Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-08-23
2002-08-06
Healy, Brian (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S124000, C385S126000
Reexamination Certificate
active
06430347
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to dispersion compensating waveguide fibers, and particularly to such fibers that are cabled or otherwise buffered and incorporated into telecommunications links designed to operate over a range of wavelengths.
2. Technical Background
Telecommunication systems using high powered lasers, high data rate transmitters and receivers, and wavelength division multiplexing (WDM) technology require optical waveguide fiber having exceptionally low total dispersion and polarization mode dispersion (PMD). The term total dispersion means all linear dispersion, including material dispersion and waveguide dispersion. In the art, total dispersion is sometimes called chromatic dispersion. In addition, the waveguide fiber must have characteristics which essentially eliminate non-linear phenomena such as self phase modulation (SPM) and four wave mixing (FWM). SPM can be limited by lowering power density, for example by increasing the mode field diameter or the effective area of the waveguide fiber. FWM is controlled by operating in a wavelength range over which dispersion of the fiber is non-zero.
Due to this requirement of non-zero dispersion, communication systems become dispersion limited at the very high bit rates. Consequently, dispersion compensating modules are typically employed to compensate for dispersion accumulated over a transmission link operating in, for example, the 1550 nm wavelength window. In order for the compensating fiber employed in the module not to become a limiting factor in the link, it should have functional parameters at least similar to the waveguide fibers in the link being compensated. For example, it is desirable for the compensating fiber to have similar effective area, cut off wavelength, and attenuation in comparison to the waveguide fiber in the link being compensated.
This desired performance is difficult to achieve, however. Usually some compromises in the form of design tradeoffs must be made. In one compensating scheme, a fiber having a very large dispersion, of sign opposite that of the dispersion of the cabled fiber forming the link, is contained within a dispersion compensating module and inserted into the link. For example, a telecommunications link comprising positive dispersion cabled fiber can be deployed in a link having regenerator spacing of about 100 km. A dispersion compensating module comprising fiber having a total dispersion at 1550 nm of about −70 ps
m-km to −100 ps
m-km typically would serve to compensate for the positive dispersion accumulated in the link. The length of fiber in the dispersion compensating module should be short in order to keep the module size relatively small and to minimize the attenuation added to the link by the dispersion compensating module. The attenuation of the fiber in the dispersion compensating module is typically much higher than that of the cabled or otherwise buffered fiber forming the link. The dispersion compensating module is essentially another link component that does not serve as a part of the link length. That is, the module does not serve as a part of the length that bridges the distance between transmitter and receiver.
Many of the high performance transmission links make use of wavelength division multiplexing (WDM) to maximize the information capacity of the links. This places an added requirement on the fiber of the dispersion compensating module in that compensation is desired over an extended wavelength range. Dispersion compensation over a wavelength range can be accomplished by control of the slope of the compensating fiber. A desirable target range of wavelengths for compensation corresponds to the wavelength range over which the gain versus wavelength curve of optical amplifiers, present in the transmission link, is relatively constant.
SUMMARY OF THE INVENTION
A first aspect of the present invention is a dispersion compensating optical waveguide fiber having a core region surrounded by a clad layer. In systems that are bandwidth limited, the waveguide fiber is preferably operated in a wavelength range over which only a single mode is propagated. Because the fiber guides light through its length, at least a portion of the clad layer must have a refractive index lower than that of at least a portion of the core region. The core region of the compensating single mode fiber preferably includes at least three segments. Substantially centered about the symmetry line of the fiber is a central segment having a relative refractive index &Dgr;
0
% in the range of about 0.6% to 1.1% . The central segment is surrounded by a first annular region of negative relative refractive index, which in turn is surrounded by a second annulus of positive relative index. The relative refractive index of the first annular segment &Dgr;
1
% is more negative than about −0.4%. The relative index of the second annular segment &Dgr;
2
% is greater than 0. The waveguide fiber so configured provides a negative total dispersion and a total dispersion slope more negative than about −0.2 ps
m
2
-km over a pre-selected wavelength range that includes the wavelength 1550 nm.
In an embodiment of this aspect, the pre-selected wavelength range is about 1500 nm to 1700 nm and preferably 1520 nm to 1650 nm.
In a further embodiment of this aspect of the invention, the core region includes a third annular segment surrounding the second annular segment and having a relative index &Dgr;
3
%≦0.2%
In yet a further embodiment of this aspect, the core region includes a fourth annular segment having relative index &Dgr;
4
% ≧0. More specifically, the relative index range of &Dgr;
2
% is about −0.4% to 0.5%, of &Dgr;
3
% is about −0.2% to 0.2%, and of &Dgr;4% is about 0 to 0.6%.
In another embodiment of this first aspect of the invention, &Dgr;
0
% is in the range of about 0.8% to 1.1%, &Dgr;
1
% is more negative than −0.5%, &Dgr;
2
% is in the range of about −0.1% to 0.1%, &Dgr;
3
% is in the range of about −0.1% to 0.1%, and &Dgr;
4
% is in the range of about 0.3% to 0.5%.
For each of these embodiments, attenuation values at 1550 nm are no greater than about 0.34 dB/km.
A second aspect of the invention is an optical waveguide fiber link, preferably using wavelength division multiplexing, that incorporates cabled waveguide fiber optically coupled to a cabled total dispersion and total dispersion slope compensating waveguide fiber (cabled compensating fiber). The definition of cabled compensating fiber is used consistently herein. Compensating over a range of wavelengths is equivalent to compensating total dispersion slope. High performance, fully or partially compensated links typically employ single mode waveguide fiber. However, the invention is applicable to two or more modes propagating in a waveguide fiber links as well.
Referring to
FIG. 3
, the link includes transmitter
58
, cabled waveguide fiber
60
, cabled compensating fiber
62
and receiver
64
. Cabled waveguide fiber
60
, preferably a single mode transmission fiber, and cabled compensating fiber
62
are optically coupled end to end in series to form the link length. The transmitter and receiver are designed for use in wavelength division multiplexed links. In one embodiment of the invention, the compensating fiber is made in accord with the first aspect of the invention and is cabled or otherwise buffered to form a portion of the link. The term cabled waveguide fiber as used throughout this application refers to the plurality of structures known in the art that may include buffered, stranded, or jacketed optical waveguide fiber. These structures may be deployed or installed essentially in the same way as any other communications cable. The link is designed so that the cabled transmission fiber is interposed between the cabled compensating fiber and a transmitter optically coupled to the cabled fiber. The transmitter is capable of launching into the cabled fiber a plurality of wavelengths selected from the particular
Cain Michael B.
Srikant V.
Chervenak William J.
Healy Brian
Song Sarah U.
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