Coupler for coupling optical energy transmitted by optical fiber to optical waveguide and method of manufacture
FIELD OF THE INVENTION 
     The present invention relates to the field of optics and more particularly to optical couplers for coupling optical energy from a fiber optic into a thin-film waveguide. 
     BACKGROUND OF THE INVENTION 
     The prior art evidences attempts to provide a satisfactory coupling device for coupling optical energy traveling in optical fibers to thin-film optical waveguides. For instance, see &#34;Interconnection of Optical Fibers to Planar Optical Integrated Circuitry,&#34; by Ash et al., appearing in IBM Technical Disclosure Bulletin, Volume 13, No. 9, pages 2529-30 (Feb. 1971). Another example of a device which is capable of coupling optical energy from an optical fiber to a thin-film waveguide is disclosed by Lean et al., in U.S. Pat. No. 3,791,715. 
     Although the devices referred to above may adequately couple optical energy from an optical fiber to a planar waveguide it is desirable to provide an improved coupler which meets one or more of the following criteria. 
     Reference to the Ash et al disclosure will illustrate that it is based on periodic structures setting up an interference pattern to cause a coupling of optical energy from a fiber to the waveguide. One of the difficulties with this approach is that it requires extremely accurate photolithography in order to transfer a pattern from a mask to a waveguide. Some of the mechanical handling requirements for performing the Ash et al. methods also do no admit of easy solution; for example, laying a fiber optic bundle along a planar waveguide with an included angle of approximately 1° is a difficult task, at best. 
     Another approach, such as that disclosed by Lean et al. requires the presence of electrical energy in order to &#34;open&#34; the coupler so that it will pass optical energy from the fiber optic to the planar waveguide. Another characteristic of Lean et al. which it may be desirable to omit is the necessity for coupling the light waves through the atmosphere, as is illustrated in FIG. 1 of Lean et al. 
     The foregoing disadvantages, and others, are typical of the prior art which can generally be characterized as requiring critical positioning and angular alignment of the fiber with respect to the waveguide substrate which makes manufacture of the coupler difficult. 
     SUMMARY OF THE INVENTION 
     Our invention provides an optical coupler for coupling optical energy from a fiber to a thin-film optical waveguide without requiring periodic structures which require accurate photolithography, without requiring the presence of electrical energy, without requiring critical positioning and angular alignment of the fiber optic and planar waveguide and without requiring the optical energy to travel outside an enclosed medium, that is such as air. In order to provide the foregoing advantages, a fiber optic is incorporated into a substrate so that a cross section of the fiber lies in the plane of the substrate surface. After suitable polishing an optical waveguide can then be provided on this surface. In such a coupler the optical energy propagating in the fiber will be trapped in the waveguide media provided the exit angle of the fiber with respect to the guide surface is greater than the critical angle for the index of refraction of the guide material. 
     One method which could be used in constructing a coupler of our invention sandwiches an optical fiber between two blocks of substrate material. A bonding material, which may be a low melting point glass powder or an epoxy, bonds the substrates and optical fiber. After the bonding step the unitized structure is cut at a correct angle in accordance with the foregoing criteria and is then polished. An optical guiding media may then be deposited on the now polished surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Our invention will be described in this specification when taken in conjunction with the drawings appended hereto. In the drawings, like reference characters identify identical apparatus, and, 
     FIG. 1A is an isometric view of a coupler in accordance with our invention; 
     FIG. 1B is a cross section of FIG. 1A taken on lines B--B showing a typical optical energy path; 
     FIGS. 2A and 2B are isometric views of different intermediate products during the course of manufacturing a coupler in accordance with our invention; and 
     FIG. 3 is an isometric view of the use of the coupler of our invention in an optical energy switching apparatus. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1A illustrates a coupler 10 in accordance with the teachings of our invention. In particular, an optical fiber 15 is shown as being incorporated into a suitable substrate 12 which may comprise quartz or other suitable material. The manner in which the optical fiber 15 is incorporated into the substrate 12 will become clear as we describe a method of manufacturing the coupler of our invention. Although FIG. 1A illustrates the coupler as including a pair of optical fibers 15, those of ordinary skill in the art will understand that any number of optical fibers may be coupled to a thin-film optical waveguide in accordance with our invention. The substrate 12 includes a pair of major surfaces 12&#39; and 12&#34;. One of the major surfaces 12&#39; is a planar surface, in which plane a cross-section 15&#39; of optical fiber 15 lies. The optical fiber 15 intersects the substrate 12 in the surface 12&#34; with a cross section of said fiber lying in the planar surface 12&#39;. As is illustrated in FIG. 1A a thin-film optical waveguide 13 has been deposited above the planar surface 12&#39; of the substrate 12. Such thin-film optical waveguide may comprise sputtered glass, doped crystals, a liquid or other material which is suitable for transmitting optical energy. In order to maintain the optical energy within the thin-film optical waveguide 13 and prevent it from entering the substrate 12, the index of refraction of the material of the substrate 12 must be less than the index of refraction of the material of the waveguide 13. 
     FIG. 1B is a partial cross section of the apparatus shown in FIG. 1A. As has been explained above, the substrate 12 has a planar surface 12&#39; in which plane a cross-section 15&#39; of the fiber optic 15 lies. This surface 12&#39; has deposited thereover a thin-film optical waveguide 13. As shown in FIG. 1B path 16 is a typical path for optical energy being coupled from the fiber optic 15 to the waveguide 13. In order to maintain the optical energy within the waveguide 15 it may be clad with a material whose index of refraction is less than the index of refraction of the material of the fiber. Alternatively, if the fiber 15 is not clad then the index of refraction of material adjacent the fiber must be less than the index of refraction of the fiber material. Otherwise, as is well known to those skilled in the art, leakage of optical energy will occur from the fiber to adjacent material. As FIG. 1B shows the optical energy, traveling on path 16, is totally reflected by the surface of the waveguide 13. In order for total reflection of this energy the angle of incidence of the energy must be greater than the critical angle for the material of the optical guide 13. In order to insure this total reflection, which is a desirable goal, the angle the fiber 15 makes with the surface of guide 13 is preferably greater than the critical angle for the material of the guide 13. 
     Those with ordinary skill in the art will understand that optical energy will be coupled from the fiber 15 to the guide 13 without traversing an unenclosed medium; without requiring the presence of an external electrical signal and without requiring accurate photolithography pattern transfers. 
     A method of manufacturing the optical coupler illustrated in FIGS. 1A and 1B will now be described with reference to FIGS. 2A and 2B. A pair of substrates 12A and 12B have sandwiched therebetween a fiber optic 15. In order to unitize these three elements a bonding agent 17 is employed. This bonding material may comprise low melting point glass powder or an epoxy or other suitable material. In particular, the fiber 15 is enclosed within surfaces 12A&#39; and 12B&#39; along with the bonding material. Although the surfaces 12A&#39; and 12B&#39; are illustrated, in FIG. 2A, as planar, those with ordinary skill in the art will understand that, if desired, the configuration of these surfaces may be made to conform more or less closely to the cross section of the fiber optic 15. Thus, surfaces 12A&#39; and 12B&#39; may be mirror images of one another whose shape conforms to the fiber cross section. If the fiber optic 15 is clad the particular material used as the bonding agent will not affect the light transmissive qualities of the fiber optic 15. However, if the fiber optic 15 is unclad then the index of refraction of the bonding material must be less than the index of refraction of the fiber optic in order to prevent light leakage; since the bonding agent is in intimate contact with the fiber optic 15. 
     In respect of the coupler illustrated in FIGS. 1A and 1B it has been pointed out that it is necessary to insure that light coupled from optic to guide be totally internally reflected and, as a corollary, the angle the fiber optic makes with the surface of the guide must be greater than the critical angle of the guide material. It is this parameter which controls the next step of the manufacturing method. The next step is a cutting operation along a plane such as 20, illustrated in FIG. 2A. The angle between the fiber 15 and the cutting plane 20 is selected so that it is greater than the critical angle of the guide material which is to be used. FIG. 2B illustrates the surface 12&#39; which is formed as a result of the cutting operation. Included in this plane is a cross-section 15&#39; of the fiber optic 15. The next step in the manufacturing operation is to polish the surface 12&#39; so as to be capable of receiving an optical guide such as optical guide 13. Finally, the optical guide is deposited on the surfaces and the coupler is complete. 
     FIG. 3 illustrates a typical application in which the coupler of our invention may be employed. In particular, a plurality of input fibers 25 are illustrated. Each of the input fibers 25 are coupled to an optical waveguide 33 in accordance with the teachings of our invention. The optical guide 33 is deposited on a suitable substrate 32 and the fibers 25 are bonded to the substrate 32 in accordance with our invention. Included in the guide 33 are a plurality of thin-film lenses 35, one such lens for each of the input fibers 25. As is well known to those skilled in the art, optical energy coupled into a planar medium such as guide 33 tends to diverge and the lenses 35 are provided to either collimate or focus the optical energy in the guide 33. Located adjacent the guide 33 is a surface acoustic wave generator 36. The construction of such surface acoustic wave generators or transducers are well known in the art and therefore detailed explanation thereof is not believed necessary. Included in the guide surface is a surface acoustic wave deflector 37 the purpose of which will become clear as this description proceeds. Along the optical path of guide 33 and opposite the input fibers 25 are provided a plurality of output fibers 40A and 40B. For each input fiber 25 there is a pair of output fibers 40A and 40B. Each of the output fibers 40A and 40B are coupled, through the substrate to the optical waveguide 33 in accordance with the teachings of our invention. In the absence of a surface acoustic wave from the surface acoustic wave generator 36, the optical paths may be so arranged that input energy from any particular input fiber 25 will be directed to one of the output fibers 40A. However, in the presence of a surface acoustic wave the surface acoustic wave deflector 37 will cause optical energy from any input fiber 25 to be coupled to an output fiber 40B. Thus, the surface acoustic wave generator 36 allows the optical energy provided at any input fiber 25 to be switched between either output fiber 40A or fiber 40B. 
     The apparatus illustrated in FIG. 3 is but one application of our invention in a system for selectably or switchably coupling optical energy. Other applications will be apparent to those of ordinary skill in the art after reading this description.