METHOD FOR FABRICATING AN OPTICAL WAVEGUIDE

Abstract: The method for fabricating an optical waveguide may include a plurality of mechanical steps, e.g., lapping, polishing, and/or dicing, and bonding steps, e.g., attaching with adhesives. In one embodiment, the method includes the steps of providing an optical material (20), thinning and polishing the optical material to form an optical waveguide core (12) comprising a plurality of longitudinally extending surfaces, providing a plurality of support substrates (30, 50, 90, 110), and attaching the plurality of support substrates to the longitudinally extending surfaces of the core (12). The plurality of support substrates may be attached to the plurality of longitudinally extending surfaces of the optical material with an adhesive. The optical material may include a high refractive index, and the plurality of support substrates and/or the adhesive may include a low refractive index.

Description: METHOD FOR FABRICATING AN OPTICAL WAVEGUIDE Cross-Reference to Related Application This application relates to the following commonly assigned, concurrently filed U.S. Patent Application: U.S. Patent Application Serial No. 09/121,454, and entitled "Optical Waveguide with Dissimilar Core and Cladding Materials, and Light Emitting Device Employing Same . " This application is hereby incorporated by reference herein in its entirety. Technical Field This invention relates in general to waveguides, and in particular to novel methods for fabricating optical waveguides. Background Information Waveguides constrain or guide the propagation of electromagnetic waves along a path defined by the physical construction of the waveguide. The use of optical channel waveguides is widespread in integrated optical circuits. In particular, an optical channel waveguide provides both vertical and lateral confinement of an optical wave while allowing low-loss signal propagation. An optical channel waveguide having small cross- sectional dimensions allows high optical power densities to be established for moderate optical input powers while the waveguiding nature provides potentially long interaction lengths. This combination of effects is extremely attractive for a variety of optical functions such as second harmonic generation, optical amplification, wavelength conversion, and phase modulation (when an appropriate electrode geometry is incorporated) . In general, a goal of waveguide fabrication is to produce waveguides which support a single guided mode of propagation of the electromagnetic waves. A number of techniques have been used with considerable success to fabricate optical channel waveguides. These include ion-exchange in glass substrates, ion indiffusion or proton exchange in LiNb0 3 substrates, pattern definition by laser ablation, photolithography of spun polymer films, and epitaxial growth and selective etching of compound semiconductor films. A drawback of these techniques is that they cannot be used with a significant number of useful optical materials, e.g., many laser crystals. Another drawback of these prior art techniques is that the equipment required to fabricate the optical waveguide is expensive. Therefore, there is a need for methods for forming optical waveguides from separate preformed optical materials in which the methods comprise a plurality of mechanical processing steps, e.g., lapping, polishing, and/or dicing, and bonding steps, e.g., attaching with adhesives. Such methods are adaptable to fabrication of optical waveguides from any, if not all, optical materials. Furthermore, such methods are suitably performed using readily available and inexpensive equipment. Summary of the Invention Pursuant to the present invention, the shortcomings of the prior art are overcome and additional advantages provided through the provision of a method for forming an optical waveguide from separate preformed materials. For example, one embodiment of the method for forming an optical waveguide comprises the steps of providing an assembly comprising an optical material between a first support substrate and a second support substrate, providing a third support substrate and a fourth support substrate, and attaching to opposite surfaces of the assembly, a third support substrate and a fourth support substrate, wherein the opposite surfaces each comprise the first support substrate, the optical material, and the second support substrate . In one expect of the invention, the step of providing the assembly comprises providing the optical material comprising a polished surface, attaching the polished surface to the first substrate, thinning and polishing a second surface of the optical material, and attaching a second support substrate to the second polished surface. In another aspect of the invention, the step of attaching opposite surfaces of the assembly between a third support substrate and a fourth support substrate comprises the steps of polishing a surface of the assembly, wherein the surface comprises the first support substrate, the optical material, and the second support substrate, attaching the polished surface of the assembly to the third support substrate, thinning and polishing an opposite surface of the assembly, wherein the opposite surface comprises the first support substrate, the optical material, and the second support substrate, and attaching the opposite polished surface to the fourth support substrate . In another aspect of the present invention, the method further comprising the step of dicing the first assembly to form a plurality of assemblies, wherein each of the plurality of assemblies is attachable to separate support structures for forming separate optical waveguides. In another embodiment of the present invention for forming a waveguide, the method comprising the steps of providing an optical material, thinning and polishing the optical material to form a core comprising a plurality of longitudinally extending surfaces, providing a plurality of support substrates, and adhesively attaching the plurality of support substrates to the longitudinally extending surfaces of the core. Desirably, the plurality of support substrates are attached to the plurality of longitudinally extending surfaces of the optical material with an adhesive. The optical material may comprise a high refractive index, and the plurality of support substrates and/or the adhesive may comprise a low refractive index. The optical waveguides fabricated according to the present invention, when the core comprises an optical gain material, are particularly suitable for lasers and amplified spontaneous emission (ASE) sources for imaging and spectroscopy applications where multi-mode fibers are used to handle high power, as well as test instrumentation for the telecommunications and cable television industries where single mode delivery is required. Additional and detailed uses of the optical waveguides of the present invention are described in the above- incorporated application. Brief Description of the Drawings The above-described objects, advantages and features of the present invention, as well as others, will be more readily understood from the following detailed description of certain proffered embodiments of the invention, when considered in connection with the accompanying drawings in which: FIG. 1 is a perspective view of an optical waveguide fabricated in accordance with the methods of the present invention; FIGS. 2A-2H are diagrammatic illustrations of one embodiment of the fabrication sequence for forming the optical waveguide shown in FIG. 1; and FIGS. 3A and 3B together provide a flowchart of the fabrication sequence of the optical waveguide shown in FIG. 2. Detailed Description of the Preferred Embodiment With reference to FIG. 1, therein illustrated is one embodiment of an optical waveguide 10 constructed in accordance with the principles of the present invention. As explained in greater detail below, a novel series of successive precision polishing and bonding steps allows quick and inexpensive fabrication of waveguide 10 having a core 12 surrounded by suitable cladding or support substrate 14. For use as an optical waveguide, core 12 comprises a high refractive index while cladding or support substrate 14 comprises a low refractive index. For example, optical material 12 may comprise a relatively expensive laser crystal, and cladding or support substrate 14 may comprise a relatively low cost glass material, e.g., fused silica. In the illustrated embodiment shown in FIG. 1, optical waveguide 10 is in the form of a channel waveguide having a substantially square cross- section. The elongated shape of core 12 provides a propagation axis a η therein which may be longitudinally aligned with the outer surfaces of the support substrate . The fabrication of optical waveguide 10 comprises a multi-step process of precision polishing and/or lapping techniques to mechanically thin a preformed optical material to form a core of the optical waveguide having a desired thickness in both the lateral and the vertical orientations. Optical adhesives are used to bond the core to preformed surrounding support substrates . The core and the cladding or support substrate may comprise dissimilar materials, e.g., materials which are structurally and/or chemically distinct, and which have been separately fabricated as physically different materials and brought together during the assembly process for the optical waveguide. Advantageously, the various methods according to the present invention may be performed by a machinist in which separate preformed optical materials are initially suitably sized for easy manipulation and fabrication according to the present invention. For example, an initially sized optical material may have a width of about 20 mm, a length of about 20 mm, and a thickness of about 0.5 mm to about 1 mm for easy manipulation, and which may be fabricated into a core of an optical waveguide in which the core has cross-sectional dimensions on the order of only tens of microns . FIGS. 2A-2H diagrammatically illustrate a sequence of steps of one embodiment according to the present invention for fabricating optical waveguide 10. FIGS. 3A and 3B together form a flowchart which describes each of the steps illustrated in FIGS. 2A- 2H in greater detail. In this illustrated and described method, core 12 (best illustrated in FIGS. 1 and 2H) of optical waveguide 10 desirably has a square cross-section that measures, e.g., 20 μm X 20 μm. Thus, the separate preformed materials used in the fabrication of optical waveguide 10 have been shown out of scale in the drawings for purposes of illustration. In this illustrated method, initially an optical material 20 (from which core 12 will be formed) is attached to a first support substrate 30. For example, optical material 20 may be planar in shape having a width of about 20 mm, a length of about 20 mm, and a thickness of about 0.5 mm to about 1 mm. First support substrate 30 may also be planar in shape having a width of about 20 mm, a length of about 20 mm, and a thickness of about 2 mm. Optical material 20 comprises a bottom surface 22 and a top surface 24. Prior to attaching optical material 20 to first support substrate 30, bottom surface 22 may be optically polished flat and smooth, e.g., so that the surface becomes transparent. Bottom surface 22 may be optically polished by standard lapping and polishing techniques, e.g., in which the optical material is moved over a flat plate on which a liquid abrasive has been poured. The process of lapping and polishing may use water-based slurries with varying particle sizes (e.g., about 0.5 μm to about 9 μm) and types of abrasives (e.g., aluminum oxide and cerium oxide) . The dimensions of the optical material can be measured using a micrometer gauge with processing being terminated upon reaching the desired surface quality and/or thickness. Accuracies of about 1 μm can be achieved. In addition, the use of high precision polishing jigs allows exceptional flatness of surfaces, as well as, surfaces being parallel (and/or perpendicular) to each other. A value of flatness suitable for surface 22 may be determined based on the wavelength of light for which the optical waveguide will be used. For example, where the optical waveguide will be used with light having a wavelength of 1 μm, a suitable flatness may be about .05 μm over, e.g., the length or the thickness. In addition, surface 22 desirably has a smooth surface quality, i.e., little, if any scratches, pits or surface damage. For example, surface 22 desirably has a scratch to dig designation of about 5-10 which is typically desired in optical components for use in laser applications. A top surface 32 of first support substrate 30 may be optically polished, e.g., by lapping and polishing, as described above with reference to surface 22 of optical material 20, so that the surface is flat and smooth. Optically polished surfaces 22 and 32 may then be adhesively attached to each other with a suitable layer of optical adhesive 40. Desirably, layer of optical adhesive 40 is formed with a thickness less than about 2 μm. With reference to FIG. 2B, in which optical material 20 is attached to first support substrate 30, top surface 24 (FIG. 2A) is thinned and optically polished, e.g., by lapping to reduce the thickness a carefully controlled amount and polishing to obtain an optically polished surface 26 as described above. In this illustrated method of fabrication, optical material 20 initially comprises a thickness of about 0.5 to about 1 mm, which is reduced in thickness to about 20 μm. A second support substrate 50 is then attached to optically polished surface 26 of the thinned optical material 20 to form an assembly 70, as shown in FIG. 2C. A bottom surface 52 of second support substrate 50 may be optically polished, e.g., by lapping and polishing as described above, so that the surface is flat and smooth. Optically polished surfaces 52 and 26 may then be adhesively attached to each other with a suitable layer of optical adhesive 60. Desirably, second support substrate 50 may be planar in shape having a width of about 20 mm, a length of about 20 mm, and a thickness of about 2 mm. Layer of optical adhesive 60 desirably has a thickness less than about 2 μm. As shown in FIG. 2D, assembly 70 (FIG. 2C) may be cut or diced through second support substrate 50, thinned optical material 20, and first support substrate 30, into a plurality of about 0.5 mm to about 1 mm thick slices 80. For example, assembly 70 may be diced using a diamond blade saw, wire saw, or wafer dicing machine. Each slice 80 may be processed into a separate waveguide according to the following method steps in which slice 80 is laid flat and sandwiched between two separate support substrates. Advantageously, simultaneous processing of slices 80 results in the production of multiple optical waveguides allowing the process to be cost effective. With reference to a single slice 80, as shown in FIG. 2E, a surface 82 of slice 80 in which surface 82 comprises first support substrate 30, optical material 20, and second support substrate 50, is optically polished, e.g., by lapping and polishing as described above, so that the surface is flat and smooth. Surface 82 of slice 80 is then attached to a third support substrate 90 as shown in FIG. 2F. A top surface 92 of third support substrate 90 may be optically polished, e.g., by lapping and polishing as described above, so that the surface is flat and smooth. Optically polished surfaces 92 and 82 may then be adhesively attached to each other with a suitable layer of optical adhesive 100. In this exemplary embodiment, third support substrate 90 may be planar in shape having a width of about 4 mm, a length of about 20 mm, and a thickness of about 2 mm. Layer of optical adhesive 100 desirably has a thickness less than about 2 μm. With reference to FIG. 2G, after slice 80 is attached to third support substrate 90, a surface 84 (FIG. 2F) is thinned and optically polished, e.g., by lapping and polishing as described above, to reduce the thickness a carefully controlled amount and to obtain a flat polished surface 86. In this illustrated method of fabrication, slice 80 initially comprises a thickness of about 0.5 to about 1 mm and is reduced in thickness to about 20 μm, so that core 12 is formed having a substantially square transverse cross-section that measures 20 μm X 20 μm. As shown in FIG. 2H, a fourth support substrate 110 is attached to optically polished surface 86 of slice 80 to form optical waveguide 10. A surface 112 of fourth support substrate 110 may be optically polished, e.g., by lapping and polishing as described above, so that the surface is flat and smooth. Optically polished surfaces 112 and 86 may then be adhesively attached to each other with a suitable layer of optical adhesive 120. In this exemplary embodiment, fourth support substrate 100 may be planar in shape having a width of about 4 mm, a length of about 20 mm, and a thickness of about 2 mm. Layer of optical adhesive 120 desirably has a thickness of less than about 2 μm. Preferably, first end 16 and second end 18 of optical waveguide 10 are optically polished. Wavelength-dependent optically reflective materials may be applied over the optical waveguide ends 16 and 18 to form an optical cavity which allows introduction of a pump energy at a predetermined wavelength into the optical waveguide and also allow radiation emission from the optical waveguide at a desired source wavelength. Desirably, the mating surfaces of the optical material and the substrates have the same surface quality, and the thickness of the layer of adhesive between each of the mating surfaces is the same. While the illustrated and disclosed method of fabricating a waveguide in which a thin, e.g., less than about 2 μm thick layer of optical adhesive is used to attach the substrates to the optical material, from the present description it will be appreciated by those skilled in the art that a layer of optical adhesive having a greater thickness may be used. For example, with a greater thickness layer of optical adhesive, e.g., greater than about 3 μm, the adhesive layer itself may provide suitable cladding to influence the waveguide properties. In this alternative embodiment, it would not be necessary to optically polish surfaces of the support substrate which mate with surfaces of optical material 20 and/or core 12. In this situation, the support substrates can be selected for their processing qualities irrespective of the refractive index. It should be noted that, for maintaining the surfaces of optical adhesive cladding layer, parallel and perpendicular to the core, and for maintaining the layer of optical adhesive at a constant thickness, it may be desirable to optically polish surfaces of the support substrates. In addition, practical issues such as edge breakage and differential polishing rates between the adhesive and core/cladding materials need to be considered in selecting appropriate optical adhesives and layer thickness. Suitable optical waveguide assemblies fabricated from the methods according to the present invention may have dimensions in the range of about 2-5 mm X about 2-5 mm in cross-section and 5-30 mm in length although greater lengths are possible. This allows easy handling and mechanical fixturing of the optical waveguide during manufacture and optical testing. Support structures 30, 50, 90, and 110 preferably comprise the same material having the same refractive index, e.g., being initially cut from a single common substrate. Layers of optical adhesive 40, 60, 100 and 120, desirably comprise the same optical adhesive having a refractive index desirably corresponding to the refractive index of the support substrates. Also desirably, the layers of optical adhesive provide adequate edge support to optical material 20 and support substrates 30, 50, 90, and 110 during the polishing and/or dicing steps so that degradation of the edges of the optical material and/or support substrates is minimized. Optical adhesives for use in the method according to the present invention may be suitable ultraviolet cured optical adhesives, e.g., Norland 61 manufactured and available from Norland Products Inc., of New Brunswick, New Jersey. In order to improve the optical properties of optical waveguide 10 (e.g., polarization dependence), preferably optically polished surface 26 is substantially parallel to optically polished surface 22, optically polished surface 82 is substantially parallel to optically polished surface 86, and each of the four adjacent surfaces, e.g., optically polished surface 26 and 82, are substantially perpendicular to each other. The fabrication process according to the present invention is also compatible with most optical materials, e.g., active materials such as laser crystals or doped glass for use as lasers, amplifiers, ASE sources and wavelength converters. Suitable active materials include LiNb0 3 , Nd:YAG, Nd: Glass, Nd:YLF, Nd:LiNb0 3 , Er:YAG, Er : Glass, Er:LiNb0 3 , ErrSilicon, Cr : Forsterite, Cr:YAG, and Ti:Al 2 0 3 . While the illustrated optical waveguide is shown as an optical channel waveguide having a square cross-section, e.g., 20 μm by 20 μm core, from the present description it will be appreciated by those skilled in the art that optical waveguides may be fabricated by the methods of the present invention to have other cross-sectional configurations depending on the particular application and the desired propagation of an optical signal within the waveguide. For example, optical waveguides fabricated according to the present invention may be in the form of planar or slab optical waveguides having a rectangular transverse cross-section. While the invention has been particularly shown and described with reference to preferred embodiment (s) thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims: CLAIMS 1. A method for forming an optical waveguide, said method comprising the steps of: providing an assembly comprising an optical material between a first support substrate and a second support substrate; providing a third support substrate and a fourth support substrate; and attaching to opposite surfaces of said assembly, said third support substrate and said fourth support substrate, wherein said opposite surfaces each comprise said first support substrate, said optical material, and said second support substrate. 2. The method according to claim 1, wherein said step of providing said assembly comprises the steps of : providing said optical material comprising a polished surface; providing said first support substrate; attaching said polished surface of said optical material to said first support structure; polishing a second surface of said optical material; providing said second support substrate; and attaching said second polished surface to said second support structure. 3. The method according to claim 2, wherein said step of polishing a second surface of said optical material comprises the step of thinning said optical material. 4. The method according to claim 2, wherein said step of attaching said optical material to said first support substrate and to said second support substrate comprises the steps of providing an adhesive, and adhesively attaching said optical material to said first and second support substrates. 5. The method according to claim 2, wherein said steps of providing said first and second substrates comprise providing said first support substrate comprising a polished surface and said second support substrate comprising a polished surface . 6. The method according to claim 1, wherein said step of providing said assembly comprises the steps of : providing said optical material; optically polishing a first surface of said optical material; providing said first support substrate; attaching said first surface of said optical material to a surface of said first support substrate; optically polishing a second surface of said optical material; providing said second support substrate; and attaching said second surface of said optical material to a surface of said second support substrate. 7. The method according to claim 6, wherein said steps of providing said first and second substrates comprise providing said first support substrate comprising a polished surface and said second support substrate comprising a polished surface . 8. The method according to claim 1, wherein said step of attaching opposite surfaces of said assembly between said third support substrate and said fourth support substrate comprises the steps of; polishing a surface of said assembly, wherein said surface comprises said first support substrate, said optical material, and said second support substrate; attaching said polished surface of said assembly to said third support substrate; polishing an opposite surface of said assembly, wherein said opposite surface comprises said first support substrate, said optical material, and said second support substrate; and attaching said opposite polished surface to said fourth support substrate. 9. The method according to claim 8, wherein said step of polishing said opposite surface of said assembly comprises the step of thinning said assembly. 10. The method according to claim 8, wherein said steps of providing said third and fourth substrates comprise providing said third support substrate comprising a polished surface and said fourth support substrate comprising a polished surface . 11. The method according to claim 10, wherein said steps of providing said third and fourth substrates comprise polishing a surface of said third support substrate and polishing a surface of said fourth support substrate. 12. The method according to claim 11, wherein said step of attaching said opposite polished surfaces to said third support substrate and said fourth support substrate comprises the steps of providing an adhesive, and adhesively attaching said opposite polished surfaces to said third and fourth support substrates . 13. The method according to claim 1, wherein said optical material and said support substrates are fabricated from separate preformed materials . 14. The method according to claim 13 , wherein said support substrates are fabricated from a common substrate material . 15. The method according to claim 1, further comprising the step of dicing said first assembly to form a plurality of assemblies, wherein each of said plurality of assemblies is attachable to separate support structures for forming separate optical waveguides. 16. The method according to claim 1, wherein said optical material comprises a crystalline material and said support substrates comprise an amorphous material . 17. The method according to claim 1, wherein said optical material comprises an active material. 18. The method according to claim 17, wherein said optical material comprises a laser crystal. 19. The method according to claim 17, wherein said optical material comprises a doped glass material . 20. The method according to claim 1, wherein said optical material comprises a high refractive index and said support substrates comprise a low refractive index. 21. The method according to claim 20, wherein said support substrates comprise similar refractive indexes. 22. The method according to claim 1, wherein said optical material is attached to said support substrates with an adhesive. 23. The method according to claim 1, wherein said waveguide comprises an optical channel waveguide having a substantially square transverse cross- section. 24. A method for forming a waveguide, said method comprising the steps of: providing an optical material; polishing a surface of said optical material; attaching said polished surface of said optical material to a first support substrate; polishing an opposite surface of said optical material; providing a second support substrate; attaching said opposite polished surface of said optical material to said second support substrate to form an assembly; dicing said assembly to provide a plurality of assemblies each of which comprises said optical material between said first and second support structures; polishing surfaces of said plurality of assemblies in which said polished surfaces comprise said first support substrate, said optical material, and said second support substrate; providing a plurality of third support substrates; attaching said polished surface of said assemblies to said plurality of third support substrates; polishing an opposite surface of each of said assemblies in which said opposite surface comprises said first support substrate, said optical material, and said second support substrate; providing a plurality of fourth support substrates; and attaching said opposite polished surfaces of said assemblies to said plurality of fourth support substrates. 25. A method for forming a waveguide, said method comprising the steps of: providing an optical material; thinning and polishing said optical material to form a core comprising a plurality of longitudinally extending surfaces; providing a plurality of support substrates; and attaching said plurality of support substrates to said longitudinally extending surfaces of said core with an adhesive. 26. The method according to claim 25, wherein said optical material comprises a high refractive index, and said plurality of support substrates comprise a low refractive index. 27. The method according to claim 25, wherein said optical material comprises a high refractive index, and said adhesive comprises a low reflective index. 28. The method according to claim 25, further comprising the step of polishing opposite ends of said core and applying a reflective material thereto. * * * * *

Inventors: LAWRENCE BRIAN L, MCCALLION KEVIN J, QUANTOCK PAUL R, SCHULZE JOHN L, WAGONER GREGORY A, LAWRENCE, BRIAN, L., MCCALLION, KEVIN, J., QUANTOCK, PAUL, R., SCHULZE, JOHN, L., WAGONER, GREGORY, A.
Applicants: MOLECULAR OPTOELECTRONICS CORP, MOLECULAR OPTOELECTRONICS CORPORATION
Application year: 1999
Main classification: G02B6
IPC codes: G02B6