Optical fiber cassette

[0002] This invention relates to the field of fiber optics, and more particularly to a cassette for storing varying lengths of excess optical fiber during the installation of optical fiber cable.

[0003] There are many applications that utilize an optical fiber network to establish optical communications between a host digital terminal (HDT) at a central office and an optical network unit (ONU) at a remote location. Typically, each ONU installation will have a unique optical fiber cable routing. Each strand of optical fiber cable follows a different path between the HDT and the ONU, and will have an excess length of optical fiber that must be stored.

[0004] The optical fiber cable consists of the optical fiber and two electrically conducting tension wires. The optical fiber itself is extremely fine, on the order of 0.25 mm (0.0098 inches) in diameter. In order that the optical fiber not become kinked, which will degrade the quality of light pulses traveling through it, the optical fiber must not be bent sharply anywhere along its path. The two tension wires accompanying the optical fiber are referred to as tension lines, as they provide mechanical resistance to stretching when the optical fiber cable is under tension. Since optical fiber cable, and in turn the tension lines, may be subject to lightening strikes, the tension lines must be grounded at tie-down points.

[0005] The joining together of two fibers end-to-end to carry a signal of light pulses is called a splice connection. The splice connection must be held in place securely.

[0006] As cutting optical fibers is a delicate process, special tools and small jigs are used to perform the whole cleaving operation. For most applications, the ends are cut close to perpendicular to the optical fiber axis. Cleanly cleaved optical fiber ends are needed for low loss fusion splices. For high-performance optical connectors and mechanical splices, the cut tip of the fiber is ground and polished. Cables that are field installed are often terminated before installation to approximately the length needed, resulting in an excess length of optical fiber that must be stored.

[0007] In accordance with the present invention, there is provided an optical fiber cassette, for use in connection with an optical domain network to store excess optical fiber cable. The optical fiber cable includes an optical fiber and a tension wire. The optical fiber cassette comprises a chassis having at least one spool for winding and storing excess optical fiber, the spool having a wall with a radius of at least a minimum bend radius.

[0008] A method is also disclosed.

[0009] A more complete understanding of the present invention may be obtained from consideration of the following description in conjunction with the drawing, in which:

[0010] FIG. 1 is a perspective view of an optical fiber cassette constructed in accordance with the invention;

[0011] FIG. 2 is a top view of the chassis of the optical fiber cassette of FIG. 1;

[0012] FIG. 3ais a simple ray model of transmission for a straight optical fiber;

[0013] FIG. 3bis a simple ray model of transmission for a bent optical fiber;

[0014] FIG. 4 is a perspective view of a splice connector receptacle of the optical fiber cassette of FIG. 1;

[0015] FIG. 5 is a cross-sectional elevational view of a splice connector receptacle and pathways, taken along lines 4—4 of FIG. 2;

[0016] FIG. 6 is a perspective view of a hinge and locking tab of the optical fiber cassette of FIG. 1;

[0017] FIG. 7 is a perspective view of a tension wire tie-down mounted in the chassis, taken along lines 6—6 of FIG. 1; and

[0018] FIG. 8 is a perspective view of the tension wire tie-down.

[0019] Referring now to the drawings, and especially to FIGS. 1 and 2, an optical fiber cassette is shown at 10, and comprises a chassis 12 having a generally planar base portion 14. A rim wall 16 generally encircles the base portion 14. The chassis 12 has integral first 18 and second 20 spools for winding and storing excess optical fiber. The first 18 and second 20 spools each have a wall 22 and 24 respectively, with a radius R of at least a minimum bend radius.

[0020] The chassis 12 includes a. plurality of path wall segments 26. The path wall segments 26, rim wall 16, and spool walls 22 and 24 define pathway walls 28 of pathways 30 aligned generally tangentially to the spool walls 22 and 24. The pathways 30 receive and store the optical fiber and guide the optical fiber toward the spools 18 and 20. The path wall segments 26 each have a radius of at least the minimum bend radius.

[0021] The minimum bend radius is the radius below which an optical fiber should not be bent. It will vary with different cable designs. Bending the cable tighter than the minimum bend radius may result in increased attenuation and broken fibers. The manufacturer should specify the minimum radius to which the cable may safely be bent during installation, and for the long term. The former is somewhat shorter than the latter. The minimum bend radius is in general also a function of tensile stresses, e.g., during installation, while being bent around a sheave while the fiber or cable is under tension. If no minimum bend radius is specified, one is usually safe in assuming a minimum long-term low-stress radius not less than 15 times the cable diameter. In practice, optical fiber bends are gradual relative to the diameter of the fiber, with curvature of a few centimeters or more compared to a 100 μm diameter of a typical optical fiber core. The minimum bend radius is typically about 30 mm (1.1 inches). Larger core optical fibers are more rigid and have a larger minimum bend radius.

[0022] Referring to FIG. 3athere is shown a simple ray model of transmission for a straight optical fiber. FIG. 3bshows a simple ray model of transmission for a bent optical fiber. The optical fiber 100 shown in longitudinal cross section has an optical core 102 which is surrounded by a cladding 104 and has a critical angle θ_c. When high order mode light rays strike a bend in the optical fiber, light can leak out if it hits the side of the fiber at an angle beyond the critical angle θ_c. This increases the loss in the optical fiber. While lower order modes are not likely to leak out, they can become transformed into higher order modes and thus leak out at the next bend. The bending of the optical cable 100 as illustrated in FIG. 3b, causes a light ray to strike the core 102 cladding 104 boundary at an angle greater than the critical angle θ_c. and a portion of the light ray will leak out. The extra, or increased angle caused by bending of the optical fiber 100 is indicated by the angle θ_bendin the inset of FIG. 3b. Accordingly, it is critical that the excess optical fiber be stored in a fashion in which any bend has a radius of at least the minimum bend radius.

[0023] Turning now to FIGS. 4 and 5, as well as FIGS. 1 and 2, a plurality of retainers 32 are disposed transversely over the pathways 30. The retainers 32 releasably retain the optical fiber in the pathways 30 and on the spools 18 and 20. The retainers 32 each have two bars 34 extending from proximal ends 36 at opposite pathway walls to distal ends 38. The distal ends 38 of the bars 34 define a slot 40 between the bars 34 at an angle to the direction of the pathway 30. Thus, upon installing the optical fiber 42 in the cassette 10, the optical fiber 42 will be turned to the angle of the slot 40 and pass through the slot 40. The optical fiber 42 will then be turned to lie in the direction of the pathway 30 and will not pass back through the slot 40, thereby being retained in the pathway 30.

[0024] The first spool 18 includes a tunnel pathway 44 defined by a pair of generally parallel tunnel walls 46 extending from a first point 48 on the spool wall 22 through the spool 18 to a second point 50 on the spool wall opposite the first point, so as to allow the optical fiber to pass through the first spool 18.

[0025] Referring now to FIG. 7, as well as FIGS. 1 and 2, a plurality of portals 52, are aligned with the pathways 30. The optical fiber cable will enter and exit the cassette 10 through the portals 52, so as to provide a variety of options for placement and direction of the optical fiber cables.

[0026] Referring now to FIGS. 1 and 6, a cover 54 is assembled to the chassis 12 by snapping a pair of keys 51 into slots 55 in the cover 54. The keys 51 each have a barb 53 on the distal end that rides up a ramp 57 and enters the slot 55. The cover 54 is attached to the chassis 12 by pivotal means, typically a hinge or a pair of hinges 56. The cover 54 is movable from an open position wherein the optical fiber cable will be placed for storage in the cassette 10, to a closed position wherein the optical fiber cable will be protected from damage. A first locking means is provided for releasably locking the cover 54 in the open position. The first locking means preferably will include a tab 58 projecting from the cover 54. The tab 58 is closely juxtaposed between the hinges 56 and the rim wall 16 when the cover 54 is in the open position. A second locking means is provided for releasably locking the cover 54 in the closed position. The second locking means preferably will include a finger 60 having a proximal end 62 attached to the cover 54. The finger 60 has a distal end 64 projecting downward from the cover 54, and a barb 66 at the distal end 64. The barb 66 engages a notch 68 in the chassis 12 when the cover 54 is in the closed position.

[0027] Turning now to FIGS. 4 and 5, as well as FIGS. 1 and 2, the chassis 12 has at least one first splice receptacle 72, and preferably a second splice receptacle 74 to releasably retain a splice connection 70. The first 72 and second 74 splice receptacles each has a pair of generally parallel, opposite splice receptacle walls 76. Each wall 76 has a top edge 78 and an inside surface 80 facing the opposite wall. The inside surfaces 80 each have a bevel 82 adjacent the top edge 78 and a groove 84 below the bevel 82. Accordingly, as the splice connection 70 is inserted into the splice receptacle 72, the bevels 82 will cause the walls 76 to move outward resiliently, the splice connection 70 will pass the bevels 82 and enter the grooves 84, and the walls 76 will move inward with bias, thereby clamping the splice connection 70 releasably in the grooves 84.

[0028] Referring now to FIGS. 7 and 8, as well as FIG. 1, a tension wire tie-down 86 is provided to clamp the optical fiber cable and electrically ground the tension wire (not shown). The tension wire tie-down comprises a base member 88 attached to the chassis, the base member 88 being electrically conductive. A plurality of clamps 90 are provided. Each clamp 90 has a bottom plate 92 and a flange 94 projecting upward from the bottom plate 92 at an obtuse angle thereto. The clamps 90 are electrically conductive, and are attached with screws 96 to the base member 88. The clamps 90 are disposed in pairs with the flanges 94 opposite each other. Each pair is adjacent a portal 52, so that the optical fiber cable will pass through the portal 52 and between the opposing flanges 94 of a pair of clamps 90. The flanges 94 will engage and hold down the optical fiber cable and make electrical contact with the tension wire, thereby grounding the tension wire to the base member 88.

[0029] Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. Details of the structure may be varied substantially without departing from the spirit of the invention and the exclusive use of all modifications which will come within the scope of the appended claims is reserved.