CA1295892C

CA1295892C - Hermetic coatings for optical fibers

Info

Publication number: CA1295892C
Application number: CA000532631A
Authority: CA
Inventors: Satyabrata Raychaudhuri; Peter C. Schultz
Original assignee: Spectran Corp
Current assignee: Spectran Corp
Priority date: 1986-03-31
Filing date: 1987-03-20
Publication date: 1992-02-18
Anticipated expiration: 2009-02-18
Also published as: DE3789867D1; US4735856A; AU7287687A; EP0301030B1; WO1987005831A1; BR8707655A; EP0301030A1; EP0301030A4; JPH01501933A; DE3789867T2; ATE105740T1

Abstract

HERMETIC COATINGS FOR OPTICAL FIBER ABSTRACT A method for hermetically sealing a silica based fiber product after the fiber product has been drawn from a melt or preform wherein a hermetic layer of boron carbide or a ceramic compound having a density of above about 4 g/cc is deposited upon the fiber product. The fiber product can be initially coated with carbon or boron carbide prior to the application of the ceramic compound. Also, a method for making such fiber products using these methods, and the resultant hermetically sealed fiber products produced thereby.

Description

HERMETIC COATINGS FOR OPTICAL FIBER

Technical Field . . . _ The invention relates to methods for providing various hermetic coatings on silica-based fiber products and to the coated fiber products produced by such methods.

Background Art Mechanical failure of optical fibers under stress over extended periods of use is a phenomenon known as static fatigue. It is well known that bare uncoated fibers are susceptible to abrasion which results in surace scratches.
These scratches can produce a significant loss oE light through the sides oE the fiber and can also result in sudden ailure through breakage oE the Eiber. optical fibers are susceptible to breakage not only because they are formed from relatively brittle materials, but in addition, the fibers typically have very small diameters and are subjected to an assortment of stresses such as bending stresses, tensile stresses, and the like.
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Optical fibers are also susceptible to stress corrosion cracking. Water or hydroxyl ions can react with a fiber under stress, thus damaging its optical proper-ties and weakening its mechanical strength and static Eatigue resistance. Microcracks in fiber surfaces present regions which are susceptible to attack by hydroxyl ions or moisture when the fiber is under stress. Such stress tends to open these~cracks thereby straining the chemical bonds at the tips of the cracks. These strained bonds are readily attacked by moisture enabling the phenomenon called stress corrosion cracking to proceed and propagate such microcracks~ Growth Oe these microcracks weakens the ~fiber continuously over a . :
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period of time until it produces sudden failure. As mentioned above, this problem is termed static fatigue.

One solution to the problem of static fatigue is to pu-t an impervious hermetic coa-ting around the fiber, so that the atmospheric moisture cannot reach the fiber surface. Various types of coatings have been investigated. A metallic seal of aluminum has been suggested as a hermetic coating ("Reduction in static fatigue of silica fibers by hermetic jacketing" -Pinnow, Robertson, Wysoski - Appl. Phys. Lett. 34~ January 1979), however, metals tend to form polycrystalline solids which can themselvés be corroded by moisture or by enhanced grain boundary diffusion. Metal coatings also provide undeslrable electrical ~aths alon~ the fiber.

Several non-metallic coatings have also been utllized.
For example, silicon nitride (U.S. Patent No. 9,028,080 to DiVita et al) has been investigated as a potential coating, but silicon nitride has been seen to weaken the fiber substantially due to residual stress in the coating. Also, it is difficult to make strong Eibers in long lengths with silicon nitride. Pyrolytic carbon has also been suggested in U.S. Patent No. 4,183,621 to Kao et al and sputter deposited carbon was suggested by Stein et al ("Ion plasma deposition of carbon-indium hermetic coatings for optical fibers" -Proceedings oE ConEerence of Laser and Electro-Optics, Washington, D.C., June 10-12, 1982). But in both cases, the crack velocity exponent N of the coating were determined to be in the range of 23-30. This means that such coatings are really not hermetic. It has recently been reported that silicon carbide is a good hermetic coating (U.S. Patent No.
~ 4,512,629 to ~anson et al). The results of investigation `' with silicon carbide indicate a N value o~ 100 or higher can ~ be obtained wlth good median strength.
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Summary of the Invention One aspect of the invention relates to a method for hermetically sealing silica based fiber products which ha~e been drawn from a melt or preform, which comprises coating the products with a hermetic layer of at least one ceramic compound having a density above 4 g/cc. Typical silica based fiber products include optical fiber or capillary tubes. The ceramic compound can be a transition metal carbide, nitride, oxide, boride, or mixtures thereof. Two or more of these compounds can be codeposited, or two or more layers oE
different ceramic compounds can be deposited.

PreE~rred ceramic compounds incJude carb~des such as tltanium carbide or chromium carbide, nitrides such as tungsten carbide, titanium nitride or chromium nitride, refractory metal oxides such as tungsten oxide, chromium oxide, or borides such as tungsten boride, titanium boride or chromium boride. Boron carbide has also been Eound useful in this invention despite that it has a densi~y oE only 2.5 gjcc. A preferred technique for coating the product with any of these compounds is by chemical vapor deposition.

~n alternate embodiment for hermetically sealing such products includes an additional step o initially coating the fiber products with a layer oE carbon or boron carbide prior to depositing thereupon a ceramic material of a carbide, nitride, oxide or boride. When compounds such as silicon carbide, silicon oxynitride, boron nitride, or the compounds mentioned above are deposited over an initial carbon layer, it was Eound that the carbon layer provides increased strength to the overall coated product. Since boron carbide has a similar density to that of carbon, it is believed that similar advantages would be provided when boron carbide is used as the initial iayer.

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The invention also relates to methods for making such hermetically sealed silica based fiber products, as well as to the sealed products produced thereby.

Brief Description of the Drawings The nature, advantages, and various other additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with the accompanying drawing figures, wherein:
FIG. l is a schematic of the equipment needed to carry out one method of the invention;
FIG. 2 is a schematic of the equipment needed to carry out multiple layer deposition accord.ing to the inv~ntion;
FIG. 3 is a graphical illust:ration oE the optical a~lng ; characterlstics in hydrogen :eor the coated Eiber products of the invention compared to the uncoated fiber products of prior art;
FIG. 4 is a schematic of the test equipment needed to perform microbend kests; and FIG. 5 is a graphical illustration of the microbend test , results for fiber products coa-ted according to the invention :; 25 compared to the uncoated fiber products of the prior art.

Detailed Description of the Invention In the description that follows, the term silica based fiber product is used to denote elongated members comprised primarily of silica. This would include optical fibers of silica or other glasses~ as well as silica capillary tubes.
~: In:describing the preferred embodiments, reference will be made to optical fiber for convenience, although thé methods of the invention are useful for coating other fiber products.

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Furthermore, the basic coating operation utilizing a chemical vapor deposition process is well known in the art and all coatings described in this invention can be deposited by this process. While it is advantageous to use the chemical vapor deposition process, the invention is operable with a number of other processes, including sputtering, vacuum deposition, or the like.

BeEore depositing any coating on optical fiber, one has to go through the sequence of operations for a regular draw ; of fiber from a melt or an optical preform. This technique is well known in the art. Once the fiber draw process has been stabil;zed at the necessary draw speed, the process oE
coating the fiber with the ceramic compounds Oe the invention can be initiated.

FIG. 1 is a schematic diagram oE the experimental arrangement used for coating optical fiber with titanium 0 carbide. A chemical vapor deposi-tion (CVD~ reactor, in the form of a quartz tube placed inside a furnace with temperature control, is used to deposit a titanium carbide coating.

The first step in this process is to raise the temperature of the furnace to the reaction temperature -which is about 1000-1100C - under a small nitrogen purge.
The fiber is passed through the reactor and, once the desired temperature is achieved, nitrogen is passed through a bubbler containing 99.9% titanium tetrachloride. The bubbler is kept inside an oil bath maintained at 60C. Nitrogen containing Ti~14 vapors enter the reactorO Hydrogen is next introduced inside the reactor, followed by gaseous n-butane. The flowrates of differen-t components appear below. Diluent ~ nitrogen is also added to the reactant stream.

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The experimental conditions are as follows:
Temperature of reactor = 1000-1100C
Flow rates:
N2 thru TiC14 bubbler = 100-500 cc/min H2 = 300-1000 cc/min N-butane = 50-100 cc/min ; diluent N2 = 300-1000 cc/min Reactor Pressure = -0.02 to -0.03 inch H20 Fiber Draw Speed = 20 m/min There is one injection point of reactants to the CVD
reactor. Consequently, all the reactant gases are mixed in ; one line be~ore enterlng the reactor. This assures homogeneity oE the reaction mixture. The reactor i~
maintained under slightly negative pressure, so that all the particulates Eormed inside the reactor are continuously removed. The reaction products are then passed through a water scrubber before being vented to the atmosphere.
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After coating the fiber with a layer of titanium carbide, a UV cured acrylate coating may be applied online using conventional methods to provide protection during routine handling.

It lS also possible to provide an initial layer of carbon beneath this titanium carbide layer to provide strength enhancement for the coated iber. In addition, it has been found that this initial carbon layer provides ; 0 strength improvement for a variety of ceramic outer layers, including nitrides or carbides, for example, silicon oxynitride,~silicon carbide, tungsten carbide, boron nitride and the like.~ For the purpose of illustrating the invention, an outer~layer of titanium carbide over the initial carbon layer wi~ll be described hereinbeIow.
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When coating the fiber with a composite of carbon and a ceramic compound, the same experimental set-up is used, except that two CVD reactors are placed in series. This arrangement is illustrated in FIG. 2. The fiber is coated with a thin carbon layer in the top CVD reactor by cracking butane at a temperature of 1100C.

The carbon coated fiber then enters the bottom reactor where a layer of titanium earbide is deposited upon the carbon layer under same conditions described above. The initial carbon layer or undercoat substantially improves the strength of the fiber, as shown in Table I~ Both reactors are maintained at slightly negative pressures so that partieulates ean be eontinuously removed from the system.

The experimental eonditions Eor depositing the dual layer earbon/eeramic eoating is as follows:

Top CVD Reaetor (earbon eoating):
Temperature of reactor = 1100C
Flow rate n-butane = 50-100 cc/min Reactor Pressure = -0.02 to -0.03 inch H20 Bottom CVD Reaetor (titanium carbide coating):

Temperature of reactor = 1000-llOO~C
Flow rates:
N2 thru TiC14 bubbler = 100-500 cc/min H2 = 300-1000 cc/min N-butane = 50-100 cc/min diluent N2 = 300-1000 cc/min Reactor Pressure = -0.02 to -0.03 inch H20 Fiber Draw Speed = 20 m/min ; ~ 35 :

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After coating the fiber with the ceramic layer (i.eO, titanium carbide), a UV cured acrylate coating may also b0 applied for protection of the coated fiber during handling.

The properties of the coated fibers of the invention are shown in Table I and were determined by the inventors in the laboratory experiments. The hydrogen aging resistance for the titanium carbide coated fiber is shown in Figure 3.

TABLE I: PROPEE~TIES OF COATED E'IBER
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Coating Coating DFTl N Value H2 Draw Material Thic ness_ Stren~th ~ DlfEusion Data Speed SioN 500 A 2Q0-250 90 90~ lO m/min ksi resistance .. . ........... .......... _ __ ; C-SioN 500 A 300-450 70 90% resistance lO m/min ksi (estimated) ~ .. __ SiC2 500 A 500 ksi 90-95 _ lO m/min 20 - _ c2 200-250 A 650 ksi 23 30% lO m/min resistance ! (double coat) . _ C-TiC 200-250 A 500-600 90 90% lO m/min ksi resistance ...._ 25TiC 200-250 A 400-500 90-lO0 90% 30 m/min ksi resistance ___ - . ...................... ._ __ control uncoa~d 600-650 23 . 30 m/min Notes:

1. DFT means dynamic fatigue strength measured on an Instron tester using 0.5m gauge length fiber.

2. Prior art coatings, not according to the invention.
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The hydrogen diffusion data were determine by placing the fiber in a chamber containing one atmosphere of hydrogen at 20C and monitoring the optical losses of the fiber over time. The term "percent resistance" refers to the optical loss change at 1250 Nm for the hermetically coated fiber of the invention compared to uncoated fiber.

The values were developed using the microbend equipment illustrated in FIG. 4. Fiber from the drawing apparatus is directed around a two meter loop before passing through the microbend apparatus and thereafter, onto the fiber spools.
The microbend test procedure is as follows:

1. measure ~ttenuation of the fiber prior to microbendlng.
2. in~ert the ~iber into the microbend device, and then close the device.

3. add a 1 kg weight to the device.

4. measure the attenuation of the fiber.

5. repeat steps 3 and 4 until attenuation data for a total of lOkg has been obtained.

The induced attenuation is the difference between the attenuation measured under the load compared to the attenuation of the fiber with no load. This value is plotted against cumulative load, and results are shown in FIG. 5.

In lieu of fused silica as the optical fiber, it is possible to use other silica based glasses, such as borosilicate glass, alkali-lead silicate glass, or fused silica that has been doped with index altering elements, such as titanium, germanium, phosphorus, boron or fluorine. In lieu of the chemical vapor deposition method, one might use ~ 35 .:

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other coating techniques such as vacuum deposition, sputtering, ion-beam or RF or DC plasma. Although the figures illustrate the use of one or two CVD reactors, it is to be understood that the appropriate number of CVD reactors can be used in series for depositing the desired number of coating layers.

It is believed that the improved moisture resistance of the sealed fiber products results from the use of the high density ceramic compounds. Such high densities (i.e., above 4 g/cc) appear to act as very effective barriers to moisture and hydrogen diffusion, thus preventing moisture from contacting the tip of the crack. Such compounds also prevent hydrogen from penetrating the Eiber core thereby increasing attenuation. The densities of certain ceramic compounds are shown below in the ~rable II.

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Table II: Ceramic Compound Densities Compound Density (g/cc) :`
titanium carbide (TiC) 4.9 titanium boride (TiB2) 4.5 titanium nitride (TiN) 5.2 titanium oxide (Tio2 anatase) 3.8 chromium carbide (Cr3C2) 6.7 chromium nitride (CrN) 5.9 chromium boride (CrB) 602 chromium oxide (Cr2o3) 5.2 , .
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tungsten carbide (WC) 15.6 tungsten nitride (WN2)>10 ~estimated) tungsten oxide ~WO2) 12.1 tungsten boride (WB2) 10.8 silicon nitride (SiN) 3.2 silicon carbide (SiC) 3.2 boron carbide (s4C) 2l5 graphite (C) 2.2 Titanium oxide, silicon nitride, silicon carbide and graphite are shown as reference compounds because their densities are below 4 g/cc. Although the density of boron carbide does not Eall within the preferred range of greater than 4 g/cc, it has nevertheless been found to be effective in these eoating applications, whether deposited alone or upon an initial earbon layer. Also, it ls believed that boron carbide can be substituted for carbon as the initial layer before depositing other ceramic compounds thereupon with similar improvements in the strength of the overall coating.

According to this invention, preferred ceramic coating materials for direct deposition upon the silica based fiber . '~1:;
products are the transition metal carbidés, nitrides, borides, oxides or mixtures thereof, provided that these materials have a density of about 4 g/cc or greater. A
transition metal, as is known by one skilled in the art, is defined in the CRC Handbook of Chemistry and Physics, 65th edition (l9a4-s) in the periodic table which appears inside ;
the front cover, as the elements appearing in columns 3b, 4b, 5b, 6b, 7b, 8, lb and 2b.~ ~

The thickness of the drawn optical fiber usually ranges from about 50 to 1500 microns in diameter, while the : : :
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' thickness of each inorganic coating can range from 100 to 1000 Angstroms. Fused quart~ capillaries usually are drawn as 500 to 700 micron diameter, thick wall hollow tube. These are used in gas chromatography equipment, and by coating such capillaries, improved resistance against the high temperature, corrosive environments of such equipment is achieved. Thus, such coated capillaries provide increased resistance against breakage due to static fatigue.

The method of the invention is advantageous in that it enables the fiber to be hermetically sealed by an inorganic or ceramic coating which is not subject to oxidation or corrosion, and which is impervious to moisture, ~ases and chemically corroslve environments. Moreover, these coatings have a relatively low thermal coe~Eicient o~ expansion which ls compatible with the thermal expansion coefficient oE the optical fiber. This results in hermetically sealed optical fibers having good thermal shock resistance.

The initial layer of carbon provides a substantial increase in the median strength of the fibers, even when the total coating thickness is between 100 and 1000 Angstroms.
As shown in Table I, an increase in median strength by about 100 ksi can be achieved with this initial carbon layer.
Similar improvement is expected with the use of boron carbide as the initial layer.

While tungsten, titanium or chromium carbides, nitrides and borides are pre~erred for direct deposition upon the optical fiber, the initial deposition of a carbon layer enables improved strength to be obtained when the outer layer is any one of the compounds mentioned above, or other ceramic compounds including carbides such as silicon carbide, nitrides such as boron nitride or silicon oxynitride, or various refractory metal oxides.

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When the ceramic compounds of -the invention are directly deposited upon the optical fiber, it is also possible to co-deposit two or more oE such compounds, if desired for particular applications. Two or more o~ these compounds can be deposited using a single CVD reactor or, as mentioned above, the use of two or more of such reactors in series.
The single CVD reactor will enable mixtures of these compounds to be deposited, while the use of a plurality of such reactors in series will enable a plurality of different ceramic layers to be deposited upon the fiber.

As mentioned above, the compounds disclosed in this invention can also be used to coat quartz capillary tubes, as well as silica based optical fibers of any cross s~ctional sha~e (i.e. round, square, trian~ular, rectanslulart ~tc.).

While it is apparent that the invention herein disclosed is well calculated to fulfill the objects above stated, it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the artj and it is intended that the appended claims cover all such modifications and embodiments as fall within the true spirit and scope of the present invention.

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Claims

-14- The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for hermetically sealing silica based fiber products after said fiber products have been drawn from a melt or preform, which comprises coating said fiber products with a hermetic layer of at least one ceramic compound having a density of above about 4 g/cc.

2. The method of claim 1 wherein the ceramic compound is a transition metal carbide, nitride, boride, oxide, or mixtures thereof.

3. The method of claim 2 wherein the ceramic compound is tungsten carbide, tungsten nitride, tungsten boride, tungsten oxide, titanium carbide, titanium nitride, titanium boride, chromium carbide, chromium nitride, chromium boride, or chromium oxide

4. The method of claim 1 wherein said fiber products are coated with said ceramic compound by chemical vapor deposition.

5. The method of claim 1 which further comprises initially coating said fiber products with a layer of carbon or boron carbide prior to applying the layer of at least one ceramic compound.

6. The method of claim 1 wherein two or more of said ceramic compounds are codeposited as mixtures upon said fiber products.

7. The method of claim 1 wherein two or more layers of different ceramic compounds are deposited upon said fiber products.

8. A method for making a hermetically sealed silica based fiber product which comprises:
drawing a silica based fiber product from a melt or preform; and coating said fiber product with a hermetic layer of at least one ceramic compound having a density of above about 4 g/cc as to form a hermetically sealed optical fiber.

9. The method of claim 8 wherein the ceramic compound is a transition metal carbide, nitride, boride, oxide, or mixtures thereof.

10. The method of claim 8 wherein the ceramic compound is tungsten carbide, tungsten nitride, tungsten boride, tungsten oxide, titanium carbide, titanium nitride, titanium boride, chromium carbide, chromium nitride, chromium boride, or chromium oxide.

11. The method of claim 8 which further comprises initially coating said fiber product with a layer of carbon or boron carbide prior to applying the layer of at least one ceramic compound

12. The method of claim 8 wherein two or more of said ceramic compounds are codeposited upon said fiber products.

13. The method of claim 8 wherein two or more layers of different ceramic compounds are deposited upon the initial carbon or boron carbide layer.

14. A method for hermetically sealing silica based fiber products after said fiber products have been drawn from a melt or preform, which comprises:
initially coating said fiber products with a layer of carbon or boron carbide; and further coating said carbon coated fibers with at least ceramic compound.

15. The method of claim 14 wherein said ceramic compound is a carbide, nitride, boride, oxide, or mixtures thereof.

16. A method for making a hermetically sealed silica based fiber product which comprises:
drawing a silica based fiber product from a melt or preform;
initially coating said fiber product with a layer of carbon or boron carbide; and further coating said carbon coated fiber product with at least one ceramic compound.

17. The method of claim 16 wherein said ceramic compound is a carbide, nitride, boride, oxide, or mixtures thereof.

18. A method for hermetically sealing silica based fiber products after said fiber product have been drawn from a melt, which comprises coating said fiber products with a hermetic layer of boron carbide.

19. The method of claim 18 which further comprises initially coating said fiber product with a layer of carbon prior to depositing the layer of boron carbide.

20. The sealed fiber product produced by the method of claim 1.

21. The sealed fiber product produced by the method of claim 8.

22. The sealed fiber product produced by the method of claim 14.

23. The sealed fiber product produced by the method of claim 16.

24. The sealed fiber product produced by the method of claim 18.

25. A silica based fiber product having an encapsulating hermetic coating comprising at least one ceramic compound having a density above about 4 g/cc.

26. The fiber product of claim 25 wherein the ceramic compound is a transition metal carbide, nitride, boride, oxide, or mixture thereof.

27. The fiber product of claim 26 wherein the ceramic compound is tungsten carbide, tungsten nitride, tungsten boride, tungsten oxide, titanium carbide, titanium nitride, titanium boride, chromium nitride, chromium boride or chromium oxide.

28. The fiber product of claim 25 wherein the coating comprises two or more of said ceramic compounds which are codeposited as a mixture.

29. The fiber products of claim 25 wherein said coating comprises two or more layers of different ceramic compounds.

30. A silica based fiber product having an encapsulating hermetic coating comprising a layer of boron carbide.

31. A silica based fiber product having an encapsulating hermetic coating comprising a first layer of carbon or boron carbide and a second layer of at least one ceramic compound upon said first layer.

32. The fiber product of claim 24 wherein the ceramic compound of the second layer is a carbide, nitride, boride, oxide, or mixtures thereof.