive index at a point r on the waveguide radius, n
1
is the refractive index on the waveguide centerline, &Dgr;=(n
1
2
−n
2
2
)/2n
1
2
, in which n
2
is a reference refractive index usually taken as the minimum value of the clad layer index, and f(r/a) is a function of r divided by the core radius a.
An &agr;-profile is a refractive index profile in which f(r/a)=(r/a)
&agr;
.
Bandwidth is a standard measure of the dispersive property of a waveguide over a range of frequencies. In particular, the bandwidth of a waveguide is the range of frequencies over which the power penalty due to dispersion is less than 3 dB where the launched power is used as the comparison base. Bandwidth may be expressed in normalized frequency units, MHz-km, which is the bandwidth of a 1 km length of waveguide. When the bandwidth units are expressed simply as MHz, the bandwidth value is representative of the total length of waveguide measured. For example, a waveguide which is 2 km in length, having a normalized bandwidth of 500 MHz -km, has an end to end bandwidth of (500 MHz-km)/2 km=250 MHz.
SUMMARY OF THE INVENTION
There is a need for a technically sound, low cost way to compensate the bandwidth at one of the two operating wavelength windows. Further, this need exists in certain applications to compensate one wavelength window without unduly sacrificing bandwidth at the other wavelength window. It is contemplated that a refractive index profile n(r) may be found which has a local maximum near a selected wavelength window to be compensated and a local maximum at another selected wavelength window of operation. The present invention meets the need for such a multimode link bandwidth compensator.
A first aspect of the invention is a dispersion compensated multimode link comprising a first multimode fiber length, which has an index profile which provides for respective preselected bandwidths at a first and second wavelength window. The multimode waveguide has a core region and a surrounding clad layer. The core region has a circular cross section of radius a, the radius measured from the waveguide centerline. In a shorthand notation, the multimode waveguide is said to have bandwidth BW
1
at wavelength &lgr;
1
and a bandwidth BW
2
at wavelength &lgr;
2
. Although the profile n
1
(r) may take many forms, the profile in general produces a bandwidth vs. wavelength curve which has a local maximum at a wavelength &lgr;
p1
. A respective target bandwidth at each of two operating windows is realized by a combination of the location of &lgr;
p1
relative to &lgr;
1
and &lgr;
2
, and the maximum bandwidth which is located at wavelength &lgr;
p1
. In order to reach the respective target bandwidths at each of the two windows, the profile is designed such that the maximum or peak bandwidth occurs at a wavelength &lgr;
p1
which lies between the center wavelengths of the two operating windows, &lgr;
1
and &lgr;
2
. The bandwidth vs. wavelength response of the waveguide may be calculated from the geometry and index profile of the waveguide. The mathematical relationships are quite complex even when mode coupling and mode mixing are not taken into account. Even using numerical methods and a computer, usually some simplifying assumptions must be made. Thus the term “mathematically derivable” is used in this document to mean a particular set of waveguide fiber properties, specifically the refractive index profile and the core and clad geometry, can be used;
to estimate relative mode delay,
to predict &lgr;
p
, or
to estimate bandwidth at &lgr;
p
.
The agreement between calculated and experimental compensation waveguide parameters, given below, demonstrates the validity of the assumptions used in this application.
The compensated link is completed by optically joining a second multimode waveguide fiber to the first fiber. The second multimode waveguide has an index profile, n
2
(r) which compensates the relative modal delays which occur in the first waveguide. One method of compensation makes use of a compensating waveguide which has maximum bandwidth at a wavelength, &lgr;
p2
. An example of such a profile is an a profile. By placing &lgr;
p2
outside the wavelength interval defined by &lgr;
1
and &lgr;
2
one of the bandwidths, that of the higher wavelength window or that of the lower wavelength window can be compensated by the second fiber. In the case in which the profile errors in the first waveguide are &agr; errors, the compensating waveguide cannot in general correct the group delay of the modes at both &lgr;
1
and &lgr;
2
so that the increase in one bandwidth is made at the expense of the other. This is because the change in a produces a change in &lgr;
p
and so shifts the bandwidth vs. wavelength curve toward a higher or the lower wavelength.
When the profile errors which reduce bandwidth are more random or non-&agr; in nature, it is possible to compensate both the high wavelength window bandwidth and the low wavelength window bandwidth with a single compensating waveguide. Thus, in general it is proper to stipulate that at least one bandwidth may be compensated. An alternative statement is that the compensating wavegui
Abbott, III John S.
Smith Greg E.
Truesdale Carlton M.
Chervenak William J.
Corning Incorporated
Lee John D.
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