Abstract:
The invention is directed to a dispersion compensation module of extremely simple design that does not rely on a spool-and-hub or similar device for holding the optical fiber used in the module, such module being here termed a “Free-Fiber dispersion compensation module”. In the inventive dispersion compensation module the optical fiber therein is in a relaxed coiled configuration having minimal tension. It has also been discovered that while coil tension is relieved by removing it from the winding spool prior to placing it in the dispersion compensation module, the tension can be further relieved by coating the coiled fiber with a finely powdered substance which will not react with or otherwise harm or damage wither the fiber or the module containing it, for example, talcum powder.

Description:
PRIORITY 
   This application claims the benefit of U.S. Provisional Patent Application No. 60/443,075, filed Jan. 28, 2003, titled “DISPERSION COMPENSATION MODULE”. 

   FIELD OF THE INVENTION 
   The invention is directed to optical communications systems and in particular to dispersion compensation modules that are used in such systems. 
   BACKGROUND OF THE INVENTION 
   The distance over which data can be transmitted in optical fibers is limited by optical power loss and spectral pulse dispersion. With the advent of erbium-doped optical fiber amplifiers this limitation has been virtually eliminated, particularly for optical communications systems operating in the 1550 nm band. To compensate for power loss and dispersion, a compensating optical fiber, as part of an amplification and/or transmission system, is typically wound on a spool and the spool is used as-is or is placed in a housing. Leads are attached to the end of the optical fiber for connecting to the optical communications systems. This entire device may be described as a “dispersion compensation module.” 
   Presently, fiber-based dispersion compensating modules made at Corning Incorporated and other manufacturers utilize a length of dispersion compensating fiber, for example, an erbium-doped fiber that is wound on a spool. Some spools are molded and cost effective, but most spools are made by attaching steel or aluminum flanges to each side of a hub. This process of spool assembly involves the costly assembly of custom parts, the exact configuration of which depends on the customer requirements. Consequently, different spools must be designed and stored for each customer. The spool is then installed in a box enclosure or, alternatively, a protective band is placed around the outer diameter of the spool to protect the fiber and the spool is used as is. (Either configuration may be termed a dispersion compensation module.) A typical spool assembly of the prior art is illustrated in  FIG. 1 . 
   When the typical spool such as that in  FIG. 1  is wound under tension with dispersion compensating fiber (DCF), a “buffer layer” of optical fiber is first laid on the hub of the spool. The exact number of layers may vary, but generally fall in the range of 5–10 layers. This buffer layer will not be connected to or utilized by the optical communications system. The purpose of the fiber buffer layer is to form a protective layer for the fiber that is actually being used in the product DCF (the “operating fiber”) from falling into the crevice that can occur between the flange and the hub of the spool. In addition, the buffer layer serves to protect the DCF from the hub during thermal excursions in which the hub will expand at a different rate than the DCF. If the operating fiber were not so protected it would rub on the hub at that location and could be damaged, resulting in a failed dispersion compensation module. Consequently, while the buffer layer is a necessary item in the typical winding operation of a spool assembly, it adds cost and complexity to the finished product. 
   Once the spool is wound, the optical fiber on the spool remains at some level of tension. This tension is believed to degrade the optical properties of the fiber over time. In addition, during thermal excursions, whether in manufacturing, testing, or field use, the fiber can be further stressed due to thermal expansion effects as the hub expands more than the fiber pack. This can cause further optical problems, and in a worst case, reliability issues such as fiber breakage can occur. 
   As a result of the foregoing problems, there exists a need for a dispersion compensation module which does not rely on a spool to hold the fiber and does not require the use of a buffer layer of costly fiber to protect the operating DCF. There is also a need for a dispersion compensation module in which the DCF if not under tension or in which the tension has been sufficient relaxed so that stress-induce problem doe not arise in the fiber with the passage of time. 
   SUMMARY OF THE INVENTION 
   The invention is directed to a dispersion compensation module of extremely simple design that does not rely on a spool-and-hub or similar device for holding the optical fiber used in the module, such module being here termed a “Free-Fiber dispersion compensation module”. 
   The invention is further directed to a dispersion compensation module in which the optical fiber therein is in a relaxed coiled configuration. 
   The invention is also directed to a method of making a Free-Fiber dispersion compensation module and a device that can be used in such method. 
   In particular the invention is directed to a dispersion compensation module for optical communication that is a take-apart cassette having, among other elements, a first part and a second part. The first part includes a first and a second shaped structure therein, the first shaped structure being located within the second shaped structure. The second part is a lid or other form of closure element for the first part and its contents. The take-apart cassette also includes a coil of optical fiber having a first fiber end and a second fiber end for attachment to other elements. The coil is located between said first and second shaped structures. The coil of fibers is separately wound on a winding spool or other element and removed from the winding spool prior to being placed between the two shaped structures. The first and second end of the coil is connected to a first and a second pigtail. The pigtails are located at the outer perimeter of said cassette for connecting the coil of optical fiber within the cassette to an optical communication system. The second shaped structure has at least two openings there through for passage of the first and second ends of said fiber coil to the first and second pigtails, respectively. 
   A further aspect of the invention is that while fiber coil tension is relieved or relaxed by removing it from the winding spool prior to placing it in the dispersion compensation module, the tension can be further relieved or relaxed by coating the coiled fiber with a finely powdered substance, for example, talcum powder or similar substance, which will not react with or otherwise harm or damage wither the fiber or the module containing it. 
   Additional advantages of the invention will be set forth in the following detailed description and the appended drawings. It is to be understood that the foregoing general description, the following detailed description and the drawings are exemplary and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a spool assembly of the prior art that is typically used in dispersion compensation modules. 
       FIG. 2  illustrates a take-apart spool fixture that is used to wind the optical fiber used in the dispersion compensation module of the invention. 
       FIG. 2A  is an exploded view of an alternative winding spool. 
       FIG. 3  illustrates the coiled optical fiber that has been removed from the spool fixture of  FIG. 2  and placed in a dispersion compensation module cassette tray. 
       FIG. 4  illustrates the coiled fiber of  FIG. 3  with pigtails attached. 
       FIG. 5  illustrates the pigtailed fiber containing cassette tray of  FIG. 4  with a resilient material placed on top of the coiled fiber to form a cover and protect the fiber. 
       FIG. 5A  is an exploded view of  FIG. 5 . 
       FIG. 6  illustrates the fully assembled dispersion compensation module of the invention with cover plate attached to the cassette tray. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The term “Free-Fiber” means a coil of fiber that is not placed on a spool or other element when placed in use in an optical device. The Free-Fiber may, however, be placed in a module, including placed around one or between two elements so that the coiled shape may be retained in the module. In such placement, the coil is loose, for example, as a is a coil of string that was first wound around a finger, removed and then placed in the palm of a hand. Generally, the coil of Free-Fiber will assume a circular or elliptical shape. Further description and understanding of the meaning of the term Free-Fiber will be attained through reading of the following text. 
   The dispersion compensation module design of the invention does not require a spool assembly for holding the optical fiber in the dispersion compensation module. However, a spool is used to wind the fiber prior to the fiber being positioned in the dispersion compensation module. In a first step, the optical fiber is wound on a take-apart spool assembly, for example, that illustrated in  FIG. 2 . While in a preferred embodiment the take-apart spool  10  illustrated in  FIG. 2  is a molded two-piece spool including a first part  20  and a second part  30 , other take-apart spools made by different methods can also be used, for example, as illustrated in  FIG. 2A . Referring to  FIG. 2 , the first part  20  has a first flange  21  of predetermined diameter; a circular (or elliptical) hub  22  of predetermined diameter smaller than that of the first flange and coaxially positioned with respect to the first flange. The hub has a selected thickness and rises a selected distance from the first flange, and has a slot  29  through the thickness of the hub. In addition, there are at least two openings  24  through the thickness of the first flange, the openings being located within the circumference of the hub and being used to position the spool on a winding device (not illustrated) for rotating the spool to wind fiber. Further, there are at least two fastening structures  26  rising from the first flange for a distance not greater than the distance of the hub  22 , the fastening structures having an internal opening for placement of fastening element, for example, a screw, to hold the second part  30  in position when the spool is fully assembled. The second part  30  is a second flange of the same diameter as the first flange, the second flange having two opening there through located such that the fastening element  28  can be inserted into the structures  26  for holding the second flange in position. 
   In operation, an end of an optical fiber is inserted into the slot  29  and lightly wound around the fastening structures  26 . This end portion winding will become a free end that later be pigtailed. The fiber is then lightly wound about the hub for a few turns. The second part  30  is then placed on the first part  20  and fastened thereto by insertion of fastening elements  28  into fastening structures  26 . The spool is then placed on a winding device and the desired length of fiber would onto the spool. When the winding is completed the fiber is cut and the spool with the wound fiber is removed from the winding device. 
   Referring to  FIG. 2A , an alternate embodiment of the winding spool, winding spool  10 A, as illustrated in the exploded drawing, is similar to that of  FIG. 2 , except that the hub  22 A is separable from the first flange  21 A; that hub  22 A is positioned in openings  25 A which are located in first and second flanges  21 A and  30 A; that there is a single opening  24 A for positioning on a winding device; and that four fastening structures  26 A are illustrated instead of two as in  FIG. 2 . Once assembled, the winding spool of  FIG. 10A  is used as described for above regarding winding spool  10 . 
   The spool containing the wound or coiled fiber is then disassembled and the coiled fiber is gently removed from the spool. When the coiled fiber is removed from the spool it has been found that it retains its coiled shape allowing it to easily be placed in a cassette tray as illustrated in  FIG. 3 . 
   The coil of fiber  70  (not illustrated) is then placed into one piece of a two-piece cassette tray  110  (shown in  FIG. 6 ) which forms the dispersion compensation module of the invention. The dispersion compensation module is illustrated in  FIG. 6  and is of substantially closed design. As illustrated in  FIG. 6 , the dispersion compensation module has openings for the insertion of fastening elements and connectors or pigtails which are used to connect the coiled fiber within the dispersion compensation module to other, external elements in a system in which the dispersion compensation module is a part. In alternate embodiments other fastening devices such as clips can be used to hold together the two pieces of the cassette tray or the two pieces can be permanently joined together, for example, by gluing. 
   Referring now to  FIGS. 3 and 6 , the cassette tray  110  has of a first part  120  for placement of the fiber and a second or lid part  112  ( FIG. 6 ) with openings  113  (not illustrated) there through for insertion of a fastening element  140  (inserted in openings  113  in  FIG. 6 ). The first part of the tray has a wall  122  rising a distance about its outer perimeter and two shaped structures  124  and  126  of different diameter within. These structures can be circular, elliptical or any other “smooth” shape lacking sharp comers. The major criterion is that the two structures be one-inside-the-other with sufficient space between for placement of the optical fiber. In the following text the word “circular” will be used throughout and is to be understood as including all such shaped structures. Circular structures are depicted in the drawing appended hereto. 
   The first part also has at least two openings  128  through the outer wall  122  for insertion of a connecting element  190  (see  FIG. 5 ) for connecting or “pigtailing” the ends  192  of the optical fiber within the tray to external leads (not illustrated) that go to some other optical element (not illustrated), for example, transmission fiber. The Free-Fiber coil  70  will be placed between the two circular structures. 
   The first circular structure  124  has a selected diameter, is preferably continuous along its entire diameter, and has a selected wall height and thickness. In a preferred design the inner structure has a plurality of vertical openings  130  within the wall thickness extending from the top of the wall for a distance into the wall for the insertion of a fastening element  40 , for example, a screw, to connect the lid  112  to the first part of the tray by inserting the fastening element through the openings in the lid and into the vertical openings of the first circular structure. 
   The second circular structure within the outer perimeter of the tray has a selected diameter greater than the diameter of the first circular structure and a plurality of vertical openings  132  within the wall thickness extending from the top of the wall for a distance into the wall for the insertion of fastening elements  140 . In addition, the second circular structure has at least one, and preferably two, vertical openings  134  through its perimeter wall to allow the ends  192  of the optical fiber to pass through the wall and be pigtailed to the connectors  190  at the outer perimeter of the cassette tray. 
   The coil  70  of Free-Fiber is placed between the two circular structures and each of the two fiber ends is connected to one of the two pigtail connectors  190 . Optionally, a thin layer of a foam or other resilient material  170 , having shape such that it will fit between the inner and outer circular structures  124  and  126 , may then be then placed over the fiber to take up the volume and cushion the lose fiber therein as is illustrated in  FIG. 3 . The coil of fiber  70  lies under the resilient material  170 . This layer of resilient material is circular with (1) a circumference approximately equal to the inner diameter or the second circular structure and (2) an inner opening whose diameter is approximately equal to outer diameter of the first circular structure. This use of this optional element is depends on the amount of fiber within the cassette. If such insert is used, then at this point the first tray having the fiber, the cushioning foam therein and the pigtails attached thereto is as illustrated is  FIG. 4 . 
   Whether the optional foam element as illustrated in  FIG. 4  is used, in the next step a layer of foam or other resilient material  180  with no inner opening and an outer diameter approximately that of the second circular structure is placed over the fiber and the top of the wall of the first circular structure as is illustrated in  FIG. 5 . In the final step, the lid  112  is placed on the tray and secured with the fastening elements  140 , for example, screws as illustrated in  FIG. 6 . As the lid draws down against the cassette under the action of the screws, the thin foam compresses, sealing any crevices that might trap the Free-Fiber, and the lid captures the pigtail terminations and seals the entire unit from external debris, contamination and protrusions. The final assembly of the Free-Fiber dispersion compensation module is as illustrated as  200  in  FIG. 6 . 
     FIG. 5A  is an exploded view of  FIG. 5  illustrating the pigtails  190 , a fiber end  192  attached to a pigtail  190  (the second fiber end  192  is not illustrated, but is similarly attached to a pigtail  190 ), the resilient material insert  170 , and the resilient material layer  180 . The coil of fiber  70  is not illustrated, but would lie between shaped structures  124  and  126  as indicated by numeral  70 . 
   A further aspect of the invention is the discovery that while tension present during optical fiber winding is relieved or relaxed by removing it from the winding spool prior to placing it in the dispersion compensation module, the fiber coil can be further relaxed by coating it with a finely powdered substance which will not react with or otherwise harm or damage either the fiber or the module containing it. Examples of such powdered substances include talc, powdered corn starch, finely powder silica, and other non-interactive substances having a particle size approximating that of talc. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.