Fiber Optic Microcable Produced With Radiation Cured Composite

The present invention generally relates to the field of fiber optic communications and more particularly to fiber optic cables. Specifically, the present invention provides a fiber optic microcable having a uniform cross-sectional dimension and a continuous length greater than conventional microcables.

A fiber optic microcable is basically comprised of a glass or plastic fiber core, a buffer, and a protective sheath. The protective sheath is typically composed of a heat polymerized organic resin impregnated with reinforcing fibers. Conventional resin materials are typically polymerized or cured at temperatures which may exceed 200.degree. C. These temperatures can damage the ultraviolet light cured buffer layers present on state-of-the-art optical fibers resulting in unacceptable degradation of optical performance. One alternative is to use heat resistant buffer layers composed of silicone rubber or thermoplastic. However, these materials greatly increase the costs of the microcable. An alternative to using expensive buffer materials is to increase the exposure time of the resin to a more moderate curing temperature for a longer period of time as compared to curing the resin at higher temperatures. This solution disadvantageously requires either a very long production oven which may be 100 to 200 feet long or a microcable production rate limited to approximately four inches per second. Both of these methods increase the microcable production costs.

A further problem associated with heat curable polymeric sheathing materials is that the sheathing tends to deform while curing, causing the microcable to become out of round. This results in a microcable having a nonuniform, noncircular cross section which creates difficulties when precision winding the microcable onto spools. Still another problem with heat curable polymeric resins is limited pot life which sets an upper limit on the continuous length of fiber optic microcable which can be fabricated in a given production run. Physical properties of heat curable resins tend to vary throughout their pot lives thereby creating additional manufacturing difficulties. All of these problems combine to increase the costs of fiber optic microcable applications and limit the maximum obtainable continuous lengths of microcable to at most 10 kilometers. Therefore, there is a continuing need to develop a fiber optic microcable which can be more readily manufactured to greater lengths within acceptable tolerances and costs.

The present invention provides a fiber optic microcable that overcomes the dimensional uniformity problems, length limitations and cost disadvantages inherent in the prior art. The fiber optic microcable includes an optical fiber core surrounded by a buffer and a protective sheath. The sheath consists of an ultraviolet light curable resin impregnated with fibers to enhance the physical strength characteristics of the microcable. The resin polymerizes almost instantaneously upon exposure to ultraviolet light. This permits the microcable to be manufactured at rates greater than 1 meter per second by processing it through an ultraviolet light processor. The length of the lamp bath is determined by the cure rate of the resin and the intensitiy of the ultraviolet light falling incident upon the resin. Because curing takes place almost immediately, there is no tendency for the resin to sag out of round as there is for microcable cured in a long oven. Therefore, the resulting microcable is uniformly round over its entire length. Another significant advantage with microcables having an outer sheath composed of an ultraviolet light curable resin is that the resin temperatures attained during polymerization are less than 100.degree. C. as opposed to temperatures greater than 200.degree. C. for heat curable resins. The lower temperatures enable the microcables to be fabricated with standard state-of-the-art telecommunications optical fiber using ultraviolet light cured buffers which are relatively inexpensive.

One advantage of the present invention is to provide a fiber optic microcable that has a uniformly round cross section over the length of the microcable. Another adavantage of the present invention is to provide a microcable that can be fabricated in lengths that exceed 10 kilometers. A very important advantage of the present invention is to provide a fiber optic microcable which can be cured at less than 100.degree. C. by incorporating a resin that is cured upon exposure to ultraviolet light. This latter advantage is important because it enables fiber optic microcables to be fabricated with optical waveguide buffers that would otherwise be damaged at the temperatures required to cure heat cured resins. Furthermore, the present invention provides a fiber optic microcable which can be manufactured at rates which exceed 1 meter per second.

Other advantages of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

FIG. 1 is a breakaway view of the fiber optic microcable.

FIG. 2 is an elevation view of a machine for manufacturing fiber optic microcable.

FIG. 3 is a plan view of a machine for manufacturing fiber optic microcable.

FIG. 4 is a view of the comb plate along line 4--4 of FIG. 2.

FIG. 5 is a plan view of an ultraviolet light curing station.

FIG. 6 is a cross-sectional end view of the ultraviolet light curing station along line 6--6 of FIG. 5.

FIG. 7 is a plan view of the two-stage ultraviolet light curing station.

FIG. 8 is a cross-sectional end view of the medium intensity ultraviolet light curing station along line 8--8 of FIG. 7.

FIG. 9 is a cross-sectional view of the low intensity ultraviolet light curing station along line 9--9 of FIG. 7.

Referring now to FIG. 1 wherein like reference numerals designate like or similar parts throughout the several views, there is illustrated a fiber optic microcable 10 for conveying pulses of light. The microcable includes an optical fiber core 12 surrounded by a buffer 14 and a protective sheath 16 composed of an ultraviolet light cured polymeric resin impregnated with reinforcing fibers 18. By way of example, fiber core 12 may be Corning single mode dispersion shifted optical fiber, and buffer 14 may be Corning CPC 3 buffer. Reference to the optical fiber core implicitly includes reference to cladding (not shown) and sub- strate (not shown) surrounding the core. It is to be understood that all hereinafter references to the core also refer to the cladding and substrate. Fibers 18, which may be fiberglass filaments grouped as yarns or rovings, enhance the resistance of the microcable to physical damage. Although the fibers have been described as being composed of fiberglass, it is to be understood that it is within the scope of this invention for the fibers to be composed of other materials, as for example, boron, nylon, carbon graphite, or aromatic polyamide polymers such as "KEVLAR," a product of the Dupont Chemical Corporation, and which may be grouped as yarns, rovings or single filaments. The fibers are radially suspended in the resin around and parallel to optical fiber core 12. By way of example, fibers 18 may be Corning fiberglass ECG150-1/0-.7Z with a 603-0 finish. The reinforcing fibers may constitute 50 to 90 percent by volume of the fiber/resin composite mixture.

Microcable 10 has a uniformly concentric cross-sectional area attributable to rapid cure of the ultraviolet light curable resin. Although fibers 18 have been described as running parallel to optical fiber core 12, it is within the scope of the invention for fibers 18 to be suspended in other patterns in the resin, as for example a helical or woven pattern around optical fiber core 12.

FIGS. 2 and 3 show a machine 20 for manufacturing fiber reinforced, ultraviolet light curable optic fiber microcable 10 fabricated by dispensing fiberglass yarns 22 from storage bobbins 26. The yarns are subjected to back tension, which may be 0.1 Newton, controlled by textile tensioners 24. The yarns individually drawn through ceramic guides 28a and then over a first set of guide pins 30a. The yarns proceed through a staggered series of ceramic pins 32 in a temperature controlled wetting pan 34 containing an ultraviolet light curable resin 36. The resin is maintained between 27.degree. and 70.degree. C. in which yarns 22 are wetted. Any air which may become entrapped in the yarns is released as the wetted yarns pass over and under ceramic pins 32. Referring to FIGS. 2, 3 and 4, collectively, the wetted yarns are drawn over a second set of guide pins 30b, exit wetting pan 34 and then individually pass through ceramic guides 28b. The wetted yarns proceed through ceramic guides 39 mounted in comb plate 38 so that yarns 22 are radially positioned around optical fiber core 12 and buffer 14. The fiber core and buffer are integrally fed from a spool 40, around sheaves 42a and 42b, and on through ceramic guide 43 mounted in comb plate 38. Although FIGS. 2, 3 and 4 depict six yarns 22 being processed, it is to be understood that FIGS. 2, 3 and 4 represent the invention by way of example only. Within the scope of the invention, fewer or greater numbers of yarns 22 may be utilized than are actually shown.

After exiting comb plate 38, yarns 22 and optical fiber core 12 with accompanying buffer 14 converge as they pass through a circular aperture 44 of a heated tungsten-carbide forming die 44a to form a matrix 46 as shown in FIGS. 2 and 3. The diameter of aperture 44 determines the diameter and fiber/resin ratio of microcable 10.

Numerical reference 47 represents alternative species of methods for curing matrix 46. The first species is encompassed within lamp housing 48 illustrated in FIGS. 5 and 6. The second species is encompassed within lamp housings 62 and 74 illustrated in FIGS. 7, 8 and 9.

A single-stage method for curing matrix 46 is illustrated in FIGS. 5 and 6 wherein after exiting aperture 44 shown in FIGS. 2 and 3, matrix 46 enters a lamp housing 48. The matrix passes through a quartz tube 50 within the lamp housing in approximately 0.5 seconds through which it is irradiated at an intensity of approximately 100,000 microwatts/cm.sup.2 by a single-stage electromagnetic radiation source 52 emitting ultraviolet light at a wavelength of anywhere from 290 to 400 nanometers. Quartz tube 50 shields matrix 46 from infrared radiation generated by electromagnetic radiation source 52 while being transparent to ultraviolet radiation. Quartz tube 50 may advantageously be filled with an inert gas such as nitrogen or helium so that matrix 46 is immersed within an inert gas atmosphere to improve cooling of the microcable while curing and to prevent undesirable chemical reactions from occuring between atmospheric oxygen and the resin. A quartz plate 53, which may be 0.32 cm thick, is mounted between electromagnetic radiation source 52 and quartz tube 50 to further shield matrix 46 from infrared radiation. The longitudinal axis of electromagnetic radiation source 52 is coincident with a focal axis a--a of a semi-elliptically shaped mirror 54. The longitudinal axis of quartz tube 50 is coincident with a focal axis b--b of a semi-elliptically shaped mirror 56. The reflective concave surface of mirror 54 faces the reflective concave surface of mirror 56 so that the reflective surfaces of both mirrors define an elliptical mirror having focal axes a--a and b--b.

Ultraviolet light from electromagnetic radiation source 52 both propagates directly towards matrix 46 and reflects off of mirror 54 to mirror 56, and then converges on focal axis b--b so that matrix 46 is irradiated from a 360.degree. field. Dry nitrogen injected into quartz tube 50 through nipples 58 displaces oxygen which can inhibit polymerization of matrix 46 and cools the matrix while it cures. Matrix 46 cures almost instantaneously upon exposure to the ultraviolet light and is thus transformed into a completed microcable 10. Of course, matrix 46 may be conveyed through two lamp housings 48 arranged in series in order to increase the ultraviolet light exposure of matrix 46 as a way of speeding up the curing process of matrix 46, providing the temperature of the curing matrix 46 does not reach a level that would degrade any of the materials that comprise matrix 46.

An alternative to the single-stage method for curing matrix 46 as described above, is a two-stage curing method illustrated in FIGS. 7, 8, and 9, collectively, wherein after exiting aperture 44 shown in FIGS. 2 and 3, matrix 46 enters lamp housing 62. Within the lamp housing, the matrix passes through a quartz tube 64 through which it is irradiated at a medium intensity of 5000 to 10,000 microwatts/cm.sup.2 by an electromagnetic radiation source 66 emitting ultraviolet light at a wavelength of approximately 290 nanometers. The ultraviolet light polymerizes an outer layer of matrix 46. Quartz tube 64 shields matrix 46 from infrared heat generated by electromagnetic radiation source 66 while being transparent to ultraviolet radiation. The longitudinal axis of electromagnetic radiation source 66 is coincident with a focal axis c--c of a semi-elliptically shaped mirror 68. The longitudinal axis of quartz tube 64 is coincident with a focal-axis d--d of a semi-elliptically shaped mirror 70. The reflective concave surface of mirror 68 faces the reflective concave surface of mirror 70 so that the reflective surfaces of both mirror define an elliptical mirror having focal axis c--c and d--d.

Ultraviolet light from electromagnetic radiation source 66 both propagate directly towards matrix 46 and reflects off of mirror 68 to mirror 70, and then converges on focal axis d--d so that matrix 46 is irradiated from a 360.degree. field. Dry nitrogen injected into quartz tube 64 through nipples 72 displaces oxygen which can inhibit polymerization of the outer layer of matrix 46 and cools the matrix while it cures.

Referring to FIGS. 7, 8, and 9, collectively, after existing lamp housing 62, partially cured matrix 46 enters lamp housing 74 where it passes between four low intensity ultraviolet lamps 76 which irradiate matrix 46 with ultraviolet light having a wavelength of about 360 nanometers at a relatively low intensity of approximately 2,000 microwatts/cm.sup.2. This second curing stage completes polymerization of matrix 46 into a completely cured microcable 10. The purpose of this two-stage process is to limit the overall temperature rise of the curing microcable matrix to no more than 100.degree. C.

Returning to FIGS. 2 and 3, after curing, completed fiber optic microcable 10 is then taken up at constant speed onto a storage spool 78 driven by means readily understood by one skilled in this art, as for example, by an electric motor 80 coupled to the spool by a "V"-belt 82. The fiber optic microcable may then be stress relieved by soaking the cooled fiber optic microcable in an approximately 70.degree. C. atmosphere for about four hours and then allowing it to air cool.

Of course, addtional layers of sheathing or jacketing, which may or may not include UV cured resin, could be applied to microcable 10, as would be well known by those skilled in this field of technology.

A suitable resin has a Young's Modulus ranging from approximately 700,000 to 2,500,000 kPa after cure, a post-cure tensile strength of approximately 28,000 to 56,000 kPa, an uncured viscosity of less than 250 centipoise within the range of 27.degree. C. to 60.degree. C., moisture absorption of less than one percent in 24 hours of water immersion after cure, strain to failure of 11/2% after cure, and a glass transition temperature from 60.degree. C. to 105.degree. C. after cure. Furthermore, resin 36 polymerizes or cures when exposed to electromagnetic radiation having a wavelength anywhere from 290 to 400 nanometers. Good results have been obtained with ultraviolet light curable resins such as DeSoto, Inc. No. 3287-5-9, Master Bond, Inc. No. 17D-1A and Borden 251-138-4.

While the invention has been described as having used ultraviolet light curable resin, it is to be understood that it is within the scope of the invention for other electromagnetic radiation curable resins to be used, as for example, resin cured upon exposure to visible light or electron beam radiation in conjunction with an electromagnetic radiation source having a wavelength capable of polymerizing the resin.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.