Graded Index Waveguide Structure and Process for Forming Same

1. Field of the invention

This invention relates generally to a process for forming planar coupler devices for optical fibers and the devices formed thereby, and, more particularly, to a process for forming a graded index optical waveguide structure and the device formed by such a process.

2. Description of the Prior Art

Optical communication systems, in which messages are transmitted by carrier waves of optical frequencies that are generated by sources such as lasers or light-emitting diodes, are of much current interest because of the advantages they offer over certain other conventional communication systems, such as a greatly increased number of channels of communication and the ability to use other materials besides expensive copper cables for transmitting messages. An optical waveguide device is used to conduct or guide waves of optical frequencies from one point to another. The operation of an optical waveguide is based on the fact that when a medium which is transparent to light is surrounded or otherwise bounded by another medium having a lower refractive index, light introduced along the inner medium's axis is highly reflected at the boundary with the surrounding medium, thus producing a light guiding effect.

As the development of optical circuits proceeded, it became necessary to have structures which could couple, divide, switch, and modulate the optical waves from one waveguide device to another. A Y-coupler is an example of such a device which is formed in a "Y" shape and couples signals together or divides them apart.

More recent developments in optical circuits have produced a graded index optical fiber, i.e., an optical fiber with a refractive index that is progressively lower from the center of the fiber to the periphery. These graded index optical fibers are highly desirable because they have an increased information bandwidth and information handling capability as compared to conventional stepped-index optical fibers. In using such graded index optical fibers, there is also the need to have structures which can couple, divide, switch, and modulate the optical waves from one such waveguide device to another.

One process for forming such graded index optical couplers makes use of an ion exchange process as described, for example, in a publication by G. H. Chartier et al, in Electronic Letters, Vol. 13, pp. 763-764, (1977). By the process of Chartier et al, an eutectic mixture comprising 80 percent lithium sulfate (Li.sub.2 SO.sub.4) and 20 percent potassium sulfate (K.sub.2 SO.sub.4) is heated in an oxygen atmosphere to 580.degree. C. A sodium glass slide is suspended over the melt for 30 minutes to allow thermal equilibration with the melt. The slide is then dipped into the melt for 20 minutes, and again suspended over the melt for 10 minutes to avoid thermal shock. Planar waveguides having a depth of 100 micrometers may be made by this process. The coupler structures are formed by appropriately masking the glass slide, for example, with a thick (1-2 micrometers) film of aluminum, prior to dipping the slide into the melt. By such an ion exchange process, lithium ions from the eutectic mixture are exchanged for the sodium ions in the soda glass substrate, thus modifying the refractive index of the ion-exchanged region of the substrate.

There are, however, certain disadvantages to the above-described ion exchange process and the optical waveguide devices formed thereby. One major disadvantage is that the coupling efficiency between a waveguide device formed by an ion exchange process and a graded index fiber is somewhat low, i.e., typically being only about 65%. In addition, there are certain difficulties encountered in carrying out the ion exchange process. For example, at the elevated temperature used for the ion exchange process, lithium sulfate decomposes to lithium oxide (Li.sub.2 O), which is corrosive to silicon dioxide. The lithium oxide reacts with the silicon dioxide to form Li.sub.2 SiO.sub.3, with a resultant thinning of the glass substrate. In addition, surface cracks often form on the glass substrate during the ion exchange process, and such cracks produce scattering in the waveguide device formed.

It is the alleviation of the problems associated with this prior art ion exchange process and, further, the formation of an improved coupling structure for use with graded index optical fibers which has a high coupling efficiency, to which the present invention is directed.

The general purpose of this invention is to provide a new and improved process for forming a graded index waveguide structure which possesses most, if not all, of the advantages of the prior art processes for forming such coupling structures, while alleviating their significant disadvantages discussed above.

In order to accomplish the above-described general purpose of this invention, I have discovered and developed a new and improved process for forming a graded index waveguide structure by first providing a substrate of a first selected optical material having a chosen index of refraction. A patterned layer of a second selected optical material is then formed on one surface of the substrate. This second optical material has a refractive index which is larger than the refractive index of the substrate. The patterned layer of the second optical material is formed in a predetermined geometry and to a predetermined thickness. Next, a layer of a third selected optical material is formed on both the patterned layer of the second optical material and the surface of the substrate to a predetermined thickness. This third optical material has a refractive index which is substantially the same as the refractive index of the substrate. Finally, the substrate with the patterned layer of the second optical material and the layer of the third optical material deposited thereon is heated to an elevated temperature for a period of time sufficient to diffuse the second optical material into the first and the third optical materials and to thus form a graded index waveguide structure.

Accordingly, it is an object of the present invention to provide a new and improved process for forming a graded index optical coupler for use with graded index optical fibers which has improved coupling efficiency.

Another object is to provide a process of the type described in which the graded index waveguide structure has a predetermined geometry.

Still another object is to provide a process of the type described which uses known lithographic procedures and is thus highly reproducible.

Another object of the present invention is to provide a new and improved graded index waveguide structure for use with graded index optical fibers.

Yet another object is to provide a device of the type described in which the portion of the structure having the graded index is substantially circular in cross-section, and thus has improved coupling efficiency with graded index optical fibers.

A further object is to provide a new and improved process for simultaneously forming a plurality of graded index waveguide structures in a chosen substrate.

These and other objects, features, and advantages of the present invention will become more readily apparent in the following description of the drawings and the preferred embodiment of the invention.

FIGS. 1(a)-1(g) illustrate, in schematic cross-section, some of the major steps in one process embodiment of the present invention.

FIGS. 2(a)-2(f) illustrate, in schematic cross-section, some of the major steps in a second process embodiment of the present invention.

FIG. 1a shows a substrate 10 of a first chosen optical material, such as fused quartz or other optically transparent glass having a softening temperature of 1000.degree. C. or higher, on which is deposited a layer 12 of a second chosen optical material, such as germanium oxide (GeO.sub.2) or lead oxide (PbO.sub.2). The layer 12 has an index of refraction that is larger than the index of refraction of the substrate, and is referred to herein as a "high index material". The refractive index of the second chosen optical material should be significantly higher than the refractive index of the first and third (to be described) optical materials, so that after undergoing the diffusion process described below, which will cause a decrease in the refractive index of the second material, the refractive index of the diffused second optical material will still be high enough to have a high light-gathering capability. The layer 12 of the second optical material is deposited to a thickness in the range of 5 to 10 micrometers, with less material being deposited for a material with a higher index of refraction (e.g. 1.80 for PbO.sub.2) than for a material with a somewhat lower index of refraction (e.g. 1.7 for GeO.sub.2). The layer 12 may be deposited by known chemical vapor deposition processes, as described, for example, by D. B. Keck, R. D. Maurer, and P. C. Schultz, in Applied Physics Letters, Vol. 22, 1973, page 307 et seq.

Using a conventional photo-etching procedure as described by, for example, William S. DeForest in the book entitled, "Photoresist: Materials and Processes," McGraw-Hill Book Company, New York, 1975, Chapter 7, the layer 12 is patterned in a predetermined geometry as illustrated in FIGS. 1b, 1c, and 1d. First, a layer 14 of photoresist material, such as commercially available Shipley 1350-J resist (obtainable from the Shipley Company of Newton, Massachusetts), is deposited to a thickness of approximately one micrometer on top of the layer 12, as shown in FIG. 1b, by known spin coating techniques. Next, the layer 14 of photoresist is exposed to selected radiation, such as an ultraviolet source, through a mask which defines the predetermined geometry in which it is desired to ultimately form the layer 12. Then, the layer of exposed resist is developed with a chemical such as commercially available Shipley 1350-J developer, to leave behind the patterned portion 14' of the photoresist which has the predetermined geometry, as shown in FIG. 1c. Next, the portion of the layer 12 exposed (not covered) by the photoresist pattern 14' is removed by known etching procedures, such as chemical etching, to form the structure illustrated in FIG. 1d. Finally, the patterned layer 14' of the photoresist is removed by the use of a selected solvent, to form the structure shown in FIG. 1e.

The geometry of the layer 12' is selected to provide the desired geometry of the graded index portion of the waveguide structure to be finally formed. If a Y-configuration is the desired geometry of the final waveguide structure, the patterned layer 12' in FIG. 1e would be formed to have a "Y" shape as seen from a top view of FIG. 1e (not shown). As shown in FIG. 1e, the layer 12' could represent the base of such a "Y" structure, with the remainder of the "Y" structure extending in a plane in a direction away from the viewer.

Next, as shown in FIG. 1f, a layer 16 of a third selected optical material that has substantially the same refractive index as the substrate 10 is deposited on the patterned layer 12' and on the surface of the substrate 10. If the substrate 10 comprises fused quartz, the layer 16 may be silicon dioxide (SiO.sub.2) which is deposited to a thickness of approximately 100 micrometers by known chemical vapor deposition processes described, for example, by Keck et al, previously referenced herein. The refractive index of the first and third optical materials must be substantially the same so that the external clad surrounding the graded index core of the coupler will be uniform on all sides and the same refractive index difference between the core and the clad of the coupler can be maintained throughout the coupler.

Finally, the structure shown in FIG. 1f is heated in a furnace to a temperature in the range of 1350.degree. C. to 1450.degree. C. for a period of approximately 5 hours, to diffuse the second optical material in the patterned layer 12' into the surrounding substrate 10 and into the surrounding third optical material of the overlayer 16. By properly controlling the diffusion conditions of temperatures and time, the core of the resultant device may be formed to have a simple parabolic refractive index decrease (proceeding outwardly from the center of the waveguide structure), which is the optimum grading profile for efficient coupling with a graded index fiber.

In addition, during the above heating for the diffusion process, the overlayer 16 becomes continuous with the substrate 10 to form the capped substrate 10', which comprises a body of optical material of essentially the same refractive index and which functions as the clad of the waveguide structure.

The resultant graded index waveguide structure shown in isometric and schematic cross-section in FIG. 1g is formed and comprises the capped substrate 10' having located within a predetermined portion thereof the body 18 of graded index optical material which has a larger index of refraction than the capped substrate 10'. It should be noted that the body 18 of the graded index optical material shown in FIG. 1g is formed to have a cross-section which is substantially circular and a diameter which is approximately equal to a selected fiber core diameter, and thus provides efficient coupling of such a structure into the core of a graded index optical fiber (not shown). The desired configuration of the body 18 is achieved by appropriate choice of the thickness of the layer 12' and the thickness of the overlayer 16.

Thus, by the process of the present invention just described, a waveguide coupler is formed in which the waveguiding layer has a graded index to provide improved coupling efficiency between the coupler and a graded index optical fiber. The waveguide coupler formed by the process of the present invention may be formed to have a graded index profile which matches the grading profile of the graded index fiber with which the waveguide structure of the present invention is to be coupled, and thus achieves optimum matching of the fiber core and the coupler core, which results in efficient coupling of light from the fiber into the coupler. The process of the present invention is particularly suited to form a waveguide coupler with a simple parabolic refractive index decrease, which is the grading profile commonly used in graded index fibers. Furthermore, the body of the graded index material is located within the clad of the coupling device structure at a position and with a geometry that matches the core of the optical fiber to be coupled and thus also enhances coupling efficiency. As indicated in FIG. 1g, the body 18 of graded index material has a substantially circular cross-section, which provides effective coupling with an optical fiber (not shown) also having a circular cross-section. In addition, the diameter of the body 18 of graded index material is approximately equal to the core diameter of the fiber to which the coupling device joins, which also provides effective coupling. Further, the process of the present invention offers the advantage that conventional planar processing using known photolithographic techniques may be employed. The standard lithographic procedures and batch process which may be used in practising the present invention provide a high degree of process control and high reproducibility.

Turning now to FIG. 2, there is shown, in schematic cross-section, some of the major steps in a second process embodiment of the present invention for forming a number of graded index optical waveguides simultaneously. FIG. 2a shows a substrate 20 of a first selected optical material, such as fused quartz, which is selectively exposed to an etchant, such as a selected chemical etchant, an ion beam etchant, or a diamond scribe, to form a patterned substrate 20' shown in FIG. 2b, with cavities 21 etched in selected portions of the substrate surface. The pattern in which the substrate 20 is etched is determined by the desired geometry of the waveguide device to be formed. For example, each cavity 21 may be in the form of a channel or a "Y". If a conventional photo-etching procedure is used, a procedure similar to that described in FIG. 1 for etching the layer 12 may be followed.

Next, a layer 22 of a second selected optical waveguiding material, such as GeO.sub.2, is formed to a predetermined thickness of the surface of the etched substrate 20' and into the cavities 21 etched therein, as shown in FIG. 2c. The layer 22 may be deposited in the manner described in FIG. 1 for the formation of the layer 12. Then, the undesired portions of the layer 22 are removed by a conventional abrasive wafer lapping procedure, for example, to leave behind the desired patterned areas 22' of the second selected optical material in the substrate 20', as shown in FIG. 2d.

Alternatively, the patterned areas 22' of the second selected optical material may be formed by first providing a mask with openings of predetermined geometry on the surface of the substrate 20 and using conventional photo-etching procedures to form the cavities 21 in the substrate 20' shown in FIG. 2b. Next, the layer 22 of the second selected optical material may be deposited over the etchant mask which was left in place and into the cavities 21 in the substrate 20'. Then, the mask may be removed by known lift-off procedures carrying with it unwanted portions of the second optical material and leaving behind the desired patterned areas 22' of the second optical material as shown in FIG. 2d.

Next, as shown in FIG. 2e, a layer 24 of a third selected optical material that has substantially the same refractive index as the substrate 20 is deposited to a predetermined thickness on the patterned areas 22' of the second selected optical material and on the surface of the substrate 20', as shown in FIG. 2e. The layer 24 may be formed in the manner described in FIG. 1 for the formation of the layer 16.

Finally, the structure shown in FIG. 2e is heated in a furnace to a temperature in the range of 1350.degree. C. to 1450.degree. C. for a period of approximately 5 hours, to diffuse the patterned areas 22' of the second optical material into the surrounding substrate 20' and into the surrounding portion of the layer 24 of the third selected optical material, to form the optical waveguide device shown in FIG. 2f.

The waveguide device formed by this second process embodiment of the present invention, as shown in FIG. 2f, comprises a multiplicity of graded index waveguide structures 26 formed simultaneously within a given substrate. As described in relation to FIG. 1, during the above heating for the diffusion process, the layer 24 becomes continuous with the substrate 20' to form the capped substrate 20" shown in FIG. 2f. The multiple waveguide structures 26 may be formed to be interconnected to each other as desired, by choosing the appropriate pattern for the etching procedure described in relation to FIG. 2b.

The waveguide structure formed by this second process embodiment of the present invention has the same advantages and improved performance characteristics as discussed in relation to the device formed by the first process embodiment of this invention shown in FIG. 1.

This example illustrates the formation of a graded index optical waveguide structure in accordance with the present invention. Following the procedure described in relation to FIG. 1, a fused quartz substrate having a refractive index of 1.47 and obtained from Corning Glass Works, of Corning, New York, was coated with a layer of germanium oxide having a refractive index of 1.7, to a thickness in the range of 5 to 10 micrometers by a known chemical vapor deposition process previously described herein. The germanium oxide layer was then patterned as described in relation to FIGS. 1b, 1c, and 1d, using Shipley 1350-J resist as the photoresist, exposing the photoresist to radiation having a wavelength in the ultraviolet range, through a mask, and developing the photoresist with Shipley 1350-J developer. The mask was formed to expose a straight channel shape in the photoresist layer. After the germanium oxide layer was patterned in a desired geometry by conventional etching, an overlayer of silicon dioxide having a refractive index of 1.47 and a thickness of 100 micrometers was formed over the germanium oxide layer and the substrate surface by a known chemical vapor deposition process previously discussed herein. Finally, the composite structure of a fused quartz substrate, a germanium oxide patterned layer, and a silicon dioxide overlayer was heated in a furnace at a temperature of approximately 1400.degree. C. for a period of 5 hours to diffuse the germanium oxide into the substrate and into the silicon dioxide overlayer and thus form the graded index waveguide structure as illustrated in FIG. 1g. (Typically, the germanium oxide diffused into the substrate to a depth of 10 to 15 micrometers.)

The graded index waveguide structure thus formed was coupled to a neon laser light source, and it was observed that the graded index waveguide structure effectively confined and transmitted the light input to a chosen optical fiber, as evidenced by the visible light path confined to the region in which the waveguide structure had been formed and by the light output obtained from the output end of the optical fiber.

While the present invention has been particularly described with respect to the preferred embodiments thereof, it will be recognized by those skilled in the art that certain other modifications in form and detail may be made without departing from the spirit and scope of the invention. In particular, the scope of this invention is not limited to the use of a fused quartz substrate, which was used merely as an example and which would be particularly useful in applications where radiation damage resistance of fibers is required, but it is intended to include any transparent glass with a softening temperature of 1000.degree. C. or higher. In addition, the present invention is not limited to the use of germanium oxide or lead oxide, but includes the use of any appropriate optical material which has index of refraction that is higher than the index of refraction of the substrate. Further, the overlayer formed is not limited to silicon dioxide, but may be formed of any optical material which has an index of refraction that is substantially the same as the index of refraction of the selected substrate.

In addition, the thickness of this high index optical material will depend upon the magnitude of the index of refraction thereof, with a thinner layer being required for a material with a higher index of refraction. Further, the deposition of both the high index material and the overlayer is not limited to the chemical vapor deposition process described herein, but includes known processes for depositing such materials which are compatible with the optical materials and processing described herein, such as dip coating of a glass substrate. Also, the process for forming the patterned high index material layer is not limited to the photolithographic procedure described herein, but may include other known lithographic procedures as well as plasma etching or ion beam sputtering. Finally, either process embodiment of the present invention described herein may be used to simultaneously form a plurality of graded index waveguide structures in a chosen substrate, which may be interconnected with each other as desired.