Polymer Coatings for Optical Fibers - American Chemical Society

38 Polymer Coatings for Optical Fibers 2

L. L. BLYLER, JR.1, and C. J. ALOISIO Bell Laboratories, Murray Hill, NJ 07974 Bell Laboratories, Norcross, GA 30071 1

Downloaded by MONASH UNIV on November 24, 2015 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch038

2

Requirements of a Coating Material Coating Application Coating Properties and Fiber Performance Fiber Strength Microbending Loss Material Considerations Microbending Loss: Relation to Coating Properties Microbending Loss: Relation to Fiber Cabling

Optical fiber technology, developed during the 1970s, has ushered in the lightwave telecommunications era of the 1980s wherein the implementation of commercial systems has begun. The optical fiber, shown schematically in Figure 1, represents the medium over which light signals, consisting of streams of digital pulses that comprise voice, video, or data information, are transmitted. The fiber consists of a central core, usually composed of a highly transparent glass, and surrounded by cladding of lower refractive index that confines the light energy to propagate within the core by total internal reflection. The cladding material is commonly another glass, but polymer-clad fibers have also been developed for short to medium distance applications. The outstanding requirement for the glass core is high transparency so that light propagation will occur with very l i t t l e energy loss by absorption or scattering processes. Losses below 0.5dB/km at selected transmission wavelengths have now been routinely achieved (1, 2). The cladding material, whether glass or polymeric, does not need to be quite so transparent, but because the light energy propagating within the core penetrates the cladding to some extent, cladding material losses below approximately 1000 dB/km are a practical necessity. This requirement rules out a l l but the most transparent polymers as cladding materials. Most fibers of commercial importance have cores composed of silica that is doped 0097-6156/85/0285-0907S07.00/0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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A P P L I E D POLYMER SCIENCE

_ n ir= I a

Figure

1.

n ladding C

n

core

Schematic of an optical fiber. A l l l i g h t rays that enter the fiber end face and strike the core-cladding interface at an angle Φ > Φ 0 (the c r i t i c a l angle), where sin φ 0 - n c i a d d i / n c o r e ; are t o t a l l y i n t e r n a l l y reflected and propagate along the core. Ine r^-ber s acceptance angle, Φ, i s given by sin Φ = (n c o r e ~ n cladding)*/^, which is called the numerical aperture (NA). (Reproduced with permission from Ref. 28. Copyright 1981 Gordon and Breach Science Publishers.)



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BLYLER A N D ALOISIO

Polymer Coatings for Optical Fibers

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with oxides of germanium and phosphorus to raise the refractive index. The cladding may be either pure s i l i c a or a doped s i l i c a such as a fluorosilicate. Fibers based on multicomponent glasses such as sodium borosilicate and soda-lime silicate systems have also been developed (3). Polymer-clad fibers usually consist of a s i l i c a core clad with either a poly(dimethy1 siloxane) resin or a fluorinated a c r y l i c polymer. The refractive index, n, of the polymer must be lower than that of the s i l i c a core (n = 1.458), a requirement that restricts the choice of available cladding polymers quite severely. Owing to their higher transmission losses and inherently lower information-carrying capacity, polymer-clad fibers have limited u t i l i t y for high-performance telecommunications applications, which are beginning to dominate the f i e l d . Consequently, we s h a l l not deal with this fiber type further, and additional information i s published elsewhere (4). We s h a l l concentrate instead on the all-glass fiber type which, as we shall see, requires a polymer coating. In this case the polymer coating has no special optical wave guiding requirement or function. Requirements of a Coating Material Polymer coatings play a crucial role in protecting the glass fiber from physical damage. Figure 2 i l l u s t r a t e s the reduction in the tensile strength of a s i l i c a fiber caused by stress concentration at a s e m i e l l i p t i c a l surface crack of radius a c (_5). The highest t e n s i l e strength that can be p r a c t i c a l l y realized in a c a r e f u l l y produced s i l i c a fiber at room temperature i s about 5.5 GN/m (800,000 psi). (This result implies that i n t r i n s i c structural defects in the glass surface are about 10"^ \im large.) However, a 1-ym flaw in the glass surface lowers the tensile strength by nearly an order of magnitude to 0.62 GN/m2 (90,000 psi). Such tiny flaws are readily produced by abrasive contact of the fiber with another solid surface. Therefore, the optical fiber must be coated as i t is drawn, before any contact with another surface can occur. Figure 3 schematically depicts a typical fiber-drawing machine (§)· A glass preform rod i s advanced into the top of a tubular high-temperature (~2200 °C) furnace at a controlled rate. The rod softens in the furnace, and i t is drawn into a fiber by a capstan at the base of the machine. The fiber passes through a diameter measurement and control-system unit (7) located directly below the furnace. This system utilizes a laser forward-scattering technique to monitor fiber diameter with high resolution (e.g., 0.2ym) at a rapid update rate (e.g. 500 Hz) and provides a signal for feedback control that adjusts the speed of the capstan to drive the fiber diameter toward the desired value. The coating apparatus is located between the fiber-diameter monitor and the capstan at the base of the machine. The logistics of the fiber-coating operation have far-reaching implications for the choice of coating materials used. For example, the coating must be applied to the fiber without damaging the glass surface. This requirement is best met when the coating is applied as a moderately low-viscosity (5-50 P) liquid. The low viscosity not only allows the coating to be applied to the glass in a gentle manner but also affords the opportunity of filtering the liquid to remove micron-sized particles that might damage the glass surface.


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APPLIED POLYMER SCIENCE

a,(GN/m2) 10

10"'

1

1

10

1

Γ


π

_z

TENSILE STRENGTH, Ο"ι (kpsi) Figure 2.

Tensile strength of a s i l i c a fiber possessing a semie l l i p t i c a l surface crack of radius, a .


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911


PREFORM FEED MECHANISM

IRCONIA INDUCTION FURNACE

-FIBER DIAMETER MONITOR COATING CONCENTRICITY ^MONITOR

•-CURING I FURNACE OR LAMPS CAPSTAN

Figure 3.

Schematic of an o p t i c a l fiber drawing machine. (Reproduced with permission from Ref. 6. Copyright 1980 IEEE.)


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A P P L I E D P O L Y M E R SCIENCE

The coating must also be s o l i d i f i e d very rapidly once i t i s applied to the fiber so that i t w i l l offer protection when the capstan is reached. This requirement rules out solvent-containing formulations for a l l but the thinnest coatings, because solvent removal is a slow process. Solvent-free coating formulations that are rapidly cross-linked by thermal activation or by UV radiation can be used very successfully. Thermoplastic hot-melt systems that solidify quickly upon cooling are also viable. The material systems of choice for this application depend upon both coating-application and coating-performance considerations.


Coating Application Figure 4 i l l u s t r a t e s a simple applicator used for fiber coating. The fiber enters the coating resin contained in the applicator reservoir where i t is wet by the liquid. The moving fiber produces a depressed meniscus in the liquid surface at the point of entry and drags the l i q u i d downward into the conical coating die located at the base of the applicator. The drag flow into this converging channel produces a hydrodynamic pressure that drives a return flow or circulation upward along the walls of the die and reservoir. The hydrodynamic pressure also tends to s t a b i l i z e the fiber position along the centerline of the die because excursions from this position produce a net restoring force. In practice this restoring force cannot be relied upon to center the fiber i f its natural path is far from the centerline. The alignment of the coating die with the fiber i s one of the central problems of fiber coating. It is not possible to guide the fiber through the center of the die, because contact with a guide, even when i t is immersed in the coating liquid, can damage the glass surface. Thus alignment of the hair-thin fiber (e.g., 125 ym od) within the die, which may be only slightly larger, requires special techniques, because the fiber i s fixed in space at two points (the preform and the capstan) that are very remote from the coating applicator. An early solution to this alignment problem was the use of flexible silicone rubber dies (8) that respond to the hydrodynamic forces generated in the converging flow f i e l d and tend to a l i g n themselves about the fiber. However, the flexible-die approach is inadequate for meeting close tolerances on coating eccentricity; therefore, a laser scattering technique (9) that has been developed enables coating concentricity to be continuously monitored by optical means. This practice allows the applicator to be mounted on a stage capable of micrometer adjustments so that the die may be accurately positioned about the fiber, and the coating eccentricity may be held within a few microns. Another coating-application problem involves the meniscus formed at the point of entry of the fiber into the coating l i q u i d at the free surface. As shown schematically in Figure 5, the a i r streamlines along the free surface of the moving l i q u i d , and the moving fiber w i l l result in a pressure increase at the tip of the meniscus. If this pressure exceeds the pressure-containment capability of the l i q u i d , an a i r column forms, collapses, and reforms, and air bubbles are entrained in the liquid. These bubbles concentrate in the l i q u i d in time, and large numbers may pass




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38.

MOVING FIBER

Figure 5.

Streamlines in the air and liquid at the coating liquid meniscus.



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through the die where they become trapped in the coating. Large bubbles within the coating are considered to be coating defects that can affect fiber properties such as transmission loss. As fiber draw speed is increased, severe i n s t a b i l i t i e s of the meniscus occur. Ultimately, the air column formed around the fiber may propagate completely through the applicator to the exit of the die, resulting in uncoated or poorly coated fiber sections. Figures 6a and 6b show the a i r column propagating toward and through the exit of a conical die, and Figures 7a and 8a show defective coatings that result from this event. Frequently, sectioning of such coating defects reveal voids produced by entrained a i r during coating (Figures 7b and 8b). A two-chamber coating applicator (Figure 9) has been developed to deal with meniscus stability problems (10). The device decouples the free surface at the top of the reservoir from the final coating operation. The f i r s t chamber (with the free surface) s t r i p s away large air bubbles. The second chamber is pressurized which produces a net flow into the first chamber and strips small bubbles from the fiber. The resultant coating is bubble free, smooth, and regular. Coating Properties and Fiber Performance Coating materials for optical fibers must be chosen with close attention paid to the manner in which their properties affect fiber performance. The a b i l i t y to protect fiber strength and to provide resistance to excess transmission losses caused by microbending are the most important functions of the coating. These items w i l l be dealt with in detail. Other important attributes of a good coating are the a b i l i t y to be stripped for s p l i c i n g and connectorization; thermal, oxidative, and hydrolytic stability; the ability to bond in cable structures; resistance to compounds such as water and gasoline; and handleability (low surface tack, toughness, abrasion resistance, etc.). Fiber Strength. The successful implementation of an optical-fiber communication system depends c r i t i c a l l y on ensuring that each individual fiber is capable of withstanding some minimum tensilestress l e v e l over i t s entire length. The stress requirement i s governed by the maximum tensile strain that the fiber w i l l encounter during fabrication operations, cabling, f i e l d i n s t a l l a t i o n , and service. The crucial importance of fiber strength is obvious when one considers that a single fiber failure w i l l normally result in the loss of at least the equivalent of several hundred telephone voice circuits. As we have seen, the f a i l u r e of glass fibers in tension i s commonly associated with surface flaws that cause stress concentrations and consequently lower the tensile strength from that of the pristine, unflawed glass. The occurrence of flaws on the glass surface i s usually associated with defective or inadequately controlled materials or processing operations. Most of the processing and material requirements important for realizing high fiber strength with an absence of c r i t i c a l l y sized flaws have been studied, and progress has been made to bring the appropriate parameters under effective control for production. The p r i n c i p a l requirements include the following items:



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Figure 6.


915

a: Photograph of the air column surrounding an optical fiber propagating toward the end of a conical die. b: Photograph of the air column penetrating through the end of the die. At this point no l i q u i d i s applied to the fiber.


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Figure 7.

a: Scanning electron micrograph of severe coating diameter fluctuations due to flow i n s t a b i l i t i e s , b: Void i n t e r n a l to the diameter fluctuation. (Reproduced with permission from Ref. 27. Copyright 1982 Society of Plastics Engineers.)




917


38.

Figure 8.

a: Scanning electron micrograph of a poorly coated fiber showing an uncoated region, b: Void within the coated region. (Reproduced with permission from Ref. 27. Copyright 1982 Society of Plastics Engineers.)


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Figure 9.

Schematic of a two-chamber applicator that produces smooth, bubble-free coatings. (Reproduced with permission from Ref. 10. Copyright 1982 Optical Society of America.)


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1. 2. 3.


4. 5.



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Use of a flaw-free glass preform. Surface flaws, such as scratches, on a glass preform may not be healed completely when the molten glass is drawn into a fiber. Maintenance of drawing-furnace cleanliness. P a r t i c l e s that s p a l l from refractory materials used in the furnace may become embedded in the molten glass. Use of s u f f i c i e n t l y high drawing-furnace temperature. Low drawing temperatures produce high glass v i s c o s i t i e s that may promote the formation of surface defects or prevent the healing of existing preform flaws. Maintenance of a dust-free drawing environment. Dust in the drawing environment may be convected into the furnace and become embedded in the molten glass. Use of a suitable coating material and technique.

With regard to item 5, the two basic considerations governing fiber strength are the avoidance of damage to the glass surface during coating application and the quality of the applied coating. Quality factors relating to the coating influence fiber strength as follows: 1. 2. 3. 4.

Coating concentricity. A highly eccentric coating that i s thin at a point on i t s circumference i s susceptible to abrasive failure during normal handling operations. Incomplete coatings. Coating flow instabilities may result in incompletely coated fiber sections, as described e a r l i e r (Figure 8a). Coating mechanical integrity. Coatings with poor abrasion resistance, such as silicones, may f a i l during normal handling unless protected with a tougher jacket. P a r t i c l e contaminants in the coating. P a r t i c l e s in the coating resin that may abrade the fiber during or after fabrication must be removed by filtration.

The importance of carrying out a l l of these operations and precautions may be seen in Figure 10. This figure shows the tensile-strength distributions (Weibull plots) for several 1-km or longer optical fibers cut and tensile tested in 20-m gauge lengths. In each case represented by the open symbols, optimum processing conditions were employed except that one item was deliberately altered from optimum, for example, high draw tension, dust in the drawing environment, or the use of a flawed preform. In each case a significant weak t a i l was observed in the strength d i s t r i b u t i o n . When a l l items were held in rigid control ( f i l l e d symbols), a 3-kra long optical fiber exhibiting only a high strength mode of failure, characteristic of an unflawed glass surface, was produced. Microbending Loss. As a result of their small diameters, optical f i b e r s bend very r e a d i l y . This feature is advantageous in that the transmission medium is very flexible and easily routed. However, when the spatial period of the bending becomes small (approx. 1 mm or less), some of the l i g h t rays normally guided by the fiber are lost through radiation. Such small period distortions may occur when a fiber is wound on a spool under tension or when i t is placed in cable structure. The phenomenon is termed microbending (11), and


920


GN/m

2

99.9 Downloaded by MONASH UNIV on November 24, 2015 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch038

99

95 -

90 80 70 60 50 40

30

2

-

20c

6

HIGH DRAW TENSION LOW QUALITY STARTING TUBE DUST IN DRAWING ENVIRONMENT WELL-CONTROLLED MATERIALS AND PROCESSING

-

-

3

Η1

UJ

or

• •

8

Η

Δ Δ

-2

10 -

3

8 Ο Ο

54 32 Ih 100

200

JL 300

Λ

400 500 600

I

I

800

Ld-5

KPSI Figure 10.

Weibull plots of tensile-strength distribution for kilometer lengths of fibers tested in 20-m gauge lengths. In each case one variable was i n t e n t i o n a l l y altered and a l l others were optimized. (Reproduced with permission from Ref. 6. Copyright 1980 IEEE.)


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in extreme cases i t can result in very large transmission losses amounting to several decibels per kilometer, thereby seriously degrading the performance of an optical-fiber transmission link. Material Considerations. Transmission losses due to microbending may be controlled or minimized through the proper choice of fiber coating materials and geometry (11, 12). Compliant coatings have been very successfully used (11-15) to cushion or buffer the fiber from stresses that induce microbending. Such coatings are best utilized in a dual coating structure in which the compliant primary coating i s surrounded by a high-modulus secondary coating. The secondary coating forms a s h e l l around the cushioned fiber and thereby further isolates the fiber from external stresses (11, 16). Thus primary polymer coatings with moduli in the rubbery range (10"10 Pa) are needed to achieve low microbending s e n s i t i v i t y . Low glass transition temperatures (T ) are also required to ensure adequate performance at low field temperatures, for example, -40 °C. Outstanding examples of such materials include s i l i c o n e rubber systems based on poly(dimethy1 siloxane) (T = -119 °C) and poly(methylphenyl siloxane) (T = -59 ° C ) . These materials are supplied as two-part liquid systems consisting of a vinyl-terminated polysiloxane resin, a t r i - or tetrafunctional silane cross-linker, and a platinum catalyst (17). After mixing, the pot l i f e of these systems is usually a few hours at room temperature, but very rapid curing is attained at high temperatures (200-400 °C). Fiber-coating operations at speeds of 1 m/s have been routinely achieved, but heat transfer difficulties are encountered at higher speeds. As mentioned e a r l i e r , UV-curable resin formulations are very attractive for fiber coating because of the rapid cross-linking rates that are achievable. Most commonly, epoxy- or urethaneacrylate resins are employed (18-22), and viscosity and cross-link density are controlled through the addition of reactive diluents. With these systems work has focused on producing low modulus, low Τ properties (20-22) through the incorporation of appropriate chemical constituents to enhance higher chain f l e x i b i l i t y , for example, ether linkages. Hot-melt thermoplastic elastomer systems (23, 24) are also effective coating materials. These materials are generally based on copolymers that are comprised of hard (crystalline or glassy) and rubbery (amorphous) segments contained in separate phases. The hard-phase regions form p h y s i c a l c r o s s - l i n k s below t h e i r c r y s t a l l i z a t i o n or v i t r i f i c a t i o n temperature, and the system therefore has elastomeric properties. The moduli and low-tem perature characteristics of these materials can be t a i l o r e d to compare reasonably well with silicone rubbers at -40 °C. However, they are limited in high-temperature a p p l i c a b i l i t y because of enhanced creep or flow due to softening. Figure 11 compares the low-temperature modulus characteristics of representative fiber-coating (buffer) materials of the three types described e a r l i e r . The figure depicts the dynamic t e n s i l e modulus as a function of frequency at -40 °C. The data were obtained on films of the materials by using a Rheometrics rotational dynamic spectrometer operated over the frequency range from 10"1 to 102 rad/s. Curves obtained at different temperatures were shifted both horizontally and vertically in accord with established linear



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Figure 11.

Master curves of the dynamic tensile moduli at -40 °C for three r e p r e s e n t a t i v e types of primary buffer coatings for optical fibers.


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viscoelastic superposition principles to produce a master curve at -40 ° C . The results i l l u s t r a t e that rubbery moduli (

Polymer Coatings for Optical Fibers - American Chemical Society

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