WO1999057585A1

WO1999057585A1 - Optical fiber outer dimension variation

Info

Publication number: WO1999057585A1
Application number: PCT/US1999/010110
Authority: WO
Inventors: Robert J. Maron; John J. Grunbeck; James R. Dunphy; Mark R. Fernald
Original assignee: Cidra Corporation
Priority date: 1998-05-06
Filing date: 1999-05-06
Publication date: 1999-11-11
Also published as: AU3891399A

Abstract

An optical waveguide (10), such as an optical fiber, having a cladding (12) and a core (14), is provided with a variation of the outer dimension (an expanded region (16) or a recessed region (8)) over a predetermined length L of the waveguide (10). The variation (16, 8) provides a low-creep high-strength means for attaching the fiber (10) to a structure, e.g., for use in an optical sensor. The variation (16, 8) may have various different shapes and sizes. There may also be a buffer layer (18) over the cladding (12) and over all or a portion of the deformation (16, 8). Also, the fiber (10) exhibits low optical loss of light propagating in the core (14) through the deformation (16, 8).

Description

Optical Fiber Outer Dimension Variation

Cross References to Related Applications

Copending U.S. Patent Applications, Serial No. (Cidra Docket No. CC- 0058), entitled "Optical Fiber Bulge", and Serial No. (Cidra Docket No. CC-

0080), entitled "Creep-Resistant Optical Fiber Attachment", both filed contemporaneously herewith, contain subject matter related to that disclosed herein.

Technical Field

This invention relates to optical fibers and more particularly to optical fiber outer dimension variations.

Background Art Sensors for the measurement of various physical parameters such as pressure and temperature often rely on the transmission of strain from an elastic structure (e.g., a diaphragm, bellows, etc.) to a sensing element. In a pressure sensor, the sensing element may be bonded to the elastic structure with a suitable adhesive. It is also known that the attachment of the sensing element to the elastic structure can be a large source of error if the attachment is not highly stable. In the case of sensors which measure static or very slowly changing parameters, the long term stability of the attachment to the structure is extremely important. A major source of such long term sensor instability is a phenomenon known as "creep", i.e., change in strain on the sensing element with no change in applied load on the elastic structure, which results in a DC shift or drift error in the sensor signal.

Certain types of fiber optic sensors for measuring static and/or quasi- static parameters require a highly stable, very low creep attachment of the optical fiber to the elastic structure. One example of a fiber optic based sensor is that described in U.S. Patent application Serial No. 08/925,598 entitled "High Sensitivity Fiber Optic Pressure Sensor for Use in Harsh Environments" to Robert J. Maron, which is incorporated herein by reference in its entirety. In that case, an optical fiber is attached to a compressible bellows at one location along the fiber and to a rigid structure at a second location along the fiber with a Bragg grating embedded within the fiber between these two fiber attachment locations. As the bellows is compressed due to an external pressure change, the strain on the fiber grating changes, which changes the wavelength of light reflected by the grating. If the attachment of the fiber to the structure is not stable, the fiber may move (or creep) relative to the structure it is attached to, and the aforementioned measurement inaccuracies occur.

One common technique for attaching the optical fiber to a structure is epoxy adhesives. It is common to restrict the use of epoxy adhesives to temperatures below the glass transition temperature of the epoxy. Above the glass transition temperature, the epoxy transitions to a soft state in which creep becomes significant and, thus, the epoxy becomes unusable for attachment of a sensing element in a precision transducer. Also, even below the glass transition temperature significant creep may occur.

Another technique is to solder the structure to a metal-coated fiber. However, it is known that solders are susceptible to creep under certain conditions. In particular, some soft solders, such as common lead-tin (PbSn) solder, have a relatively low melting point temperature and are thus relatively unsuitable for use in transducers that are used at elevated temperatures and/or at high levels of stress in the solder attachment. The use of "hard" solders with higher melting temperatures, such as gold-germanium (AuGe) and gold-silicon (AuSi), can reduce the problem; however, at elevated temperatures and/or high stress at the solder attachment, these hard solders also exhibit creep. In addition, the high melting temperature of such solders may damage the metal coating and/or damage the bond between the metal coating and glass fiber.

- 2 Summary of the Invention

Objects of the present invention include provision of a creep-resistant high-strength technique for attaching a structure to optical fiber.

According to the present invention, an optical waveguide comprises a cladding; a core within the cladding; and the cladding having a variation of an outer dimension of the cladding.

According further to the present invention, the variation comprises an expanded region. According still further to the present invention, the variation comprises a recessed region. According further to the present invention, the waveguide is an optical fiber.

The present invention provides a significant improvement over the prior art by providing an optical fiber with an outer dimension variation which is expanded and/or recessed and which allows for many options for attachment of the optical fiber to a structure. This geometry is created while providing low optical loss of the light being transmitted through the core of the fiber.

The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof.

Brief Description of the Drawings

Fig. 1 is a side view cross-section of an optical fiber showing a technique for creating an increased outer diameter region in an optical fiber, in accordance with the present invention.

Fig. 2 is a side view cross-section of an optical fiber showing an alternative technique for creating an increased outer diameter region in an optical fiber, in accordance with the present invention.

Fig. 3 is a side view cross-section of an optical fiber showing an alternative technique for creating an increased outer diameter region in an optical fiber, in accordance with the present invention.

- 3 Fig. 4 is a side view cross-section of an optical fiber showing a decreased outer diameter region in an optical fiber, in accordance with the present invention.

Best Mode for Carrying Out the Invention

Referring to Fig. 1, an optical waveguide 10, e.g., a known single mode optical fiber, having a cladding 12 with an outer diameter dl of about 125 microns and a core 14 having a diameter d2 of approximately 7-10 microns (e.g., 9 mircons), has a region 16 with an increased (or expanded) outer diameter (or dimension), in accordance with the present invention. The fiber 10 is designed to propagate light along the core 14 of the fiber 10. The cladding 12 and the core 14 are made of fused silica glass or doped silica glasses. Other materials for the optical fiber or waveguide may be used if desired. The region 16 has a length L of about 500 microns, and an outer diameter d3 of about 200 microns. Other dimensions of the cladding 12, the core 14, and the region 16 may be used if desired, provided the diameter d3 of the region 16 is greater than the diameter dl .

One technique for making the expanded region 16 is to use a fiber (or fiber section) which has an enlarged diameter d4 substantially equal to or greater than the diameter d3 of the region 16. The fiber section may be made using a suitable glass pre-form with a cladding/core diameter ratio that can be drawn down using conventional techniques to achieve the desired core size but has a cladding outer diameter d4 which is greater than the desired value for the final optical fiber. To create the expanded region 16, the diameter d4 of the fiber 10 is reduced to the desired diameter by eliminating an outer portion 15 of the cladding by conventional (or yet to be developed) glass manufacturing techniques, e.g., grinding, etching, polishing, etc. If desired, some of the outer diameter of the region 16 may also be removed. Using chemical etching (e.g., with hydrofluoric acid or other chemical etches), laser etching, or laser enhanced chemical etching are some techniques which reduce the fiber outer diameter without applying direct contact force as is required by grinding and

- 4 - polishing. Certain types of etching may produce a sharper vertical edge 17 on the region 16, or an angled or curved edge 13. Also, selective etching may produce a notch 11 (or more than one notch) in the region 16. Also, the etching may produce the sharp edge 17 at one side (e.g., the left side) of the region 16 and the curved geometry 13 on the other side (e.g., the right side) of the region

16.

Fire polishing using conventional techniques, i.e., applying heat for a predetermined time across the region 16, may be performed after the etching to smooth any rough surfaces that may be left by the etching process (as rough surfaces may increase stress levels and reduce fatigue life in dynamically loaded fibers). The fiber section may then be optically connected, e.g., by fusion splicing, by an optical connector, etc, to a standard-sized fiber (not shown) having a cladding and core which match the final fiber section described hereinbefore. Referring to Fig. 2, alternatively, instead of the region 16 being made using a single axially continuous fiber, a fiber 4, having a length L and an outer diameter dy, e.g., 125 microns, is fusion spliced between two fibers 3 having an outer diameter dx, e.g., 80 microns, at interfaces 5,6. The fibers 3,4 have the same core 14 diameter, e.g., 9 microns, and may be fusion spliced using known splicing techniques. Other diameters for the claddings and cores of the fibers

3,4 may be used. The edge 17 may be a vertical edge or may be a curved edge as shown by the dashed lines 13. Depending on the application, it may be desirable and/or acceptable to have only one change in the outer dimension of the fiber (or two changes located a long distance apart). In that case, there would be one splice, e.g., at the interface 5, between the fibers 3,4 and the fiber

4 would be longer than that shown in Fig. 2.

Referring to Fig. 3, alternatively, a glass/ceramic tube (or sleeve) 7 may surround the fiber 10 to create the expanded region 16. In that case, the tube 7 is heated to the melting or softening temperature of the tube 7 such that the tube 7 is fused to or becomes part of the cladding 12. The tube 7 has a softening temperature which is the same as or slightly lower than that of the fiber 10. Any form of heating may be used, e.g., oven, torch, laser, filament, etc. The tube 7 may be a single cylindrical piece or have multiple pieces to surround the fiber 10. To help keep the tube concentric with the fiber, the process may be performed with the fiber held vertically. Also, more than one concentric tube may be used around the fiber if desired, each tube being melted onto an inner tube at the same time or successively.

Referring to Fig. 4, alternatively, instead of the region 16 being an expanded outer dimension (or diameter), the region 16 may comprise a decreased outer dimension (or recess or depression or notch) 8 in the waveguide 10. The recess 8 may be created by numerous techniques, such as by reducing the outer diameter of the fiber 10 using the techniques discussed hereinbefore with Fig. 1 (e.g., grinding, etching, polishing, etc.), by splicing a smaller diameter fiber between two larger diameter fibers, such as that discussed hereinbefore with Fig. 2, or by heating and stretching the desired region of the fiber by pulling on one or both ends of the fiber 10 (i.e., putting the fiber 10 in tension) using a technique similar to that for heating and compressing the fiber to create a bulge in the fiber 10 (i.e., stretching instead of compressing), such as is described in copending U.S. Patent Application Serial No. (Cidra Docket No. CC-0058), entitled "Optical Fiber Bulge", filed contemporaneously herewith, which is incorporated herein by reference. Etching the fiber 10 may create recessed vertical edges 2 (into the fiber 10) or a curved or angled recessed geometry 9, and heating and stretching the fiber 10 creates the curved geometry 9. The depth d8 of the recess 8 may be the same as the distance the expanded region 16 in Figs. 1-3 extends from the cladding 12 diameter, e.g., about 75 microns. Other depths may be used.

If heating and stretching is used to create the recessed region 8, such a process may be performed with the longitudinal axis of the fiber 10 aligned horizontally or vertically or with other orientations. One advantage to vertical orientation is that it minimizes axial distortions caused by gravitational effects of heating a fiber. Alternatively, the fiber may be rotated during heating and stretching to minimize gravity effects.

- 6 - For any of the embodiments described herein, precise symmetry (axial or cross-sectional) of the region 16 (for either expanded or recessed regions) are not required for the present invention. For example, the lower portion of the regions 16,8 may be slightly larger or smaller than the upper portion, or vise versa. However, the core 14 should retain axial alignment along both sides of the region 16 to minimize optical losses from the core 14 as light travels through the region 16. The better the axial alignment of the core 14, the lower the optical loss. Although the core 14 at the region 16 are shown as being straight, it should be understood that there may be some small amount of deformation of the core 14. The less deformation of the core 14 at the region 16, the lower the amount of optical loss. Also, the strength of the fiber 10 will depend on the settings and method used to make the region 16.

Also, for any of the embodiments described herein, instead of an optical fiber, any optical waveguide having a core and cladding may be used, e.g., a flat or planar waveguide, on which the region 16 can be created. In the case of a flat or planar waveguide, the region 16 may be on the upper and/or lower surfaces of the waveguide. Also, a multi-mode optical waveguide may be used if desired.

The region 16 may have other shapes (or geometries) than those described herein, provided at least a portion of the optical waveguide has a variation, deformation or change (expanded and/or recessed) of the outer dimension of the waveguide.

Also, a combination of any of the above techniques for creating the region 16 may be used. For example, the etching technique discussed with Fig. 1, may be used to alter the geometries described with Figs. 2-4. Other techniques than those described herein may be used if desired to create the region 16.

Also, the regions 16 described with Figs. 1-4 may be combined to provide both an expanded outer diameter region and a reduced diameter region. Further, more than one of the regions 16 may be provided along a given optical fiber if desired.

- 7 After the regions 16 are made, the cladding 12 may be coated or re- coated with a protective overcoat or buffer layer (not shown), such as a metal, polymer, teflon, and/or carbon, or other materials, which may be used to protect the fiber and/or enhance attachment to the fiber. The region 16 allows the fiber 10 to be attached to a structure in many different ways for many different applications, by providing a mechanical stop to reduce or eliminate creep, such as is discussed in copending U.S. Patent Application, Serial No. (Cidra Docket No. CC-0080), filed contemporaneously herewith. It should be understood that the Figs, shown herein are not drawn to scale.

It should be understood that any of the features, characteristics, alternatives or modifications described regarding a particular embodiment herein may also be applied, used, or incorporated with any other embodiment described herein.

- 8 -

Claims

Claims What is claimed is:

1. An optical waveguide, comprising: a cladding; a core within said cladding; and said cladding having a variation of an outer dimension of said cladding.

2. The apparatus of claim 1 wherein said variation is over a predetermined length of the waveguide.

3. The apparatus of claim 1 wherein said variation comprises an expanded region.

4. The apparatus of claim 1 wherein said variation comprises a recessed region.

5. The apparatus of claim 1 wherein said waveguide is axially continuous near said variation.

6. The apparatus of claim 1 wherein said waveguide is an optical fiber.

7. A method for making a variation in an outer dimension of an optical waveguide, comprising the steps of: a) obtaining an optical waveguide having a cladding and a core; and b) creating a variation in an outer dimension of the optical waveguide.

8. The method of claim 7 wherein said step of creating said variation comprises reducing a predetermined portion of the outer dimension of said cladding over a predetermined region.

9. The method of claim 8 where said step of reducing is performed on a region of the waveguide where said variation is located.

- 9 -

10. The method of claim 8 where said step of reducing is performed on a region of the waveguide around said variation.

11. The method of claim 7 wherein said step of creating said variation comprises splicing together two waveguides having different outer dimensions.

12. The method of claim 7 wherein said step of creating said variation comprises fusing a tube to the waveguide.

13. The method of claim 7 wherein said step of creating said variation comprises heating and stretching the waveguide.

14. The method of claim 7 wherein said step of creating said variation comprises etching the waveguide.

15. The method of claim 7 wherein the waveguide is an optical fiber.

- 10