Abstract:
Micromodule breakout cables are constructed to pass selected burn tests while maintaining a desired degree of accessibility and durability. The micromodule cables can be incorporated in data centers and are robust enough to serve as furcation legs while allowing hand accessibility. The cables can incorporate optical fibers with low delta attenuation and can have low skew.

Description:
PRIORITY APPLICATIONS 
     This application claims the benefit of U.S. App. No. 61/152,860, filed Feb. 16, 2009, the entire contents of which are hereby incorporated by reference as if presented herein. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an optical cable with micromodules and the use of micromodule cables. 
     BACKGROUND 
     Micromodule cables are high density optical cables having a number individual micromodules. The micromodules typically include optical fibers arranged in a sheath for transmitting optical data. The micromodules are arranged within the cable jacket. Conventional micromodule cables are capable of transmitting large amounts of data and are typically designed for outdoor applications. 
     One of the problems in planning data centers is the large number of cables required to transfer data between racks of electronic equipment. The cables may become tightly packed, which restricts cooling air flow in and around the equipment. Micromodule cables have the ability to transmit large amounts of data, but existing micromodule cables lack many properties that render them suitable for applications such as data centers. For example, prior art high density cables have used relatively flimsy subunit materials such as chalk-filled EVA or polyester-based thermoplastic elastomers. The subunits have had wall thicknesses of about 0.1 mm, which provided only minimum protection for the optical fibers. Furcation of these cables required that the subunits be placed within protective furcation legs so that they were robust enough for field use. Existing micromodule cables may also lack the ability to be bent around corners or other tight spaces without undue attenuation of the optical data. Further, conventional micromodule cables may not be designed to pass burn specifications such as plenum-ratings. 
     SUMMARY 
     According to one embodiment, a breakout cable comprises a jacket and a plurality of micromodules enclosed within the jacket, each micromodule having a plurality of optical waveguides and a polymer sheath surrounding the waveguides. The polymer sheath has a thickness in the range of 0.2 mm and 0.3 mm. The breakout cable may have exceptionally low attenuation under various test criteria, such as when subjected to a corner bend load of six kilograms, delta attenuation at 850 nm in the cable is less than 0.2 dB. The polymer sheath surrounding the waveguides can allow access to the optical waveguides by hand tearing, while being robust enough so that they can serve as furcation legs. According to one aspect, the polymer sheath has a thickness in the range of 0.2 mm-0.3 mm and still allows for hand access to the optical waveguides. 
     According to another embodiment, a plurality of the breakout cables are enclosed within an outer jacket to form a micromodule breakout cable. 
     According to one aspect, a method of calculating jacket thickness for a cable is used to provide a breakout cable with robustness, compliance with plenum burn requirements, and with hand accessibility. The method determines a maximum jacket thickness based on material modulus. 
     Those skilled in the art will appreciate the above stated advantages and other advantages and benefits of various additional embodiments reading the following detailed description of the embodiments with reference to the below-listed drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The present embodiments are explained in more detail below with reference to figures which show the exemplary embodiments. 
         FIG. 1  is a cross section of a micromodule cable according to a first embodiment. 
         FIG. 2  is a sectional view of a breakout unit or cable of the cable of  FIG. 1 . 
         FIG. 3  is a sectional view of a micromodule of the cable of  FIG. 1 . 
         FIG. 4  is a sectional view of a cable according to a second embodiment. 
         FIG. 5  is a plot illustrating various design constraints in designing a cable for strength, accessibility and for plenum burn characteristics. 
         FIG. 6  is a schematic representation (not to scale) of the refractive index profile of a cross-section of the glass portion of an exemplary embodiment of multimode optical fiber disclosed herein wherein the depressed-index annular portion is offset from the core and is surrounded by an outer annular portion. 
         FIG. 7  is a schematic representation (not to scale) of a cross-sectional view of the optical waveguide fiber of  FIG. 6 . 
         FIG. 8  illustrates a cable according to a third embodiment undergoing a corner tie down bend. 
         FIG. 9  is a plot of delta attenuation at 850 nm for the cable of  FIG. 8  in a corner tie down bend test. 
         FIG. 10  is a plot of delta attenuation in a corner bend under load test. 
         FIG. 11  is a plot of delta attenuation for one embodiment of the invention in small diameter mandrel wrap tests. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a cross section of an optical micromodule cable  10  according to a first embodiment and having a generally circular cross-section. The optical cable  10  comprises a plurality of breakout units or cables  20  arranged (e.g. stranded) around a central member  30  in an interior  34  of the cable  10 . The central member  30  can be, for example, a relatively stiff member  36  of fiber or glass-reinforced plastic, or a relatively flexible combination of aramid fiber that may include an overcoating of plastic material  38 . The breakout units  20  are enclosed within the cable&#39;s outer jacket  50 . The outer jacket  50  can be formed from, for example, a polymer material, and may be reinforced with fibers, etc. (not shown), and has a thickness  52 . Each breakout unit  20  includes a plurality of micromodule subunits  60 , or simply “micromodules”, that each contain at least one optical waveguide  66 . The exemplary micromodules  60  are not stranded within the breakout units  20 , although stranding may be used for certain applications. For example, the micromodules  60  can be twisted in helical fashion with respect to one another, in particular when a plurality of or all of the micromodules  60  are arranged in such a way that they are rotated with a specified lay length. 
     In the illustrated embodiment, the micromodule breakout cable  10  has eight breakout units  20 , each breakout unit  20  having twelve micromodules  60 , with each micromodule  60  including twelve optical waveguides  66 . The total number of optical waveguides  66  for the cable is therefore 1152. Other numbers of breakout units  20 , micromodules  60 , and optical waveguides  66  can be employed for various applications, however. The micromodule cable  10 , the breakout units  20  and the micromodules  60  all have generally circular cross-sections, although other cross-sections may be used. 
       FIG. 2  is a cross section of one of the exemplary breakout units  20  having diameter  22 . Each breakout unit  20  has a jacket  70  of thickness  72  that encloses its micromodules  60 . The thickness  72  will not be completely uniform and thicknesses, as well as diameters described in this specification refer to nominal or average values. A strain-relief element  76  may be disposed adjacent to the interior of the jacket  70  and surrounding the micromodules  60 . The strain-relief element  76  may be, for example, a layer of yarns (e.g. aramid yarn) for absorbing tensile loads on the cable  10 . In  FIG. 2 , the exemplary strain-relief element is  76  are illustrated schematically as a layer of yarn disposed adjacent to the interior of outer jacket  70 . The layer of strain-relief element  76  is shown with a non-uniform thickness because the locations of the micromodules  60  may cause the strain-relief element to compress at various locations along the length of the breakout unit  20 . 
       FIG. 3  is a cross section of a micromodule subunit  60  having a diameter  62 . The optical waveguides  66  of the micromodules  60  are enclosed in a polymeric sheath  80  of thickness  82 . 
     According to one aspect of the first embodiment, the micromodule cable  10  can be constructed to achieve desired properties. For example, the components of the cable  10  can be constructed of selected materials of selected thicknesses such that the cable  10  achieves plenum burn ratings according to desired specifications. The micromodules  60  can also be constructed so that they are relatively robust, such that they are suitable for field use, while also providing a desired degree of accessibility. For example, the micromodules  60  according to the present embodiments can be constructed with thicker sheaths  80 , on the order of 0.2 mm or more, which provide sufficient protection for the fibers such that the micromodules  60  may be used as a furcation leg. The cable jacket  50 , the breakout unit jackets  70 , and the micromodule sheaths  80  can also be formed from fire-retardant materials to obtain a desired plenum burn rating. For example, highly-filled PVC of a specified thicknesses can be used to form the micromodule sheaths  80 . One well known plenum burn standard is the National Fire Protection Act Standards (NFPA) 262 burn test. NFPA 262 prescribes the methodology to measure flame travel distance and optical density of smoke for insulated, jacketed, or both, electrical wires and cables and optical fiber cables that are to be installed in plenums and other spaces used to transport environmental air without being enclosed in raceways. Cables according to the present embodiments may also be constructed to be low skew within the micromodules  60  so that they are suitable for use in parallel optic transmission systems. Skew is generally defined as the difference in the time of flight of optical signals for the fibers within a module and has units of picoseconds per meter (ps/m). 
       FIG. 4  is a section view of a cable  200  according to a second embodiment having a diameter  202 . The cable  200  is generally similar to the breakout units  20  of the cable  10 , but the cable  200  may have a jacket  270  of greater thickness  272  than the thickness  72  of the breakout unit jacket  70  to provided added robustness to the cable  200 . The interior of the cable  200  may also allow for greater spacing of the micromodules  260 . In  FIG. 4 , elements of like or identical construction to those of  FIGS. 1-3  are indicated with like reference numbers, with the elements in  FIG. 4  being preceded by a “2.” The micromodules  260  illustrated in  FIG. 4  may, for example, be identical to the micromodules  60  illustrated in  FIGS. 1-3 . As in the case of the cable  10 , the cable  200  can be constructed to have desirable burn properties as well as a selected degree of durability and hand accessibility. Exemplary methodologies for constructing cables according to the present embodiments are discussed in detail below. 
     The following discussion is addressed to the components of the cable  200  illustrated in  FIG. 4 , although the principles discussed herein apply to the cable  10  ( FIGS. 1-3 ). The micromodule sheath  280  thickness  282  may be increased or decreased to adjust the properties of the cable  200 . For example, the micromodules  260  may be made with a thicker sheath  280  to make a more robust unit, or it may be made with a thinner sheath wall in order to reduce size and material costs. However, according to the present embodiments, additional constraints may be placed on the materials and dimension of the micromodule sheath  280  thickness. One desirable property is accessibility, or the ability to easily remove the micromodule sheath  80  from around the optical waveguides  66  by simple tearing with the fingers. The axial strength of the micromodule sheath  280  is the cross sectional area of the sheath  280  times the modulus of the material. A plenum grade PVC was tested as the micromodule sheath  280  material having a modulus of 2500 psi with sheath thicknesses of 0.1 mm, 0.2 mm, and 0.3 mm. A sheath  280  thickness of 0.2 mm allowed for easy removal of the sheath  280 , but at 0.3 mm it became much more difficult to remove the sheath. A micromodule sheath  280  of thickness in the range of 0.2 mm to 0.3 mm accordingly provides highly desirable properties not available in existing cables. Using these values as design constraints, the maximum sheath thickness for a given material modulus may be calculated using the following Equation 1 (wall thickness in millimeters and modulus in psi):
 
Material_Modulus=590(Wall_thickness) −1.2 .
 
     The constraints discussed above define a desired operating region. The DESIRED OPERATING REGION is represented on a plot of wall thickness versus modulus as shown in  FIG. 5 . 
     The methodologies set forth above may be applied to alternative forms of packaging. Equation 1 establishes a relationship and practical limit for any packaging that is intended for easy opening by hand. For example, it could be used in the design of a vessel accommodating foodstuffs. In  FIG. 5 , the minimum wall (or sheath) thickness is determined by the plenum burn test; however, for a bag of foodstuffs, the minimum wall thickness can be determined by the minimum thickness to maintain freshness or some other constraint. Another direct application of Equation 1 is in the design of packaging for emergency medical equipment in which the package must maintain a sterile environment inside but be easily opened by the medical technician during an emergency. Equation 1 can be used to determine the maximum wall thickness, and the minimum wall thickness can be determined by the thickness required to maintain the contents in a sterile environment. 
     According to another aspect of the present embodiments, the cables  10 ,  200  may be used in data center applications, such as in a data information transfer system. In data centers, optical signals are transmitted and received in blade servers. Common blade servers have from 16 to 48 optical ports. Each optical port has a transmit fiber and a receive fiber. In parallel optic systems, the transmit fiber would be replaced by 12 fibers and the receive fiber would be replaced by 12 fibers in a 120 Gb/s Ethernet blade server. Thus a parallel optic system with a 48 port blade servers would need 1152 optical fibers. An example cable suitable for use in a data center is described below: 
     Example 1 
     According to one example embodiment, a cable  10  as shown in  FIG. 1  includes twelve optical waveguides  66 , in the form of optical fibers, in each micromodule  60 . The micromodule diameter  62  is 1.6 mm. Twelve micromodules  60  are placed together in a  144  fiber breakout unit  20  having a diameter  22  of about 8.6 mm. Eight breakout units  20  are placed together in an 1152 fiber cable that is less than 36 mm in diameter  12 . 
     According to the above example, if desired, the fiber count can be increased to 1728 fibers in a plenum cable of less than 40 mm in diameter by using twelve breakout units stranded in a 9-over-3 pattern. 
     According to the above embodiments, the cable  200  may comply with NFPA 262 while having a diameter  202  of less than 13.1 mm for a 144 fiber cable. The cables  10 ,  200  may have low skew for parallel optic systems of less than 2.0 ps/m. 
     In the above embodiments, low skew may be obtained in the subunits by using OM3 grade or better multimode fibers and precise control of the fiber tension during processing. The maximum allowed difference in fiber tension during processing of the subunit should be less than 50 g in order to achieve a skew less than 0.75 ns in a 300 meter cable assembly. The numerical aperture (NA) of the optical fiber should also be controlled. The fibers for the micromodules can be selected so that the difference in NA for the fibers within a subunit are less than 0.08 to achieve a skew less than 0.75 ns in a 300 m cable assembly. 
     One exemplary fiber suitable for use in the above cables is a bend resistant multimode optical fibers comprising a graded-index core region and a cladding region surrounding and directly adjacent to the core region, the cladding region comprising a depressed-index annular portion comprising a depressed relative refractive index relative to another portion of the cladding. The depressed-index annular portion of the cladding is preferably spaced apart from the core. Preferably, the refractive index profile of the core has a parabolic or substantially curved shape. The depressed-index annular portion may, for example, comprise a) glass comprising a plurality of voids, or b) glass doped with one or more downdopants such as fluorine, boron, individually or mixtures thereof The depressed-index annular portion may have a refractive index delta less than about −0.2% and a width of at least about 1 micron, said depressed-index annular portion being spaced from said core by at least about 0.5 microns. In some embodiments that comprise a cladding with voids, the voids in some preferred embodiments are non-periodically located within the depressed-index annular portion. By “non-periodically located” we mean that when one takes a cross section (such as a cross section perpendicular to the longitudinal axis) of the optical fiber, the non-periodically disposed voids are randomly or non-periodically distributed across a portion of the fiber (e.g. within the depressed-index annular region). Similar cross sections taken at different points along the length of the fiber will reveal different randomly distributed cross-sectional hole patterns, i.e., various cross sections will have different hole patterns, wherein the distributions of voids and sizes of voids do not exactly match for each such cross section. That is, the voids are non-periodic, i.e., they are not periodically disposed within the fiber structure. These voids are stretched (elongated) along the length (i.e. generally parallel to the longitudinal axis) of the optical fiber, but do not extend the entire length of the entire fiber for typical lengths of transmission fiber. It is believed that the voids extend along the length of the fiber a distance less than about 20 meters, more preferably less than about 10 meters, even more preferably less than about 5 meters, and in some embodiments less than 1 meter. The multimode optical fiber disclosed herein exhibits very low bend induced attenuation, in particular very low macrobending induced attenuation. In some embodiments, high bandwidth is provided by low maximum relative refractive index in the core, and low bend losses are also provided. Consequently, the multimode optical fiber may comprise a graded index glass core; and an inner cladding surrounding and in contact with the core, and a second cladding comprising a depressed-index annular portion surrounding the inner cladding, said depressed-index annular portion having a refractive index delta less than about −0.2% and a width of at least 1 micron, wherein the width of said inner cladding is at least about 0.5 microns and the fiber further exhibits a 1 turn, 10 mm diameter mandrel wrap attenuation increase of less than or equal to about 0.4 dB/turn at 850 nm, a numerical aperture of greater than 0.14, more preferably greater than 0.17, even more preferably greater than 0.18, and most preferably greater than 0.185, and an overfilled bandwidth greater than 1.5 GHz-km at 850 nm 50 micron diameter core multimode fibers can be made which provide (a) an overfilled (OFL) bandwidth of greater than 1.5 GHz-km, more preferably greater than 2.0 GHz-km, even more preferably greater than 3.0 GHz-km, and most preferably greater than 4.0 GHz-km at an 850 nm wavelength. These high bandwidths can be achieved while still maintaining a 1 turn, 10 mm diameter mandrel wrap attenuation increase at an 850 nm wavelength of less than 0.5 dB, more preferably less than 0.3 dB, even more preferably less than 0.2 dB, and most preferably less than 0.15 dB. These high bandwidths can also be achieved while also maintaining a 1 turn, 20 mm diameter mandrel wrap attenuation increase at an 850 nm wavelength of less than 0.2 dB, more preferably less than 0.1 dB, and most preferably less than 0.05 dB, and a 1 turn, 15 mm diameter mandrel wrap attenuation increase at an 850 nm wavelength, of less than 0.2 dB, preferably less than 0.1 dB, and more preferably less than 0.05 dB. Such fibers are further capable of providing a numerical aperture (NA) greater than 0.17, more preferably greater than 0.18, and most preferably greater than 0.185. Such fibers are further simultaneously capable of exhibiting an OFL bandwidth at 1300 nm which is greater than about 500 MHz-km, more preferably greater than about 600 MHz-km, even more preferably greater than about 700 MHz-km. Such fibers are further simultaneously capable of exhibiting minimum calculated effective modal bandwidth (Min EMBc) bandwidth of greater than about 1.5 MHz-km, more preferably greater than about 1.8 MHz-km and most preferably greater than about 2.0 MHz-km at 850 nm. Preferably, the multimode optical fiber disclosed herein exhibits a spectral attenuation of less than 3 dB/km at 850 nm, preferably less than 2.5 dB/km at 850 nm, even more preferably less than 2.4 dB/km at 850 nm and still more preferably less than 2.3 dB/km at 850 nm. Preferably, the multimode optical fiber disclosed herein exhibits a spectral attenuation of less than 1.0 dB/km at 1300 nm, preferably less than 0.8 dB/km at 1300 nm, even more preferably less than 0.6 dB/km at 1300 nm. In some embodiments, the numerical aperture (“NA”) of the optical fiber is preferably less than 0.23 and greater than 0.17, more preferably greater than 0.18, and most preferably less than 0.215 and greater than 0.185. In some embodiments, the core extends radially outwardly from the centerline to a radius R 1 , wherein 10≦R 1 ≦40 microns, more preferably 20≦R 1 ≦40 microns. In some embodiments, 22≦R 1 ≦34 microns. In some preferred embodiments, the outer radius of the core is between about 22 to 28 microns. In some other preferred embodiments, the outer radius of the core is between about 28 to 34 microns. In some embodiments, the core has a maximum relative refractive index, less than or equal to 1.2% and greater than 0.5%, more preferably greater than 0.8%. In other embodiments, the core has a maximum relative refractive index, less than or equal to 1.1% and greater than 0.9%. In some embodiments, the optical fiber exhibits a 1 turn, 10 mm diameter mandrel attenuation increase of no more than 1.0 dB, preferably no more than 0.6 dB, more preferably no more than 0.4 dB, even more preferably no more than 0.2 dB, and still more preferably no more than 0.1 dB, at all wavelengths between 800 and 1400 nm. 
       FIG. 6  is a schematic representation of the refractive index profile of a cross-section of the glass portion of an embodiment of a multimode optical fiber  400  comprising a glass core  420  and a glass cladding  500 , the cladding comprising an inner annular portion  530 , a depressed-index annular portion  550 , and an outer annular portion  560 .  FIG. 7  is a schematic representation (not to scale) of a cross-sectional view of the optical waveguide fiber of  FIG. 6 . The core  420  has outer radius R 1  and maximum refractive index delta Δ1MAX. The inner annular portion  530  has width W 2  and outer radius R 2 . Depressed-index annular portion  550  has minimum refractive index delta percent Δ3MIN, width W 3  and outer radius R 3 . The depressed-index annular portion  550  is shown offset, or spaced away, from the core  420  by the inner annular portion  530 . The annular portion  550  surrounds and contacts the inner annular portion  530 . The outer annular portion  560  surrounds and contacts the annular portion  550 . The clad layer  500  is surrounded by at least one coating  510 , which may in some embodiments comprise a low modulus primary coating and a high modulus secondary coating. The inner annular portion  530  has a refractive index profile Δ2(r) with a maximum relative refractive index Δ2MAX, and a minimum relative refractive index Δ2MIN, where in some embodiments Δ2MAX=Δ2MIN. The depressed-index annular portion  550  has a refractive index profile Δ3(r) with a minimum relative refractive index Δ3MIN. The outer annular portion  560  has a refractive index profile Δ4(r) with a maximum relative refractive index Δ4MAX, and a minimum relative refractive index Δ4MIN, where in some embodiments Δ4MAX=Δ4MIN. Preferably, Δ1MAX&gt;Δ2MAX&gt;Δ3MIN. In some embodiments, the inner annular portion  530  has a substantially constant refractive index profile, as shown in  FIG. 6  with a constant Δ2(r); in some of these embodiments, Δ2(r)=0%. In some embodiments, the outer annular portion  560  has a substantially constant refractive index profile, as shown in  FIG. 6  with a constant Δ4(r); in some of these embodiments, Δ4(r)=0%. The core  420  has an entirely positive refractive index profile, where Δ1(r)&gt;0%. R 1  is defined as the radius at which the refractive index delta of the core first reaches value of 0.05%, going radially outwardly from the centerline. Preferably, the core  420  contains substantially no fluorine, and more preferably the core  420  contains no fluorine. In some embodiments, the inner annular portion  530  preferably has a relative refractive index profile Δ2(r) having a maximum absolute magnitude less than 0.05%, and Δ2MAX&lt;0.05% and Δ2MIN&gt;−0.05%, and the depressed-index annular portion  550  begins where the relative refractive index of the cladding first reaches a value of less than −0.05%, going radially outwardly from the centerline. In some embodiments, the outer annular portion  560  has a relative refractive index profile Δ4(r) having a maximum absolute magnitude less than 0.05%, and Δ4MAX&lt;0.05% and Δ4MIN&gt;−0.05%, and the depressed-index annular portion  550  ends where the relative refractive index of the cladding first reaches a value of greater than −0.05%, going radially outwardly from the radius where Δ3MIN is found. 
       FIG. 8  illustrates a cable  600  according to a third embodiment of the present invention tied down to experience a 90 degree corner bend of approximately three-fourths inch (19 mm) radius. The cable  600  can be tied down to experience any number of corner bends. The cable  600  is generally similar to the cable  200  illustrated in  FIG. 4 , and incorporates 48 optical fibers of a configuration as discussed in the immediately preceding paragraphs. The cable  600  has a jacket  650 , and includes 4 micromodules, each micromodule having 12 fibers. 
       FIG. 9  is a plot of corner tie down delta attenuation data at 850 nm for the cable  600  undergoing varying numbers of corner bends, as shown in  FIG. 8 , of approximately one inch (25.4 mm) radius. Measured data points for the cable  600 , of relatively low attenuation, are generally indicated by reference numeral  655 . For reference purposes, higher delta attenuation data for similar cables utilizing alternative multimode fibers are also illustrated. As shown in  FIG. 9 , the delta attenuation values in group  655  over two, four, six and even eight bends are below 0.05 dB. More specifically, delta attenuation values for each of the data values in group  655  shown in  FIG. 9  are below 0.05 dB, and even as low as 0.03 dB or less. 
     Referring now to  FIG. 10 , the cable  600  can also be subjected to a corner bend under load. In a corner bend under load test, a weight is hung from an end of the cable that hangs over the corner of a surface. The corner may have a small radius, such as 1 mm. The other end of the cable is secured on the surface. Test delta attenuation data for the corner bend under load are shown in  FIG. 10 . The delta attenuation data, generally indicated by the bracket  660 , for the cable  600  was much lower than similar cables utilizing alternative fibers. For loads of up to 8 kg, delta attenuation was less than about 0.05 dB. For loads of up to 6 kb, delta attenuation was less than about 0.03 dB. 
       FIG. 11  is a plot of minimum bend radius test data for the cable  600  that contains twelve bend insensitive multimode optical fibers and has an outer diameter of 4.4 mm The standard cable bend requirements is that the cable have low attenuation when wrapped around a mandrel that is ten times the cable outside diameter.  FIG. 11  demonstrates that the cable  600  is capable of achieving bend diameters much smaller than the current 10× standard by having low attenuation at five times and even as low as three times the cable diameter. For example, for five wraps around a mandrel having a diameter of five times the cable diameter, delta attenuation due to the wraps is less than 0.15 dB. For one wrap around a mandrel having a diameter of three times the cable diameter, delta attenuation due to the wraps is less than 0.10 dB. 
     The present cable embodiments may utilize tensile yarns to form a separation layer between the modules and the outer jacket and thus prevent the modules from sticking to the jacket during extrusion of the jacket. The tensile yarns also provide strength to the cables. A preferred material for the tensile yarns is aramid (e.g., KEVLAR®), but other materials such as fiberglass yarn may also be used. The yarns may be stranded to improve cable performance. In the illustrated cables, the jackets may be sized such that the micromodules have a sufficient degree of lateral movement to reduce fiber stresses and optical attenuation when the cable is exposed to external forces such as tension, torsion, bending, or compression. The void fraction within the cable jackets may be about 56%. 
     In one particular set of parameters, cables according to the present embodiments may contain from four to twelve optical fibers within each micromodule. The breakout units  20  or the cables  200 ,  600  may contain from 2 to 24 micromodules within the cable for a range of fiber counts of 8 to 288. The dimensions of the micromodule may be adjusted based on the number of fibers within the module. The fibers may be loosely disposed within the module in an essentially parallel array. The fibers may be coated with a thin film of powder, such as chalk or talc, which forms a separation layer that prevents the fibers from sticking to the molten sheath material during extrusion. The outer jackets may be made of a fire retardant PVC material or a PVDF material to achieve a plenum burn performance rating. The cables may be further encased in an interlocking armor for enhanced crush resistance. 
     Many modifications and other embodiments of the present invention, within the scope of the claims will be apparent to those skilled in the art. For instance, the concepts of the present invention can be used with any suitable fiber optic cable design and/or method of manufacture. For instance, the embodiments shown can include other suitable cable components such as an armor layer, coupling elements, different cross-sectional shapes, or the like. Thus, it is intended that this invention covers these modifications and embodiments as well those also apparent to those skilled in the art.