Water-blocked fiber optic ribbon cable

[0002] The present invention relates to an improved optical fiber communications cable. More particularly, the invention relates to a fiber optic ribbon cable in which water is prevented from longitudinally flowing along the interior of the fiber optic ribbon cable.

[0003] In order to overcome axial water penetration through a buffer tube in a fiber optic cable, it has been the practice in prior cable designs to flood the buffer tubes with a grease or gel in order to fill the interstices of the buffer tube(s) of a fiber optic cable so as to prevent the flow of water along the axis of the cable. Most conventional fiber optic cables for outdoor use that include fiber optic ribbon cables and loose tubes utilize the aforementioned grease or gel flooded buffer tube technology.

[0004] As is also well known to those skilled in the fiber optic cable art, the flooding of the buffer tube(s) of a fiber optic cable with grease or gel to prevent axial water penetration through a buffer tube suffers from many shortcomings and drawbacks. The most serious shortcoming is that a craftsman handling the optical fiber communications cable must remove the grease or gel from the buffer tube(s) and clean all of the elements therein before beginning whatever work is necessary for the craftsman to perform. The cleaning process is both time consuming and unpleasant for the craftsman. Also, the grease or gel adds a significant amount of weight to the fiber optic communications cable which results in limiting its potential uses. For these and other reasons, it is desirable to eliminate the need to flood the buffer tube(s) of a fiber optic cable with grease and gel to prevent axial water penetration through the buffer tube(s) of the fiber optic cable.

[0005] Also, efforts have been made to deal with the water penetration and water migration problem of fiber optic cable by using superabsorbent compounds which are added to an optical fiber cable in the form of treated yarns, tapes and/or core elements as well as applying thin coatings and powders selectively to the cable construction. U.S. Pat. Nos. 4,703,998; 4,913,517; 5,133,034; and 5,642,452 (assigned to the assignee of the present invention) are representative patents relating to the use of superabsorbent compounds in optical fiber cable constructions to impart water blocking capabilities to the optical fiber cables. However, the prior efforts to utilize superabsorbent compounds have not all proven to be highly effective at a modest cost for reasons that are well known to those skilled in the fiber optic cable art.

[0006] The present invention is believed to overcome the foregoing problems of the prior art by providing for a novel fiber optic ribbon cable construction as described herein.

[0007] In accordance with the present invention, applicants provide a water-blocked optical fiber loose tube cable possessing enhanced performance characteristics. The cable comprises a plurality of stacked fiber optic ribbons having a plurality of water blocking yarns extending generally along the length of the stack of fiber optic ribbons and positioned around at least a portion of the circumference of the stack wherein the plurality of water blocking yarns possess water swellable characteristics. The stack of fiber optic ribbons and the plurality of water blocking yarns extending along the length of the stack of fiber optic ribbons are all loosely disposed in a buffer tube having an interior channel larger than the stack of fiber optic ribbons. The swell capacity (M_capacity) of the plurality of water blocking yarns should exceed the critical mass of water (M_critical) that could enter the buffer tube by a factor of 2.0 or more wherein the swell capacity is determined by the formula:

[0008] M_capacity=(N)(d)(B)

[0009] wherein

[0010] N=number of water blocking yarns

[0011] d=yarn denier

[0012] B=absorbency

[0013] and the critical mass of water is determined by the formula:

[0014] M_critical=(A_tubeopen)(P_water)

[0015] wherein

[0016] A_tubeopen=open area of buffer tube

[0017] P_water=density of water

[0018] Optionally, a suberabsorbent powder may also be applied between and/or on the stack of fiber optic ribbons loosely disposed in the buffer tube of the optical fiber loose tube cable.

[0019] It is therefore an object of the present invention to provide an improved water-blocked optical fiber communications cable.

[0020] It is another object of the present invention to provide an improved water-blocked fiber optic ribbon cable wherein cable weight is significantly reduced vis-à-vis grease or gel cable construction so as to enhance installation of the cable by a craftsman and allow installation of longer cable lengths.

[0021] It is another object of the present invention to provide an improved water-blocked fiber optic ribbon cable that can be used for outside plant data communications and telecommunications applications without using grease or gel filling compounds within the buffer tube(s).

[0022] Some of the objects of the invention having been stated hereinabove, other objects will become evident as the description proceeds, when taken in connection with the accompanying drawing as best described hereinbelow.

[0023] FIG. 1 shows a vertical cross-sectional view of a preferred single buffer tube embodiment of the cable of the present invention; and

[0024] FIG. 2 shows a vertical-cross sectional view of a preferred multiple buffer tube embodiment of the cable of the present invention.

[0025] Referring now to FIG. 1, there is shown therein a water-blocked fiber optic ribbon cable constructed in accordance with the preferred embodiment of the present invention and generally designated 10. Cable 10 is a water-blocked fiber optic ribbon cable which includes a buffer tube 12 which extends along the longitudinal axis of cable 10 and is most suitably formed of a plastic such as polyoelefin or engineered thermoplastic. Fiber optic ribbons 14 comprising one or more optical fibers each are stacked one on top of the other to form a stack of fiber optic ribbons 14 which are loosely positioned in buffer tube 12. Water blocking yarns 16 are provided within buffer tube 12 and extend generally along the length of the stack of fiber optic ribbons 14, and water blocking yarns 16 possess water swellable characteristics as will be explained in more detail below. Water blocking yarns 16 may be positioned around at least a portion but preferably the entire circumference of the stack of fiber optic ribbons 14, and water blocking yarns 16 may extend generally lengthwise along the length of the stack of fiber optic ribbons 14 or helically along and around the length of the stack of fiber optic ribbons 14.

[0026] Water blocking yarns 16 are preferably a blended yarn of poly(acrylamide) and polyester staple fibers yarns although other yarns that incorporate induced dipoles such as those based on poly(acrylic acid) may be used in cable 10. Further, as will be explained in more detail hereinafter, water blocking yarns 16 are characterized by a swell capacity that exceeds the critical mass of water that could enter cable 10 by a factor of at least 2.0 and preferably about 4.0. Optionally, a superabsorbent powder 18 may also be applied between and/or on the stack of fiber optic ribbons 14. The superabsorbent powder is most suitably stabilized poly(acrylic acid) although other superabsorbent powders may be used and are contemplated as being within the scope of the present invention.

[0027] Still referring to FIG. 1, it should be understood that water blocking yarns 16 are most suitably manufactured from or treated with superabsorbent compounds such as PAA (polyacrylic acid) so as to be water swellable when water comes into contact therewith. Superabsorbent yarns may completely consist of spun PAA or a suitable PAA may be applied to a staple yarn. Although PAA is the preferred superabsorbent compound in the preferred embodiment of the invention, the invention contemplates that other superabsorbent material can be used to treat water blocking yarns 16 such as polyacrylamide, blends of polyacrylic acid and polyacrylamide, salts of polyacrylic acid and polyacrylamide, and copolymers of polyacrylic acid and polyacrylamide. Finally, a cable jacket 20 is provided around buffer tube 12 in a conventional manner that would be well known to one skilled in the art.

[0028] Although cable 10 is shown for illustrative purposes in FIG. 1 as comprising four (4) fiber optic ribbons 14 that each comprise twelve (12) optical fibers and twelve (12) water blocking yarns 16 surrounding fiber optic ribbons 14 within buffer tube 12 that is in turn surrounded by cable jacket 20, applicants contemplate that cable 10 may comprise from one (1) to forty-eight (48) fiber optic ribbons 14 containing from one (1) to forty-eight (48) optical fibers in each of the ribbons, and from one (1) to twenty-four (24) water blocking yarns 16 within buffer tube 12. Further, cable 10 can be formed either with or without water blocking powder 18. If water blocking powder 18 is used, it can be applied between and/or on the stack of fiber optic ribbons 14 loosely positioned in buffer tube 12.

[0029] Although FIG. 1 illustrates cable 10 comprising a single tube 12 therein, it should be understood and appreciated that cable 10 may include two or more tubes 12 therein as a matter of design choice as illustrated in FIG. 2. The multiple buffer tube cable shown in FIG. 2 in generally designated 50 and is contemplated as being within the scope of the present invention as defined by the claims appended hereto. Cable 50 as a matter of design choice may include strength member S and up to twenty-four (24) tubes 12 therein.

[0030] Ratio of Swell Capacity of Water Blocking Yarns to Critical Mass of Water

[0031] To satisfy the water penetration requirements for fiber optic ribbon cable 10 at least one and optionally two technologies may be employed. The first and most important is water blocking yarns 16. These yarns will swell and increase their size greatly upon contact with water. The second (and optional) technology is water blocking superabsorbent powder 18. Powder 18 swells a great deal and is easy to apply to areas that are typically difficult to reach.

[0032] To determine the placement and amount of water swellable yarns 16 to be used in cable 10 the theoretical calculations set forth below were performed followed by testing to validate the calculations.

[0033] To determine the required amount of water blocking material it is first necessary to determine the open area of the tube that water may migrate through with the following equation: $A_{tub{e}_{open}}=\frac{\pi D_{tube}^2}{4}-A_{ribbon}-A_{yams}-A_{powder}$

[0034] For a given length of tube 12 there then remains a determinable mass of water than must be blocked or absorbed in order to maintain the integrity of cable 10. This mass may be determined by multiplying the open area of tube 12 by the density of water and is expressed in mass per unit length in the following equation:

[0035] M_critical=(A_tube)(P_water)

[0036] For each water blocking element in cable 10 it is necessary to determine its swell capacity, or when exposed to water how much mass it will absorb. For water blocking yarns 16 this absorbency number is given as a function of the yarn Denier and expressed as swell mass per yarn mass. Thus, for a given number of yarns N, of Denier d, and absorbency B the total capacity of water absorption expressed in mass per unit length is given by the equation:

[0037] M_capacity=(N)(d)(B)

[0038] Therefore to maintain integrity of cable 10 and block water penetration in tube 12 with yarns 16 the following equation must be satisfied:

[0039] M_capacity>M_critical

[0040] In most cases it is important that the swell capacity of yarns 16 exceeds the critical mass of water by a factor of 2.0 or more. This is to ensure that an open tube 12 exposed to a 1.0 meter head of water will not allow water to migrate through a 1.0 meter tube in 24 hours as required by a number of cable industry standards. It is also important to note that while the swell rate is exceptionally quick, the swell speed and capacity of water blocking compounds of yarns 16 will degrade when being processed and after exposure to temperature and humidity extremes in operational environments and may necessitate an additional safety factor.

[0041] In fiber optic ribbon cables 10 it is necessary to ensure that water does not migrate between fiber optic ribbons 14. In most applications with small fiber optic ribbon stacks the water will be absorbed by the gel formed by water blocking yarns 16, but it remains an optional safety precaution to add water blocking powder 18 to the fiber optic ribbon stack. In addition to impeding water migration in the fiber optic ribbon stack, this also adds an additional safety factor to the overall water blocking ability of tube 12. Water blocking powder 18 also aids in the bending properties of cable 10, allowing the fiber optic ribbons to move more freely in buffer tube 12.

[0042] Repeatedly solving the earlier stated equations for a 6.5 mm inner diameter tube 12 containing four (4) twelve (12) optical fiber ribbons 14 yielded a solution with twelve (12) 2000 denier 1.0 mm water blocking yarns 16 and optional water blocking powder 18. While powder 18 has a significant water blocking capacity (300 g/g plus) it was not included in the calculations since the preferred embodiment of cable 10 includes only yarns 16 and not powder 18. Instead the primary function of powder 18 was left to block water migration down the fiber optic ribbon stack and thus to provide additional water blocking safety factor for tube 12.

[0043] Substituting these numbers gives: $D_{tube}=6.5\mathrm{mm}$ $A_{ribbon}=\left(4\right)\left(0.30\mathrm{mm}\right)\left(3.05\mathrm{mm}\right)$ $A_{yams}=\left(12\right)\frac{{\pi \left(1.00\mathrm{mm}\right)}^2}{4}$ $A_{powder}\approx 0$ $With,A_{tub{e}_{open}}=\frac{{\pi \left(6.5\mathrm{mm}\right)}^2}{4}-3.66{\mathrm{mm}}^2-9.42{\mathrm{mm}}^2=20.10{\mathrm{mm}}^2$

[0044] This gives

[0045] M_critical=(20.10 mm²)(1000 kg/m³)=20.10 g/m

[0046] Now calculating the absorption capacity

[0047] M_capacity=(12)(2000d)(30 g/g)=80 g/m

[0048] This gives a safety factor of $M_{\mathrm{safety}}=\frac{M_{\mathrm{capacity}}}{M_{\mathrm{critical}}}=3.98$

[0049] These results were verified with experiments on tube samples. The results are set forth in Table 1 below wherein the water amounts indicated were absorbed by yarns 16 assisted by optional superabsorbent powder 18.

[0050] It will be understood that various details of the invention may be changed without departing from the scope of the invention. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation—the invention being defined by the claims.