CA1293878C - Depressed cladding optical fiber cable - Google Patents

Abstract

DEPRESSED CLADDING OPTICAL FIBER CABLE
Abstract An optical fiber cable includes a core comprising plurality of units Each unit is formed by a plurality of optical fibers which are assembled together without intended stranding. Each of the optical fibers includes a core, and inner and outer claddings with the inner cladding characterized by an index of refraction depressed from that of the outer cladding The ratio of the inner cladding diameter to the core diameter and the ratio of the difference in the indices of refraction of the inner and outer claddings to the difference in indices of refraction between the core and the inner cladding are such that each optical fiber is capable of operation in a single mode fashion at a predetermined wavelength. Also, the difference between the indices of refraction of the core and the inner cladding is sufficiently high to cause eachfiber to be substantially insensitive to microbending. The plurality of units are enclosed in a common tube which provides a predetermined packing density.
In one embodiment, a waterblocking material is disposed within the tube to fill the interstices between the optical fibers and between the units. The waterblocking material is such that its critical yield stress does not exceed about 70 Pa at 20°C and such that it has a shear modulus of less than about 13 KPa at 20°C. The common tube is enclosed with a sheath system. This arrangement is such that the optical performance of the cable is not degraded at temperatures as low as -40°F.

Description

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DEPRESSED CLADDING OPTICAL FIE~ER CABLE
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Techn_cal Field 2 This invention relates to a depressed cladding optical fiber cable 3 which includes a pluraiity of units each comprising a plurality of optical fibers 4 with each of the optical fibers h~ving an inner cladlding which has an index of refraction that is less than that of an outer cladding. The plurality of units is 6 disposed in a common tube enclosed in a sheath system.
7 Background of the Invention 8 Although desired for their large baç~dwidth capabilities and small g size, light-transmitting optical fibers are mechanically fragile, exhibiting brittle fracture under tensile loading and degraded light transmission when the 11 fiber is bent. A cable for use in a duct must be capable of withstandincl tensile 12 loads applied when the cabte is pulled into the duct and stresses caused by1 3 bends which may be frequent in loop plant in urban areas. As a result, cable 14 siructures have been developed to proteci- mechanically the optical fibers.1 5 Cable structures which have been developed for optical f ibers 16 include loose tube, ribbon and stranded cables. See U. S. patent 4,153,332.17 Ribbon cable comprises a core of one or more ribbons with each including a 18 plurality of optical fibers disposed generally in a planar array. The core is 19 surrounded by a loose-fitting plastic inner tubular jacket and an outer jacket reinforced with strength members. Another optical communications cable 21 which is suitable for use in duct systems is disclosed in U. S. patent 4,241,979.
22 The cclble includes two separate layers of strength members, which ~are 23 wrapped helically in opposite directions. Under a sustained tensile load, these 24 two iayers of strength members produce equal but oppositely djrected torques about ihe cable to insure the absence of twisting. In another type of optical 26 communications cable, a plurality of optical fibers are enclosed in an extruded 27 plastic tube to form a unit and a plurality of these tubed units are enclosed in 28 a common extruded plastic tube which is enciosed in a sheath system. The 29 optical fibers which are enclosed in each unit tube are stranded together about a central strength member.
31 Generally, optical fiber cables of the prior art, such as ribbon and 32 stranded and loose tube, suffer from the disadvantage of having the ribbons, ~2~ 7~

the stranded units or the tubes manufactured on a separate line. In stranded 2 cable, for example, a plurality oF units which priorly have been enclosed 3 individually in tubes and stranded are fed into a line which applies the common 4 tube and the outer jacket. Because the ribbon or tubed core is general Iy stranded with a predetermined lay, its manufacture and the assembly of the 6 ribbons or tubes into the core involve the use of relatively heavy rotating 7 apparatus which may be undesirabie from a manufacturing standpoint.
8 Also, in an optical fiber cable, perturbations along the axes of the 9 optical fibers, which are referred to as microbends, ccm cause optical loss by lû allowing power to escape through the cladding. The degradation in 11 transmission which results from this type of bending is known as microbending 12 loss. For a discussion of microbending loss, see S.E. Miller et al, Optical Fiber 13 Telecommunications, Acadernic Press, New York, (1979) pp. 158-161; H~ G.1 4 Unger, Planar Optical Waveauides and Fibers, Clarendon Press, Oxford~
Chapter 6, pp. 552-64~; and D. Marcuse "Microdeformation Losses of Single 16 Mode Fiber", Applied Optics, vol. 23 no. 7, April 1, 1984, pp. 1082-1091. This 17 problem may occur, for example, when a waterblocking filling material is 18 introduced into the cable core in order to prevent the incursion of water.
19 Typicaily, waterblocking materials of the prior art do not yield under strains experienced when the cable is made or handled. This prevents the movement 21 of the optical fibers within the cable and the fibers bucl<le because they 22 contact, with a relatively small periodicity, a surface of the unyielding filling 23 material. This is overcome somewhat by stranding the fibers which allows the 2~ fibers under stress to form new helices to avoid microbending losses.
However, as is well known, stranding requires the use of a lower line speed.
26 With the fibers being enclosed in coatings, these microbending losses 27 are not discernible at room temperature. The coatings absorb perturbations at 28 room temperature, allowing the fiber to remain substantially unperturbed.
29 However, at relatively low temperatures, that is in the range of -~ûF, the 3û coating material experiences thermal changes thereby causing the optical 31 Fiber axes in the cable to bend. Because of the properties of the coating 32 material and of the cable, the coating mater;al may only partially absorb these 33 perturbations and some are transFerred through to the optical fibers.
3~ What is needed is an optical fiber cable structure which is relatively easy and inexpensive to make and which inhibits the introduction of undue ~31~7~3 stresses that lead to microbending basis in the optical fibers. Also 2 improvement in the consistency of its performance at relatively low 3 temperatures in the range of about -40F is desired. What is needed is an 4 optical fiber cable which is compact and which inhibits the introduction of undue stresses that could lead to microbending losses in the optical fibers over6 commonly accepted temperature ranges.
7 Summary of the !nvention 8 The foregoing problems have been overcome by the optical fiber cable 9 of this invention. An optical fiber cable of this invention includes a plurality of optical fibers which are assembled together without intended stranding to 11 form a unit which extends in a direction along the longitudinal axis of the 12 cable. The cable may include a plurality of units. A length of tubing which is 13 made of a plastic material encloses the plurality of optical fibers and is14 parallel to the longitudinal axis of the cable. All of the units are disposed in a common plastic tube ins~ead of having each unit disposed within an associated 16 individual tube. Each of the optical fibers includes a core, an inner cladding 17 and outer cladding. The inner cladding has an index of refraction which is less 18 than that of the outer cladding. The overall refrqctive index difference of 19 each of the optical fibers, that is the difference between the indices of the core and the inner cladding is sufficiently high so that each optical fiber is 21 substantially insensitive to microbending. Also9 the ratio of the inner cladding 22 diameter to the core diameter and the ratio of the difference in the indices of 23 refraction of the inner and outer claddings to the difference in the refractive 24 indices of the inner cladding and the core are such that each optical fiber is capable of operation in a single mode fashion at a predetermined wavelength.
26 Further, the ratio of the cross-sectional area of the plurality of optical fibers 27 to the cross-sectional area within the tubing does not exceed a predetermined 28 value which in a preferred embodiment in which the optical fibers are coated 29 is about 0.5.
The cable also includes at ieast one strength member and a jacket 31 which is made of a plastic material and which encloses the length of tubina. In 32 one embodiment, a waterblocking material which is disposed within the tubing 33 and which fills substantially the interstices between the fibers has a critical 34 yield stress which is not greqter thqn about ~û Pa at 2ûC. Each unit, if the -3~

~2~78 cable includes a plurality of units, is separated from the other units only by 2 the waterblocking material and the plurality of units are enclosed in a common3 length of tubing instead of in individual tubes as in the prior art.
4 Brief Description of the Drawinas FIG. I is a perspective view of an optical fiber cable of this 6 invention;
7 FIG. 2 is an end view of the cable of FIG. I;
8 FIG. 3 is qn end view of a coated optical fiber;
9 FIGS. 4A and 4B are profile representations of the refractive index 1 û configuration of a matched cladding optical fiber and of a preferred 1 l embodiment depressed cladding fiber of the cablo of this invention, 1 2 respectively;
3 FiG. 5 is an exemplary curve of applied stress versus strain for a 4 waterblocl<ing material;
FIGS. 6A-6B are graphs which depict added losses associated with 16 cables of this invention and of the prior art, respectively, at one operating 1 7 wavelength;
18 FIGS. 7A-7B are graphs which depict added losses associated with 19 cabtes of this invention and of the prior art, respectively, at another operating 2Q wavelength;
21 FIG. 8 is a perspective view of an alternative embodirnent of a 22 cable of this invention; and 23 FIG. 9 is an end \~iew of the cable of FIG. 8.
24` Detaiied Description Referring now to FIGS. I and 2, there is shown a preferred embodiment 26 of a cable 2û of this invention. It includes a core 21 comprising a plurality of 27 units each of which is designated generally by the numeral 22 and includes a 28 plurality of individual coated optical fibers 24-24. Further, each of the coated 29 optical fibers 24-24 includes a fiber ~5, which comprises a core 26 and cladding designated generally by the numeral 27, and one or more coatings 28-31 28~(see FIG. 3j. it should be understood that herein the term optical fiber32 refers to the fiber itself and any coating applied thereto. Typically, for a 33 single mode optical fiber, the core 26 has a diameter in the range of 8-9,~m 34 and an outer cladding diameter of about 125 IJm. Each of the units 22-22 .

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either is stranded or non-stranded, that is the unit extends generally parallel 2 to a longitudinal axis 29 of the cable, or is formed with an oscillating lay.
3 It should be understood that the opticai fibers 2~-24 which are included 4 in each of the units 22-22 of the preferred embodiment are assembled without being stranded together and futhermore that the unit itself is assembled with 6 an infinite lay length. The optical fibers may be undulated along portions of 7 the unit which will cause each of the optical fibers to have a length which is at 8 least slightly greater thqn the length of any enclosing sheath system. This will 9 prevent undue strain from being imparted to the optical fibers 24-24 during1 û manufacture, instal lation and service of the cable.
11 As is further seen in FIGS. I and 2, the core 21 comprises a plurality of 12 units which are individually bound by a binder 32 qnd which are enclosed in a 13 tube 34. The tube 34 which is made of a plastic material such as polyvinyl 14 chloride or polyethylene, for exannple, contains tlle inclividwlly untubed, bound uni~s and extends generally parallel to the longitudinal axis 29 of the cqble 20.
16 An important characteristic of the cable of this invention is its packing 17 density. Packing density is defined as the ratio between the cross-sectional 18 area of the optical fibers and any coatings thereon to the total cross-sectional 19 area enclosed by the tube 34. If the packing density is too high, optical fibers 2û within the core may experience relatively high stress and could break during 21 handling of the cable. This occurs when the packing density is too high 22 because as with the use of waterblocking materials which yield at a relatively 23 high stress, the optical fibers cannot move sufficiently within the tube to24 relieve stresses such as would occur in bending. In a preferred embodiment, the packing density does not exceed a value of about 0.5 26 As mentioned hereinbefore, small perturbations in the axis of an optical 27 fiber which are referred to as microbends can cause optical loss by allowing 28 power to escape through the cladding. The degree of confinement of the 29 optical power and thus the susceptibility to microbending - induced optical loss 3û can be characterized by the spot size, also referred to as the mode f ield31 diameter, and the effective index of the fundamental propagating mode.
32 These well known parameters as well as others used hereinafter are defined,33 for example, in the hereinbefore~identified Miller, Unger and Marcuse 34 publications. Critical parameters which affect microbending loss are the ~L2~ 7~

diameter, d, of the core and the difference in the indices of refraction, nC and2 ncL, of the core and the cladding, respectively. This difference generally is 3 expressed as a percentage difference qnd is design(lted ~. See for example, a 4 representation 36 of the index of refraction profile of q typical single mode opticql fiber as shown in FIG. 4A. An optical fiber having an index of 6 refraction profile as shown in FIG. 4A is referred to as a matched cladding 7 optical fiber. There, the core has a diameter, d, and an index of refraction nC
8 designated by the numeral 37 and the cladding hqs an index of refraction nCL
g designated by the numeral 38 which is uniform throughout the cladding. A
1 û typicql matched cladding opticql fiber has q ~of 0.30%. These parameters, d 11 and ~, determine, ~t q given wavelength, the spot size and the effective index.
12 A small spot size and high effective index qssure tight confinement of the 13 optical power to the region of the fiber core and thus high resistance to14 microbending induced loss.
1 5 Although improved microbending performance cqn be obtqined by 16 increasing ~ and decreasing d, leading to a reduced spot size, it is known thqt 17 the difficulty of achieving low fiber splice loss increqses as the spot size 18 decreases. Further, the wavelength of zero dispersion increases as the core 19 diameter decreases. As the zero dispersion wavelength moves above the 2û operating wavelength, the f iber dispersion increqses and the maximum 21 bandwidth at the operating wavelength decreases. These adverse effects, 22 increasing fiber splice loss and increasing fiber dispersion, limit the minimum 23 fiber core si~e for optimal operation at 1.3~m.
24 The dependence of the spot size ~nd zero dispersion wavelength on a is less pronounced. For example, an optical fiber having a relatively high~ may 26 be used to provide microbending resistance, while maintaining a spot size27 which is suitable for low loss splicing. However, inc~eqsing Q to improve the 28 microbending performance will increase the cutoff wavelength of the fiber.
29 As is well known, the cutoff wavelength is that wavelength below which higher order rnodes may be propagated. Inasmuch as the bqndwidth of the fiber is 31 drastically reduced if higher order modes are present, any increase in cutoff 32 wavelength must be controlled to preserve single mode operation at the 33 system wqvelength.
3~

7~1 This problem is overcome by causing the opticql fibers 24-~ which are 2 used to provide the units 22-22 to be characterized by an inner cladding l~0 3 (see FIG. 2) having an index of refr~ction which is depressed relative to that of 4 an outer cladding ~2. Such a fiber is said to have a depressed inner cladding.5 It has been found that (l depressed cladding optical fiber is advantageous in 6 that such a fiber can be designed with a relatively high resistance to optical7 loss which is induced by microbending~ As shown in FIG. 4B~ the core 26 has a 8 diameter, d, and an index of refraction 44 which has a relative refractive g index difference ~+ with respect to a reference line 45 corresponding to the 10 index of refraGtion nCL of the outer cladding 42. The inner cladding 40 has a11 diameter, D, and an index of refraction ncL; which is designated by the 12 numeral 43 and which has a relative refractive index difference ~ with respect 13 to the same reference line ~5. The overall difference in index of refraction 14 from that of the core to that of the inner cladding is designated~.
An advantage of a depressed cladding opiical fiber is that the cutoff 16 wavelength, in addition to its dependence on the core diameter, d, and overall 17 refractive index dif ference ~, depends on the ratio of the inner cladding 18 diameter to the core diameter, D/d, and the ratio of the inner c!adding index 19 depression to the total index difference"~ . For example, as D/d decreqses, the cutoff wavelength decreases. A nominal value of D/d is 6.5, but a useful 21 range may include values as low as about 3. Because the spot size, effective22 index and zero dispersion wqvelength are insensitive to these rqtios in the 23 contemplated range thereof, the higher values of ~ desired for the improved 24 microbending performance can be offset by choosing appropriate values of D/dand j/~ to control the cutoff wavelength. Typical values of Dld and 1~
26 result in a lowering of the cutoff wavelength by lO0 nm or more and thereFore 27 single mode operation is achieved in a relatively high ~ microbending loss 28 resistqnt f iber.
29 What is needed is ~a ,~ and D/d which are appropriate to provide a cutoff wavelength which is substantiaily reduced from that of a matched 31 cladding fiber with the same core size and overall refractive index difference.
32 What is provided is an optical fiber having parameters which hav~ been chosen 33 in such a way that a substantially microbending insensitive fiber is capable of 34 operation in single mode fashion at a predetermined wavelength. The cable of this invention facilit~tes operation at a relatively high total index of 2 refraction difference,~. This allows single mode operation which normally 3 could not be achieved at a wavelength of 1.3,1Jm, for exarnple, and assures high 4 resistance to microbending induced loss even if the system is operated at ahigher wavelength of 1.55~m, for example. In a preferred embodiment, the 6 predetermined wavelength of single mode operation is 1.3 ,orn with the 7 additional capability of low loss operation at 1.55~m.
8 The cable of this invention includes low dispersion, low loss fibers g having a ~, a core diameter, D/d and ~/~ which provide cutoff at sufficiently low wavelengths to guarantee single mode operation at 1.3,1)m. A
11 fiber having an up~doped core with a down-doped cladding can provide a high12 and low material dispersion in the vicinity of 1.3,um. As set forth in U. S.
1 3 patent 4,439,û07, a relatively high ~ does not necessarily result in high 14 material dispersion in ihe vicinity of 1.3~m when reliance is had on a down-doped cladding. SuFficiently low cutoff wavelengths, low dispersion and a spot 16 size suitable for low loss splicing are obtained by the appropriate choice of d, 17 D/d and ~ with a relatively high ~which results in low packaging loss.
18 Typical values of ~- range between 10 and 40% of ~. In a preferred 19 embodiment, d = 8.3,um, D = 5~m, .b~ = 0.25%, ~ = 0.12% and ~= 0.37%.
2û In the embodiment shown in FIGS. I and 2, the units 22-22 and the core21 between the units and the tube 34 are filled with a suitable waterblocking 22 material 46. It has been determined that in an optical fiber cable, a filling 23 composition must also function to maintain the optical fibers in a relatively 24 low state of stress.
A cable filling or waterproofing material, especially an optical fiber 26 cable filling compound, should meet a variety of requirements. Among them is 27 the requirement that the physical properties of the cable remain within 28 acceptable limits over a rather wide temperature range, e. g. from about -40 29 to about 76C. It is also desirable that the filling material be relatively free of syneresis over the aforementioned temperature range. Syneresis is the 31 separation of oil from the gel under applied stress. Filling materials for use in 32 optical fiber cables also should have a relatively low shear modulus.
33 According to the prior art, the shear modulus is a critical material parameter of optical fiber cable filling materials because it is believed to be directly 3~7~

related to the amount of microbending loss. Typicaily, microbending loss is 2 more difficult to control at long wavelengths than at short ones. Thus, it is 3 important to be able to produce optical fiber cable that has no significant 4 cabling-induced losses at long wavelengths such as, for example, 1.55 ~m.
The preferrecl waterblocking material is a cornposition which comprises 6 two major constituenfs, narnely oii, and a gel!ing agent such as colloidal 7 particles, and, optionally, a bleed inhibitor as a third major constituent. Other 8 constituents such as Q thermal oxidative stabilizer, f<)r example, are optional.
g Among the oils useful in the waterblocking material are polybutene oilslo having a minimum specific gravity of about 0.83 and a maximum pour point, as11 per ASTM D97, of less than about 18C, or ASTM type i03, 104A, or 104B, or 12 mixtures thereof, per ASTM D-226 test, of naphthenic or paraMnic oils havinga minimum specific gravity of about 0.86, and a maximum pour point, per ASTM D97, of less than about -4C. Specific examples of oils useful in the cable of ~he invention are a polybutene oil, which is a synthetic hydrocarbon 16 oil having a pour point per ASTM D97 oF -35C, an SUS viscosity of 1005 at 17 99C, a specific gravity of G.8509, and an average molecular weight of 460. It 18 is available from the Amoco*Chemical Corporation, Texas City, Texas, under lg the trade designation L-100. Another example oil is a white mineral oil, having a pour point per ASTM D97 of -25C, an SUS viscosity of 53.7 at 99~C, 21 an average specific gravity of 0.884, and maximum aromatic oils 1% by we;gh$22 (b.w.). The latter is available from Penreco of Butler* Pennsylvanic, under the 23 designation Drakeol 35. 3ther oils inciude triglyceride-based vegetable oils24 such as castor oii and other synthetic hydrocarbon oils such as polypropylene oils. For applications requiring fire-retardant properties, chlorinated paraffin26 oils havin~ a chlorine content of about 30-75% b.w. and a viscosity at 25C of 27 between 100 and 10,000 cps are useful. An example of such oil is Paroil 15~,28 which is available from the Dover Chemical Company of Dover, Ohio.
29 Polymerized esters of acrylic acid or similar materia!s are useful as pour-point 3û depressants at addition levels below 59~ b.w. An example is ECA 7955, 31 ~ available from the Exxon~:hemical Company.
32 Colloidal filler particles in oil gel the oil by bonding surface hydroxyl 33 groups to form a network. Such gels are capable of supporting a load below a34 critical value of stress. Above this stress levelJ the network is disrupted, and * trade -mark _9_ 3~

the material assumes a liquid-like character and flows under stress. Such 2 behavior is often referred to as thixotropic.
3 Colloidal filiers useful in the cable of Jhe invention include colloidql 4 silica, either hydrophilic or hydrophobic, preferably a hydrophobic fumed silica having a BET surface area between about 50 anci about 40û m2/gm. An 6 example of o hydrophobic fumed silica is a polydimethylsiloxane-coated fumed 7 silica having a BET surface area of about 8û-120 m2/gm, containing about 5%
8 b.w. carbon, and being available from the Cabot Corporation of Tuscola, 9 Illinois under the trade designation Cab-O-Sil N70-TS. An exemptary hydrophilic colloidal material is fumed silica with a BET surface areq of about 11 175 225 m2/gm, nominal particle size of û.012,um, and a specific gravity of 12 2.29 available from the Cabot Corporation under the designation Cab-O-Sil M-1 3 5* Other colloidal fillers useful in the practice of the invention are 14 precipitated silicas and clays such as bentonites, with or without surface 1 5 treatment.
16 Oil-retention of the inventive greases may be improved by the addition 17 of one or more bleed inhibitors to the composition. The bleed inhibitor can be 18 a rubber block copolymer, a relatively high viscosity semiliquid, sometimes 19 referred to aS semisolid, rubber, or other appropriate rubber. Block copolymers and serniliquid rubbers will be referred to collectively as rubber 21 polymers. Incorporating a rubber polymer into the grease composition allows a 22 reduction in the amount of colloidal particles that must be added to the23 mixture to prevent syneresis of the gei. This reduction can result in cost 24 savings. Furthermore, it makes possible the formulation of nonbleeding compositions having q relatively low critical yield stress.
26 Arnong the rubber block copolymers that can be used in waterblocking27 compositions for the cable of the invention are styrene-rubber and styrene-28 rubber-styrene block copolymers having a ~ styrene/rubber ratio between 29 approximately 0.1 and 0.8 and a molecular weight, as indicuted by viscosity in 3û toluene at 25C, of from about 10û cps in a 20% b.w. rubber solution to about 31 20û0 cps in a 15% b.w. rubber solution. Exemp1ary block rubbers are a) a 32 styrene-ethylene propylene block ~opolymer (SEP), unplasticized, having a 33 styrene/rubber ratio of about 0.59, a specific grov;ty of about 0.93, a break 34 strength per ASTM D-412 of 3ûû psi, and beina available from the Sheii *tra~e - mark ,~j . '~ ~,l, 7~3 Chemical Company of Houston, Texas, under the trade designation Kraton 2 G 1701; b) a styrene-ethylene butyiene block copolymer (SEB), having a 3 styrene/rubber ratio about 0.41, and being availabie from the Shell*Chemical4 Company under the designation TRW-7- 1 5 11 , and c~ a styrene-ethylene butylene-styrene block copolymer (SEBS), unplasticized, and having a 6 styrene/rubber ratio of about 0.16, a specif ic gravity of about 0.9û, 750%
7 elongation, 300% modulus per ASTM D-412 of 350 psi, and being available 8 from the Shell Chemical Corporation under the trade designation Kraton 9 G1657. Other styrene-rubber or styrene-rubber-styrene block copolymers are styrene-isoprene rubber (Sl) and styrene-isoprene-styrene (SIS) rubber, styrene-11 butadiene (SB) ~nd styrene-butadiene-styrene (SBS) rubber. An example of SIS12 is Kraton Dl 107, and an example of SBS is Kraton Dl 102, both available from 13 the Shell Chemical Company.
14 Among the semiliquid rubbers found useful in the pr~ctice of the invention are high viscosity polyisobutylenes having a Flory molecular weight 16 between about 2û,000 and 7û,00û. Exemplary thereof is a polyisobutylene 17 having a Flory molecular weight of about 42,600-46,10û, a specific gravity of 18 about 0.91, and a Brookfield viscosity at 3S0F (about 177C) of about 26,000-19 35,000 cps, cnd avai lable from the Exxon Chemical Compcmy of Houston, 2~ Texas vnder the trade designation Vistanex LM-MS. Other rubbers which are 21 considered to be useful are butyl rubber, ethylene-propylene rubber (EPR), 22 ethylene-propylene dimer rubber (EPDM), and chlorinated butyl rubber huv;ng 23 a Mooney viscosity ML 1+8 at 100C per ASTM D-1646 of between about 20 24 and 90. Examples of the above are Butyl 077, Vistalon 4û4, Vistalon 37û8, ~nd Chlorobutyl 1066, respectively, al~ a~ailable from the Exxon Chemical 26 CQmpany. Also useful are depolymerized rubbers having a viscosity of 27 between about 40,000 and 40û,000 cps at 38C. An example thereof is DPR 7 28 available from Hardman, Inc. of Belleville, New Jersey.
29 The composition of the waterblocking material 46 is intended to block 3û effectively entry of water into the core 21 while minimizing the added loss to 31 the cable in order to provide excellent optical performance. Although the oil 32 retention characteristic of the composition is a concern, the most important33 property is the optical performance of the cable 20.
34 Table i shows the eiFfect of several different bleed inh;bitors on oil separation, for two different oils, Drakeol 35 and L-10û. The three block *trade -mark ~3~

copolymer-containing compositions comprise 92% b.w. oil, 6% b.w. Ca~O-Sil 2 N70-TS colloidal filler, and 2% b.w. inhibitor. The semiliquid rubber-3 containing compositions L~-MS cornprise 6% b.w. N70-TS colloidal fiiler~ the 4 indicated amounts of the inhibitor, and 89 cmd 84% b.w. of Drakeol 35. Tabie I
6 Oil Separation 8 Inhibitor C)rakeol-35 L- 100 9 % Separation 9~ Separation 1 û 2% SEP ~.5 0.7 11 2% SEB I I 3.5 12 2% SEBS 5 2 13 5~ LM-MS 7 14 10% LM-MS 2 16 Table ll shows data on oil separation for several compositions that17 do not include bleed inhibitors. It should be evident that the qddition of a 18 bleed inhibitor is more effective than increasing the colloidal particle content 19 of the cornposition in preventing oil separation or drip. Also, increasing the 2û colloidal particle-content of a grease to the point where syneresis is avoided 21 results in increased critical yield stress. Th~s to slvoid syneresis altogether, ~22 the low values of critical yield stresses needed in some instances may be 23 ~ unobtainable without use of bleed inhibitors. The data of Table ll was obtained 24 with N70-TS colloidal filler and Drakeol 35 oil~
Table l l 26 Oil Separation 27 fumed silica (% b.w.) 6 7 8 10 28~ oil separation (% b.w.) 36 28 20 14 30 ~ FIG. 5 shows a generalized stress-strain curve 47 at constant 31 strain rate for a thixotropic material such as that used as the waterblocking 32 material 46, and identifies several important parameters. In segment 48 of 33 the stress~-strain curve 47, the material acts essentially as an elastic solid.
34 The segment extends from ;zero stress to the critical yield stress~. The *trade -marlc ~3~

strain corresponding to l;sc is identified as ~, the critical shear strain. By 2 definition, the coordinates 6~, ~cindicate the onset of yielding and the 3 quantity ~Sc /~; (or ~t~/d ~ for 6 ~ is known as the shear modulus 4 (Ge) of the material.
The prior art teaches that filling materials for optical fiber 6 cable need to have low values of Ge. However~ it has been determined that, at 7 least for some applications, a low value of Ge of th~e filling material is not 8 sufficient to assure low cabling loss, and that a further parameter, the critical g yield stress, 6~, also needs to be controlled. Typically, the critical yield 1 û stress of material according to the invention is not greater than about 70 Pa, 11 measured at 20C whereas the sheur modulus is less thun about 13 kPa at 1 2 20C.
13 A segment 49 of the stress-strain curve of FIG. 5 represents 1 4 increasing values of incremental strain for increasing stress. The stress 6~ is the maximum vulue of stress sustainable by ~he material at u given strain rate 16 with ~ being the corresponding strain. For strains in excess of ~y, the stress 17 at first decreases as shown by segment 50, becoming substantially 18 independent of strain for still greater values of strain as shown by the segment 19 51. The waterblock;ng materia! thus exhibits a liquid like behavior for ~ > ~.
2û A filling composition for a filled cable 20 typically comprises between Z1 about 77 and about 95% b.w. oil. If a bleed inh7bitor is present and the 2Z~ inhibitor is a r~Jbber block copolymer, then the oil content typicqlly is between 23 about 90 and about 95% b.w. On the other hand, if the bleed inhibitor is a24 semiliquid rubber, then the oil content typically is between about 77 and about 91% b.w. The composition further cornprises at most 15% b.w., preferably at 26 most 10% b.w., of colloidal particles. If the colloidal particles are fumed 27 silica, then a typical range is from 2 to about 10% b.w., with 5-8h b.w. being 28 currently preferred for some applications. The bleed inhibitor content of the 29 cornpositjon is typically between about 0.5 and 15%, with the currentiy preferred range for block copolymer rubbers being between abovt 0.5 and 31 about 5% b.w., and for semiliquid rubbers being between about 3 and about 32 15% b~wo Optionully, the composition may also comprise minor amounts of an33 oxidative stabilizer and other additives. An exemplary stabilizer is tetrakis 34 methane, available from Ciba-Geigy under the trade designation Irganox 1010.

*t rade-mark . ~, .

~ D3~78 Typically the oil, colloidal particles, and, if used, a bleed inhibitor, account for 2 about 99% b.w. or more of the total composition.
3 Exemplary compositions that were studied are shown in Table 111 4 in parts by weight. The compositions were prepared by known methods, typically comprising blending oil, bleed inhibitor, antioxidant, and colloidal particle material first at ambient temperature and pressure, then at ambient 7 temperature under a partial vacuum (typically less than about 30û Torr). Some 8 compositions, e.g. E, were heated to about 150C while being stirred, and 9 maintained clt that temperature for about 4 hours. The resulting compositions lû were evaluated, including a determination of 6c ancl Ge of some by cone-and-11 plate rheometry. An exemplary summary of the properties also is presented in 12 Table 111 with all measurements f~c and Ge being at 2ûC.
13 Of the example compositions disclosed in Table 111, example A is 14 preferred. The stress values designated (a) were determined without aging while those designatecl ~b) were aged for the tirne indicatecl. Notwithstanding 16 the use of bleed inhibitors in many of the examples of Table 111, some may not pass the drip test. However, cables filled with any of the compositions of Table 111 meet the requirernents for optical performance.

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The mechanical properties of the inventive composition are a function of the colloidal particle contentO For example, it has been 3 determined that ~ as wel I as Ge decreases with decreasing particulate 4 content.
Advantageously, the waterblocking material 46 which is used to 6 fill the core of a cable of this invention yields at a low enough stress so that 7 the optical fibers 24-24 and units 22-22 are capable of moving within the core 8 when the cable is loaded or bent. The yielding filiing material allows the 9 optical fibers to move within the tube 34 which reduces the stress therein and lengthens the life of the optical fibers.
1l As mentioned hereinbefore, the cable of this invention may be 12 made with the units not being stranded together, as in the preferred 3 embodiment, or stranded or with an oscillating lay. Of course, the non-4 stranded is preferred inasmuch as the stranding apparatus may be eliminated1 5 and I ine speeds increased.
16 The tube 34 may be considered as one elemeni of a sheath 7 system 52 of the cable 20. Returning now to FIGS. I and 2, it is seen that over 18 the tube 34 are disposed other elements of a sheath system comprising a 19 bedding layer 53 and a group of reinforcing strength members 58-58, an intermediate jacket 60 of polyethylene, another bedding layer 62 and another 21 group of strength members 66-66 and an outer jacket 68. Both jackets are 22 made of polyethylene although other plastic materials may be used. Further,23 the materials for the jackets may differ. The strength members are steel 24 wires in the preferred embodiment. However, i i is apparent that other materials, metallic and non-metallic, may be used for those members.
26 Referring now to FIGS. 6A and 7A, there are shown graphical 27 representations of the added loss due to packaging in the optical fibers of the 28 cable of this invention at two relatively low temperature levels and at two 29 different operating wavelengths, 1.55 and 1.3~m, respectively. In FIGS. 6B
3û and 7B, there are shown graphical representations of the added loss at the31 temperatures shown and at operating wavelengths of 1.55JJm and 1.3ym, 32 respectively, in prior art cables which include optical fibers having the 33 matched cladding refractive index profile shown in FIG. 4B. Each rectangular 34 box represents the middle 50% of the sample with the horizontal line in each 3~
-being the median. Broken iines extending ver~ically from each box represent 2 the top and lower quartiles of the sample. As can be seen, at an operating 3 wavelength of 1.3,um, the median added loss for a cable including matched 4 cladding optical fibers af -40F is 0.05 dB/km whereas for the cable of thisinvention, there is none. At an operqting wavelength of 1.551Jm, the added loss 6 of matched cladding fibers at -4ûF is about 0.15 dE3/km greater than that for 7 the cable of this invention.
8 It should be clear that sheath systems other than that shown in 9 FIGS. I and 2 may be used for cables of this invention. For the example, a cable 70 depicted in FIGS. 8 and 9 includes a core 21 and the tubular member 11 34. The tubular member 34 is enclosed by a plastic jacket 74. Interposed 12 between the tubular member 34 and the jacket is a strength member system 80 13 which includes a layer 82 of strength members 84-84 and a layer 86 comprising 14 a plurality of strength members 88-88 as well as several of the strength members 84-84. Each of the strength members 8~-84 is a glass roving which 16 has been impregnated with a polyurethane material, for example, whereas 17 each of the strength mernbers 88-88 is a glass yarn which has been 18 impregnated with an epoxy material. The strength members 88-88 are capable 19 of resisting compressive as well as tensile stresses.
2û It is to be understood that the above-described arrangements are 21 simply illustrative of the invention. Other arrangements may be devised by 22 those skilled in the art which will embody the principies of the invention and 23 fall within the spirit and scope thereof.

Claims (10)

1. An optical fiber cable, which comprises:
a plurality of optical fibers which are assembled together without intended stranding to form a unit which extends in a direction substantially along a longitudinal axis of the cable, each of said optical fibers including a core, a outer cladding, and an inner cladding;
a tube which is made of a plastic material and which encloses the plurality of optical fibers with the ratio of the cross-sectional area of the plurality of optical fibers to the cross-sectional area within the tube not exceeding a predetermined value, said tube being substantially parallel to the longitudinal axis of the cable;
a strength member system; and a jacket which is made of q plastic material and which encloses said tube;
said cable being characterized in that said inner cladding of each of said fibers has an index of refraction which is less than that of the outer cladding thereof and each of said fibers is characterized by a difference between the indices of refraction of said core and of said inner cladding which is sufficiently high to cause saideach optical fiber to be substantially insensitive to microbending with the ratio of the inner cladding diameter to the core diameter and the ratio of the difference in the indices of refraction of the inner and the outer claddings to the difference in indices of refraction between the core and the inner cladding being such that each said optical fiber is capable of operation in a single modefashion at a predetermined wavelength.

2. The cable of claim 1, wherein each of the optical fibers is provided with a coating and wherein the ratio of the cross-sectional area of the plurality of coated optical fibers to the cross-sectional area within the tube does not exceed a value of about 0.5.

3. The optical fiber cable of claim 1, wherein the difference between the indices of refraction of the inner and outer claddings is in the range of about 10 to 40% of the difference between the indices of refraction of the core and the inner cladding, and wherein the ratio of the inner cladding diameter to the core diameter is not less than about 3.

4. The optical fiber cable of claim 1, which comprises a plurality of said units with said tube enclosing the plurality of units without any intermediate tubes separating said units from one another, each unit comprising a plurality of optical fibers being wrapped with a binder and with the ratio of the cross-sectional area of the plurality of optical fibers to the cross-sectional area within the tube not exceeding a predetermined value, said tube being substantially parallel to the longitudinal axis of the cable, and wherein said cable also includes:
a waterblocking material which is disposed within the tube and which fills substantially the interstices between the optical fibers of the units within the tube and between the units and the tube, the waterblocking material having a critical yield stress and a shear modulus which allow movement of the units within the core when the waterblocking material is subjected to a predetermined stress, wherein the tube is a common tube which encloses the plurality of units with the units being separated from one another only by the waterblocking material, wherein each of the optical fibers is provided with a coating, wherein the predetermined value is 0.5 and wherein the length of the tube is no greater than the length of any fiber in each unit.

5. The cable of claim 47 wherein the units are stranded together.

6. The optical fiber cable of claim 4, wherein the waterblocking material has a critical yield stress which is not greater than about 70 Pa at 20°C and a shear modulus less than about 13 KPa at 20°C.

7. The cable of claim 6, wherein the waterblocking material is a composition of matter which comprises:
a) 77 to 95% by weight of an oil selected from the group consisting of i. paraffinic oil having n minimum specific gravity of about 0.86 and a pour point less than -4°C and being of ASTM type 103, 104 A
or 104B;
ii. naphthenic oil having a minimum specific gravity of about 0.86 and a pour point less that -4°C and being of ASTM type 103, 104A
or 104B;
iii. polybutene oil having a minimum specific gravity of about 0.83 and a pour point less than 18°C; and iv. any mixture thereof; and b) 2 to 15% by weight of hydrophobic fumed silica colloidal particles.

8. The cable of claim 6, wherein the waterblocking material is a composition of matter comprising:
a) 77 to 95% by weight of an oil selected from the group consisting of:
i. paraffinic oil having a minimum specific gravity of about 0.86 and a pour point of less that -4°C and being of ASTM type 103, 104 A or 104B;
ii. naphthenic oil having a minimum specific gravity of about 0.86 and a pour point less that -4°C and being of ASTM type 103, 104A
or 104B;
iii. polybutene oil having a minimum specific gravity of about 0.83 and a pour point of less than 18°C;
iv. triglyceride-based vegetable oil;
v. polypropylene oil;
vi. chlorinated paraffin oil having a chlorine content between about 30 and 75% by weight and a viscosity at 25°C of between 100and 10,000 cps;
vii. polymerized esters, and viii. any mixture thereof; and b) 2 to 15% by weight colloidal particles selected from the group consisting of hydrophobic fused silica, hydrophilic fused silica, precipitated silica, and clay, the colloidal particles having a BET surface areain the range from about 50 to about 400m2/g.

9. The cable of claim 8, wherein the composition of matter further comprises up to 15% by weight of a bleed inhibitor selected from the group consisting: of styrene-rubber and styrene-rubber-styrene block copolymers having a styrene/rubber ratio between about 0.1 and 0.8, semiliquid rubber having a Flory molecular weight between 20,000 and 70,000, butyl rubber, ethylene-propylene rubber, ethylene-propylene dimer rubber, chlorinated butyl rubber having a Mooney viscosity at 100°C between about 20 and 90, and depolymerized rubber having a viscosity at 38°C between 40,000 and 400,000 cps, and wherein the oil, the colloidal particles, and the bleed inhibitor comprise at least 99% by weight of the composition of matter.

10. The cable of claim 9, wherein said composition comprises about 90 to 95% b.w. of oil and about 2 to 10% b.w. of colloidal particles.