Patent Publication Number: US-2003231849-A1

Title: Closures and cassettes for housing bridge and/or transition optical fibers in dispersion-managed networks

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates generally to fiber optic cable closures for optical fibers, splices, and/or connections. More specifically, the invention relates to fiber optic cable closures and fiber optic cassettes for housing bridge and/or transition optical fibers for use in a dispersion-managed network (DMN).  
       BACKGROUND OF THE INVENTION  
       [0002] Fiber optic cables include optical waveguides such as optical fibers that transmit optical signals may include voice, video, and/or data information. Optical fibers of different fiber optic cables can be optically connected together, thereby forming an optical pathway. These optical pathways can form a portion of a fiber optic network that can span long distances. Fiber optic networks typically include closures at splice locations such as distribution hubs in the fiber optic network. In the field, craftsmen route fiber optic cables into a closure at a hub location and can splice the optical fibers of the cables together, thereby forming a portion of a fiber optic network. The amount of data information that a fiber optic network can accommodate is called bandwidth. Bandwidth is usually measured in Gigabits per second (Gbps).  
       [0003] One way to increase bandwidth is by wavelength-division multiplexing (WDM). WDM is sending multiple optical signals, where each signal has a slightly different wavelength, through a single optical fiber. Another way to increase the bandwidth of a fiber optic network is to transmit the data at a faster rate. However, there are limits on the bandwidth that a fiber optic network can accommodate due to the optical degradation of the optical signal as it travels along the optical fiber. Optical signals degrade due to optical performance characteristics of the optical fibers such as optical attenuation, optical connection loss, and/or chromatic dispersion.  
       [0004] Generally speaking, optical attenuation is a loss in transmitted power. Optical attenuation is typically due to absorption, scattering, and leakage of light from the optical waveguide and is customarily measured in a fiber, or cable, as a loss in transmitted power per unit length such as dB/km. A loss in transmitted power is undesirable because a weak optical signal is difficult to detect. Additionally, optical connections between optical fibers such as splices can have a loss in transmitted power, but optical connection losses are generally measured in decibels (dB).  
       [0005] On the other hand, chromatic dispersion is the differential transit time of adjacent wavelengths in an optical fiber of a WDM system. Chromatic dispersion results in pulse spreading of the optical signals, which makes the optical signals difficult to detect. Chromatic dispersion in fiber optic waveguides is the sum of material and waveguide dispersions and is generally measured in picoseconds of pulse spreading per nanometer of source per kilometer of optical fiber length (ps/(nm·Km)).  
       [0006] Material dispersion results from the differences in refractive index with respect to wavelengths being transmitted in the optical waveguide. For silica-based glass used for optical fibers, material dispersion increases with the wavelength being transmitted in the commonly used communication range of about 0.9 μm to 1.6 μm. Material dispersion can have a negative or a positive sign depending on the wavelength.  
       [0007] Conversely, waveguide dispersion results from light traveling in both the core and cladding of an optical fiber. Waveguide dispersion is a function of wavelength and the refractive index profile of the optical fiber. For example, a predetermined refractive index profile of the optical fiber can be selected to influence the wavelength dependency of wavelength dispersion therein, thereby influencing the chromatic dispersion at a predetermined wavelength.  
       [0008] Wavelength and material dispersion effects can be influenced to yield an overall positive or negative chromatic dispersion characteristic in a given optical fiber at a given wavelength. As a result of these optical characteristics, optical fibers and/or networks are generally designed to minimize the chromatic dispersion characteristic at a specific wavelength such as 1.3 μm to inhibit degradation of the optical signal at that wavelength. However, using wavelengths other than the intended wavelength, or spanning distances greater than intended, can result in appreciable chromatic dispersion that effects optical performance.  
       [0009] One way to overcome chromatic dispersion characteristics is by using additional network equipment such as repeaters and/or regenerators to preserve the optical signal. However, the additional equipment adds undesirable expense and labor to the fiber optic network.  
       [0010] Other methods that compensate for chromatic dispersion can avoid the additional costs of repeaters and/or regenerators; however, these methods have other difficulties. For instance, U.S. Pat. No. 5,933,561 (&#39;561) discloses splicing a length of an optical fiber section to an existing single mode optical fiber pathway. The existing single mode optical fiber pathway is designed to have relatively low chromatic dispersion at a wavelength of 1.3 μm, but has appreciable chromatic dispersion at a wavelength of 1.55 μm. By splicing a length of an optical fiber section to the existing single mode optical fiber pathway, the chromatic dispersion at 1.55 μm of the existing optical fiber pathway can be offset, thereby allowing the use of a 1.55 μm wavelength while maintaining acceptable performance.  
       [0011] In particular, the &#39;561 patent discloses a dispersion compensating optical fiber connector body having three different optical fibers with specific optical characteristics. Sizing the three optical fibers as disclosed the &#39;561 patent can reduce the total connection loss of the connector body compared with directly splicing the positive dispersion and dispersion compensating optical fibers together. However, the connector body is still subject to the optical performance constraints associated with splice losses. The splicing operation between the three optical fibers of the connector body requires special, expensive, and sensitive splice equipment to precisely align the different types of optical fibers. This type of splicing equipment is not desirable for use in the field by the craftsman. In particular, the connector body requires an active alignment splicing device capable of moving an aligning base in at least two dimensions to precisely align the optical fibers, thereby minimizing the splice loss in the connector body.  
       [0012] On the other hand, the craftsman in the field desires to minimize the equipment he carries in size, quantity, and complexity. For example, a craftsman prefers to carry small, lightweight, and ruggedized equipment. Moreover, it is expensive to train and equip each craftsman with special equipment that will be subject to field use and abuse.  
       SUMMARY OF THE INVENTION  
       [0013] The present invention is directed to a fiber optic cable closure for containing optical fibers of a dispersion-managed network. The fiber optic cable closure includes a housing having a cavity, and at least one bridge optical fiber disposed within the cavity. The bridge optical fiber has a first end and a second end. The first end being configured to optically connect to a first optical fiber having a first dispersion characteristic, and the second end being configured to optically connect to a second optical fiber having a second dispersion characteristic.  
       [0014] The present invention is also directed to a fiber optic cassette for containing optical fibers of a dispersion-managed network. The fiber optic cassette including a first storage area, and at least one bridge optical fiber having a first and second end. At least a portion of the bridge optical fiber is disposed within the first storage area. The first end is configured to optically connect with a first optical fiber having a first dispersion characteristic, and the second end is configured to optically connect with a second optical fiber having a second dispersion characteristic.  
       [0015] The present invention is further directed to a fiber optic cassette for containing optical fibers of a dispersion-managed network. The fiber optic cassette including a first storage area, a second storage area, at least one bridge optical fiber, a first optical fiber, and a second optical fiber. The bridge optical fiber has a first end and a second end, and at least a portion of the at least one bridge optical fiber is disposed in the first storage area. The first optical fiber is a transition optical fiber having a positive dispersion (D+) characteristic and is in optical communication with the first end of the at least one bridge optical fiber. The second optical fiber being a transition optical fiber having a negative dispersion (D−) characteristic and is in optical communication with the second end of the at least one bridge optical fiber. At least a portion of the first and second optical fibers is disposed in the second storage area.  
       [0016] Additionally, the present invention is directed to a dispersion-managed network including a first fiber optic cable, a second fiber optic cable, a fiber optic cable closure, and at least one bridge optical fiber. The first fiber optic cable has at least one positive dispersion (D+) optical fiber. The second fiber optic cable has at least one negative dispersion (D−) optical fiber. The fiber optic cable closure having a housing with a cavity, and at least a portion of the first and second fiber optic cables is disposed within the cavity. The at least one bridge optical fiber having a first and second end disposed within the cavity of the housing with the at least one D+ and D− optical fibers being in optical communication with the at least one bridge optical fiber. 
     
    
    
     BRIEF DESCRIPTION OF THE FIGS.  
     [0017]FIG. 1 is a partial schematic view of a portion of a dispersion-managed network (DMN) according to one embodiment of the present invention.  
     [0018]FIG. 2 is a partially exploded, partially perspective view of a butt-type fiber optic cable closure for containing optical fibers, splices, and/or connections according to one embodiment of the present invention.  
     [0019]FIG. 3 is a rear perspective view of portions of the fiber optic cable closure of FIG. 2.  
     [0020]FIG. 4 is a front perspective view of portions of the fiber optic cable closure of FIG. 2.  
     [0021]FIG. 5 is a partially exploded, partially perspective view of the end cap and portion of the support frame of the fiber optic cable closure of FIG. 2.  
     [0022]FIG. 6 is a front perspective view of the end cap and portion of the support frame of the fiber optic cable closure of FIG. 2  
     [0023]FIG. 7 is a side view of the end cap, a portion of the support frame, and other components that can be used with the fiber optic cable closure of FIG. 2.  
     [0024]FIG. 8 is a partially exploded perspective view of a portion of fiber optic cassette assembly that can be used in the fiber optic cable closure of FIG. 2.  
     [0025]FIG. 9 is a partially exploded perspective view of the fiber optic cassette assembly of the fiber optic cable closure of FIG. 2 having optical fibers therein.  
     [0026]FIG. 10 is a perspective view of the assembled fiber optic cassette assembly of FIG. 9.  
     [0027]FIG. 11 is a side view of a portion of fiber optic cassette assembly of FIG. 10 with the plate removed.  
     [0028]FIG. 12 is another side view of a portion of fiber optic cassette assembly of FIG. 10 with the plate removed.  
     [0029]FIG. 13 a  is a schematic plan view of a routing of scheme for the bridge optical fiber disposed in the first storage area of the fiber optic cassette assembly of FIG. 9.  
     [0030]FIG. 13 b  is a schematic plan view of a routing scheme for the first transition optical fiber disposed in the second storage area of the fiber optic cassette assembly of FIG. 9.  
     [0031]FIG. 13 c  is a schematic plan view of a routing scheme for the first optical fiber disposed in the second storage area of the fiber optic cassette assembly of FIG. 9.  
     [0032]FIG. 13 d  is a schematic plan view of a routing scheme for the second transition optical fiber disposed in the second storage area of the fiber optic cassette assembly of FIG. 9.  
     [0033]FIG. 13 e  is a schematic plan view of a routing scheme for the second optical fiber disposed in the second storage area of the fiber optic cassette assembly of FIG. 9.  
     [0034]FIG. 14 is an exploded perspective view of another fiber optic cassette assembly according to the present invention.  
     [0035]FIG. 15 is an exploded perspective view of another fiber optic cassette assembly according to the present invention.  
     [0036]FIG. 15 a  is a schematic plan view of a routing scheme for optical fibers disposed in a storage area of the fiber optic cassette assembly of FIG. 15.  
     [0037]FIG. 16 is a perspective view of the assembled fiber optic cassette assembly of FIG. 15.  
     [0038]FIG. 17 is an exploded perspective view of another fiber optic cassette assembly according to the present invention.  
     [0039]FIG. 17 a  is a schematic plan view of a routing scheme for a  30  bridge optical fiber disposed in a storage area of the fiber optic cassette assembly of FIG. 17.  
     [0040]FIG. 18 is a perspective view of the assembled fiber optic cassette assembly of FIG. 17.  
     [0041]FIG. 19 is a perspective view of a subassembly of an in-line fiber optic cable closure according to another embodiment of the present invention.  
     [0042]FIG. 20 is a top plan view of the subassembly of FIG. 19.  
     [0043]FIG. 21 is a partially exploded, partially perspective view of a butt-type fiber optic cable closure using the cassette of FIG. 15 according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0044] Illustrated in FIG. 1 is a portion of a dispersion-mananged network (DMN)  1  according to the present invention. DMN  1  includes a first fiber optic cable  2  having at least one optical fiber  2   a  with a first dispersion characteristic, a second fiber optic cable  3  having at least one optical fiber  3   a  with a second dispersion characteristic, at least one bridge fiber  42 , and a fiber optic cable closure  10 . However, the DMN can include additional fiber optic cables, fiber optic cable closures, transition optical fibers and/or other suitable components. The DMN offsets the dispersion characteristics of the optical fibers of an optical pathway so that the DMN has a relatively low net dispersion, preferably an essentially zero net dispersion. Thus, DMNs according to the present invention allow for increased bandwidth and/or longer transmission distances with improved optical performance.  
     [0045] As used herein, the first and second dispersion characteristics of optical fibers  2   a , 3   a  mean different dispersion characteristics measured at the same reference wavelength.  
     [0046] Different dispersion characteristics mean dispersion characteristics associated with, for example, different types of optical fibers having different optical characteristics such as mode-field diameters (MFD), core diameters, cladding diameters, refractive index profiles, and/or refractive indices, rather than dispersion characteristics associated with the same, or similar, types of optical fibers due to manufacturing variances.  
     [0047] For example, at a reference wavelength of  1550  nm, optical fiber  2   a  may have a positive dispersion (D+) characteristic and optical fiber  3   a  may have a negative dispersion (D−) characteristic. While traveling through a D+ optical fiber, an optical pulse signal stretches, thereby increasing its duration compared with the original optical signal. On the other hand, while traveling through a D− optical fiber, the optical pulse is shortened, thereby decreasing its duration compared with the original optical signal. Thus, by optically connecting suitable lengths of D+ and D− optical fibers the net dispersion of a DMN can be manipulated to have a relatively low net dispersion.  
     [0048] However, a direct optical connection between optical fibers having extremely different first and second dispersion characteristics can undesirably result in a relatively high splice loss. For example, directly connecting D+ and D− optical fibers having MFDs of about 11.5 μm and 6.0 μm, respectively, results in a relatively high splice loss. To overcome this relatively high splice loss, the DMNs of the present invention optically connect D+ and D− optical fibers in a fiber optic cable closure with bridge fiber  42  therebetween. In one embodiment, the bridge fiber acts as gradual change in MFD between the D+ and D− optical fibers, thereby allowing for a relatively low splice loss. Even though bridge fiber  42  has different optical characteristics than either the D+ or D− optical fibers, the splice loss is advantageously reduced compared with a direct D+ to D− splice. Illustratively, the MFD of bridge fiber  42  is less than or about equal to the MFD of the D+ optical fiber and greater than or about equal to the MFD of the D− optical fiber. For instance, a bridge fiber with a MFD of about 8.6 μm can be used with the D+ and D− optical fibers described above, thereby making a gradual change in MFD. However, other bridge fibers having other suitable MFDs and/or other suitable optical characteristics can be used. Additionally, one or more of the optical characteristics of bridge fiber  42  may be the same, or similar, to either, or both of optical fibers  2   a , 3   a.    
     [0049]FIG. 2 depicts an explanatory butt-type fiber optic cable closure  10  according to one embodiment of the present invention. Fiber optic cable closure  10  is suitable for containing optical fibers, optical fiber splices, other suitable optical connections and/or components of a dispersion-managed network (DMN). Fiber optic cable closure  10  (hereinafter closure) preferably includes a subassembly  11  having an end cap  12  with a frame assembly  16  attached thereto, at least one fiber optic cassette assembly  40 , an O-ring  30 , a collar  35 , and a housing  50 .  
     [0050] As shown in FIGS.  3 - 6 , end cap  12  has a predetermined diameter and includes a first end  12   a  and a second end  12   b . A plurality of ports  14  extending through the end cap from the first end  12   a  to the second end  12   b  of end cap  12  and are configured for receiving fiber optic cables. As shown in FIG. 6, ports  14  may be sealed at the second end  12   b  by a plurality of respective covers  13  that can be molded into end cap  12 , thereby sealing the ports  14  when not occupied by a fiber optic cable. Thus, in this embodiment cover  13  must be removed before using the respective port  14 . For example in FIG. 6, removing two covers  13  opens the two respective ports  14 , thereby allowing the ports to receive first and second fiber optic cables  2 , 3 . In other configurations, plugs may be used to seal ports  14 .  
     [0051] End cap  12  can also include a marking indicia  12   c  to aid the craftsman in identifying, and positioning, fiber optic cables  2 , 3  in predetermined ports  14  of end cap  12 . For example, adjacent the respective ports  14  of end cap  12  are the indicia “D+” and “D−” suggesting ports  14  for respective fiber optic cables. However, other suitable marking indicia  12   c  can be used and/or be located on other suitable locations of end cap  12 , or on other suitable locations of closure  10 . In other embodiments, fiber optic cables  2 , 3  can be marked with an indicia to identify the optical fibers therein.  
     [0052] Attached to the first end  12   a  of end cap  12  is frame assembly  16 . Frame assembly  16  includes support frame  17 , strain relief brackets  17   a  and cassette stackers  19  (See FIG. 3). Support frame  17  is attached to end cap  12  by a fastening elements  17   b , such as a pair of threaded bolts, that extend through apertures in support frame  17  (FIG. 5) and attach adjacent to first end  12   a  of end cap  12 . Other suitable fastening elements such as quarter-turn screws, or snap-fits can be used. Frame assembly  16  has a storage means or portion for routing and storing buffer tubes  2   b , 3   b  (FIG. 2) of fiber optic cables  2 , 3 . For example, buffer tubes  2   b , 3   b  can protect the optical fibers of the fiber optic cables as they are routed to a predetermined fiber optic cassette  40  for optical connection therein.  
     [0053] Preferably, the sheath and the strength member of a fiber optic cable are clamped to a strain relief bracket  17   a  that is attached to support frame  17  by an attachment element  19 . Attachment element  19  can be any suitable fastener such as bolts, rivets, welds, etc. Clamping of the sheath of the fiber optic cable to the strain relief bracket can be accomplished, for example, with a hose clamp  18 , however, other suitable elements may be used. Clamping the sheath to strain relief bracket inhibits the fiber optic cable from being pulled out of the closure. Likewise, the central strength member of the fiber optic cable can be restrained (not shown) to inhibit the same from pistoning into closure  10  during temperature variations.  
     [0054] As shown in FIG. 2, housing  50  preferably has a generally cylindrical shape about a longitudinal axis A-A with a first end  51  and a second end  52 . Adjacent to first end  51 , housing  50  includes an opening  56  to a cavity  54 , and a circumferential flange  57  adjacent to the opening  56 , whereas, second end  52  of housing  50  is closed. When closure  10  is assembled, a majority of subassembly  11  fits within housing  50  with end cap  12  being adjacent to opening  56  to generally close the same, thereby protecting that portion of subassembly  11  within cavity  54  from environmental elements. Preferably, closure  10  includes a split annular collar  35  that securely engages circumferential flange  57  of housing  50  and a circumferential flange  12   d  (FIG. 5) of end cap  12  to secure the end cap  12  to housing  50 . Collar  35  and circumferential flanges  57 , 12   d  cooperate with O-ring  30  that is received in a circumferential channel  12   e  defined by end cap  12 . However, end cap  12  may be secured to housing  50  in other suitable manners as known to those skilled in the art. Additionally, the concepts of the present invention can be practiced with closures, housings, and/or other suitable environmentally sealed devices having other suitable shapes, sizes, and configurations. For example, other sealed devices may include above-grade, below-grade, or aerial closures; however, housings such as pedestals can also be used.  
     [0055]FIG. 8 illustrates a portion of an explanatory fiber optic cassette assembly  40  (hereinafter cassette) for use with fiber optic cable closure  10 . Cassette  40  can be made from, for example, metal, dielectric, or any other suitable material and is capable of being removably attached to frame assembly  16 . FIG. 9 illustrates cassette  40  with a portion of the optical pathway contained therein. In this embodiment, cassette  40  has a friction fit within cassette stackers  19  of frame assembly  16  (FIG. 3). However, other suitable elements can be used to secure cassette  40  to frame assembly  16 . For example, cassette  40  can be removably secured with Velcro® straps, friction fits using such features as dimples or resilient members, bolts, or other suitable elements.  
     [0056] As depicted in FIGS. 8 and 9, cassette  40  includes a first tray  43  having a first storage area  43   a  and a second tray  44  having a second storage area  44   a , at least one fastening element  45 , a plate  46 , and a pair of transition tubes  47 , 48 . First tray  43  and second tray  44  can be removably secured together by fastening elements  45 , for example, threaded fasteners such as a bolts, or quarter-turn screws. Thus, when secured together, removal of fastening elements  45  is required to access first storage area  43   a . In one embodiment, fastening elements  45  can have a non-standard head that requires a special engagement tool to drive the same. In other embodiments, fastening elements  45  can be latches, snap-fitting portions, or clamps.  
     [0057] Specifically, fastening elements  45  such as bolts are received through a pair of apertures in first tray  43  that have a pair of standoffs  43   b  that maintain second tray  44  at a predetermined distance away from storage area  43   a . Fastening elements  45  are received in a pair of threaded bores in second tray  44  to secure trays  43  and  44  together. On the other hand, plate  46  removably attaches to second tray  44  with a friction fit, thereby allowing relatively easy access to second storage area  44   a  the for reasons which will be discussed herein. The threaded bores of second tray  44  for engaging fastening elements  45  may be within a pair of standoffs  44   b . Additionally, standoffs  44   b  inhibit plate  46  from entering storage area  44   a . Preferably, trays  43 , 44  contain rails  43   c , 44   c  adjacent to the tray edges to retain optical fibers within storage areas  43   a , 44   b ; however, other suitable elements can be used. A plurality of splice organizers  49  are disposed within second storage area  44   a . Splice organizers  49  define a plurality of parallel grooves for receiving and organizing optical splices therein.  
     [0058]FIG. 10 illustrates an assembled cassette having buffer tubes  2   b , 3   b  entering second storage area  44   a . In other embodiments, buffer tubes  2   b , 3   b  can enter the second storage area  44   a  at other suitable locations, for example, the same side of tray  44 , but at opposite ends. In the field, the craftsman would route buffer tubes  2   b  and  3   b  having optical fibers  2   a , 3   a  of fiber optic cables  2 , 3  to the second tray  44  for optical connection. The optical connection is with a pair of transition optical fibers T 1 ,T 2  as will be discussed. Buffer tubes  2   b , 3   b  can be removably secured to second tray  44  by crimping tab  41   c  therearound. Additionally, other suitable elements can be used to secure buffer tubes  2   b , 3   b  such as tie wraps secured to apertures of second tray  44 .  
     [0059] Bridge fiber  42 , first transition optical fiber T 1 , second transition optical fiber T 2 , and optical fibers  2   a , 3   a  are schematically represented in FIGS. 13 a - 13   e  to illustrate an exemplary routing and splicing of the same within cassette  40 . For clarity of the optical fiber routing and splicing, first storage area  43   a  is depicted in FIG. 13 a  and second storage area  44   a  is depicted in FIGS. 13 b - 13   e . More specifically, FIG. 13 a  illustrates a splice S 1  between a first end  42   a  of bridge optical fiber  42  and a first end E 1  of transition optical fiber T 1 . Likewise, FIG. 13 a  also illustrates a splice S 2  between a second end  42   b  of bridge optical fiber  42  and a first end E 2  of transition optical fiber T 2 . Respectively, FIGS. 13 b ,  13   c ,  13   d  and  13   e  illustrate the routing of one optical fiber to a splice as follows: first transition optical fiber T 1  to a splice S 3 ; optical fiber  2   a  of fiber optic cable  2  to splice S 3 ; second transition optical fiber T 2  to a splice S 4 ; and optical fiber  3   a  of fiber optic cable  3  to splice S 4 . Splices of the present invention can include, for example, a plastic shrink wrap therearound for protection of the same from stress and/or strain. For purposes of clarity only one optical pathway is shown in cassette  40 ; however, cassette  40  can contain a plurality of optical pathways, for example,  24  separate optical pathways.  
     [0060] As assembled, cassette  40  houses a portion of at least one bridge fiber  42  therein. More specifically, as depicted in FIG. 13 a , at least a portion of bridge fiber  42  is disposed within first storage area  43   a  of first tray  43 . First storage area  43   a  is sized so that when bridge fiber  42  is coiled and stored therein it does fall below a minimum bend radius. In one embodiment, ends  42   a , 42   b  of bridge fiber  42  are capable of being optically connected with respective first ends E 1 ,E 2  of a pair of transition optical fibers T 1 ,T 2  (FIG. 13 a  ) having predetermined, dispersion characteristics, which can be different. For example, first transition optical fiber T 1  can be a D+ optical fiber, having optical characteristics similar to optical fiber  2   a . On the other hand, second transition optical fiber T 2  can be a D-optical fiber, having optical characteristics similar to optical fiber  3   a . The optical connections between ends  42   a , 42   b  of bridge fiber  42  and respective first ends E 1 ,E 2  of transition optical fibers T 1 ,T 2 , for example, splices S 1 ,S 2  (FIG. 13 a  ) are stored in first storage area  43   a  of first tray  43 . In other words, splices S 1 ,S 2  are housed in first storage area  43   a  so that they cannot be accidentally or easily accessed by the craftsman, i.e., the bolts would have to be removed to access the same. Additionally, cassette  40  can be marked by suitable means notifying the craftsman not to access and/or tamper with splices S 1 ,S 2 .  
     [0061] Preferably, bridge fiber  42  has a predetermined length so that it can be stored in first storage area  43   a  with a predetermined number of coils therein. For example, bridge fiber  42  has a length of about 1-5 meters; however, any other suitable length can be used. Using a predetermined length within first storage area  43   a  allows splices S 1 ,S 2  to be located at a predetermined position within first storage area  43   a . Preferably, splices S 1 ,S 2  lie along the longest length of first storage area  43   a , thereby maximizing the bend radius along splices S 1 ,S 2 . In other words, splices S 1 ,S 2  preferably are not appreciably bent, thereby inhibiting stresses and/or strains on splices S 1 ,S 2 .  
     [0062] Advantageously, in one embodiment splices S 1 ,S 2  are performed in a factory environment with precision alignment splicing equipment so that splice losses between transition optical fibers T 1 ,T 2  and bridge fiber  42  are minimized. Transition optical fibers T 1 ,T 2 , are preferably selected to have the same, or similar, optical characteristics as optical fibers of the optical fiber cables that are intended to enter closure  10 . However, any other suitable transition fibers can be used. Performing splices S 1 ,S 2  in the factory allows the craftsman to splice together the same, or similar, optical fibers in the field, rather performing splices between bridge fiber  42  and transition optical fibers T 1 ,T 2 , which may require special equipment, procedures, and/or training. Additionally, performing splices S 1 ,S 2  in the factory is more efficient, facilitates testing of the splices, and does not require special and expensive equipment to be carried in the field by the craftsman.  
     [0063] Conversely, a pair of other ends O 1 ,O 2  (FIGS. 13 b  and  13   d ) of transition optical fibers T 1 ,T 2  are intended for optical connection in the field by the craftsmen. Specifically, other ends O 1 ,O 2  are capable of being optically connected with optical fibers  2   a , 3   a  of respective fiber optic cables  2 , 3  having predetermined dispersion characteristics that are, respectively, similar to transition optical fibers T 1 ,T 2 . In one embodiment, other ends O 1 ,O 2  are disposed in second storage area  44   a . The optical connection between O 1 ,O 2  and optical fibers  2   a , 3   a  can be, for example, splices S 3 ,S 4 , however, other suitable optical connections can be used. Since, optical fibers  2   a , 3   a  preferably have the same, or similar, characteristics compared with transition optical fibers T 1 ,T 2 , the splicing operation therebetween is relatively easy and can be performed in the field. Thus, splices having suitable optical performance can efficiently be made in the field by the craftsman, thereby forming an optical pathway capable of transmitting optical signals with a relatively low splice-loss between optical fibers  2   a , 3   a  of respective fiber optic cables.  
     [0064] As illustrated in the embodiment of FIG. 9, a portion of transition optical fibers T 1 , T 2  are disposed in respective transition sections  47 , 48 . Transitions sections  47 , 48  protect transition optical fibers T 1 , T 2  from undue stress and/or strain during, for example, routing the transition optical fibers T 1 ,T 2  from first tray  43  to second tray  44  of cassette  40 . Generally speaking, transition sections  47 , 48  inhibit pinching or snagging of transition optical fibers T 1 ,T 2  during routing between the trays, which can damage the same and/or degrade optical performance. Transition sections  47 , 48  can be, for example, transition tubes as depicted in FIGS. 11 and 12, which shows the orientation of the transition tubes between first tray  43  and second tray  44 . Transition sections  47 , 48  can be securely held in place by respective tabs  41   a , 41   b  of first and second trays  43 , 44 . However, other suitable orientations of transition sections and/or securement of the same can be used such as clamps or tie wraps. In still other embodiments, transition sections  47 , 48  may not be required or can have other configurations such as a plastic spiral tube or a tape. However, the optical fibers should not have a bend radius below a suitable minimum bend radius in order to preserve optical performance.  
     [0065] Additionally, transition sections  47 , 48  can include a marking indicia  47   a , 48   a  to aid the craftsman in identifying the different transition optical fibers T 1 ,T 2 . For example, the tubes can be different colors such as a blue tube for a D+ transition optical fiber and a green tube for a D− optical fiber. For purposes of illustration, the marking indicia  47   a , 48   a  are respectively represented in the drawings with a shaded tube and a non-shaded tube. However, other suitable marking indicia  47   a ,  48   a  can be used, for example, stripes, decals, embossing, or the like. Moreover, marking indicia  47   a , 48   a  can be located on other suitable components of cassette  40  such as a stamping on second storage area  44   a  adjacent to where the transition fibers T 1 ,T 2  enter. Additionally, the concepts of the present invention can be practiced with cassettes having other suitable configurations and/or other numbers of trays. FIG. 14 illustrates a portion of another explanatory cassette assembly  140  according to the present inventions that is similar to cassette  40 . Cassette  140  includes first and second trays  143 , 144  having respective first and second storage areas  143   a , 144   a , at least one fastening element  145 , a first plate  146   a , a pair of transition tubes similar to those shown in FIG. 9, a plurality of splice organizers  149 , and a second plate  146   b . Cassette  140  is essentially the same as cassette  40 , except first plate  146  covers first storage area  143   a . Otherwise, the features and operations are essentially the similar to cassette  40 .  
     [0066] Additionally, the concepts of the present invention can also be practiced without transition optical fibers T 1 ,T 2 . In other words, bridge optical fiber  42  would be optically connected directly to optical fibers  2   a , 3   a  within closure  10 . Other modifications of the present invention can include more than one optical fiber interposed between the bridge optical fiber and optical fibers of the cables. In one embodiment, a cassette having a single storage area can house bridge fiber  42  and the optical connections. In other embodiments, bridge fiber  42  can be used with cassettes  40 , 140  and bridge fiber  42  can be routed to the second storage area for direct optical connection with optical fibers  2   a , 3   a . Additionally, other suitable configurations can be practiced with the concepts of the present invention.  
     [0067]FIG. 15 illustrates an another cassette assembly  240  (hereinafter cassette) for housing bridge fiber  42  according to the present invention. Cassette  240  includes tray  243  and plate  246 . Tray  243  includes a raceway  241  for routing optical fibers therein, a pair of fastening elements  245 , and a standoff  250  having a bore therethrough. Plate  246  includes a pair of rectangular apertures  245   a  for receiving fastening elements  245  and an aperture  252 . FIG. 16 illustrates cassette  240  after assembly. Plate  246  is secured to tray  243  by fastening elements  245  and the bore of standoff  250  and aperture  252  are aligned, thereby allowing a securing element  255  to pass therethrough.  
     [0068]FIG. 15 a  depicts bridge fiber  42  and splices S 1 ,S 2  disposed in cassette  240 . In this embodiment, the optical connection, for example, splices S 3 ,S 4  between optical fibers  2   a , 3   a  and transition optical fibers T 1 ,T 2  occurs in a second cassette  280  as shown in FIG. 21. Closures using cassette  240  route ends O 1 ,O 2  of transition optical fibers T 1 ,T 2 , or ends of a bridge optical fiber, to a second cassette  280  disposed within a closure. For example, as depicted in FIG. 21, a plurality of cassettes  240  are located on one side of a frame assembly  216  and a plurality of second cassettes are located on the other side of frame assembly  216 . A pair of transition sections  247 , 248 , for example, tubes route a portion of transition optical fibers T 1 ,T 2  from cassette  240  to second cassette  280 . Second cassette  280  includes a tray  280   a  and plate  280   b . However, second cassette can include other suitable components such as splice organizers. Cassettes  240  and second cassettes  280  can be secured to frame assembly  216  using suitable means. For example, cassettes  240  can be secured to frame assembly by bolts  255  and second cassettes  280  can be secured with Velcro® straps.  
     [0069] Ends of transition sections  247 , 248  can be attached to respective portions of cassette  240  and second cassette  280  as described herein. For example, transition section  247  can be attached to apertures of tray  243  using tie straps. In other embodiments, bridge fibers  42  from several cassettes  240  can be routed a single second cassette  280 . Cassette  240  and second cassette  280  can be, for example, different shapes, colors and/or marked to indicate to the craftsman that the optical connection between optical fiber  2   a , 3   a  and transition optical fibers T 1 ,T 2  is intended to occur in field cassette  250 . Additionally, cassettes  240  can be marked and/or made tamper-resistant to prevent the craftsman from accessing the same.  
     [0070]FIG. 17 illustrates an another cassette assembly  340  (hereinafter cassette), similar to cassette  240 , for housing bridge fiber  42  according to the present inventions. Cassette  340  includes a tray  343  a first plate  346   a , and a second plate  346   b . Cassette  340  incorporates features similar to cassette  240  and essentially operates in the same manner. FIG. 15 a  depicts bridge fiber  42  disposed in cassette  240 . In this embodiment, bridge fiber  42  is routed to through transition sections to a second cassette for direct optical connection with optical fibers of a pair of optical fibers; however, transition optical fibers could also be used. FIG. 18 illustrates cassette  340  after assembly.  
     [0071]FIGS. 19 and 20 depict an exemplary in-line closure  200  with its housing removed. Closure  200  is similar to closure  10 , except instead of the housing being closed on one end it has an additional end cap. More specifically, a pair of end caps  212  are respectively positioned at opposite ends of closure  200  and are intended to close respective openings on opposite ends of the housing (not shown). This type of closure allows fiber optic cables to enter and/or exit the in-line splice closure from either or both ends. Otherwise, closure  200  operates in a manner similar to closure  10 . For example, each end of frame assembly  216  is attached to a respective end cap  212  and at least one fiber optic cassette assembly  40  can be removable secured in one of a plurality of cassette stackers  219 . As discussed with respect to closure  10 , each end cap  212  can be secured to a respective flange on the housing using O-rings and collar, or with other suitable elements.  
     [0072] Many modifications and other embodiments of the present invention, within the scope of the appended claims, will become apparent to a skilled artisan. For example, the bridge fiber can be disposed in other configurations within the cavity of the closure while still employing the concepts of the present invention, rather than at least partially disposed within a cassette. Additionally, fiber optic cable closures of the present invention can include other suitable components and/or configurations. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments may be made within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. The invention has been described with reference to splices between optical fibers, but the inventive concepts of the present invention are applicable to other suitable optical connections as well.