Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2002-06-12
2003-10-28
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S028000, C385S037000, C385S045000
Reexamination Certificate
active
06640031
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an optical waveguide fiber and optical components. More particularly, the invention relates to a dispersion compensating module and a mode converter, coupler and dispersion compensating optical waveguide fiber useable therewith.
BACKGROUND OF THE INVENTION
Dispersion compensating fibers used in telecommunications systems
10
, such as illustrated in
FIG. 1
, correct for the unwanted effects of dispersion of the transmission fiber
12
. Transmission fibers
12
preferably have a large effective area (e.g., >60 &mgr;m
2
, and more preferably greater than 70) and propagate light signals in the fundamental mode (LP
01
). An example of a transmission fiber is LEAF® optical fiber manufactured by Corning Incorporated of Corning, N.Y., which is designed to operate primarily at about the 1550 nm operating window. In some systems, compensation takes place within a module
11
having a length of Dispersion Compensating (DC) fiber housed within it. A section
13
of transmission fiber terminates at the module
11
and is coupled with the DC fiber. After being dispersion compensated, the DC fiber is again coupled with the transmission fiber
12
and the signal continues along a continuing portion
14
of the transmission system
10
.
FIG. 1
illustrates a simple system deployment. It should be understood that typical transmissions systems include other devices such as amplifiers before and after the module, add/drop devices, etc.
One solution described in U.S. Pat. No. 5,185,827 and shown in
FIG. 2
, compensates for the dispersion of the transmission fiber by providing a dispersive waveguide element which transmits the light signal at a higher-order LP
11
mode. An optical mode converter is utilized to convert the incoming signal from the fundamental mode carried by the transmission fiber to the higher-order mode LP
11
, mode that is supported and carried by the dispersive waveguide element. Similarly, once the dispersion compensation is achieved, a second optical mode converter converts the light signal back to the fundamental mode (LP
01
). However, transmission in the LP
11
mode has a problem that the signal may be split into multiple modes due to slight imperfections in the fiber's circular geometry. This has the effect of undesirably distorting the transmitted signal.
Thus, it should be recognized that the properties of the DC fiber used in the dispersion compensating module are vitally important to the performance of the overall optical transmission system.
SUMMARY OF THE INVENTION
According to a first embodiment of the invention, an optical waveguide fiber suitable for use as a dispersion compensating fiber is provided with improved properties such that it may advantageously support light propagation in an LP
02
mode. Preferably, propagation is supported at a wavelength of about 1550 nm and for a sufficient distance to compensate for dispersion of another fiber, for example an optical transmission fiber.
According to another embodiment of the invention, a Dispersion Compensating (DC) waveguide fiber is provided comprising a plurality of core segments. The refractive index profile of the DC fiber is selected to exhibit properties such that an LP
02
mode is supported and propagated thereby at a wavelength of about 1550 nm. Upon conversion to the LP
02
mode, preferably by an all-fiber mode converter according to another embodiment of the invention, the incoming signal is propagated within the DC fiber for an appropriate length (generally about 0.5-3.0 km, depending on the transmission fiber used). The DC fiber is designed to compensate LP
02
mode for the dispersion effects of the transmission optical waveguide (the primary fiber transmitting in an LP
01
mode).
Preferably, the transmission waveguide, for which dispersion correction is occurring, has a length greater than 25 km, and more typically on the order of between about 50 km-100 km. The invention described herein advantageously allows for a very short segment of DC fiber to accomplish the dispersion compensation. For example, in one embodiment, less than {fraction (1/100)}
th
of the length of the transmission fiber may be required for compensation of certain transmission fibers, for example Corning's LEAF® optical fiber.
In accordance with another aspect of the invention, the DC optical waveguide fiber exhibits a kappa value between about 10 nm and about 500 nm; where kappa is the ratio of dispersion in the LP
02
mode at about 1550 nm divided by the dispersion slope in the LP
02
mode at about 1550 nm. In accordance with a more preferred embodiment, the kappa value is in the range between about 30 nm and about 70 nm. According to another embodiment, the DC waveguide preferably has an effective area greater than about 30 &mgr;m
2
at about 1550 nm, more preferably greater than about 60 &mgr;m
2
, and more preferably yet between about 30 &mgr;m
2
and 150 &mgr;m
2
, and most preferably between about 50 &mgr;m
2
and about 90 &mgr;m
2
.
In a preferred embodiment of the invention, the fiber comprises a plurality of, preferably at least three core segments. Preferably, first and third segments of the plurality of segments comprise a dopant such as germanium to raise the index of refraction of the core a sufficient amount with respect to the cladding to achieve the desired &Dgr;%. Alternatively, any other suitable dopants such as phosphorous may be employed. Moreover, fluorine doping may be employed to lower the refractive index of a second core region and/or the clad region as compared to the core.
The geometry of the refractive index profile of the DC fiber is selected accordingly to enable transmission of the LP
02
mode over substantial distances (e.g., >0.5 km). For example, the structure, i.e., the radius of the various segments, their width dimensions, and their &Dgr;% values are selected in accordance with the invention as described in the several examples provided herein.
In accordance with one preferred embodiment, the waveguide comprises a structure with:
(a) a first core segment having an outer radius in the range between about 3 &mgr;m and 8 &mgr;m and a &Dgr;% peak in the range between about 1.0% and 2.5%,
(b) a second core segment having an outer radius in the range between about 7 &mgr;m and 13 &mgr;m and a &Dgr;% peak in the range between about 0.3% and −0.5%, and
(c) a third core segment having an outer radius between about 10 &mgr;m and 20 &mgr;m and a &Dgr;% peak in the range between about 0.2% and 1.0%.
Other embodiments and more preferred values of radii, &Dgr;% or combinations thereof are described more fully in the specification and appended claims. Fibers with these ranges of radii and &Dgr;% enable transmission in the LP
02
mode.
In accordance with another preferred embodiment, the waveguide fiber comprises:
(a) an effective area in the range between about 50 &mgr;m
2
and 90 &mgr;m
2
at about 1550 nm and in the LP
02
mode,
(b) a dispersion value at about 1550 nm and in the LP
02
mode between about −50 and −400 ps
m/km, and
(c) a dispersion slope value at about 1550 nm and in the LP
02
mode between about −0.01 and −20 ps
m
2
/km.
Other preferred values of effective area, dispersion, dispersion slope, kappa or combinations thereof are more fully described in the specification and appended claims.
According to another embodiment of the invention, a dispersion compensating optical waveguide includes a plurality of core segments, the refractive index profile of which is selected to exhibit an effective area between about 30 &mgr;m
2
and 150 &mgr;m
2
wherein the dispersion compensating optical waveguide is capable of propagating light in the LP
02
mode a sufficient distance at about 1550 nm, upon being converted from an LP
01
mode, to be capable of compensating for dispersion of a length of fiber transmitting in the LP
01
mode. Preferably, the fiber transmitting in the LP
01
, mode is a long-haul waveguide having a length greater than about 25 km. More preferably, the transmission fiber may be a fibe
Dong Liang
Qi Gang
Weidman David L.
Corning Incorporated
Rojas Omar
Sanghavi Hemang
Wayland Randall S.
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