METHOD OF MAKING AN INTEGRATED OPTICAL MODE-INDEX ELEMENT

Abstract: There is disclosed an improved method for making a mode-index optical device on a waveguide. The method is of the type comprising the step of vacuum depositing material through a mask onto the surface of the waveguide to form a built up area of said material on the surface. The improvement is that the mask is moved during the deposition so that the built up area forming the optical element has tapered edges.

Description: METHOD OF MAKING AN INTEGRATED OPTICAL MODE-INDEX ELEMENT Technical Field The present invention relates to methods for forming mode-index optical elements on the surface of optical waveguides. Background Art Integrated optical systems are attractive because of their small size, permanent alignment (after fabrication), and their potential use as remote optical processing systems. Waveguide lenses are recognized as key components of integrated optical systems, used primarily to collimate the output from a point source (i.e., a semiconductor laser) or focus a parallel beam to a diffraction limited spot. As a result, several varieties of lenses are actively being explored within this field of technology. Waveguide lenses are classed into four types, mode-index (including the Lunegerg type), geodesic, Fresnel, and Bragg-reflection. The lenses of interest here, mode-index type, are the result of physical changes to a waveguide, either by the addition of material (overlay lens) or chemical modification, to form patterns of the effective refractive index (Neff) which is similar to a change in the real refractive index for conventional geometric optics. A mode-index lens is most similar to three dimensional lenses that focus by refraction at a curved interface between regions of differing refractive indices. These lenses can be generated by physical vapor deposition or use of micro- lithographic patterning techniques. The concept of geometric optics in slab waveguides was first proposed and studied by R. Shubert and J. H. Harris, Opt. S c. Am., £1, 154 (1971) and R. ϋlrich and R. J. Martin, Appl. Opt., 10. 2077 (1971). Shubert and Harris first discuss the use of a thickness taper as a transition between regions of differing thickness (i.e. a change in the Neff). Lens and prism structures were fabricated by vacuum deposition to form thicker regions. In addition, thickness-profiled layers were examined as a technique of lensing. Thin—film lenses were fabricated with f— umbers of 2 to 3. The performance of the lenses were not discussed, the paper was only a demonstration of optical principles and no details of deposition or masking is presented. Ulrich and Martin demonstrated geometric optics in thin films again by vacuum deposition of materials to pattern the waveguide by thickness changes. The taper between these regions is mentioned as being smooth and extending over a distance much greater than the vacuum wavelength of light in the guide. The step in film thickness was produced by first coating the entire substrate with a film of uniform thickness then masking the substrate and performing a second deposition. The mask was kept at a distance of approximately 0.1 mm during the second deposition so that the taper is attributed to surface diffusion and the natural angular spread of the incident vapor stream that produced a taper length of 50 microns. P. K. Tien, S. Riva-Sanseverino, R. J. Martin and G. Smolinsky, Appl. Phys. Lett., 24, 647 (1974), discussed the construction of integrated optical thin-film prisms, lenses, and reflectors. The devices were vacuum deposited through masks onto the substrate and then overcoated by a continuous layer formed by a gas discharge polymerization process. Tapers of 50 microns in the length were considered to produce low levels of conversion of the waveguid-e propagation modes. L. P. Boivin, Applied Optics, 12, 391 (1974), was the first to investigate the generation of a lens by micro-lithographic techniques in his work to fabricate a thin—film laser— o—fiber coupler. The lenses were made by first depositing thick guides of silicon oxynitride, then coating a photoresist layer, exposing with the lens pattern, and finally the waveguide region was reduced in thickness by reactive etching. Thus, many researchers have recognized the value of mode-index elements and have recognized the desirability of providing tapered edges. However, relying on vacuum diffusion and etching to produce the taper is less than desirable. These processes are difficult to control. More particularly, it is difficult to attain a predetermined uniform taper using these methods which can be used in a model of the optical device. Disclosure of the Invention In accordance with the present invention there is provided an improved method for producing a mode-index optical element on an optical waveguide, said method comprising the step of vacuum depositing material through a mask onto the surface of said waveguide to form a builtup area of said material on said surface. The improvement wherein said mask is moved relative to said surface during said deposition so that said builtup area has tapered edges. Brief Description of the Drawing Figure 1 is a schematic representation of the method of the invention. Detailed Description of the Invention In accordance with the method of the present invention, a variety of optical elements can be produced on the surface of an optical waveguide. Since a mode-index lens is the most common and importaπt type of element that is formed, it will be described in detail. It will be understood, however, that other elements can be formed, including for example, channel waveguides; beam splitters (e.g. prisms) and other common elements as well as tapers useful for facilitating the prism coupling of light into a waveguide. The linear thickness taper produced by the method of the invention can be parallel with the optical axis of the element at all locations of the aperture. Linear translation with deposition thickness produces a symmetrically identical taper on the tapered edges. The advantages of well defined tapers are: (1) decreased mode conversion (mode conversion causes the light to enter propagation modes the lens was not designed for); (2) decreased scatter losses through the linear taper, i.e. light is not converted to radiative modes; (3) a gradual taper produces a gradient Neff which, serves as an antireflection boundary; and (4) good control of taper allows accurate modeling of the optical element. According to the present method, material is vacuum deposited on the surface of a waveguide. The waveguide and the material that is deposited on the waveguide to produce the optical element can be conventional. Thus, the waveguide can be a glass layer having additional glass deposited on its surface to form the mode-index element. As discussed more completely below, the preferred material is a low molecular weight organic compound having low propagation losses. The improvement according to the invention is that the mask is moved relative to the deposition surface during the deposition process. The exact rate at which the mask is moved is determined by the taper length desired; the rate of deposition of the material; and the total thickness deposited through the mask. Once the theoretical amount of taper is determined, this taper can be achieved by simple experiment using the present specification as a guide. For example, moving the mask at a rate of 2.0 microns movement per nanometer of deposited thickness produces a taper length of 1.0 mm when the total deposited thickness is 500 nanometers with the deposition rate being usually less than 2.0 nano¬ meters per second. A taper of 1.0 mm serves to convolute the natural angular spread of the incident vapor stream and surface diffusion of the material deposited. The deposited thickness of the tapered region can be optically monitored and the mask is moved according to this thickness as described above. The exact thickness is determined by comparing a reflection signal with a modeled thickness versus reflection signal relationship. This process produces less than 1% error in deposited thickness for any given position. The thickness can also be monitored by a piezoelectric film thickness monitor. Apparatus for moving the mask (or the deposition surface) during deposition is easily designed. In the example below, a conventional translating stage that is used in microelectronics manufacture was connected to a microcomputer controller that was programmed to move the stage during deposition. Other equivalent apparatus will be apparent to those skilled in the art. The vacuum apparatus that is used to carry out the method of the invention is conventional. A typical vacuum coating apparatus will include a vacuum chamber which is connected to a mechanical vacuum pump which typically provides a pressure as _3 low as about 10 mm Hg. In addition, a diffusion pump is provided to reduce the vacuum further, typically down to about 10~ mm Hg. Inside the chamber, there is provided an evaporation source for the material. The container is typically covered, the cover having an opening to direct the flow of material. The substrate to be coated is usually above the container. The uniformity of the coating can be improved by increasing the distance between container and the support. The quality of the element that is produced by the invention is determined by the control of the regional effective refractive index which in turn is dependent on the careful control of the thickness once the adjacent layers of the layered assembly are defined (i.e. thickness and optical constants). The present invention provides for careful control of the taper. It is also important to control the thickness of the non-tapered regions. This can be accomplished by monitoring the deposition of the additional material by optical reflection or transmission of the deposit as the deposition is being made or crystal monitoring techniques. Both methods are well—known to those skilled in the art. The method of the invention can be understood by reference to Figure 1. There is shown a waveguide 10 on support 12, and a deposition mask 20. Material is vacuum deposited through the opening 22 in mask 20 as shown by arrows 24 so as to produce a mode—index element 30 on the surface of waveguide 10. Because of the movement of the mask 20 as shown by the direction of the arrow, a tapered area 32 is formed in the element 30. The waveguide material can be any material allowing light propagation when in a waveguide configuration that can be deposited under high vacuum. For example glass, silicon oxinitride and arsenic trisulfide. As noted above, the preferred material for forming the waveguide and the mode-index element is a low molecular weight organic compound. Compounds that are useful in the present method have a glass transition temperature of greater than about 50*C. The glass transition temperature is measured using conventional techniques. For example differential scanning calorimetry. The measurement should be taken from amorphous bulk material that is substantially free from residual solvents and decomposition products since that is the condition of the materials when they are vacuum coated according to the process aspects of the invention. The low molecular weight organic compounds that form the transparent layers and mode—index elements can be the "multicyclic aromatic nucleus" compounds described in U. S. Patent 4,499,165 or derivatives thereof. A "multicyclic aromatic nucleus" is a nucleus comprising at least two cyclic groups one of which is aromatic, including aromatic heterocyclic ring groups. The cyclic group may be substituted with substituents such as aliphatic hydrocarbons, including cycloaliphatic hydrocarbons, other aromatic ring groups such as aryl, and heterocyclic ring groups such as substituted or fused thiazole oxazole, imide, pyrazole, triazole, oxadiazole, pyridine, pyrimidine, pyrazine, triazine, tetrazine and quinoline groups. The substituents are fused or non-fused and mono or polycyclic. Examples of multicyclic aromatic nuclei include 9,9-bis(4—hydroxy-3,5-dichlorophenyl)fluorene, 4,4'-hexahydro-4,7-methanoindan-5-ylidenebis(2,6-dichlo rophenol); 9,9-bis(4-hydroxy-3,5-dibromophenyl)- fluorene, 4,4'-hexahydro-4,7-methanoindan-5-ylidene- bis(2,6-dibromophenol); 3' ,3",5' ,5"-tetrabromo- phenolphthalein, 9,9-bis(4-aminophenyl)fluorene, phenylindandiols; l,l'-spirobiindandiols, 1,1'—spirobiindandiamines, 2,2'-spirobichromans; 7,7-dimethyl—7H-dibenzo[c,h]xanthenediol; 9,9-dimethylxanthene- »6-bis(oxyacetic acids); 4,4'-C3-phenyl-l-indanylidene)-diphenol and other bisphenols; 9-phenyl-3-oxo-2,6,7—trihydroxyxanthene; and the like. The support onto which the organic compound is coated is not critical. The choice of support will be determined by the use of the device. The support can be, for example, glass in the case of a lens; silicon or quartz. Other inorganic layers can be provided either between the organic compound layer and the support or over the organic compound layer. The following example is presented for a further understanding of the invention. Example The mask used to prepare an integrated optics lens was etched using Photoflex™ available from Eastman Kodak. The shape of the aperture was designed using Super-Oslo software available from Sinclair Optics. Photoflex™ is rolled copper with an etching resist on one side, and a photosensitive support plastic on the opposite side. The mask was created by etching a hole the shape of a lens through the copper, through which the organic material could be deposited. The copper was 0.0356 mm thick, and was coated with a novolak based etching resist. For this lens, 17 (y,z) coordinate pairs were calculated using the equation that defines a lens interface. The mask position was selected digitally by a microcomputer controlling a translating stage, the Compumotor 150 Indexer, with an incremental step resolution of 3.39 μm and a total translation range of 25 cm. The input to the microcomputer for positioning, as the coating proceeded, was a piezoelectric film thickness monitor. For the lens preparation, the substrate mask was translated linearly with a target thickness taper of 0.913 nm thickness/μm movement. The taper resulting from controlled movement of the mask along the central optical axis was measured. The slope of the taper was determined to be 0.905 nm thickness/μm distance. The small difference between the programmed and fabricated taper, less than the error in measurement, indicated the high level of control for taper fabrication. Nonlinear regions are seen on the taper at the top and base over a length of 100 μm. The effect of these regions on lens performance is minor. The focal length of the lens was determined. Two collimated beams were input at an aperture radius of 1.5 mm. A photodiode was mounted with micropositioning along the optical axis and an axis perpendicular to the optical axis. The photodiode was used to detect the intersection of the rays from the lens. The focal length was determined to be about 13.9 ± 0.3 mm. Industrial Applicability The present invention can be used to make a variety of mode-index optical elements on an optical waveguide.

Claims: CLAIMS : 1. In a method for producing a mode—index optical element on an optical waveguide, said method comprising the step of vacuum depositing material through a mask onto the surface of said waveguide to form a builtup area of said material on said surface, the improvement wherein said mask is moved relative to said surface during said deposition so that said builtup area has tapered edges. 2. A method according to claim 1 wherein said mask is moved at a rate of about 2.0 microns movement per nanometer of deposited thickness while the deposition of said material is at a rate of about 2.0 nanometers per second. 3. A method according to claim 1 wherein said material that is deposited is a low molecular weight organic compound. 4. A method according to claim 1 wherein more than one mode—index optical element are made simultaneously. 5. A method according to claim 1 wherein movement of said mask is controlled by optical monitoring of the deposited thickness.

Inventors: BRAZAS JOHN CHARLES JR, MACHIELE DELWYN EARL, BRAZAS, JOHN, CHARLES, JR., MACHIELE, DELWYN, EARL
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1989
Main classification: G02B6
IPC codes: G02B6