CA1191029A - Reduction of strength loss during fiber processing - Google Patents

Abstract

REDUCTION OF STRENGTH LOSS
DURING FIBER PROCESSING

Abstract Glass fiber strength is improved by removal or avoidance of water derived species within the fiber such as result from exposure of coated or uncoated fiber to ordinary air. Of particular significance is retention of greater strength during a step following thermal processing such as during splicing. Strength improvements result from a procedure, e.g., protection of fiber to avoid exposure to water-bearing ambient or use of medium which chemically alters such water-derived species in the fiber. (FIG. 2)

Description

~E~UC'I`lON O~ ST~NGT~1 LO~S
~UKIL~'G ~'IBLR P~O~SSING

~acKyround of tne ~ ention -l. Field of the Inverltion ~ e ir-vention is concer1led Witi1 glass fiber oE a form referred to dS "silica" based. E~iber of conse~uer1ce fro1m tne standpoil1t ot ~1lis invenLiorl is ottnerwise irnbued ~i~h certdln cnaracteristics assvciated witt~ optical grade 10 Illaterlal used in con~ unications. Characteristics so ~ lied in~lude a "~risti1le" strength, thdt is a tensile strenytr1 uyon drawiny, wnich is typically at a level of 41.37 x l0~Pa (60~,000 p~i) or hi~her.

2. De~cription of tne Prior Ar~
'1'he rapid growth of fiber lightwave com1nunicatior1s from its be~innings a decade ago to a signiflcant indusLry is well documented. Terrestrial and underwater syste1ns, some base on 1i1ultimode fibers, some on single 111ode, carryir1y voice as well as data are in com111ercial use~ More arnbitious plans are in the formati~e stages, and it is reliably estifl~ated that optical fioer will largely replace copper conductor and become the domir1ar1t factor in communications.
Man~ of ti1e initial problems in fiber ~5 fabrication: coatiny, sneathin~3, spliclr1g, etc., have been overcome. Fibefs in regular ~ro~uction today are cnardcterized by low insertion loss, high bandwidtt1, high stren9t1l, and generally in characteristics that already result in eco1lomic advantage over traditional rra1lsinission :~0 Inedia~
As the technology matures attention is being ~irecte~ to secondary problems, some concerned with economic considerations such as fiber yield and throughput.
~th~rs concerr- mecr1anical ~roperties of particular interest ~5 duriny inst~llatio1l. E~Or example, ~ensile streng~h is a particularly slgnificar1t parameter during layiny of 2~

submarine cable from shipboard.
It has been recognized for some time that pristine fiber strength characteristically at a level of 55.16 x 108Pa (800,000 psi) is substantially reduced in practical cabled fiber. Studies have indicated that some part of tensile strength loss is associated with high temperature processing. Even before optical fiber became a practical reality it had been observed that annealing, expected to increase strength, in fact resulted in strength loss. See, for exampie, Proceedin~ _ the Royal Society of London, Vol. 297, pp. 534-551, 1967. The conclusion reached in that reference assigned loss of strength to surface contamination by dust particles.
Subsequent experience continues to show loss in strength with thermal processing. Studies attributed strength loss to a variety of causes, e.g., surface damage from coating and handling, localized devitrification from surface contamination, as well as mechanical strain. See 17 Electronic Letters, page 23~ (1981) and references cited therein.
The situation was clarified early in 1981 when it was shown that tensile strength values below about 27.58 x 108Pa (400,000 psi) were the result of mechanical handling.
Electronic Letters, Vol. 17, pp. 232-233 (1981). This work attributes major loss to mechanical stripping of the usual organic coatings normally produced as an in-line step following fiber drawing. Usual organic coatings must be removed to permit thermal processing at temperatures at which such coatings degrade. The problem is complicated by the fact that as-drawn fiber is not of uniform strength resulting in the practice of removing weak spots and fusion splicing to join remaining sections. Fusion splicing is a form of thermal processing which itself degrades strength.
The need for splicing i5 graphically illustrated by consideration of distributing of strength values. It is convenient to refer to pristine fiber strength at, for example, a nominal value of 55.16 x 108Pa (300,000 psi).
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in fact, this strength is a median strength with distribution including as much as lO percent of fiber length at less than 13.79 x 103Pa (200,000 psi) as tested on lengths greater than 5 ~ilometers. ~imilarly, nominal splice strength of 27.58 x 108Pa (400,000 psi) resulting from elimination of poor mechanical handling practice is also a median value with a distribution of perhaps 1 percent of all splices at 6.89 x 103Pa (300,000 psi).
In general, industry has yet to fully appreciate and to incorporate the most recent findings. As a result, fiber systems are generally designed for fiber strengths of the order of 6.89 x 108Pa ~lO0,000 psi), a value well below nominal pristine strength. Annealing processes which show promise for substantial strength recovery have gener-ally not been used commercially, to some extent due to lack of reproducibility and sometimes to actual strength loss.
Summary of the Invention According to the invention there is provided preparation of glass fiber which is permissibly coated with a water-pervious coating said glass fiber having a glass surface composed of at least 95 weight percent silica and having a tensile strength of at least 600 ksi as drawn in which said preparation includes thermal processing defined as resulting in an attained elevated temperature at least for a portion of the said fiber with said elevated temperature equal to at least lO0 degrees C for a temper-ature and time equivalent to 600 degrees C for a period of at least lO seconds in accordance with an Arrhenius relationship characterized in that a procedure is introduced to lessen water-derived species on said fiber at least within the said portion at least during final thermal pro-cessing for at least that interval of time over which the said portion is heated under the said temperature-time conditions except that any portion undergoing viscous flow during such final thermal processing need be exposed to ,,.v ~

- 3a -said procedure only during that interval within which the said temperature-time conditions are met subsequent said flow said water-derived species being defined as molecular water or other product present in said ~iber resulting from exposure to molecular water.
Improved fiber strength results from avoidance of fiber contact with water or by removal of water-derived species resulting from exposure. Strength improvement is the result of reduced thermal degradation, that is, degradation in strength at a temperature-dependent rate.
Latent damage results even from short term exposure of coated or uncoated fiber to ordinary ambient air. Since damage due to exposure proceeds most rapidly at elevated temperature, it is important that fiber be protected at all times prior to thermal processing.
The alternative of removin~ water-derived species is Eor the same reason most desirably practiced prior to attainment of maximum elevated temperatures during thermal processing. Since expedient removal is a kinetic process, it is most effectively carried out at elevated temperatures.
It has been found that glass flow conditions, as during fusion, heal "thermally damaged" fiber. While it is expected that commercial practice will entail protection of fiber continuously from fabrication at least through final thermal processing, protection may be omitted prior to attaining flow conditions.
Fiber protection may take the form of water exclusion, e~g., by use of vacuum, protective inert gas, or dried ambient, or alternatively, use may take the form of gaseous chlorine SOC12, HCl, or other medium which chemically displaces water or water-derived species in the fiber. Removal of water-derived species depends again on use of a medium which displaces water-derived species. A
prime example is chlorine. Similar results may be obtained by relatively long term annealing under dry conditions, e.g., in vacuum.
The invention is concerned with avoidance or removal of water-derived species (which constitute latent strength degradation). It is not directed to recovery of strength in already degraded fiber. Since degradation is a temperature dependent phenomenon, desired term of protection may depend upon conditions of use. Strength requirements sufficient to permit installation, e.g., of submarine cable, may result from water exclusion or water-derived species removal, as described. Long term strength may require continued protection during use. Assuming proper practice of the invention during processing, long term strength is assured, for example, by metal or other impervious coating or in properly designed filled or protective cable structures. An alternative coating may take form of "low melting point" inorganic glass which may be self-healing at temperatures of interest. Examples include chalcogenide glasses.
An important aspect of the inventive teaching involves so-called "pristine" strength values. It has been commonly assumed that median strengths of 55.16 x 108Pa (800 ksi) measured on "as drawn" fiber represents the inherent strength of the fiber. In fact, normal fabrication includes drawing in air thereby resulting in strength degradation -- some as measured upon drawing, so~e latent to result in further degradation at a rate depending upon temperature. Exclusion of water during drawing results in improved strength. Real fiber strengths in excess of 68.95 x 108Pa (1,000 ksi) have been measured. Other experiMents serve as the basis for estimations as high as 137.9 x 108Pa (2,000 ksi). Ul~imate fiber strength re~uires total or near total exclusion of "water-derived species". Normally such species results from exposure to water. Other possibilities include -OH introduction from organic coatings as during crosslinking.
Brief Description of the Drawinq Figures 1 and 2, on coordinates of probability of failure (upon tensile testing) and strength in units of thousands of pounds per square inch (ksi) [1 ksi =
~,894,757Pa] r depict data for fiber: as drawn (FI~. 1, curve 1; FIG. 2, curve 10), as conventionally spliced with mechanical stripping (FIG. l, curve 2), as thermally processed without the application of the inventive teaching ~FIG. 1, curve 3), as spliced with chemical stripping (FIG.
2, curve 11), and finally, fiber as treated to remove water~derived species during splicing (FIG. 2, curve 12).
Detailed Description 1. GLOSSARY - While all terms used in the description of the invention are known, it is useful to assign specific, sometimes more quantitative meanings.
1. ~ Silica Fiber - This term which encompasses fiber, desirably processed, in accordance with the invention, is characterized by a surface under or before any protective coating which contains at least 95 percent by weight silica, whether physically admixed or chemically combined. For these purposes, "surface"
connotes a thickness corresponding with total surface roughness in turn definng the maximum depth of contact by ambient gas. Typical thickness is a micrometer or less.
Usual communications-grade optical fiber structures include core regions which may contain as much as ~0 weight percent or more of germania or other dopant material with silica, and clad regions of silica sometimes containing small amounts of dopant such as fluorine or boron oxide.
While communications-grade fiber certainly con-stitutes the aLea of greater significance from the inventive standpoint, the invention is successfully practiced on fiber designed for other purposes, for example on reinforcing fiber to be included in plastic composites.
2. High Stren~th Fiber - Fiber of a pristine median strength of at least 41.37 x 103Pa (600 ksi).
Contemplated fiber has been drawn and permissibly coated.
Coating as contemplated is generally by organic polymeric material. Permissible coatings may be cured as by irradiation cross-linking. Some types of commercial optical fiber are coated with two or more layers having differing elastic moduli.
Strengths reported in this description may be obtained from failure on direct ~dynamic) tensile testing or on proof testing which involves winding from one drum to another under prescribed tension. Appropriate procedures are described in Optical Fiber Telecommunications, Academic Press, Inc., 1979, edited by A.G. Chynoweth and S.E. Miller, Chapter 12.
In general, fiber of lesser pristine strength is not beneficially treated by the invention since failure is dominated by other mechanisms.

3. Thermal Processing - Processing in which the fiber, or at least some surface region of the fiber, attains a temperature of at least 600 C for a period of at least 10 seconds or equivalent. The temperature-time relationship if Arrhenius 1 in K so that an temperature equivalent time for a temperature of 700 C is 50 milli-seconds. In fact, real splicing generally requires tem-peratures approaching 2000 C at times of 10 seconds.
Experimentally, it has been determined that these conditions reult in maximum damage. (Further increase in time does not result in greater damage).

4. Fusion Splicing - Procedure by which fiber ___ ends are joined by contacting at sufficiently high X

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temperature to result in flow and finally by joinder upon cooling. Sufficient flow occurs within a period no greater than a few minutes. For pure silica, splicing is accom-plished at a tempera~ure of 1800 C or higher. In general, high silica fiber as contemplated attains a temperature of at least 1800 C, at least at the surface during splicing.
Fusion splicing may be accomplished by a variety of tech-niques, for example, by torch, arc or laser heating.

5. Fiber - Elongated body of a major cross-sectional dimension -- usually diameter of up to 300 micrometers. Usual communications fiber has a thickness of from 100 to 2no ~m. Strength fiber may be somewhat thinner, for example, of a diameter of 50 micrometers or less. Fi~er may be manufactured by a number of methods usually involving drawing from a preform body which in turn may be produced by deposition within a tube, e.g., by CVD or MCVD, or by deposition of hydrous particles ("soot"), by deposition on the outside or end of a rod. An alternative involves continuous formation from a melt as by double crucible.
Strength member fiber is sometimes produced by drawing from a single crucible.

6. "Water-Derived Species" - Chemically detectable compositional change observable at a glass fiber surface by infrared spectroscopy due to exposure to water vapor.
Detected species include molecular water, isolated Si-OH, hydrogen bonded Si-OH. Description is here in terms of "adsorption" by silica, the main constituent of the glass fiber at the surface. Inclusion by ingredients other than silica within the specified 5 weight percent limit may result in other species.

7. Water-Derived Stren~_ - Tensile strength resulting from thermal treatment as defined for fiber containing water-derived species without application of the inventive procedure. the same strength may result after longer times at lower temperature. Such strength to be meaningful is measured on fiber which has not been subjected to mechanical damage, for example, damaged by mechanical stripping. Water-derived strength is typically at a median value of about 27.58 x 103Pa (400 ksi).
Strengths of a median value of less than about 20.68 x 108Pa (300 ksi) indicate substantial mechanical damage and do not benefit substantially from applicati~n of the invention.
In a preferred embodiment the fiber is protected from water at least through final thermal processing so that the water-derived strength is not a measurable value.

8. Improved Stren~h Level - Medium tensile strength representing improvement over "water-derived strength" due to practice of the invention. Use of an inventive procedure on exposed fibers following thermal processing results in return to a 'Irecovery strength".
Recovery strength is at least 20 percent higher than water-derived strength. Typical values of improved strength are found to be at or above 34.47 x 108Pa ~500 ksi) and preferred species of the invention are so defined.

9 Threshold Temperature - That temperature .
attained during thermal processing which results in measur-able loss of strength in the absence of the invention. For these purposes time is assumed to be the shortest practical interval for such processing. This time may vary somewhat depending on the type of thermal processing; as an example, arc splicing perhaps one of the rapid processes contemplated requires three to four seconds to complete a splice as presently practiced. Strength loss, as well as improvement realized by the practice of the invention are measurable for thermal processing in which the fiber is maintained at 600 C for three to four seconds, although maximum strength loss at this temperature is bound to occur only after perhaps four minutes.

10. ~ Temperature - Temperature above which some part of strength loss exemplified by water-derived strength is decreased resulting in recovery of some X

part of the pristine strength. Again identifying a tempera-ture level implies a corresponding time. For temperatures effective in healing silica rich compositions (temperatures of 1500 C or higher) times as short as one second or less suffice. "Strength loss" is a fiction for aspects of the invention in which treatment avoids introduction of water-derived species.
3. The Drawing FIG. 1 on Coordinates of Probability of Failure (~) on the ordinate and fracture stress in ksi (1 ksi =
6894.757 x 103Pa) on the abscissa is a plot of data reflecting the relationship of these two parameters for three coated fibers which are identical in composition and cross-section, but have undergone different processing histories. Curve 1 contains data for fiber following usual drawing in air; that is for a fiber of "pristine" strength.
Curve 2 corresponds with a fiber which has been damaged by mechanical stripping of organic coating preparatory to fusion splicing. Curve 3 corresponds with a fiber which has been thermally processed (without application of the inventive teaching).
Median strengths for the three fibers are: 55.16 x 108, 4.83 x 108 and 27.58 x 108Pa (>800 ksi, 70 ksi, and 400 ksi), respectively. While distribution is characteristically different for the fibers as evidenced by the different slopes of the three curves, it is clear that meaningful strength loss results from mechanical handling (Curve 2). Loss for fiber which is thermally processed while avoiding mechanical damage also shows a statistical strength loss which is quite significant. Distribution is tightest for pristine fiber (Curve 1). From Curve 2, it is seen that loss due to thermal processing of mechanically damaged fiber is largely the result of mechanical damage itself. The median strength of approximately 4.83 x 108Pa (70 ksi) is not significantly reduced upon thermal processing following mechanical damaging.

FIG. 2 again contains three Curves; Curve 10 is identical to Curve 1 of FIG. 1. Again median s~rength is 55.16 x 108Pa (800 ksi). Measurements were again made on fiber which had been coated with an organic composition by an in-line process before reeling. The fiber of Curve 11 has a median strength similar to that of Curve 3 of FIG 1.
The fiber of Curve 11 had been thermally processed following stripping in hot concentrated sulfuric acid during which mechanical stripping was avoided. Curve 12~considered exemplary of processes in accordance with the invention is plotted from values of recovery strength. For one set of experiments such recovery strength represents a real improvement from the median strength of the water-derived strength valvue of FIG. 11 [median strength about 27.58 x 10 Pa (~00 ksi)] to a median value of about 41.37 x 108Pa (600 ksi). In this set of experiments the difference between the values represented by Curves 12 and

11 resulted from inclusion of chlorine during oxyhydrogen torch fusion splicing.
4. Inventive Procedure (a) General - Extensive study has characterized the relative parameters of and resulting strength loss due to thermal processing. Discussion in these terms is useful for purposes of description but is not limiting. For example, as discussed, an important aspect of the invention involves substantial avoidance of water and therefore of such loss so that fiber being processed has never been reduced to its water-derived strength (strength of exposed fiber resulting from thermal processing.
Strength loss to which the invention is directed occurs at a temperature of at least threshold value under conditions which are noted. Since some substantial part of the damage may be healed by attainment of higher tempera-tures, damage may be greatest only over fiber regions maintained with certain temperature minima and maxima (corresponding with threshold value and flow). In an exemplary process, e.g. fusion splicing, maximum X

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temperature is, by definitiont sufficient to bring about flow. Damage prior to fusion is retained only for cooler fiber portions (although exposure to air during cooling introduces damage -- usually maximum damage -- at the position of the splice).
For certain types of thermal processing, parti-cularly where heating of substantial fiber length is essentially uniform, points of greatest stength loss are randomly located while for other types of thermal processing maximum strength loss occurs for fiber positions corres-ponding with temperatures between noted minima and maxima.
This latter category is described in terms of "edge" effect.
This most significant strength loss -- usual point of failure -- occurs at some location remote from the position lS of highest temperature attainment during thermal processing.
Regardless of the form of thermal processing damage to which the invention is directed occurs at or above some threshold value as defined in the glossary. For usual purposes this threshold value is 600 C. All processes in accordance with the invention require treatment of all fiber regions which attain threshold temperature during thermal processing. Continued strength require continued protection, as well.
Treatment in accordance with the inven~ion, e.g., use of chlorine during fusion splicing is generally continued through the entirety of the thermal processing in which the concerned regions of the fiber are at or above threshold although inventive treatment may permissibly be discontinued for regions of the fiber during attainment of temperatures sufficient to result in healing and if healing is adequate, for the same regions prior to attainment.
The inventive procedure invariable takes a form resulting in lessening of introduced water-derived species.
From strength measurements it has been found that exposure of fiber at any normally encountered ambient conditions, i.e., to air at room temperature leads to deterioration g

- 12 -upon thermal processing. Relative humidity values as low as 10 percent or lower results in measurable strength loss upon thermal processing. Procedures in accordance with the invention contemplate avoidance of or lessening of introduced water-derived species and are best implemented by continuous protecting of fiber from drawing at least through attainment of threshold temperature durinq final thermal processing (and desirably beyond~. In usual processing, treatment by an inventive process is initiated before or on attainment of threshold and is continued through cooling to threshold.
Exame~
Examples set forth below are chosen to be of comparable structure processiny history. Fibers are of the general structure and composition of commercial interest, iOe.l are high strength, high silica of outer diameter from 50 to 300 ~m. Some fibers were communications-grade structures with high index core regions. Others were uniform compositions test structures.
Strength improvement realized in accordance with the invention has not been found dependent on such inner structure provided, of course, that the fiber is of a pristine median strength of 41.37 x 10 Pa (600 ksi) thigh strength fiber as defined).
Examples 1 and 2 were conducted on communications-grade mul~imode graded index fiber of an outer diameter 125 um with an outer surface fo better than 99 percent by weight silica and with a core region of approximately 55 ~m diameter, graded with germania, to a maximum value of about 15 weight percent and having a "pristine strength", defined as the tensile strength realized upon drawing in air, o~
approximate~y 55.16 x 108Pa (800 ksi). In both examples, splices were produced by fusion splicing using an oxyhydro-gen torch with maximum attained temperature about 2000 C
as indicated by an optical radiation pyrometer. In both instances, organic coating produced by an in line coating procedure was chemically removed without abrading for a

- 13 -distance of approximately 2 cm. Splicing was accomplished in the usual manner by butting the unheated fiber ends and heating with splicing occurring in a period of between 5 and 30 seconds.
Example 1 Fiber, as described above, spliced in a conven-tional oxyhydrogen torch operated in air, was found-to have a median strength of approximately 27.58 x 108Pa (400 ksi), with a distribution of the form shown in FIG. 1, Curve 3.
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The procedure of Example 1 was repeated, however, with an outer mantle of chlorine affixed during splicing.
The heat zone in Example 2 (as in Example 1) as defined by visible radiation extended for a distance of approximately 2 mm in each direction from center (defining an overall heat zone of approximately 4 mm). This zone also defined the fiber region at or above threshold temperature (600 C).
The chlorine mantle contacted the fiber over a region at least equal to the heat zone. Tensile strength, as measured dynamically, was at a median level of 41.37 x 10 Pa (600 ksi) with a distribution as shown in FIG. 2, Curve 12.
Example 3, 4 Fiber as in Examples 1 and 2, was spliced by means of a commercial 3 watt laser (CO2). Continuous heating times were of the order of 5 seconds duration. Splicing was attained with a single duration. Resulting median strengths were 27.58 x 108Pa (400 ksi) and 41.37 x 108Pa (600 ksi) without and with a heated chlorine mantle, respectively. The chlorine was heated to a temperature of about 600 C to simulate conditions inherent to oxyhydrogen splicing. Unheated chlorine was essentially ineffectual for this very brief splice time for the specimen fiber which had been exposed during drawing.
Example 5 Fiber was drawn from a 2 mm diameter silica rod by use of a laser resulting in a temperature of X

approximately 2000 C in a vacuum chamber (100 ~m) (so that air was excluded during the entire temperature tranverse from maximum to room temperature). Fiber diameter was from 60 to 120 ~m. The fiber was then reheated without having previously been exposed to air to approximately 1700 C, again with a vacuum applied over the entire temperature range during cooling to approximately room temperature~
Tensile strength measurement in air resulted in retention or pristine strength.
Example 6 The procedure of Example 5 was repeated, however, with reheating in air resulting in a tensile strength of approximately 24~13 x 108Pa (350 ksi).
Example 7 Example 6 was repeated, however, with fiber drawn in air resulting in a substantially unchanged pristine strength as drawn [approximately 55.16 x 108Pa (800 ksi)]
but in reduced strength upon testing following reheating [(approximately 24.13 x 108Pa (350 ksi) ] .
Exam~le 8 Example 7 was repeated, however, with reheating conducted in vacuum. Final strength realized after reheating was approximately 2~.13 x 108Pa (350 ksi).
Example 9 --Fiber of a diameter from 150-250 ~m was drawn from 2mm rod of fused silica of purity of at least 99 weight percent by use of an oxyhydrogen torch resulting in a maximum attained temperature of at least 2000 C.
Fiber was tensile strength tested as drawn to result in a measured value of at least 55.16 x 10 Pa (800 ksi).
Example 10 Fiber as produced in Example 9 was reheated to a temperature of approximately 650 C in air for a period of about 30 minutes resulting in a water-derived strength of approximately 32.41 x 108Pa (470 ksi).

Example 11 Fiber as drawn in Example 10 was subsequently heated in a vacuum of approximately 100 ~m to a temperature of from 750-850 C for a period of about 30 minutes resulting in a minimum recovery strength of approximately 46.54 x 108Pa (675 ksi).
Example 12 Example 11 was repeated except that final heating was conducted in a chamber which had first been evacuated and then backfilled with chlorine. Temperature and time were as set forth in Example 11. Measured recovery strength was approximately 46.88 x 108Pa (680 ksi).
Example 12 Example 2 was repeated substituting an HCl gas mantle for the chlorine mantle. Strength realized was agian approximately 41.37 x 103Pa (600 ksi).
Example 13 Example 1 was repeated, however, substituting a chlorohydrogen torch with a result that tensile strength realized upon splicing was approximately 41.37 x 10 Pa (600 ksi).

Conversion Fact _ 1 psi = 6894.757 Pa

Claims (19)

Claims

1. Preparation of glass fiber which is permissibly coated with a water-pervious coating said glass fiber having a glass surface composed of at least 95 weight percent silica and having a tensile strenght of at least 600 ksi as drawn in which said preparation includes thermal processing defined as resulting in an attained elevated temperature at least for a portion of the said fiber with said elevated temperature equal to at least 100 degrees C
for a temperature and time equivalent to 600 degrees C for a period of at least 10 seconds in accordance with an Arrhenius relationship CHARACTERIZED IN THAT a procedure is introduced to lessen water-derived species on said fiber at least within the said portion at least during final thermal processing for at least that interval of time over which the said portion is heated under the said temperature-time conditions except that any portion undergoing viscous flow during such final thermal processing need be exposed to said procedure only during that interval within which the said temperature-time conditions are met subsequent said flow said water-derived species being defined as molecular water or other product present in said fiber resulting from exposure to molecular water.

2. Preparation of claim 1 in which the said procedure consists essentially of exclusion of water in contact with the said fiber portion.

3. Preparation of claim 2 in which exclusion results from substantially reduced pressure.

4. Preparation of claim 2 in which exclusion results from use of a non-water containing gas.

5. Preparation of claim 2 in which said exclusion results from use of an ambient material which chemically displaces water.

6. Preparation of claim 5 in which said ambient material comprises gaseous chlorine.

7. Preparation of claim 1 in which said fiber is exposed to water-containing ambient prior to the said thermal processing and the said procedure comprises con-tacting at least the said portion to an ambient material which chemically alters said water derived species to thereby effectively reduce the amount of said species.

8. Preparation of claim 7 in which said ambient containing gaseous chlorine is maintained at a temperature of at least 600 degrees C for a period of at least 10 seconds or equivalent in accordance with the Arrhenius relationship.

9. Preparation of claim 1 in accordance with which said procedure is applied with the possible exception noted during every thermal processing.

10. Preparation in accordance with claim 9 in which said procedure is continued for a substantial period prior to the said thermal processing.

11. Preparation of claim 10 in which a said procedure is continuously applied to said fiber during that portion of fabrication which at least encompasses the interval from attainment of a temperature of 600 degrees C
upon cooling during drawing.

12. Preparation of claim 11 in which a procedure as defined is applied over an interval extending beyond the said final thermal processing.

13. Preparation of claim 12 in which the said pro-cedure at least during the interval following final thermal processing provides for a moisture-impervious coating in intimate contact with the said fiber as otherwise fabricated.

14. Preparation of claim 13 in which the said impervious coating is metallic.

15. Preparation of claim 14 in which the said impervious coating is in intimate contact with an organic coating.

16. Preparation of claim 12 in which the said procedure comprising encompassing the said fiber within a sheathing which is water-impervious.

17. Preparation of claim 16 in which the procedure comprises designation of a cable filling material.

18. Preparation of claim 1 in which the said thermal processing corresponds with fusion splicing.

19. product produced in accordance with the preparation of claim 1.