WO2001042851A1

WO2001042851A1 - Metallic glass hermetic coating for an optical fiber and method of making an optical fiber hermetically coated with metallic glass

Info

Publication number: WO2001042851A1
Application number: PCT/US2000/029213
Authority: WO
Inventors: Bruce G. Aitken; Ji Wang
Original assignee: Corning Incorporated
Priority date: 1999-12-07
Filing date: 2000-10-23
Publication date: 2001-06-14
Also published as: AU1340601A

Abstract

In one approach to manufacturing hermetically-coated optical fibers, a metallic glass jacket [22] is fabricated over a preform [26] having the desired core and overclad profile for the finished fiber. The jacketed preform is drawn to form the hermetically sealed optical fiber [34]. In a further embodiment of the invention, a freezing technique is used.

Description

METALLIC GLASS HERMETIC COATING FOR AN

OPTICAL FIBER AND METHOD OF MAKING AN OPTICAL

FIBER HERMETICALLY COATED WITH METALLIC GLASS

Background Of The Invention

1. Field of the Invention

The present invention relates generally to improvements in the field of optical fiber, and more particularly to hermetic coatings for optical fiber and methods for hermetically coating optical fibers.

2. Technical Background

There are currently being developed a variety of multicomponent and soft glass fibers, such as GeAs sulfide 1.3 μm, for use in a variety of devices, including optical amplifiers operating in the 1.3 to 1.5 μm wavelength range. These multicomponent and soft glasses are typically much more susceptible to moisture or water attack than fibers fabricated from silica. This susceptibility to moisture not only weakens these specialty fibers gradually over time, but can also have a detrimental effect on the performance of amplifiers using these fibers. This phenomenon has already been observed in silica fibers doped with erbium (Er). Thus, there is a need for a hermetic (i.e., airtight and watertight) coating to be applied to such fibers.

Currently, there are two basic types of hermetic coating materials that are used for silica fibers, namely inorganic and metallic compounds. Examples of the former include SiC, TiN, BN, SiON and C (carbon). These inorganic coatings are typically thin (on the order of 50-100 nm), and conventionally applied by chemical vapor deposition (CND) or a similar process performed in a reaction vessel as (or just after) the preform is drawn to form the optical fiber. Although inorganic hermetic coatings have been effectively applied to silica fibers and have shown good hermeticity, it is known that some inorganic hermetic coatings can weaken the strength of silica fibers by as much as one-third after coating.

Representative metals that have been used as silica fiber coatings include Al, Au, In, Νi, and Zn. These are applied using a so-called "freezing" technique (i.e., in which the coating is deposited onto a fiber by drawing the fiber through a heated metal melt). Most of these coatings are applied at relatively high temperatures (i.e., 1000°C or above). Thus, they cannot be easily applied to lower-temperature multicomponent and soft glass fibers. Further, metal-coated fibers exhibit substantial added optical losses. The cause of this detrimental effect is not yet precisely understood, but is generally believed to be related to two factors. First, the high application temperature causes a reaction at the interface between the coating and the fiber. Second, the non- homogeneous microstructure of the coating materials ~ as most of them are polycrystalline in nature — causes microbending loss and defects that weaken the fiber.

There is thus a need for a material to use in hermetically coating multicomponent and soft-glass fibers that does not significantly weaken the fiber or lead to unacceptable added optical losses, and may be applied using a commercially feasible process without adversely affecting the optical fiber's performance.

Summary Of The Invention

One aspect of the present invention relates to a method for manufacturing hermetically coated optical fiber in which a metallic glass jacket is fabricated over a preform having the desired core and overclad profile for the finished fiber. The jacketed preform is drawn to form the hermetically-sealed optical fiber. In a further embodiment of the invention, a freezing technique is used.

A more complete understanding of the present invention, as well as further features and advantages of the invention, will be apparent from the following detailed description and the accompanying drawings. Brief Description Of The Drawings

Fig. 1 shows a flowchart of a method according to the present invention for applying a metallic glass hermetic coating to an optical fiber using a pre form -jacketing technique;

Fig. 2 shows a diagrammatic perspective view of a metallic glass jacket used in the method illustrated in Fig. 1 ;

Fig. 3 shows a diagrammatic perspective view of a jacketed preform used in the method of the present invention;

Fig. 4 shows a schematic representation of the jacketed preform of Fig. 3 loaded into a draw tower in accordance with the method of the present invention;

Fig. 5 shows a flowchart of a method according to the present invention for applying a metallic glass hermetic coating to an optical fiber using a freezing technique; Fig. 6 shows a diagram of a fiber coating system for performing the freezing method of the present invention; and

Fig. 7 shows a diagram of an alternative embodiment of the fiber coating system of Fig. 6.

Detailed Description Of The Preferred Embodiments

The present invention now will be described more fully with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. However, the described invention may be embodied in various forms and should not be construed as limited to the exemplary embodiments set forth herein.

Rather, these representative embodiments are described in detail so that this disclosure will be thorough and complete, and will fully convey the structure, operation, functionality, and potential scope of applicability of the invention to those skilled in the art. According to the present invention, a metallic glass is used to hermetically coat optical fibers. Metallic glass, also known as amorphous metal, is composed of a metal alloy not having a crystalline structure. Metallic glass is formed by melting the metal alloy and then cooling it quickly enough so that heterogeneous nucleation is bypassed. The critical cooling rate (that is, the rate at which the molten metal must be cooled so as to avoid crystallization) depends upon the particular alloy being amorphized. Because the atoms within metallic glass do not lie on an orderly crystal lattice, metallic glass typically displays significantly different physical properties than those exhibited by the corresponding metal alloy. These properties are discussed further below.

Metallic glasses can be applied on soft glass fibers as well as on harder conventional silica fibers, and have many advantages over the hermetic coating materials currently used on silica fibers. In contrast with standard fiber coatings, metallic glasses can be applied at a much lower temperature, and are microscopically homogeneous and isotropic, because they are amorphous in nature.

Metallic glasses exhibit superb mechanical strength and extremely high resistance to fatigue and corrosion. Many alloy systems can be amorphized at cooling rates as low as 1 °C/sec. Although early methods produced micrometer-thick, melt-spun ribbons, current methods and alloys can produce metallic glass in large bulk specimens. One alloy system that is particularly suitable for use as a hermetic coating for optical fiber is the Zr-Ti-Cu-Ni-Be glass system, which includes the particularly thermally-stable glasses Zr _{6 75}Ti _{8 2}iCu_{7 5}Niι₀Be_{2 5} andZr - ₂Ti ₎₃ Cu-_{2 5}NiιoBe_{22 5}. Another alloy system that is also suitable for use as a hermetic coating for optical fiber is the Pd-Ni-Cu-P glass system, which includes the thermally-stable glasses Pd oNi₄oP₂o and Pd ₀NiιoCu oP₂₀.

There are several advantages for using metallic glasses as hermetic-coating materials. First, they can be applied to both soft-glass fibers and conventional silica fibers (due to metallic glass offering a much lower temperature process). For example, the Zr-Ti-Cu-Ni-Be glass system can have a T_g as low as approximately 310°C, and thermal characteristics that approximate those of a GeAs sulfide glass for use in a 1.3 μm amplifier fiber. Second, the lower coating temperature minimizes the interaction at the fiber/coating interface. Third, metallic glasses are highly engineerable in order to meet specific needs, as there is considerable flexibility in choice of composition. Finally, metallic glasses provide the combined advantages of both metal and glass, overcoming the brittleness of most ceramic materials and the nonuniform microstructure of conventional metals. The particular method to be used for applying the hermetic metallic-glass coating will depend on the fiber to be coated. For soft glass fiber, a direct preform- jacketing technique may be preferable. This technique is illustrated in Figs. 1-4. For silica and other higher temperature fibers, the "freezing" technique currently used for metal coatings may be preferable. This method is illustrated in Figs. 5-7.

Fig. 1 shows a flowchart of a method 10 according to the present invention in which a direct preform jacketing technique is used. In step 12, also illustrated in Fig. 2, a jacket or sleeve 22 is fabricated out of metallic glass. Fig. 2 shows a perspective view (not to scale) of the sleeve 22, which is essentially a hollow cylinder with an interior channel 24 dimensioned to fit around a glass preform 26. The sleeve 22 can be machined, extruded, or cast, as desired.

In step 14, and as illustrated in Fig. 3, the jacket or sleeve 22 is placed over a preform 26 having the desired core and overclad profile of the finished optical fiber. Fig. 3 shows a perspective view (not drawn to scale) of the jacketed preform 26. The preform 26 is fabricated using any of a number of currently-used or hereafter-developed techniques, including rod-in-tube, double-crucible, extrusion, outside vapor deposition (OND), vapor axial deposition (VAD), and modified chemical vapor deposition (CVD) to name a few. The metallic glass jacket 22 and the preform 26 have similar thermal properties so that they will enter a softened state and return to a rigid state at approximately the same temperature and in approximately the same amount of time.

In step 16, illustrated in the schematic representation shown in Fig. 4, the jacketed preform 26 is loaded into a draw tower 28. In step 18, a "hot zone" 30 in the draw tower 28 is heated to a temperature sufficiently high to soften the lower portion of the jacketed preform 26. In order to close any gap between the jacket 22 and the preform 26, a vacuum 32 is applied. After the lower portion of the jacketed preform 26 has become sufficiently fluid, a "gob" of the fluid preform then drops off, drawing behind it a trail of fluid fiber 34, which cools to room temperature and hardens almost immediately upon leaving the hot zone. The resulting hermetically sealed fiber is then collected and wound onto a bulk takeup spool. Fig. 5 shows a flowchart of a method 36 according to the present invention in which a freezing technique is used to apply a metallic glass hermetic coating to an optical fiber. As discussed above, this method is more suitable for use with silica optical fibers. The steps of the method 36 are illustrated in Fig. 6, which is a schematic representation of a system 43 for performing the freezing process. In step 38, metallic glass 44 is heated in a reservoir 46 until it is fluid. In step 40, optical fiber 48 is unwound from a bulk spool 50 and drawn through the fluid metallic glass 44, such that the fiber 48 is coated with the fluid metallic glass 44. In step 42, the coated fiber 48 is then cooled in a lower-temperature region 52, allowing the fluid coating to harden, thereby hermetically sealing the optical fiber 48. Finally, the coated fiber is wound onto a takeup spool 54.

Fig. 7 shows an alternative embodiment of the system shown in Fig. 6. In the Fig. 7 system 43a, the source of the optical fiber 48a that is fed through the fluid metallic glass 44a in the reservoir 46a is a draw tower 28a, rather than a bulk spool. The coated fiber 48a is again passed through a cooling zone 52a, allowing the fluid metallic glass coating to form a hermetic seal around the fiber 48a, and the coated fiber 48a is then wound onto a takeup spool 54a. The Fig. 7 system would be used to hermetically seal an optical fiber, such as silica, that becomes fluid at a significantly higher temperature than the metallic glass used for the hermetic coating.

While the currently preferred embodiments of the present invention have been described, it will be readily appreciated by those skilled in the art that other embodiments may be practiced according to the present invention, such as the use of an optical fiber selected from the silica or silica-germania glass system, or where the metallic glass jacket 22 is formed on or over the preform 26 using a deposition process such as CVD, MCND, OND, or NAD.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the present invention. Thus, it is intended that the present patent cover the modifications and variations of this invention, provided that they come within the scope of the appended claims and their equivalents.

Claims

What is claimed is:

1. A method for manufacturing an optical fiber having a hermetic coating, said optical fiber being drawn from a preform having a core and a cladding, said method comprising the steps of: providing a jacket over the preform, the jacket being fabricated from a metallic glass; heating a portion of the jacket and a portion of the preform until each becomes deformable; and drawing the deformable portion of the preform and the deformable portion of the jacket into the optical fiber such that the jacket forms the hermetic coating.

2. The method of claim 1 wherein the step of drawing includes heating the deformable portion of the preform and the deformable portion of the jacket until each becomes generally fluid, and wherein the step of drawing further includes applying a vacuum so as to substantially eliminate the formation of a gap between the deformable portion of the preform and the deformable portion of the jacket which are generally fluid.

3. The method of claim 1 wherein the metallic glass is selected from the Zr-Ti-Cu-Ni-

Be glass system.

4. The method of claim 3 wherein the metallic glass selected from a group consisting of Zr_{46 75}Ti s ₂₅Cu_{7 5}Ni|₀Be₂₇5 or Zr_{41 2}Ti _{I3 8}Cuι_{2 5}Niι₀Be₂₂5.

5. The method of claim 1 wherein the metallic glass is selected from the Pd-Ni-P or the Pd-Ni-Cu-P glass systems.

6. The method of claim 5 wherein the metallic glass is Pd ₀Ni oP₂o or Pd₄₀NiιoCu oP₂o-

7. The method of claim 1 wherein the preform is fabricated from a sulfide glass.

8. The method of claim 7 wherein the preform is fabricated from a GeAs sulfide glass.

9. The method of claim 1 wherein the jacket and the preform have generally similar thermal properties.

10. A method for manufacturing an optical fiber having a hermetic coating comprising the steps of: heating a metallic glass in a reservoir until the metallic glass becomes generally fluid; drawing the optical fiber through the metallic glass such that the optical fiber is coated with a molten layer of the metallic glass; and allowing the molten layer of the metallic glass to cool such that the optical fiber is hermetically sealed within the hermetic coating of the metallic glass.

1 1. The method of claim 10 wherein the metallic glass is selected from the Zr-Ti-Cu-

Ni-Be glass system.

12. The method of claim 11 wherein the metallic glass is selected from a group consisting of Zr _{6 75}Ti _{8 25}Cu_{7 5}Niι₀Be _{7 5} or Zr_{4) 2}Ti _{13 8}Cu_{!2 5}Niι₀Be_{22 5}.

13. The method of claim 10 wherein the metallic glass is selected from the Pd-Ni-P or the Pd-Ni-Cu-P glass systems.

14. The method of claim 13 wherein the metallic glass is Pd₄oNi oP₂o or Pd₄₀Niι₀Cu₃₀P2c-

15. A hermetically-coated optical fiber comprising: an optical fiber; and a hermetic coating surrounding the optical fiber, the hermetic coating being fabricated from a metallic glass.

16. The hermetically-coated optical fiber of claim 15 wherein the optical fiber is fabricated from a GeAs sulfide glass.

17. The hermetically-coated optical fiber of claim 15 wherein the metallic glass is selected from the Zr-Ti-Cu-Ni-Be glass system.

18. The hermetically-coated optical fiber of claim 17 wherein the metallic glass is selected from the group consisting of Zr_{46 75}Ti _{8 2}sCu₇ sNi-oBe_{2 5} or Zr₄ι ₂Ti ι_{3 8}Cuι ₅NiιoBe_{22 5}.

19. The hermetically-coated optical fiber of claim 15 wherein the metallic glass is selected from the Pd-Ni-P or the Pd-Ni-Cu-P glass systems.

20. The hermetically-coated optical fiber of claim 22 wherein the metallic glass is Pd₄₀Ni₄₀P₂o or Pd₄₀Niι₀Cu₃₀P₂o.