Optical waveguides – With splice – Fusion splicing
Reexamination Certificate
2002-05-31
2004-09-14
Lee, John D. (Department: 2874)
Optical waveguides
With splice
Fusion splicing
C385S027000, C385S030000, C385S095000
Reexamination Certificate
active
06789960
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the connection of optical fibers having different optical properties. More particularly, the present invention relates to an optical fiber which can be used as a bridge to connect two other optical fibers. While the invention may be suitable for a wide range of connectivity applications in telecommunications systems, a fiber disclosed herein is especially well suited for connecting step index single mode fibers to dispersion compensation fibers having complex refractive index profiles, and will be particularly described in that regard.
2. Technical Background
Dispersion compensation techniques may be successfully used in telecommunications systems or links. Total dispersion, or chromatic dispersion, may be compensated by an appropriately designed waveguide fiber contained in a dispersion compensating module that can be inserted into the link at an access point such as an end of the link. The compensating waveguide fiber can be designed to allow operation in, for example, the 1550 nm operating wavelength window of a link that was originally designed for the 1310 nm operating window. A disadvantage of compensating with a module is that attenuation and nonlinear penalties are added to the link without increasing the useful link length. Furthermore, refractive index profile designs for such dispersion compensation are typically more complex, more difficult to manufacture, and may have higher attenuation than the so-called transmissive fibers making up the link. In some designs, the compensation parameters may be achieved at the expense of the effective area of the optical fiber, but a lower effective area compensating waveguide fiber is more susceptible to non-linear effects.
Another approach to dispersion compensation is providing both positive and negative dispersion fibers in the cables of the link, wherein both the positive and negative dispersion fibers are transmissive fibers, that is, the compensative fiber or fibers are not wound around bobbins inside a module, but rather add to the useful link length. Each cable can contain both positive and negative total dispersion waveguide fibers, or the link can be formed using cables having only positive dispersion in conjunction with cables having only negative dispersion. However, as in the dispersion compensating module approach, a relatively high attenuation and low effective area of the negative dispersion fiber can be problematic in this approach as well. Furthermore, cable inventory must typically be managed carefully, because replacing or repairing a cable may involve tracking of one or more other variables, such as the sign of the dispersion of fibers in the cable. In certain profile designs, a mismatch of mode fields between the positive and negative total dispersion fibers may exist, resulting in relatively large or excessive splice or connecting losses.
In the latter approach, uncoiled lengths of optical fibers such as single mode fibers and compensative fibers such as dispersion compensating fibers can be effectively used together in optical systems, although to achieve desirable or necessary system performance, such fibers typically need to be connected or spliced by other optical fibers or optical devices without exhibiting excessive connection losses or “splice losses”. Typically, the positive dispersion single mode transmissive fibers have mode field diameters that differ in size and in other aspects from the mode field diameters of the negative dispersion compensative fibers to which the single mode fibers will be connected. The direct connection of fibers having such mismatched mode field diameters generally results in excessive splice loss.
A number of techniques have been developed over the years to limit the adverse effect of splice loss resulting from mode field diameter mismatch. Heretofore, physical tapering, in-line optical devices, and thermally diffused expanded core (“TEC”) methods have been employed in an attempt to adequately match the mode fields of fibers and other devices having different mode field diameters. Physical tapering includes both down-tapering and up-tapering. TEC methods include those methods used to expand the mode field diameter via diffusion.
In the down-tapering method, the optical fiber is first fusion-spliced by conventional methods, and the spliced portion of the fiber is thereafter heated so that it can be stretched by pulling. In this way, the softened spliced portion develops a tapered shape. The reduced core misalignment due to the tapered shape and the spreading of the mode field diameter in the smaller core diameter fiber typically result in lower splice loss when compared to the original non-tapered splice. However, the tapers fabricated by this method are sensitive to physical perturbations or external refractive-index change because the mode field is no longer tightly bound to the core. In addition, the outer diameter of the tapered fiber changes during the drawing process, thus special fiber plugs are typically required for connections.
An up-taper is fabricated at the stage of drawing a preform and results in an enlarged core. The enlargement of the core results in an expanded mode field diameter. This method is typically applicable for mechanical splicing, bonded splicing, or connectors between an erbium-doped fiber (“EDF”) and a single mode fiber. However, this method also requires special plugs for the connectors, and additionally requires a special preform.
The thermally diffused expanded core method entails heating a fiber to cause dopant diffusion to expand the mode field diameter. Generally, the fusion connection of two fibers with different mode field diameters with this method is made by continuously or adiabatically varying the core diameters of one or both fibers so that the mode field diameters match at their boundaries. During the process of dopant diffusion, the core diameter locally becomes large, and the relative refractive index difference locally becomes small, resulting in a tapered core and a tapered mode field diameter along the length of the fiber. Accordingly, the thermally diffused expanded core method can be an effective method for locally expanding the fiber mode field diameter. However, implementation of the thermally diffused expanded core technique generally involves either heat-treating a smaller mode field diameter fiber in a furnace or a gas burner, then fusion-connecting the expanded fiber with the larger mode field diameter fiber, or first fusion-connecting the two fibers, then applying additional heat to diffuse the fused region.
When furnaces or microburners are employed to provide the heat for the diffusion, the process typically takes several hours to complete, due to the temperature limits of most furnaces, and may require the application of a carbon coating once the primary coating has been stripped from the fiber to reduce the heat exposure time required to properly diffuse the dopant. However, application of a carbon coating is expensive and time consuming. Furthermore, long periods of exposure to a gas flame tends to make a fiber brittle even if temperatures within the furnace are not extreme. Thus, for example, open-ended furnaces having a maximum temperature of approximately 1300° are employed to treat the fiber in such a process. Using such an open-ended furnace generally requires exposing a fiber having a 1% maximum relative refractive index difference for more than ten (10) hours, for example. Typically, the low temperature gradient in an open-ended furnace allows the fiber core to expand slowly. Typically, however, the relatively long heat-treated section of the fiber has relatively low mechanical strength and requires extra protection and packaging. Moreover, the first implementation of the thermally diffused expanded core technique is generally either not available or not practical in the field, where many of the fiber splices must be made, because of the large size of the required furnace and microburner systems.
The second impleme
Bickham Scott R.
Cain Michael B.
Hajcak Pamela A.
Hempstead Martin
Hepburn Lisa L.
Corning Incorporated
Homa Joseph M.
Lee John D.
Lin Tina M
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