Abstract:
A fiber optic cable suitable for indoor applications includes a core tube surrounding a plurality of coated optical fibers; a jacket formed of flame-retardant polymer material surrounding the core tube; and at least one layer of strength members disposed between said core tube and said jacket. The jacket has an outer diameter of not greater than about seven mm and the coated optical fibers experience a short-term increase in signal attenuation of no more than about 0.01 dB when the cable is looped in a radius of 5 centimeters.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to optical fiber cables which are suitable for use within building vertical shafts and also are provided with protection against moisture internal migration. In particular, this invention relates to single-tube design optical fiber cables suitable for such uses. 
     Drop cables are outdoor cables which bring telephone service to buildings, whereas riser cables provide telephone service within buildings. Riser cables extend upwards from basement vaults to wiring closets located on upper floors. 
     Drop cables must be able to withstand the changing conditions found in the outdoor environment. Most materials increase in length and volume with increases in temperature and decrease in length and volume with decreases in temperature. Each material may have a different rate of change of length given a specified change in temperature. Such a rate is called the coefficient of thermal expansion for a material. Because different materials in a cable may have different coefficients of thermal expansion, temperature changes may induce strains in the cable components. For this reason, changes in optical fiber attenuation over different temperatures are measured in cables intended for outdoor use. Successful cables must not experience unacceptable increases in optical fiber attenuation caused by cable strains induced by temperature-related conditions. 
     Drop cables also must be protected against migration of moisture within the cable. Although cable jackets are intended to prevent the ingress of water into the cable, no plastic material perfectly stops the ingress of moisture. Furthermore, water may enter a cable at points where the cable jacket has been damaged, or at the end of the cable. Therefore, longitudinal movement of water along the inside of the cable must be prevented. For this reason, water-blocking or water-absorptive material is provided in cable interstices which otherwise could act as conduits for moisture internal migration. Types of materials which may be used for this purpose are gel-like filling and flooding compounds. Filling compounds are disposed alongside the optical fibers within buffer tubes, while flooding compounds are disposed in spaces between the cable jacket and the buffer tubes holding the optical fibers. Many filling and flooding compounds are oil or grease-based. As a result, most filling and flooding compounds provide fuel for combustion. However, most cables intended for outdoor use are not required to be flame-retardant. 
     Other types of materials are becoming more widely used in outdoor use cables for protection against cable moisture migration. Examples include water-absorptive polymers, which may be inserted into a cable as loose powders or incorporated into tapes which are wrapped about other cable components. Another example is water blocking strength members, as disclosed in U.S. Pat. Nos. 4,913,517 and 5,389,442. 
     Cables intended for use within buildings normally are not exposed to the moisture and extreme temperature conditions experienced by cables intended for outdoor use. However, building cables are required by the National Electrical Code to meet criteria indicating that the cables will not act to spread fires within a building. The most well-known test standard for riser-rated cables is Underwriters Laboratories (UL) Standard 1666, &#34;Test for flame propagation height of electrical and Optical-fiber Cables installed vertically in Shafts (Second Edition, Jan. 22, 1991). The second edition of this standard is referred to herein as UL Standard 1666. 
     An optical fiber service cable designed to be suitable for both indoor and outdoor use is disclosed in U.S. Pat. No. 5,566,266, which issued on Oct. 15, 1996 in the names of Nave and McDowell. However, the disclosed cable is designed for use with a rather high optical fiber count and discloses an inner tube which itself has an outer diameter of 10.2 mm. Such a cable could not be connectorized using standard buffer tube fanout kits. The cable also employs a tape formed from materials such as a polyimide. Such tapes significantly add to the cost of the cable, and it is necessary to process and splice such tapes. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a riser-rated cable having a small diameter and low minimum bend radius which also is formed using low-cost materials. 
     These and other objects are provided, according to the present invention, by a fiber optic cable suitable for both outdoor and indoor applications, comprising: a core tube surrounding a plurality of coated optical fibers; a jacket formed of UV-resistant flame-retardant polymer material surrounding said core tube; and at least one layer of strength members disposed between said core tube and said jacket, characterized in that said jacket has an outer diameter of not greater than about seven mm and said coated optical fibers experience a short-term increase in signal attenuation of no more than about 0.01 dB when the cable is looped in a radius of 5 centimeters. 
     In a preferred embodiment, two layers of strength members are wrapped around the core tube in opposite directions and the set of two strength member layers is disposed between and directly contiguous to said core tube and said jacket. The cable is capable of meeting the flame retardance requirements set out in UL Standard 1666 in the absence of a flame-resistant tape. The strength members may be impregnated with a water blocking material. 
     The cable may have an average weight not exceeding 53 kg/km while being capable of withstanding a short-term tensile load of 1320N. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of the invention are described in the several drawings, in which: 
     FIG. 1 is a cut-back perspective view of a cable according to a preferred embodiment; and, 
     FIG. 2 is a cross-sectional view of the cable of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which one or more preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that the disclosure will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. The drawings are not necessarily drawn to scale but are configured to clearly illustrate the invention. 
     A cable 10 according to a preferred embodiment is shown in FIGS. 1 and 2. Cable 10 is designed to include from two to twelve coated and colored 250 μm optical fibers 11. Optical fibers 11 may be either single-mode or multimode fibers, or a mixture of single-mode and multimode fibers. Core tube 12 also may contain a filling compound 17 disposed in the space within the core tube not occupied by the optical fibers. The optical fibers 11 typically are not stranded with each other, and have a length which is from 0% to 0.2% greater than the length of core tube 12. Core tube 12 itself may be formed from a flexible plastic material such as polypropylene. In the preferred embodiment, the core tube 12 need not be formed from flame-retardant material. 
     Core tube 12 may have an inner diameter of 1.8 mm and an outer diameter of 3.0 mm, and loosely contains the optical fibers 11. Buffer tubes having an outer diameter of 3.0 mm are widely used, so that the buffer tube may be connectorized using equipment which is already available to the industry. 
     An inside layer 13 and an outside layer 14 of flexible strength members are stranded in opposite directions about core tube 12. In a cable according to the preferred embodiment, eight yarns form outside layer 14 and six yarns form inside layer 13. Inside strength member layer 13 is contiguous to core tube 12; outside strength member layer 14 is directly contiguous to inside strength member layer 13; and jacket 16 is directly contiguous to outside strength member layer 13. A polyester ripcord 15 lies at the inner surface of jacket 16. 
     The strength members forming layers 13 and 14 are chosen to be highly flexible. In a preferred embodiment, the strength members forming layers 13 and 14 may be Advantex™ reinforcements, available from Owens Corning, which are fiberglass yarns including a superabsorbent polymer. The reinforcements swell up to five times their own weight in deionized water, providing effective water-blocking protection to the space between jacket 16 and buffer tube 12. 
     These strength members are chosen to provide sufficient anti-buckling and tensile strength to the cable. The Advantex™ reinforcements have a tensile modulus of elasticity of 7×10 4  MPa, allowing the cable according to the preferred embodiment to have a maximum tensile loading during installation of 1320N, and a long term maximum tensile load of 330N. 
     The outer jacket 16 may be formed from polyvinyl chloride material which is both ultraviolet resistant and flame retardant, adapting the cable according to the preferred embodiment for both indoor and outdoor use. The average outer diameter of the outer jacket 16 may be 7.0 mm. This makes the cable according to the preferred embodiment small enough to be placed in existing ducts where space is at a premium. 
     A sample cable having a length of 20 m containing three multimode fibers and nine single-mode fibers was tested for optical fiber attenuation at low bend radius. The cable excess fiber length percentage was 0.2%. The single-mode fibers were concatenated and terminated separately from the multimode fibers. Attenuation test sets operating at 1300 nm for multimode fibers and 1550 nm for single-mode fibers were connected to the concatenated fibers. The cable sample was placed in a loop using a template with a possible radii range of 8 cm to 2 cm. The loop diameter was slowly decreased from 8 cm down to 2 cm while the optical attenuation (ΔdB) was measured. Table 1sets out the results. (Results at 2 cm are not reproduced , as the optical fiber itself has a minimum bend radius of about 2.5 cm for long-term mechanical reliability). Due to the nature of the test, single-mode values are to be divided by nine, and multimode values are to be divided by 3 to obtain average values for individual optical fibers. 
     
                       TABLE 1______________________________________Change in attenuation (Δ dB) in concatenated optical fibers atdifferent cable bend radii; results given separately forsingle-mode (SM) and multimode (MM)Bend Radius    8 cm   7 cm     6 cm 5 cm   4 cm 3 cm______________________________________Trial 1 SM    0.00   0.00     0.02 0.02   0.04 0.09Trial 1 MM    0.00   0.01     0.01 0.02   0.04 0.07Trial 2 SM    0.00   0.00     0.00 0.01   0.03 0.03Trial 2 MM    0.00   0.00     0.01 0.03   0.05 0.05Trial 3 SM    0.00   0.01     0.01 0.01   0.02 0.03Trial 3 MM    0.00   0.01     0.01 0.01   0.02 0.05______________________________________ 
    
     When the proper division is made, it is seen that both the single-mode and the multimode individual optical fibers had a change of signal attenuation of 0.01 dB or less at a cable bend radius of 5 cm. Furthermore, the cable did not kink even at a bend radius of 2 cm. 
     The cable according to the preferred embodiment performs very well during stripping, handling, and bending. Because of the small outside diameter and flexibility of the cable, a ring cut is difficult to make in the jacket using a hook blade. Use of a straight blade for this purpose therefore is recommended. 
     Cables according to the preferred embodiment may be used in interbuilding and intrabuilding backbones in aerial, duct, or riser applications. These cables have a specified operating temperature of -40° C. to +70° C. These cables are UL 1666 listed and meet the requirements of ICEA-596. 
     The cable core comprising tube 12, filling compound 17 and coated optical fibers 11 may be made using either a vertical or horizontal buffering line as known to the prior art. Spinners may be used to apply strength reinforcement member layers 13 and 14. The tension applied to the strength members may be 350 g, and their lay length may be 250 mm. In jacketing cables according to the preferred embodiment, a tip diameter of 5.25 mm and a die diameter of 7.0 mm may be used. Six inches may separate the die orifice and a cooling water vat, and the extruder temperature profile used in forming the outer jacket may cover the range 142°-185° C. A line speed of 25 m/min. may be employed. 
     Aramid fiber yarns coated with a swellable powder or film are alternative strength members which may be used. A flame-retardant polyethylene material may be used as a jacket material for zero halogen, low smoke applications. 
     It is to be understood that the invention is not limited to the exact details of the construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art without departing from the scope of the invention.