Apparatus and Method for Embedding Optical Fibers in Metal

This invention relates in general to the field of fiber optics. More particularly, the present invention relates to apparatus and method for embedding optical fibers and optical fiber sensors in metal.

Ever since the introduction of the fiber optics technology, the optical fiber has proven to be a most versatile and promising means of transmitting data and information. Recent advanced developments in fiber optics sensor technology have spurred considerable research on embedding optical fibers and optical fiber sensors in various materials and structures. Studies have been conducted on embedding optical fibers in graphite-epoxy composites, plastics, concrete, and solder metals with relatively low melting temperature of 190.degree. C.

Optical fibers and sensors embedded in metals with much higher melting temperatures and strengths are desirable in numerous applications. For example, if metal-embedding were possible, an optical fiber sensor can then be cast into the cylinder head of an internal combustion engine to monitor its performance. In addition, metal-embedded fibers are applicable to connectors and feedthroughs in, for example, vacuum systems, undersea vehicles, undersea repeaters, chemical processing plants, and steam power plants. Metal-embedded fiber sensors may also find application in smart structures, such as aircraft wings and fuselages, ships, spacecraft, bridges, and buildings.

Previous attempts to embed optical fibers in metals with high melting temperatures have all resulted in failure. The fragile fibers invariably break either at or near the air-metal interface. The fiber breakage has been attributed to the stress inflicted on the fiber by the contraction of the surrounding metal as it cools from its melting temperature to room temperature. Therefore, it is obvious that resolution of the fiber breakage problem in embedding optical fibers in metals has broad implications. The present invention is a novel and unique solution to this problem.

In accordance with the present invention, apparatus and method for embedding optical fibers in metal are provided which substantially eliminate or reduce disadvantages and problems associated with prior apparatus and methods.

In one aspect of the present invention, a method for embedding optical fibers in metals uses stress-relieving tubes at the air-metal interfaces. The optical fiber is inserted through the tubes, which are positioned partially extending into the mold. The ends of the fiber extend beyond the tubes. Molten metal is then introduced into the mold, embedding the optical fiber and the stress-relieving tubes.

In another aspect of the present invention, one or more optical fibers are embedded in a metal component or structure by using stress-relieving tubes at the air-metal interface of the fibers. The stress-relieving tubes extend at least partially into the metal component.

For a better understanding of the present invention, reference may be made to the accompanying drawings, in which:

FIG. 1 is a simplified diagram illustrating a preferred method for embedding optical fibers in a metal structure;

FIGS. 2 and 3 are elevational views of metal-embedded embedded optical fibers in several arrangements;

FIG. 4 is an elevational view of an embedded fiber bolt assembly;

FIG. 5 is an elevational view of an embedded fiber feedthrough component; and

FIG. 6 is an elevational view of an alternate embodiment of the present invention.

With reference to the drawings, FIG. 1 illustrates a setup 10 for embedding optical fibers in metal in accordance with the teachings of the present invention. Setup 10 includes a stand 12 with fiber guides 14 and 15. A mold 16 constructed of investment, sand, graphite, or other materials known in the foundry art is held by a holder 18 attached to stand 12. The shape and dimension of the mold cavity generally determine the outer shape and dimension of the resulting metal component.

Mold 18 has an opening 20 for receiving a molten metal 22, and a small opening 24 at the bottom. The size of opening 24 is such that it permits a stress-relieving tube 26 to be inserted therethrough, but to leave minimal gap between opening 24 and the outer surface of tube 26 so that molten metal in the mold cavity cannot escape through it. In addition, the inside diameter of tube 26 is such that it accommodates one or more optical fibers or sensors and yet minimizes the entry of molten metal. For example, tubes having an outside diameter of 1.6 millimeters and an inside diameter of 0.5 millimeters have been shown to properly serve this purpose. Another tube 28 of similar construction and size is similarly positioned at the top opening 20 of mold 16, shown held in place by a holder 30. Tubes 26 and 28 may be constructed of metals, such as stainless steel, or ceramic and other suitable materials. Tubes 26 and 28 may, as an example, extend approximately one centimeter into the mold cavity. The length of each tube 26 and 28 is arbitrary but could, for example, be approximately two inches, and the tube lengths need not be the same. The length of tubes 26 and 28 preferably depends on the nature of the resultant metal part or structure. It is also contemplated that the portions of tubes 26 and 28 that extend beyond the metal part or structure may be removed after molten metal 22 has cooled.

An optical fiber or a fiber containing an internal sensor 32 is then inserted through tubes 26 and 28, and therefore the mold cavity. If fiber 32 is wrapped within a protective shield or buffer (not shown), it is removed from the portion of fiber 32 to be embedded in metal. In setup 10, weights 34 and 35 are attached to the respective ends of optical fiber 32 to effect a tensile load thereon. The magnitude of this load is a matter of engineering design, but could be approximately 20g, for example. Proper alignment of fiber 32 and tubes 26 and 28 are held and maintained by fiber guides 14 and 15 on stand 12. The tensile force ensures that the portion of fiber 32 within the metal part is straight. Those fibers having internal sensory structures such as the fiber Fabry-Perot interferometer (not shown) should be positioned so that the portion of the fiber containing the sensory structure is situated within the mold cavity.

After stress-relieving tubes 26 and 28 and optical fiber 32 have been positioned properly, molten metal 22 may be poured into the mold cavity. Molten metal 22 may be aluminum, aluminum alloys or other suitable metals or metal alloys. The only constraint for the purpose of embedding optical fibers is the melting temperature of the metal used. The melting point of molten metal 22 should be less than that of fiber 32, which is approximately 1600.degree. C. for glass fibers and over 2000.degree. C. for sapphire fibers.

Molten metal 22 is allowed to cool to or near room temperature before removing the resultant embedded optical fiber metal component from mold 16. It is important to note that setup 10 as shown in FIG. 1 is merely illustrative to point to the necessary steps for embedding optical fibers in metal and the present invention is not necessarily so limited.

Referring to FIG. 2, a more detailed elevational view of a resultant embedded optical fiber metal component 40 is shown. An optical fiber 42 is embedded in a metal part 44 threaded through partially-embedded stress-relieving tubes 46 and 47. Optical fiber 42 may be wrapped within a protective shield or buffer 48, but shield 48 is removed from the portion of fiber 42 buried in metal part 44. FIG. 3 serves to illustrate the unrestricted application of the instant invention. A metal component or structure 50 may have more than one embedded fiber in a number of possible arrangements 52-56. Arrangements 52 and 56 each have multiple optical fibers inserted through a pair of stress-relieving tubes. Arrangement 54 has one fiber embedded through one pair of stress-relieving tubes.

Referring to FIG. 4, a possible implementation of an optical fiber feedthrough 60 in accordance with the present invention is shown. Optical fiber 62 is embedded, along with partially embedded stress-relieving tubes 64 and 65, in a metal part that has been machined into a bolt assembly 68. Bolt assembly 68 has been threaded so that it can be screwed or fastened into a larger structure. Another implementation of an optical feedthrough 70 is shown in FIG. 5. An optical fiber 72 is fed through connectors 74 and 75 also partially embedded in metal part 78. Connectors 74 and 75 may be prefabricated prior to the fiber embedding process, or can be fashioned after the process. As shown, connectors 74 and 75 are of the decouplable type. Metal part 78 has been machined and threaded. A washer or spacer 80 and a nut 82 are also provided for attachment to part 78. Metal part 78 is further constructed for accommodating a vacuum seal or O-ring 84 to effect a hermetic seal. It can be seen that by using the teachings of the instant invention, optical fibers can be embedded into metals that are commonly used in structural constructions, and therefore the optical fibers can be cast directly into an existing component of a structure or system, such as an engine cylinder head.

FIG. 6 illustrates an alternate embodiment of an embedded optical fiber or sensor 90. Fiber or sensor 90 is inserted through a stress-relieving tube 92 partially embedded in a metal component or structure 94. Rather than positioning the fiber or sensor 90 so that both of its ends extend beyond the mold for constructing metal component 94, fiber or sensor 90 terminates in an embedded end 96. Constructed in this manner, optical signals may be delivered in fiber or sensor 90 and signals reflected by end 96 may be received and analyzed.

From the foregoing, the metal-embedded fiber and fiber feedthroughs can be made much smaller, cheaper and more effective in eliminating leakage than conventional O-ring feedthroughs. The embedded fiber sensors, such as the fiber Fabry-Perot interferometer, find wide application in monitoring a myriad structures and systems.

Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made thereto without departing from the spirit and scope of the present invention as defined by the appended claims.