CA1168412A - Method for coating fiber waveguides - Google Patents

Abstract

Abstract The specification describes techniques for coating fiber waveguides by passing the fiber through a reservoir of coating material while preventing the gross accumulation of bubbles in the reservoir. Preventing accumulation of bubbles improves coating concentricity and quality.

Description

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IM~ROVED METHOD ~OR COATING
E'IBER WAV~:~;UrOES

Background of the Invention S rt is naw well known tha~ long leng~hs of glass fibers have conside{able poten~ial s~rength but the strength is ~alized only if ~he pristine fiber is protected wi~h a coa~ing the insta~t it is drawn.
Considerable efort is being expended to develo~ efective techniques for coating glass fiber waveguides. ~ost of this activity involves coating the fiber with prepolymer material and curing the prepolymer using heat or light.
Coating the fiber with a hot thermo~lastic resin is also being investigated extensively. In each case the coating material is typically applied by immersion of the pristine fiber in a reservoir containing fluid prepolymer or polymer ma~erial. Typically, the fiber en~ers the ~oatin~ fluid through a free surace, and exits through a small die at the bottom of the reservoir. ~he coating ma~erial is cured or cooled to a solid state, and ~le fiber is taken up by a suita~le capstan and reel.
Tllis general technique has heen used widely and successfully for coating considerable lengths of high quality glass fiber waveguide. ~owever, signiicant

2~ economical ~dvantage is still to be realized by increasing the draw rate of commercially produced fiber. We have found new difficulties in the coati-ng process as the rate ex~eeds the conventional rate of approximately one meter per second. For example, ~e have found that a fiber traveling at a high speed as it enters the free surface of the coating fluid draws a considerable quantity of air into the fluid medium. As coating progresses, the ai-r accumulates in ~he reservoir in the orm of air bubbles.
As the quantity of these bu~bles builds, some tend to pass through the die and remain in the fiber coating. The number of bu~bles or voids found in a given length o-f - 2 ~ ~ ~6~

coated ~ber is believed to be related directly or indirectly to the concentration o bubbles in th~
reservoir. Moreover, we have ound that these bubbles move rapidly with the streamlines in the fluid and interact mechanically with the fiber causing instabilities in the fiber alignment w~th respec~ to the die. It is known to ~e important ~o avoid excursions of the fiber as it passes ~rough the die. In addition instabilities associat~d with air bubble en~rain~ent can ~roduce fiber misalignment within the ~oating as well as coating ~iame~er varia~ions~
In summary we have identified a new and important o~s acle to ~igh-speed ~anufacture of glass iber waveguide.
Summary of the rnvention We have discovered that the problem of co~ious : ~u~ble eormation an~ consequent entrainment of bub~les in the fiber coating or misalignment of the fiber within the coating can ~e alleviated or substantially eliminate~ by properly employing a baffle in the reservoir of coating ~aterial. We have demonstrated that a properly designed ~afle effectively separates the bubbles from the region where the fiber traverses and exits the reservoir. We have also obse~ved s~ripping of bubbles from this region which we a~ribute to a hydrodynamic increase in the fluid pressure as the fiber passes through the constriction in the fluid path caused by the presence of the baffle. The stripping of the bubbles reduces the inci~ence o~ voids in the coa~ing.
According to the invention there is provided a method for the manufacture of coa*ed fiber comprising the steps of: pulling the fiber from a fiber source, passing the pulled fiber through a reservoir of liquid coating mater-ial and there~fter through an exit die in the reservoir, ; the invention characterized in that the fiber is passed through the liquid coating material at a rate rapid enough to cause entrainment of bubbles where the fiber passes into the liquid coating material, and characterized further by the step of exposing the fi~er ~o an abrupt hydrodynamic - 2a -pressure change while the fiber is within the reservoir and prior to its entry into the exit die.
Other aspects of this invention are claimed in our co-pending application serial no. 398,130 filed on March 11, 1982, of which the present application is a division.
Brief Description of the Drawing FIG. 1 is a sche~atic view cf a fiber drawing appacatus illustrating how the problem to which the invention is addressed develops;
1~ FIG. 2 is a schematic representation o~ the use of a baffle in accordance wi-th the invention ~o preven-t entrainment of gas bubbles in the fiber coating;
FIG. 3 is a sectional view taken as shown through the middle of the baffle of FIG. 2;

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FIGS. 4, 5, 6 and 7 illustrate various alternative baffle configurations useful to provide results similar to that obtainable with the baffle o FIG. 2~
FIG. 8 illustrates a circulating arrangement for eliminating bubbles ~rom the coating reservoir.
Detailed Description The problem to w~ich the invention is address~d develops in a typical coating a~a~a~us in the manner shown schematically in-~G. 1. The fibe~ 10 is pulled from a preform, cruci-~le or other sui-ta~le source (not shGwn), passes through the coa~ting reservoir 11, and e~its the rese~Y~ir ~rough coatin~ die 12. The coa~ed fi~er is then cu~ed, for example by UV lamps 14, an~ taken up on a suitable capstan 15 and reel 16. The free surface of the coating fluid 12 typically assumes the shape indicated, with a downwardly ex~ending meniscus 13 clearly evident.
T~e size of the meniscus varies with several parameters, ~ut nearly always forms if the s~eed of ~ra~el of the fiber through the coating fluid is sufficiently rapid. The presence of this meniscus indicates a posi~ive flow of air into the region o the fiber around ~he point of entry of the iber into the 1uid. Air inevitably is drawn into the fluid medium where it forms bubbles during the coating operation. As ~he number of bubbles increases, the likelihood increases ~that some will become entrained in the final coating. It should be noted that the coa-ting fluid in the reservoir develops a st-rong flow pattern under the influe~ce of ~he rapidly movin~ fiber. The flow patte~rn is evident from the motion o~ bubbles that develops, as sho~m in ~rG. 1. Bubbles are formed near the meniscus 13 and travel with the fiber downward through the fluid. Some bubbles exit with the fiber but a consi~era-ble number flow outward from the fiber and into the ci-rcular flow ~attern shown in the ~igure. However, as the num~ber of bubbles increases, the inci~ence also increases of bubbles drawn from the lowe~ reaches of the ~low pattern throuqh the exit die, where they become entrained in the fiber coating. It ' .

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has not been established whether the number of bubbles present in the fluid directly or indirectly indicates the number entrained in the fiber coating. We believe that some bubbles, at least, proceed directly from the meniscus, S along the fiber, through the exit die. However, we have es~ablished that such events are accompanied by the fo~ma~ion of copious bubbles in the fluid.
Usin~ video and photographic methods, we have studied the dynamics of the coa~ing fluid in considerable detail. T~e understanding we have gained leads us to the solution that we now describe.
FIG. 2 shows schem~tically ~he same coating aplpa~a~us ~hat ap~ears in FI~. 1 but with t~e important addition of a baffle 20. A baffle in this con~eKt is lS defined as a physical mernber designed ~o restrict the flow of bubbles frorn one ~egion o a fluid reservoir ~o another.
ln ~his case ~he 10w of bubbles is restricted between the main ~ody o the coating material reservoir and the region w~e~e the Eiber ~raverses the flùid. rn some embodiments, particular attention is directed to preventing the flow of bub~es ~o the region of the reservoir near the exit die.
The ~af1e exSends around the fiber, for an appresiable dis~ance outward into the reservoir. It is not necessary that the baffle close with the walls o the reservoir. It is necessary that the ba~fle be arranged so as to avoid contac~ wi~h ~the glass fiber. The effect of the baffle is illustrated in the ~i~ure. It serves to confine the bubbles to the upper region of the reservoir, thus reducing substantially the risk that bub-bles will exit the coating die 12, and becom~ imbedded in the fiber coating.
An exemplary baffle design is shown in F~G. 3.
The openings 30 provide for circulation of coating fluid from the lower chamber of the reservoir to the upper chamber. ~luid is drawn by the rapidly moving fiber down through the hole 30 into the lower chamber, and it is advantageous to provide a return path to the upper chamber.
However, the opening 31 can be made large enough to ~ake 4~

that unnecessary. The actual size of the opening 30 depends on several considerations including the draw speed, which in large part establishes the severity of the problem, and the effectiveness with ~Jhich the bubbles need S to be stripped. The diameter o~ the fiber is also a consideration alt~ough that typically cemains in the range of 50 to 500 microns. We have suscessully used a baffle with an opening of 80~ ~m to coat a fibe{ with a nominal diameter of 125 ~m.
An important consideration in ~he choice of the size o t~e opening in the baf~le is ~he pressure g~adient ~eveloped in the 1uid in the vicinity of the bafle opening. ~e believe that the hydrodynamic for~es generated using a baffle in the fi~er coaSing arrangemen~ described are eeective in stripping bubbles on or near the iber sur~ace and preventing the inclusion o~ these bubhles in ~the final coating. Thus the baf~le,-depending on its design, ~ay serv~ two important functions. The ~i~st, as men~ioned, is the prevention o accumulation of copious quantities of bubbles in the 1uid reservoir, ~hus reducing s~oradic lateral physical displacement of the fiber during the coating operation, and attendant lac~ of concentricity o ~he fiber and the coating. The second advantage of the baf~le, which is regarded as optional depending on the 2S choice of the process designer, is the stripping o bubbles from t~e surface, or from near the surface, of the fiber prior to reaching the exit die. The exit die can be considered a form of baffle in the sense used here, and is itself useful ~or stripping bu~bles in the manner just described. However, we have found that the single stripping stage represented by the exit die ty-pically does not eliminate bubble inclusions in the final coating when operating at high coating speeds. A separate bubble s-tripping s~ta~e, as described here, is an importan~
addition in coating pro~esses where bubble inclusions are otherwise prevalent. Situations may even arise whece multiple, i.e., two or more, ba1es will be useful ~or more complete stripping.
It is evident that for most eEfective stripping, t~e opening in the baffle through which the fiber passes should be small enough to impede the flow o bubbles through the baEfle. Openings only slightly lsrger than the fiber are optimum rom this standpoint, and thus we recommend openings smaller than a few, i.e., three, times the iber diameter.
It should be apprecia~ed that large~ diameters are useful ~or a~hieving goal numb~r 1 described above, and are useul also for some degree of bu~ble stripping. We ~elieve that bubble stripping results rom abrupt hydrodynamic pressure changes as ~he iber t~averses the fluid. From our experiments and observations, we calcula~e that useul s-tripping of bubbles from the fiber occu~s if the iber expe~iences a pressure change equivalent to 300 psi/sec prior to reaching the vicinity of ~he exit die.
Thus ~he fiber, during this coating process, undersoes two distinct hydrodynamic pressure changes. The ~ressure change defined here is in terms of ~equivalent~ pressure change since the actual change in pressure is dependent on the design of the ~af1e and the rate of speed of ~he fiber. The minimum Oe 300 psi/sec quoted a~ove is calculated based on a fiber speed of 1/3 meter/sec and a 2~ bafle design in ~hich the pressure change occurs over a distance o approximately 1 cm. In this case -the pressure difference experienced by the fiber while passing through the baffle is of the order of 10 psi.
other baffle arrangements can be designed -to achieve results simi~ar to ~hose just discussed. Three advantageo4s configurations are shown in FIGS. 4, 5, 5 and 7. The baffle configuration of FIGS. 4 and 5 conorm somewhat to the flow ~attern of the fluid in the upæer chamber. ~he arrangement of F-IG. 6 affords the advanta~e of ease in threading ~e fiber ~hrough the reservoir during startup. The use of multiple~baffles is illustrated in FIG. 7.

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Having recognized the need to eliminate accumulation of bubbles in the coating fluid reservoir at a location that risks entrainment of the bubbles in the fiber coating, we devised the alternative of eliminating the S bubbles by circulating the coatlng fluid into and out o the reservois. In this manner one is effectively simply continuously replenishing the reservoir with bubble-free coating fluid. An arrangemenS for achieving this is shown in FIG. 8. This particular arrangement employs a combinatton o~ a bafle with recirculating means stripping bubbles from the reservoir. The baffle is shown at 80, the means for elL~inatlng the bu~bles ~{om the reservoir is outlet 81, and the means for introducing bubble-free coa~ing material is shown at ~2. Material from outlet 81 may be processed by filtering, centrifuqing vacuum degassing or the like to remove the bubbles, and then returned to ~he reservoir via inlet 82. ~he ba~fle aids in the efectiveness of this apparatus, but is not essential.
Ci~culating the fluid, with replenishment with bubble free mate{ial, can alone prevent accumulation of bubbles in the reservoir.
As mentioned at the outset, the presence of these rapidly moving bubbles in the coating 1uid physically dis~laces the fiber in an erratic manner and leads to poor centering of the fiber. The resulting coating is non-uniform around ~he fiber and this reduces the effectiveness of the fiber when used for lightwave transmission. Thus, the use of the baffle not only ~roduces a iber with a smooth, bubble-free coating, but one ~hat is well centered within the coating.
It s~lould ~e evident that continuous stripping of bubbles from the coatinq apparatus according to the teachings of the invention is an importan~ expedient leading to good quality coa-tin~s produced a~ high fiber - 35 dra~ing ra~es. The value o~ the invention cannot therefore be measured in terms o~ improve~en~s in the coating apparatus per se, bu~ is measured more appropriately in terms of an im~roved method for producing coated fiber waveguide.
The coating process of the invention is suitable for primary coating of glass or plastic waveguide, or for S secondary coating of such waveguide that is already coated.
The process may be applied to either or both steps of a dual- or multiple-coating in-line process or to any step of a sequential multiple coating process where the ~iber is reeled between coating steps. The process is also useful for the application of fluids to the fiber for surface modiication prior to coatin~, foc application of dyes or colorants for color coding, or applieation o ~luid materials for other purposes. It is also useful for eoa~ing films eomposed of matecials other than glass such as polymer fibers, crystal fibers and metal fibers.
Various additional modifieations and extensions of this invention will beeome apparen-t to those skilled in the art. All such variations and deviations which basi-cally rely on the teaehings through whieh this invention has advanced the art are properly eonsidered to be within the spirit and scope of this invention.

Claims (5)

Claims:

1. Method for the manufacture of coated fiber comprising the steps of: pulling the fiber from a fiber source, passing the pulled fiber through a reservoir of liquid coating material and thereafter through an exit die in the reservoir, the invention characterized in that the fiber is passed through the liquid coating material at a rate rapid enough to cause entrainment of bubbles where the fiber passes into the liquid coating material, and char-acterized further by the step of exposing the fiber to an abrupt hydrodynamic pressure change while the fiber is within the reservoir and prior to its entry into the exit die.

2. The method of claim 1 in which the pressure change is equivalent to at least 300 psi/sec.

3. The method of claim 2 in which the fiber is passed through the liquid coating material at a rate exceeding 1/3 meter/sec.

4. The method of claim 1 in which the abrupt pressure change is created by passing the fiber through a baffle.

5. The method of claim 4 in which the opening in the baffle is less than three times the diameter of the uncoated fiber.