FIBER OPTIC COIL AND HUB HAVING MATCHING COEFFICIENTS OF THERMAL EXPANSION

Abstract: A fiber optic device is disclosed having a hub supporting a coil. The coil is wound from an optical fiber having a coefficient of thermal expansion. The hub is formed from a material having a coefficient of thermal expansion substantially matching the coefficient of thermal expansion of the optical fiber.

Description: FIBER OPTIC COIL AND HUB HAVING MATCHING COEFFICIENTS OF THERMAL EXPANSION TECHNICAL FIELD OF THE INVENTION The present invention relates to fiber optic devices such as fiber optic rate sensors. BACKGROUND OF THE INVENTION A fiber optic rate sensor is frequently used in advanced global positioning and inertial guidance systems to sense rotation. A fiber optic rate sensor ordinarily comprises an interferometer which includes a light source, a beam splitter, a detector, and an optical path which is mounted on a platform. Light from the light source is split by the beam splitter into two light beams which are directed to opposite ends of the optical path. The two light beams counterpropagate around the optical path and, as the light beams exit the optical path, they are recombined. The recombined light beams are applied to a detector. If the optical path rotates, the distance traveled by one of the light beams is greater than distance traveled by the other light beam, so that there will be a phase difference between the two light beams at their optical path exit points. A sensing circuit connected to the detector determines this phase difference as an indication of the extent and direction of rotation. The optical path of a fiber optic rate sensor is provided by an optical fiber which is coiled around a spool or hub to form a winding configuration. The winding configuration usually has multiple layers where each layer contains multiple turns. Although many different winding configurations are known, coils used in fiber optic rotation sensors are typically wound as quadrupoles. In order to form a quadrupole, a first end of a continuous optical fiber is wound onto a first intermediate spool, and a second end of the continuous optical fiber is wound onto a second intermediate spool. Then, the optical fiber on the first intermediate spool is used to wind a first layer of turns in a clockwise direction around the hub, the optical fiber on the second intermediate spool is used to wind a second layer of turns in a counterclockwise direction over the first layer, the optical fiber on the second intermediate spool is used to wind a third layer of turns over the second layer of turns, and the optical fiber on the first intermediate spool is used to wind a fourth layer of turns over the third layer of turns. If "+" and "-" are used to designate the first and second ends of the optical fiber, respectively, the resulting quadrupole winding pattern has a + - - + winding configuration, where + indicates a layer wound from the first end of the optical fiber and - indicates a layer wound from the second end of the optical fiber. Ideally, the length of optical fiber in the "+" layers is equal to the length of optical fiber in the "-" layers. This quadrupole winding pattern may be repeated as often as desired for a fiber optic rate sensor. Accordingly, if a second quadrupole is wound with + - - + layers about the first quadrupole, the resulting two quadrupole arrangement has a + - - + + - - + winding pattern. It is also known to wind a reverse quadrupole from the "+" and "-" ends of the optical fiber. In this case, the reverse quadrupole has a + - - + - + + - winding pattern and is generally referred to as an octupole. This octupole winding pattern may be repeated as often as desired for a fiber optic rotation sensor. Indeed, a reverse octupole may be wound according to the following winding pattern: + - - + - + + - - + + - + - - +. In order to form a coil having an interleaved winding pattern, one or more layers of the coil are wound as alternating turns from first and second ends of an optical fiber. Accordingly, in such a layer, odd numbered turns are wound from a first end of the optical fiber, and even numbered turns are wound from a second end of the optical fiber. The result of such winding is that each turn (other than the outer turns) of an interleaved layer is wound from one end of an optical fiber and is sandwiched between two turns wound from the other end of the optical fiber. Not all layers of a coil having an interleaved winding pattern are required to be wound with the interleaved winding pattern. For example, all of the turns of the innermost layer of the coil can be wound from the same end of the optical fiber, or one or more groups of adjacent turns of the innermost layer of the coil can be wound from the first end of the optical fiber and one or more other groups of adjacent turns of the innermost layer of the coil can be wound from the second end of the optical fiber. The direction of the axis running through the hub and about which the coil is wound is generally referred to as the axial direction of the coil, and the direction perpendicular to the axial direction is generally referred to as the radial direction of the coil. Coils of optical fiber typically have a large thermal expansion rate in the axial direction and a much smaller thermal expansion rate in the radial direction. If these thermal expansion rates are not matched by the structure, such as the hub, which supports the coil, performance of a fiber optic coil may be significantly degraded. The present invention is directed to an arrangement where a coil and the structure that supports the coil have matching thermal expansion rates. SUMMARY OF THE INVENTION In accordance with one aspect of the present invention, a fiber optic device comprises a support structure and a coil supported by the support structure. The coil is wound from an optical fiber, and the support structure and the coil have substantially matching coefficients of thermal expansion. In accordance with another aspect of the present invention, a fiber optic device comprises a hub and a coil. The hub is formed at least partially from glass. The coil is supported on the hub, the coil is wound from an optical fiber, and the glass of the hub and the optical fiber of the coil have substantially matching thermal expansion rates. In accordance with yet another aspect of the present invention, a method of making a fiber optic sensor having a coil supported by a hub comprises the following steps: a) winding an optical fiber into the coil, wherein the optical fiber has a coefficient of thermal expansion; and, b) forming the hub so that the hub is formed from a material having a coefficient of thermal expansion substantially matching the coefficient of thermal expansion of the optical fiber and so that the hub supports the coil. In accordance with still another aspect of the present invention, a fiber optic device comprises a low thermal expansion rate support structure, a fiber optic coil supported by the low thermal expansion rate support structure, and a compliant joint between the low thermal expansion rate support structure and the fiber optic coil. BRIEF DESCRIPTION OF THE DRAWINGS These and other features and advantages of the present invention will become more apparent from a detailed consideration of the invention when taken in conjunction with the drawings in which: Figure 1 illustrates a first embodiment of the present invention in which a fiber optic coil is attached externally to a hub having thermal expansion rates substantially matching the thermal expansion rates of the fiber optic coil; Figure 2 illustrates an end view of the first embodiment of the present invention illustrated in Figure 1 ; Figure 3 illustrates a second embodiment of the present invention in which a fiber optic coil is attached internally to a hub having thermal expansion rates substantially matching the thermal expansion rates of the fiber optic coil; Figure 4 illustrates an end view of the second embodiment of the present invention illustrated in Figure 3; Figure 5 illustrates a third embodiment of the present invention in which a fiber optic coil is attached externally to a hub having thermal expansion rates substantially matching the thermal expansion rates of the fiber optic coil; Figure 6 illustrates a fourth embodiment of the present invention in which a fiber optic coil is attached to a hub having a glass fiber outer cylinder bonded with compliant adhesive to a low expansion inner support structure such that the hub's external interface has thermal expansion rates substantially matching the thermal expansion rates of the fiber optic coil; Figure 7 illustrates a first exemplary winding pattern which may be used in connection with the fiber optic coils shown in Figures 1 - 6; Figure 8 illustrates a second exemplary winding pattern which may be used in connection with the fiber optic coils shown in Figures 1 - 6; and, Figure 9 illustrates a third exemplary winding pattern which may be used in connection with the fiber optic coils shown in Figures 1 - 6. DETAILED DESCRIPTION As shown in Figures 1 and 2, a fiber optic rate sensor 10 includes a sensing coil 12 wound around a hub 14 in any predetermined winding configuration. The fiber optic rate sensor 10 has an axial direction 16 and a radial direction 18. An optical fiber is used to wind the sensing coil 12 in multiple layers with each layer having multiple turns. The coil 12 is suitably attached to the hub 14. The hub 14 is formed from materials so as to have coefficients of thermal expansion substantially matching the coefficients of thermal expansion of the sensing coil 12 in both the axial direction 16 and the radial direction 18. For example, the hub 14 may be a mounting structure wound from a glass fiber which is the same as, or substantially similar to, the optical fiber that is used to wind the sensing coil 12. The glass fiber used to wind the hub 14 may be coated with a buffer material and treated with adhesive so that, after curing, the hub 14 will be a rigid support structure upon which the sensing coil 12 can be wound. Alternatively, the hub 14 may be a mounting structure having a plurality of glass laminations bonded together by epoxy. Accordingly, the hub 14 has a thermal expansion rate which is the same as, or similar to, the thermal expansion rate of the sensing coil 12 in both the axial direction 16 and the radial direction 18. Because the thermal expansion rates of the sensing coil 12 and the hub 14 substantially match, performance of the fiber optic rate sensor 10 is not significantly degraded due to changing temperature conditions along the axial direction 16 and/or the radial direction 18. An adhesive layer may be provided at the interface between the sensing coil 12 and the hub 14 in order to bond the sensing coil 12 to the hub 14. Figures 3 and 4 show a second embodiment of the present invention in the form of a fiber optic rate sensor 20. The fiber optic rate sensor 20 has a sensing coil 22 attached internally to a hub 24. As in the case of the fiber optic rate sensor 10, the hub 24 may be formed from materials similar to the materials used to form the hub 14 of the fiber optic rate sensor 10. Accordingly, the thermal expansion rates of the sensing coil 22 and the hub 24 substantially match. An adhesive layer may be provided at the interface between the sensing coil 22 and the hub 24 in order to bond the sensing coil 22 to the hub 24. Figure 5 shows a third embodiment of the present invention in the form of a fiber optic rate sensor 30. The fiber optic rate sensor 30 includes a sensing coil 32 wound about a hub 34. In this case, the hub 34 is in the shape of a spool having end flanges 36 and 38 and a center cylindrical section 40. An adhesive layer may be provided between the sensing coil 32 and the end flanges 36 and 38 in order to bond the sensing coil 32 to the hub 34. Alternatively, the adhesive layer may be provided between the sensing coil 32 and the center cylindrical section 40 of the hub 34 in order to bond the sensing coil 32 to the hub 34. As a still further alternative, an adhesive layer may be provided between the sensing coil 32 and the end flanges 36 and 38 as well as the center cylindrical section 40. As in the case of the fiber optic rate sensors 10 and 20, the hub 34 may be formed from materials similar to the materials used to form the hubs 14 and 24. Accordingly, the sensing coil 32 and the hub 34 have substantially matching thermal expansion rates. Figure 6 shows a fourth embodiment of the present invention in the form of a fiber optic rate sensor 50. The fiber optic rate sensor 50 includes a sensing coil 52 wound about a hub having an inner hub 54 and an outer hub 56. The outer hub 56 may be formed by winding a glass fiber around the inner hub 54. The optical fiber used to wind the outer hub 56 is preferably the same as, or substantially similar to, the optical fiber that is used to wind the sensing coil 52. Also, the optical fiber used to wind the outer hub 56 may be bonded to the inner hub 54 with a compliant adhesive. The inner hub 54 may have a low thermal expansion rate and may be fabricated using (i) a sintered powder such as copper and tungsten, or copper and molybdenum, or the like, (ii) an alloy such as Monel™ or stainless steel or titanium, or the like, or (iii) co-fired ceramics such as Mycor™ or the like. Accordingly, the sensing coil 52 and the hub 54/56 have substantially matching thermal expansion rates. Hub and coil configurations other than those shown in Figures 1-6 may be provided according to the present invention. Moreover, other materials may be used for the hub of a fiber optic rate sensor in accordance with the present invention as long as the thermal expansion rates of the hub substantially matches the thermal expansion rates of the coil of such fiber optic rate sensor. The sensing coils of a fiber optic rate sensor, such as the sensing coils 12, 22, 32, and 52 shown in Figures 1-6, may have various winding configurations. Three such winding configurations are shown by way of example in Figures 7, 8, and 9. A winding configuration 60 shown in Figure 7 is generally referred to as a quadrupole winding arrangement. The winding configuration 60 specifically comprises a plurality of quadrupoles wound sequentially about a center line 62. Each layer of the winding configuration 60 represents a plurality of turns wound from an optical fiber. The turns in a layer without x's represent turns wound from one end of the optical fiber, and the turns in a layer with x's represent turns wound from the other end of the optical fiber. Accordingly, the turns of a first layer 64 are wound from a first end of an optical fiber, the turns of a second layer 66 are wound from a second end of the optical fiber, the turns of a third layer 68 are wound from the second end of the optical fiber, and the turns of a fourth layer 70 are wound from the first end of the optical fiber to form a first quadrupole of the winding configuration 60. A second quadrupole is wound about the first quadrupole. The second quadrupole includes layers 72, 74, 76, and 78. As can be seen from Figure 7, the layers 72, 74, 76, and 78 are wound in the same configuration as the layers 64, 66, 68, and 70. That is, the turns in the layer 72 are wound from the first end of the optical fiber, the turns in the layer 74 are wound from the second end of the optical fiber, the turns in the layer 76 are wound from the second end of the optical fiber, and the turns in the layer 78 are wound from the first end of the optical fiber. A winding configuration 80 is shown in Figure 8 and is an octupole winding configuration. An octupole winding configuration generally has a first four layers wound as a conventional quadrupole, and a second four layer wound as a reverse quadrupole. Accordingly, the winding configuration 80 has layers 82, 84, 86, 88, 90, 92, 94, and 96. Each layer comprises a plurality of turns wound from an optical fiber having first and second ends. As shown in Figure 8, the turns of the layer 82 are wound from a first end of the optical fiber, the turns of the layer 84 are wound from a second end of the optical fiber, the turns of the layer 86 are wound from the second end of the optical fiber, the turns of the layer 88 are wound from the first end of the optical fiber, the turns of the layer 90 are wound from the second end of the optical fiber, the turns of the layer 92 are wound from the first end of the optical fiber, the turns of the layer 94 are wound from the first end of the optical fiber, and the turns of the layer 96 are wound from the second end of the optical fiber. Additional octupoles may be wound around the winding configuration 80 shown in Figure 8. Indeed, as discussed above, a reverse octupole may be added to the octupole shown in Figure 8. A winding configuration 130 is shown in Figure 9 and is an interleaved winding configuration. The winding configuration 130 includes layers 132, 134, 136, 138, 140, 142, 144, 146, and 148. The turns of the layer 132 are wound from a first optical fiber, and the turns of the layers 134, 136, 138, 140, 142, 144, 146, and 148 are wound from a second optical fiber. Accordingly, the turns in the layer 132 are not a functional part of the winding configuration 130, although the turns in the layer 132 could be functional. The first optical fiber that is used to wind the turns of the layer 132 has an outer diameter that is larger than the outer diameter of the second optical fiber which is used to wind the layers 134, 136, 138, 140, 142, 144, 146, and 148. As shown in Figure 9, the layers 134-148 include alternate turns wound from the first and second ends of the second optical fiber. A specific interleaved winding pattern for the layers 134-148 is shown in Figure 9, although other interleaved winding patterns can be employed. Examples of interleaved winding patterns are taught in U.S. Application 08/668,485, which was filed on June 21, 1996, and which has been allowed by the U.S. patent and Trademark Office. The disclosure of U.S. Application 08/668,485 is incorporated by reference herein. Certain modifications of the present invention have been discussed above. Other modifications will occur to those practicing in the art of the present inven- tion. For example, the present invention has been described in terms of a particular type of fiber optic device, i.e., a fiber optic rate sensor. However, the present invention may be used in connection with other types of fiber optic devices. Moreover, as discussed above, hub and coil configurations other than those shown in Figures 1-6 may be provided according to the present invention. For example, a sensing coil could be wound onto a low thermal expansion rate hub using a compliant joint between the sensing coil and the hub. This compliant joint reduces stress on the sensing coil caused by differences in axial thermal expansion rates between the sensing coil and the hub. The compliant joint may be a compliant adhesive. Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.

Claims: WHAT IS CLAIMED IS: 1. A fiber optic device comprising: a support structure; and, a coil supported by the support structure, wherein the coil is wound from an optical fiber, and wherein the support structure and the coil have substantially matching coefficients of thermal expansion. 2. The fiber optic device of claim 1 wherein the support structure comprises a fiber support structure formed from an optical fiber having substantially the same coefficient of thermal expansion as the coefficient of thermal expansion of the optical fiber used to wind the coil. 3. The fiber optic device of claim 2 wherein the optical fiber of the fiber support structure is bonded by an adhesive. 4. The fiber optic device of claim 1 wherein the support structure comprises glass embedded in an adhesive, and wherein the glass has substantially the same coefficient of thermal expansion as the coefficient of thermal expansion of the optical fiber used to wind the coil. 5. The fiber optic device of claim 4 wherein the glass is laminated glass. 6. The fiber optic device of claim 1 wherein the support structure comprises inner and outer hubs, and wherein the outer hub is formed of glass fiber to have substantially the same coefficient of thermal expansion as the coefficient of thermal expansion of the optical fiber used to wind the coil. 7. The fiber optic device of claim 6 wherein the inner hub is formed of a low thermal expansion rate material. 8. A fiber optic device comprising: a hub, wherein the hub is formed at least partially from glass; and, a coil supported on the hub, wherein the coil is wound from an optical fiber, and wherein the glass of the hub and the optical fiber of the coil have substantially matching thermal expansion rates. 9. The fiber optic sensor of claim 8 wherein the glass of the hub comprises an optical fiber. 10. The fiber optic sensor of claim 9 wherein the optical fiber of the hub is embedded in an adhesive. 11. The fiber optic sensor of claim 8 wherein the glass of the hub is laminated glass. 12. The fiber optic sensor of claim 11 wherein the laminated glass is embedded in an adhesive. 13. The fiber optic sensor of claim 8 wherein the hub comprises inner and outer hubs, and wherein the outer hub is formed of glass fiber to have substantially the same coefficient of thermal expansion as the coefficient of thermal expansion of the optical fiber used to wind the coil. 14. The fiber optic sensor of claim 13 wherein the inner hub is formed of a low thermal expansion rate material. 15. A method of making a fiber optic sensor having a coil and a hub, wherein the method comprises the following steps: a) winding an optical fiber into the coil, wherein the optical fiber has a coefficient of thermal expansion; and, b) forming the hub so that the hub is formed from a material having a coefficient of thermal expansion substantially matching the coefficient of thermal expansion of the optical fiber and so that the hub supports the coil. 16. The method of claim 15 wherein step b) comprises the step of forming the hub from an optical fiber having substantially the same coefficient of thermal expansion as the coefficient of thermal expansion of the optical fiber used to wind the coil in step a). 17. The method of claim 16 wherein step b) comprises the step of embedding the optical fiber of the hub in an adhesive. 18. The method of claim 15 wherein step b) comprises the step forming the hub from laminated glass having substantially the same coefficient of thermal expansion as the coefficient of thermal expansion of the optical fiber used to wind the coil in step a). 19. The method of claim 18 wherein step b) comprises the step of embedding the laminated glass in an adhesive. 20. The method of claim 15 wherein step b) is performed before step a) so that the optical fiber is wound onto the hub to form the coil. 21. The method of claim 20 wherein step b) comprises the step of forming the hub from an optical fiber having substantially the same coefficient of thermal expansion as the coefficient of thermal expansion of the optical fiber used to wind the coil in step a). 22. The method of claim 21 wherein step b) comprises the step of embedding the optical fiber of the hub in an adhesive. 23. The method of claim 20 wherein step b) comprises the step forming the hub from laminated glass having substantially the same coefficient of thermal expansion as the coefficient of thermal expansion of the optical fiber used to wind the coil in step a). 24. The method of claim 23 wherein step b) comprises the step of embedding the laminated glass in an adhesive. 25. The method of claim 15 wherein step a) is performed before step b) so that the optical fiber is wound into the coil and is then applied to the hub. 26. The method of claim 15 wherein step a) comprises the step of winding the coil so that four layers of the coil are wound with a + - - + winding configuration. 27. The method of claim 15 wherein step a) comprises the step of winding the coil so that eight layers of the coil are wound with a + - - + + - - + winding configuration. 28. The method of claim 15 wherein step a) comprises the step of winding the coil so that eight layers of the coil are wound with a + - - + - + + - winding configuration. 29. The method of claim 15 wherein step a) comprises the step of winding the coil so that at least one layer of the coil has an interleaved winding configuration. 30. The method of claim 15 wherein step b) comprises the step of forming an outer hub around an inner hub so that the outer hub is formed from a material having a coefficient of thermal expansion substantially matching the coefficient of thermal expansion of the optical fiber. 31. The method of claim 30 wherein the inner hub is formed from a material having a low rate of thermal expansion. 32. The method of claim 31 wherein the material is a sintered powder. 33. The method of claim 31 wherein the material is a sintered powder of copper and tungsten. 34. The method of claim 31 wherein the material is a sintered powder of copper and molybdenum. 35. The method of claim 31 wherein the material is an alloy. 36. The method of claim 31 wherein the material is a stainless steel alloy. 37. The method of claim 31 wherein the material is a titanium alloy. 38. The method of claim 31 wherein the material is a co-fired ceramic. 39. The method of claim 15 further comprising the step of bonding the coil to the hub. 40. A fiber optic device comprising: a low thermal expansion rate support structure; a fiber optic coil supported by the low thermal expansion rate support structure; and, a compliant joint between the low thermal expansion rate support structure and the fiber optic coil.

Inventors: KALISZEK ANDREW W, KALISZEK, ANDREW, W.
Applicants: HONEYWELL INC, HONEYWELL INC.
Application year: 1999
Main classification: G02B6
IPC codes: G01C19, G01P9, G02B6