CA2179456C - Method for and encapsulation of an optical fiber - Google Patents

Abstract

A method and device is provided wherein a reinforcement is provided for one or more optical fibres. One or more fibres are inserted into a sleeve made of a material that is substantially the same as the material of the cladding of the one or more optical fibres. The bore of the sleeve is sized to accommodate the one or more optical fibers; After the one or more optical fibres is inserted into the sleeve sufficient heat is applied to the sleeve for a duration to collapse the sleeve onto the one or more optical fibres. Preferably, the sleeve is a glass pre-form consisting substantially of 90% or greater silica.

Description

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Method for and li'r , ' '- ) of an Optical Fiber Field of the Invention This invention relates to an optical system and method for epoxyless coupling of a pre-formed ferrule to an optical fiber. More particularly, an aspect of the invention relates to hermetically enveloping at least a portion of an optical fiber with a pre-formed ferrule of a similar and/or compatible material.

Ba.4. ~ . ~ of the Invention Optical fibers are used in a wide variety of applications ranging from tPI~."" " ", 1, I;ca~h)ll~ to medical technology and optical ~ JUII~ . Because of their unique structure, optical fibers are capable of highly accurate transmission of light, which is relatively unaffected by i~ rc~cll~,c, diffusion, and other signal de-enhancing rhPnr nnPn~ Ho~vever, for optical fibers to function at their optimum potential they must be structurally intact and free of scratches, cracks, or leaks.
Optical fibers consist of a core material that is surrounded by a cladding. The difference between the indexes of refraction of the core and cladding materials (which, in some cases, are simply different types of fused silica glass) allows the optical fiber to function. Most commercially available optical fibers, in addition, have an external "buffer or jacket". The jacket is a thin coating (usually a plastic, other polymer, or metal) which is applied to the fiber to protect it from being scratched during handling ând to limit the amount of water than can come into contact with the fiber. Scratching or contact with water or moisture can deleteriously affect both the optical properties and the strength of I

21 7q~
the glass fiber. In addition to shielding the fiber's surface, the buffer also operates to help maintain the high tensile strength and the bending capability of the glass optical fibers.
A number of fiber optic d~ liulls require that one terminus of the fiber be s located in an ~IIVil~JlUll~llL isolated from the other terminus. Tbis implies the use of a connector, coupling device, or "feedthrough" which serves as the point of , ."""",.,i..,.lion between the distinct environments. Oftentimes, it is necessary or desirable for the point of ~ iorl between the ellVilUlllll~ i to be completely sealed except for the presence of the optical fiber. Herein arises the need for a satisfactory 0 method to hermetically seal optical fibers within metal fittings.
Fabrication of hermetic fiber optic-to-metal C~ has until recently been difficult due to a number of factors. Principal among these is the large thermal expansion mismatch between the very low coefficient of expansion of the optical fibers (most commonly made of fused silica glass) and the high coefficient of expansion of the metal shell to which the optical fibers are attached. This difference can cause severe stressing of the fiber optic ~,ullllJu~l~.lb, especially where fabrication methods use application of heat, which, in turn, can cause ull~ ilable cracks and leaks in the optical fibers.
A United States patent 5,143,531 in the name of Kramer issued September I, 1992 and assigned to the United States of America as represented by the United States Depa~tment of Energy, discloses a glass-to-glass hermetic sealing technique which can be used to splice lengths of glass fibers together. A solid glass pre-form is inserted into the cavity of a metal component which is then heated to melt the glass. An end of an optical fiber is then advanced into the molten glass and the entire structure is cooled to solidify the glass in sealing ~"~".,J~",. ,I with the optical fiber end and the metal cavity.

Another U.S. patent 5,337,387 in the name of the same inventor issued August 9, 1994 and relates to a method of the continuous processing of hermetic flber optic ~,U~ and the resultant fiber optic-to-metal C~ Ullci~ by assembling and fixturing elements comprising a metal shell, a glass pre-form and a metal-coated fiber 5 optic into desired relative positions and then sealing said fixtured elements, preferably using a continuous heating process.
Although Kramer's inventions for hermetically sealing optical fibers may performtheir intended functions, the general approach is believed to be a relatively costly and 10 somewhat complex.
Methods are known for placing and affixing optical fibers in ferrules and sleeves of different types for the purposes of providing a protective sheath for reducing damage to optical fibers that would otherwise be exposed, and for attempting to provide a housing 15 for optical fibers. Furthermore, such ferrules or sleeves have been used as housings in which optical fibers are fused together. In many of these applications an a&esive such as epoxy is placed in the ferrule with the optical fiber to provide a bonded seal between the fiber and the ferrule.
In one U.S. patent 5,094,518 issued March l 0 1992 in the name of Musk a method of making an opto-electronic component comprises inserting a pre-assembled device carrier into a mould, filling the mould with a light and/or thermally curable material.
In yet another U.S. patent 5,061,034 in the name of Fujikawa et al issued October 29, 1991, a permanent connector for optical fibers comprises a protective glass tube, a capillary tube received therein and er~ntrirAlly joined thereto; the two tubes are made of ultraviolet-~ glass; an a&esive-passing groove is formed in the middle 2~ 7~56 portion of the capillary tube and opening on the side opposite to the side where they are joined together. An ultraviolet-curing type adhesive agent is cllarged into the permanent connector for optical fibers and the ends of the fibers are inserted thereinto. Fujikawa's device and Musl;'s device are both relativel~ complex and do not appear to be optimal 5 solutions for hermetically sealing an optical fiber.
Therefore, it is an object of this invention to provide a method of ~nr~rs~ ti n~ an optical flber that is practicable for hermetic optical fiber applications.
It is a further object ofthe in~ention to provide an i~l~A~ ;ve and reliable optical fiber [~dllu~ugl~ for hermetic optical fiber applications.
It is yet a further object of the invention to provide an epoxyless method of reinforcing an optical fiber by fusing it to a stiffening sleeve in tlle presence of heat.
Summary of the Invention In accordance with the invention, a method of ~ r~ t~ at least a portion of an optical fiber is provided comprising the steps of: providing a ferrule having a bore, the 20 ferrule at least about and deflning the bore comprising a material that is similar to and fusible with an optical fiber sized to tightly flt into the bore; inserting at least a portion of the optical fiber into the bore; heating the ferrule sufficiently to fuse at least a portion of the optical fiber inserted into the bore with the ferrule;
In accordance ~vith the invention there is further provided, a method of rnr~rs~ ting at least a portion of an optical fiber comprising the steps of placing an optical fib~r into a tube having an inner wall made of a material that will fuse to the fiber 2 ~ 7~ 5~
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in th~ presence of suitable heat; and, vitrifying and collapsing said tube onto the fiber to encapsulate a portion of optical fiber within the tube.
In accordance with another aspect of the invention there is provided an optical 5 fiber having an outer cladding sllhst~nti:~lly consisting of silica, the cladding on at least a portion of the optical fiber being r~ I by and fused with a sleeve having an inner wall defining a bore into which the fiber is disposed, sllhst~nt~ y consisting of silica.
Ad~ ly this invention provides a method of thickening a section of lo optical fiber by collapsing and fusing a tube of compatible material onto it. The thickened section comprising the at least fused fiber and tube can then be soldered thereby providing a hermetically sealed feedthrough. In particular, this method has been shown to exhibit significantly less stress damage and crack formation than hermetic fiber optic ~U~ made using other techniques Brief Description of the Drawings Exemplary embodiments of the invention will now be described in conjunction with th~ drawings, in which:
Fig. I is a cross sectional view of an optical fiber placed into an opening a fenule prior to the application of heat ror collapsing the ferrule;
Fig. Ia is a is a cross sectional view of an optical fiber placed into an opening a 25 ferrule having a stepped down opening for allowing a jacketed fibre to be inserted part way into the rerrule prior to the application of heat for collapsing the ferrule;

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Fig. 2 is a cross sectional view of the optical fiber ferrule ~ 11 Ig~ shown in Fig.l after the application of heat whereby the ferrule is shown collapsed onto the optical fiber;
Fig. 3 is a cross sectional view of the optical fiber "" ,., I~ shown in Fig. 2,5 wherein the mode field diameter within the core of the optical fiber within the ferrule is thermally expanded;
Fig. 4a is a cross sectional view of an alternative embodiment of a sleeve having flared ends for use in accordance with the invention;

Fig. 4b is a pictorial view of a sleeve having a rectangular bore for accommodating a pair of optical fibers;
Fig. 4c is an end view of the sleeve shown in Fig. 4b;
Fig. 4d is an end view of a sleeve having a bore with a triangular cross section;
Fig. 5a is an end view of the sleeve shown in Fig. I, collapsed on an optical fiber as is shown in Fig 2;
~o Fig. ~b is an end view of an optical fiber glued into a sleeve in a conventionalmarLner; and, Fig. 6 is a side view of a sleeve shown with two optical fibers placed within prior 25 to collapsing the sleeve on the optical fib~rs.
Detailed Description 21 ;79~56 .
The terms "sleeve, felrule, and tube" shall be used illlt~ dbly in this description to represent a housing having a bore at least partially defined therethrough.
Referring now to Fig. 1, a conventional optical fiber 10 is shown having a portion of its lengtn inserted into a flber tube or sleeve 18 consisting 51~hct~nfi~1ly of silica. Tbe sleeve 18 can be a precision sleeve having a wall 13 deflning an inner diameter as small as 125.1 ~Lm to ,~ t. an optical fiber having an outer diameter of 125 llm or the inner diameter of the sleeve can be larger than 126 ,um. The outer diameter of the sleeve is not conflned to, but may be in the range of I mm. The optical flber 10 has a core 12, lo and a cladding 14 that both consist substantially of silica having different refractive indexes; an outer protective jacket 16 is sho~vn on a portion of the fiber outside the sleeve 18. Typical dimensions of t~1e core 12 for single mode fiber are in the range of 6 to 12 ~Lm. The outer diameter of the cladding 14 is generally about 125 llm and the protective jacket 16 outer diameter typically can ranges from 250 to 900 llm.
In an attempt to obviate known problems in the fabrication of hermetic fiber optic components related to t~lermal expansion mismatch between the very low coefficient of expansion of the fused silica glass optical fibers and the differing coefficient of expansion of the other mate~ials, this invention provides a sleeve of a compatible material (silica) that is sllh~t~nti:~lly matched in coefficient of expansion to that of optical fiber, thereby reducing stressing of the optical fiber in the application of heat which otherwise can cause le~ila~le cracks and leaks in the optical fibers.
Two materials that provide a match to optical fiber are silica ~llhst~nti~lly consisting of SiO2 (nearly 100%) and Vycor TM (about 95% silica and partially consisting of Boron Oxide) available from the Corning Glass Co. Sleeves made of either of these materials will fuse to the cladding 14 that substantially consists of pure silica at 21 7945~
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UUC~ of about 1800 C. Thus, it is preferred if the sleeve 18 consists of or at least substantially consists of the same material as the cladding 14.
Referring to Fig. I a a ferrule 1 8a is shown having a stepped down inner wall 1 3a s for allowing a jac~eted fiber to be inserted part way into the opening. The stripped optical flber is inserted tllrough the ferrule 1 8a. Heat is provided to one end (not shown) and one portion of th~ tube is collapsed upon the fiber.
Cu~v~ lly and optionally, the opening at the end of the sleeve 18 is flared to lo ease th~ entry of a fiber end into the end of the sleeve 18. Referring now to Fig. 4a, a sleeve 48 is shown having a flared end 47 and having bore extending only part~way through providing an opening into which a fiber end may be inserted, rather thanproviding a through-hole as shown in Fig. 1. In an alternative embodiment, bores having a cross-section other than circular can be provided. For example, and referring now to 5 Fig. 4c, a substantially rectangular bore cu~ ltly ~ r~ two optical fibers for fusion with the bore in the presence of sufficient ~l~at. A triangular bore is shown in Fig. 4d for ~ o"",,~,.l"li"g 3 optical fibers.
When tlle glass tube 18 is heated to a sufficient ltl~lp~ldlul~ external surfacezo tension on the tube shrinks and collapses the diameter of the tube 18. Fig. 2 illustrates this process whereby a micro-flame burner 20 heats the silica tube 18 to about 1800 C
until th~ tube 18 collapses and at least partially fuses with the cladding 14. Fig. 5a illustrates complete fusion of the cladding and the tube inner diameter; the cladding and the tube are not dirr~l~lllial,le and only the core of the fiber can be di~ ,ui~ d. In 25 contrast to this, Fig. 5b shows a conventional .~ where an optical fiber withadhesive in the form of epoxy is placed into a bore of a sleeve 58. The cross section shows a ring 57 defining a boundary of epoxy at the interface between the fiber cladding and the wall of the bore.

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Referring once more to Fig. 2, advallla~vu~ly by heâting the tube 18 and ~.vl15~ y the fiber 10 inside the tube at such a high temperature, ~he mode field 12a of the core 12 expands, yielding a hermetically sealed optical fiber end with a thermally 5 expanded core (TEC). Alternatively, the fiber can be placed in the sleeve, heated slowly (for a few hours) at a temperature of âbOut 1300 C to expand the mode field diameter of the core, and later can be heated at suhif ~nti~lly higher temperatures to collapse the tube 18 onto the optical fiber cladding 1~. Advantageously, and more importantly in applications ~vhere tolerances are very small, when the fiber is heated and fused to the o sleeve, the core of the fiber self-centers within the sleeve. This simplifies later alignment of the reinforced fused fiber and sleeve.
Optionally, the tube 18 of the resulting r~ optical fiber can be soldered or dipped in metal bath to coat the outer periphery of the device. Of course other coating and depositing methods may be envisaged to apply a metal to the outside of the tube 18 for hermetic sealing with a metal package.
In an alternative embodiment of this invention shown in Fig. 6, the silica tube 18 described heretofore, can be used in a similar manner to reinforce and hold optical fibers, 20 hermetically sealing them, however this ~mh-)~1imrnt provides for two optical fibers 10a and I Ob to be held, reinforced, and fused end-to-end together inside the tube 18. First, ends of the fibers are placed into the tube 18 such that they are in contact to one another.
Sufficient heat is then applied until the fibers have fused. Optionally, silica glass soot may be inserted into the tube prior to the placement of either fiber in the tube or after the placement of a first fiber and prior to the insertion of the second optical fiber. The soot, having a lower melting point than the optical fiber fuses and solders the adjacent ends of tlle fibers together inside the tube in the presence of heat applied to the tube 18.

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Of course, numerous other ~ may be envisaged, without departing from the spirit and scope of the invention.

Claims (19)

Claims What we claim is:

1. A method of encapsulating at least a portion of a single optical fiber having a predefined diameter, the method comprising the steps of:

providing a ferrule having a bore closely accommodating the diameter of the single optical fiber, the ferrule at least about and defining the bore comprising a material that has essentially a same coefficient of thermal expansion, is similar to and fusible with the single optical fiber sized to fit into the bore;

inserting at least a portion of the single optical fiber into the bore;

heating the ferrule sufficiently to fuse at least a portion of the optical fiber inserted into the bore with the ferrule.

2. A method of claim 1, wherein the ferrule has a bore extending partially therethrough.

3. A method as defined in claim 1 or 2 further comprising the step of metalizing at least some of the outer periphery of the ferrule.

4. A method as defined in claim 1, 2 or 3, wherein the step of applying a suitable heat is performed at a temperature and for a duration sufficient to expand the mode field diameter of the single optical fiber placed into the ferrule such that the diameter of the expanded core is greater than the diameter of the same fiber proximate to but external to the ferrule.

5. A method as defined in any one of claims 1 to 4, wherein the ferrule when cooled is placed into a metal housing thereby providing a feedthrough to said housing.

6. A method as defined in any one of claims 1 to 5, wherein the step of applying a suitable heat comprises heating the ferrule to a temperature substantially about 1800° C
or greater.

7. A method as defined in any one of claims 1 to 6, wherein the inner wall of the ferrule is comprised of at least 95% SiO2.

8. A method as defined in any one of claims 1 to 7, wherein the bore of the ferrule has a non-circular cross-section that substantially conforms to the single optical fiber.

9. A method as defined in claim 1 to 8, wherein the bore has a substantially rectangular cross-section.

10. The method as defined in any one of claims 1 to 9, wherein the bore is a stepped down bore, a portion of which has a first diameter and a portion of which has a smaller second diameter, for accommodating a jacketed fiber and a stripped fiber respectively.

11. An encapsulated optical fiber comprising:
a sleeve having a central bore defined by an inner wall extending at least partly therethrough surrounding a single optical fiber, the inner wall of the tube, substantially matched in coefficient of thermal expansion to that of the optical fiber, being fused to the optical fiber to provide reinforcement.

12. An encapsulated optical fiber as defined in claim 11, wherein the material of the inner wall of the sleeve comprises at least 95% SiO2.

13. An encapsulated optical fiber comprising a sleeve made essentially of a material that is at least similar to a clad portion of a single optical fiber, the sleeve enveloping and being collapsed upon the clad portion of the optical fiber and being in optical contact therewith.

14. An encapsulated optical fiber as defined in claim 13 wherein the optical fiber and the collapsed sleeve are fused together.

15. An encapsulated optical fiber as defined in claim 13 or 14, wherein the clad portion and the sleeve are each comprised of at least 95% silica.

16. An encapsulated optical fiber as defined in claim 13, 14 or 15, wherein an at least a portion of the optical fiber fused to the sleeve has a core at least a portion of which has an expanding mode field diameter along its longitudinal axis, said at least portion of the core being larger in diameter than the core of other portions of the optical fiber not reinforced.

17. A method of encapsulating two optical fiber ends in an end-to-end relationship, the method comprising the steps of:
providing a first optical fiber having an end with exposed cladding;
providing a second optical fiber having an end with exposed cladding;
providing a sleeve having a first open end and a second open end and a bore extending therethrough from the first open end to the second open end, wherein the sleeve is made of a material that is substantially the same as the material of the cladding, the bore being sized to closely accommodate the diameter of the first and second optical fiber;
inserting the end of the first optical fiber into the first end of the bore;
inserting the end of the second optical fiber into the second end of the bore such that the first and second ends of the optical fibers are substantially adjacent one another end-to-end inside the bore;
applying sufficient heat to the sleeve for a duration to collapse the sleeve onto the first and second fibers.

18. The method as defined in claim 17, further comprising the step of placing a material in the form of a soot into the bore that will assist in the fusion of the first and second optical fiber ends, in the presence of the sufficient heat, prior to said heating step.

19. The method as defined in claim 16, wherein the optical fibers and the sleeve are made of a material that has a low melting point that is less than 1000°
C.