Polymer Materials for Optical Fiber Coating - American Chemical Society

Chapter 34 Polymer Materials for Optical Fiber Coating L. L. Blyler, Jr., F. V. DiMarcello, A. C. Hart, and R. G. Huff

Downloaded by UNIV ILLINOIS URBANA on March 8, 2013 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch034

AT&T Bell Laboratories, Murray Hill, NJ 07974 In order to protect their surface from damage caused by abrasion, optical fibers must be coated with one or more polymers as they are drawn. The properties of the coating are crucial to the optical and mechanical performance of the fiber in a lightwave transmission system. The role played by the coating in determiningfiberperformance is described. The performance properties of opticalfibersused in telecommunications applications are strongly influenced by the polymer coating applied duringfiberdrawing. The properties most affected by the coating arefiberstrength and transmission loss. In addition manufacturing economy and productivity are directly linked with the coating process, which usually imposes the major limitation to the attainment of high drawing speeds (0. Fiber Drawing and Coating Afiberdrawing and coating tower is shown schematically in Figure 1. The glass preform rod, manufactured in a separate process, is fed into the top of a high-temperature (2200 C) furnace at a controlled rate. The preform softens in the cylindrical furnace and thefiberis freely drawn from the molten end of the preform rod by means of a capstan located at the base of the tower. Thefiberpasses through a diameter monitor and control apparatus immediately below the furnace. The device provides a high resolution, e.g., 0.1 Mm, measurement of the fiber diameter at a rapid update rate, e.g., 500 Hz, and generates a signal for use in a feedback control loop which adjusts capstan speed to controlfiberdiameter. Typically,fiberdiameter can be held to a standard deviation of 0.2% (2), an important concern forfibersplicing and connectorization. Because the surface of the silica glassfiberis very susceptible to damage caused by abrasion, it is necessary to coat thefiberin-line, as it is drawn, before it comes into contact with any solid surface. Prior to coating, however, thefibermust be allowed to cool. If its temperature is too high, it cannot be coated in a stable fashion (1,3). Hence, in present practice, tall draw towers, 7 meters or more in height, are employed in order to provide sufficient distance for convective cooling of thefiber(1). Forced convective cooling with appropriate gaseous heat transfer media such as helium, is also practiced (4). Once thefiberis sufficiently cool, e.g. below 80 C (0, it can be coated, generally with one or two layers of organic polymers. Because the application method must not damage the glass surface, the polymers are applied in the liquid state, commonly as reactive prepolymer or hot melt formulations. The coating diameter and concentricity are monitored and controlled via suitable techniques (5). Once applied the coating must solidify very rapidly, before the fiber e

e

0097-6156/87/0346-0410$06.00/0 © 1987 American Chemical Society

In Polymers for High Technology; Bowden, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

34.

BLYLER ET A L .

Polymer Materials

for

Optical Fiber

Coating

411

reaches the capstan at the base of the tower. Photocuring polymer formulations are therefore very advantageous and drawing speeds of greater than 10 m/sec have been achieved with these systems (6,7). A s mentioned earlier, the optical fiber performance properties most affected by the polymer coating are strength and transmission loss. The relationship between the coating properties and fiber properties will now be described.


Fiber Strength The integrity of optical fibers deployed in the field depends upon the assurance that their strength is maintained over their entire length, which may be several kilometers. While silica glass possesses a very high intrinsic tensile strength, its strength is severely degraded by the presence of bulk or surface defects which act as stress concentrators (8). Such defects act as local weak points along the fiber. It is essential to eliminate sources of these defects in the materials used to fabricate optical fibers and in the fiber manufacturing processes as well. Low strength fiber defects may arise from a number of sources. Surface flaws such as scratches or abrasions on the glass preform surface, which may not be healed by the fiber drawing process, can be removed by fire polishing (9). Solid particles emanating from the drawing furnace, or convected into the furnace from the drawing environment, may become lodged in the molten glass at the end of the preform. These particles are drawn into the fiber as surface particles whose size and shape determine the severity of the flaw produced. Thus the drawing environment must be as particle-free as possible, and the drawing furnace must be scrupulously maintained to avoid particle generation. Another source of particles is the polymer coating material itself. Particles present in the material may locate at the coating-glass interface, where they may produce flaws by contact or abrasion. Therefore it is necessary to filter fiber coating materials to remove particles with sizes in the micron range. A s an illustration of the relationship between coating particles and fiber strength, Figure 2 shows strength data for fibers coated with materials containing intentionally-added alumina particle contamination (10). Tensile strengths are high (—6 GPa) for fiber coatings with no added particles (control) as well as for those with particles in the 1-3 μπι size range. Larger particles, however, degrade the strengths to 1.4-3.5 G P a (200500 kpsi). Similar results are obtained for coatings having high or low modulus values, providing they have relatively high adherence to the fiber. If the adhesion is poor, however, relative movement can occur between the fiber and coating, leading to increased abrasion between the fiber surface and any particles located at the interface. Figure 3 displays a plot of the number of tensile failures per kilometer of a fiber in a prooftest (continuous tensile test of a fiber over its entire length) versus the force required to pull a 1 cm length of the fiber out of its coating. This force is a function of fiber-to-coating adhesion. The number of prooftest failures increases with decreasing adhesion for coatings containing particles in the 6 - 1 0 Mm range. Significant failure frequencies also occur for fiber coatings with very low adhesion containing particles in the 1—3 μπι size range. This range lies below the critical size for highly adherent coatings, typified by those used to produce the data of Figure 2. Therefore to ensure against fiber failure due to particle contamination of the coating, highly adherent coatings, filtered to remove all particles larger than 3 μπι in size, should be suitable for most applications. Coating defects which may expose the fiber to subsequent damage arise primarily from improper coating application. Defects such as large bubbles or voids, highly non-concentric coatings with unacceptably thin regions, or intermittent coatings, can be overcome through proper applicator design and process controls. Pressurized applicator designs have been successful in eliminating bubble incorporation in the coating (11). Coating concentricity can be monitored by optical techniques, e.g., laser forward scattering, and the applicator positioned so as to apply the coating concentrically (5). Intermittent coating is overcome by insuring the fiber is suitably cool at the point of its entry into the coating applicator so as to avoid coating flow instabilities ( O .


POLYMERS FOR HIGH T E C H N O L O G Y

412

- —

Preform Feed Mechanism

a

Preform

Furnace I


Q

I Fiber Diameter Monitor

Convective Cooling Region Coating Applicator I

I Concentricity Monitor Curing Station I

cjC^) Figure 1

I Coating Diameter Monitor Capstan

Schematic of an optical fiber drawing tower. GPa

2

99.9

τ—

99 h A

95 90 80 70 60 50 40 30 FAILURES

I

i A • A •

20

A •

10

A •

A • • A

J 100 Figure 2

CONTROL 1-3 MICRON 6-10 MICRON 15-20 MICRON

L

200 300 400 500 FAILURE STRESS (ksl)

700 900

Failure stress distribution for fibers with coatings having various particle size ranges. (Reproduced with permission from Reference 10. Copyright 1985 I E E E ) .


34.

BLYLER ET AL.

Polymer Materials

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Optical Fiber

413

Coating

Transmission Loss - Microbending


Optical fibers are susceptible to a transmission loss mechanism known as microbending (12,13). Since the fibers are thin and flexible, they are readily bent when subjected to mechanical stresses, such as those encountered during placement in a cable or when the cabled fiber is exposed to varying temperature environments or mechanical handling. If the stresses placed on the fiber result in a random bending distortion of the fiber axis with periodic components in the millimeter range, light rays, or modes, propagating in the fiber may escape from the core. These losses, termed microbending losses, may be very large, often many times the intrinsic loss of the fiber itself. Thus the fiber must be isolated from stresses which cause microbending. The properties of the fiber coating play a major role in providing this isolation, with coating geometry, modulus and thermal expansion coefficient being the most important factors (3). Two types of coating geometries, displayed in Figure 4, are commonly used. Single coatings, employing a high modulus, e.g. 10 Pa, or an intermediate modulus, e.g. 10* Pa, are most easily produced and are used in applications requiring high fiber strengths or in cables which employ units, e.g., buffer tubes (15), where fiber sensitivity to microbending is not a significant problem. Dual coatings (12,14), consisting of a low modulus (ΙΟ —10 Pa), elastomeric primary coating surrounded by a high modulus secondary coating, are used for more sensitive applications. This structure isolates the fiber very well from external stresses which would tend to cause local bending. Such stresses may be imposed in two distinct ways. First, non-uniform lateral stresses, imposed by the cable structure surrounding the fiber, may cause bending with periodic components in the microbending regime. The dual coating serves to cushion the fiber via the primary layer and to distribute the imposed forces via the secondary layer, so as to isolate the fiber from bending moments. Second, axial compressive loading of the fiber occurs when the surrounding cable components contract relative to the fiber. Such contraction results from both the differential thermal contraction of the cable materials relative to the glass fiber and from the viscoelastic recovery of residual orientation present in the cable materials. If the axial compressive load imposed on the fiber becomes large enough, the fiber will respond by bending or buckling (16). The low modulus primary coating is effective in promoting long bending periods for the fiber which are outside the microbending range. 9

6

7

A variety of techniques have been employed to induce microbending losses in fibers so that they may be studied. Some of the methods used include tensioning the fiber on a microscopically rough drum (9,13), pressing the fiber between blocks (17), and winding the fiber over itself under tension in a multilevel fashion on a spool (18). The microbending losses induced thereby decay with time owing to viscoelastic stress relaxation of the fiber coating. Figure 5 illustrates the time-dependence of the transmission losses of an optical fiber determined in the spool test. The loss changes parallel the time-dependence of the relaxation modulus of the fiber coating, also shown in Figure 5. These and other studies (9) reveal that the microbending losses decay to their minimum value when the coating modulus relaxes to a value of approximately 10 M P a (1500 psi) or below. Thus the equilibrium modulus of the primary coating used in a dual coating structure is desirably kept below 10 M P a . The most effective range is generally taken to be 0.5 to 5.0 M P a (75 to 750 psi), with moduli below 0.5 M P a regarded as too low for adequate physical protection. The primary coating should possess a low glass transition temperature interval as well as a low modulus. The low glass transition temperature range insures that the primary coating will provide adequate resistance to microbending at the lowest temperatures encountered in field environments, e.g. - 4 0 C or lower. The secondary coating forms a tough, elastic shell around the primary coating which provides the fiber with suitable handling characteristics, e.g. abrasion resistance, low friction. Its glass transition (or melting temperature for crystalline thermoplastics) should lie well above normal field environment temperatures. e

Coating Materials Several types of primary coating materials have been employed commercially. The earliest materials used in large scale production were thermally-cured silicones (14). They are notable


POLYMERS FOR HIGH T E C H N O L O G Y

414 150

Δ 6-10/am PARTICLES • 1-3 /am PARTICLES 100 h NO. OF FAILURES PER km 50


Oh 0 1 POOR ADHESION Figure 3

2 3 PULLOUT FORCE - kg

4 GOOD ADHESION

Effect of coating-to-fiber adhesion on prooftest failure frequency. with permission from Reference 10. Copyright 1985 I E E E ) .

(Reproduced

Hard Secondary · Hard or Tough Coating

Single C o a t i n g Figure 4

Dual Coating

Schematic of single and dual coatings for optical fibers.

1 0

3

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^COATING

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RELAXATION

a

m S

1 0

-

8

to

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_ SPOOLED 140 g

ω to ο

0 FIBER

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103

Polymer Materials for Optical Fiber Coating - American Chemical Society

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