Optical Fiber-to-Channel Waveguide Coupler

Abstract

A micromanipulator and UV curing adhesive allows a precise end-on coupling of an optical fiber to a film optical waveguide on a substrate. Such a coupling facilitates the optical processing of data on active or passive optoelectronic chips with the inherent advantages of parallel, high speed capability. After coarse The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. Technological advances in optical communications and in optical data processing have seen the emergence of thin-film optical and electro-optical logic elements in conjunction with optical fibers and have led to high speed signal processing concepts wherein fibers would be used as interconnecting optical channels, delay lines or shift registers. Thin film guided wave devices are strong candidates to perform terminal functions in optical fiber communication systems. These applications require practical fiber-to-thin film coupling methods and in some cases precise specification and control over the optical delay from one integrated optical element through an optical fiber to another integrated optical element via some coupling regions. Consequently, a decisive need has been recognized for a reliable permanent coupling of single mode optical fibers to channel thin-film optical waveguides and efforts are underway to develop the technique for fiber-film channel waveguide coupling with an emphasis on suitability to different waveguide materials, stability and producibility. These desirable characteristics must further be carefully chosen to assure compatibility with existing methods to produce maximum coupling efficiency and with waveguide device requirements such as electrical access. Some single mode optical fibers and thin-film channel waveguides can be used to perform in the 800-1600 nm wavelength region although other regions can be selected as a particular application dictates. Typically channel waveguides have been fabricated in LiNbO.sub.3 by Ti indiffusion (Ti:LiNbO.sub.3) or in other transparent substrates with rib or with strip-loaded waveguides since such structures are widely used for active and passive thin film devices. Designers have two options for coupling the energy from the fiber to the thin-film waveguides, namely via evanescent wave coupling between the two waveguides or by end-on launching. End-on coupling has an appeal due to its applicability to different waveguide materials such as GaAs and Si.sub.3 N.sub.4 and for simplicity. In fact, in recent years more research has been devoted to the end-on configuration which is practical and not material specific. Coupling efficiency is vital and clearly a challenge since the optical waveguides are microns in size, mode patterns from two different types of waveguides must be matched, and film and fiber edge conditions must be controlled. Several publications which show the use of fibers and thin-film optical logic elements and the coupling efficiencies of such structures are available, for example the article by G. M. Dillard et al entitled "Fiber and Integrated Optics Techniques for Radar and Communications Signal Processing," National Telecommunications Conference Record VIII, pages 37.5-1 to 37.5-5 (December 1976) and 0. G. Ramer's article "Single Mode Fiber-To-Channel Waveguide Coupling," appearing in J. Opt. Comm. 2, 122, (1981) and R. C. Alferness's article "Efficient Single-Mode Fiber to Titanium Diffused Lithium Niobate Waveguide Coupling for Lambda Equals 1.32 Micron," IEEE J. Ouantum Electron. QE-18, 1807, (1982). The best figures known to be reported for Ti:LiNbO.sub.3 waveguides are ninety-five percent (-0.2dB) coupling efficiency when Lambda equals 633 nm and a fiber waveguide fiber insertion loss of 1dB for a 1 cm long waveguide when Lambda equals 1.32 microns. These figures were arrived at by R. C. Alferness et al in the article cited above and by M. Papuchon et al in their article entitled "High Coupling Efficiencies Between Single Mode Optical Fibers and Ti Diffused LiNbO.sub.3 Waveguides" Paper FB3-1 at the Topical Meeting on Integrated Guided Wave Optics, Pacific Grove, Calif., Jan. 6-8, 1982. Micromanipulators were used to position the fibers in the experiments of these last two articles. The figures of Alferness and Papuchon exceeded theoretical estimates for this type of coupling, see the article by W. K. Burns et al entitled "End Fire Coupling Between Optical Fibers and Diffused Channel Waveguides" Applied Optics 16, 2048, (1977). The couplings of the Burns article involved calculation of the overlap integral of the fiber and channel waveguide modes. The transverse fields of these optical modes are modeled as circular and rectangular Gaussian functions respectively. Optimum conditions for coupling were calculated as well as the adverse effects of waveguide tilt, off-set and end separation. For example, to keep each of these loss contributions below ten percent (-0.5 dB) for typical Ti:LiNbO.sub.3 waveguides and fibers that tilt should be less than about one degree, the off-set less than about 0.8 microns and the end separation less than about twenty microns, note 0. G. Ramer's "Controlled Fiber Optic Switching" Hughes Research Laboratory Air Force Technical Report, AFWAL-TR-81-1002 December 1980. Although low loss, adjustable connectors have been reported for fiber-film coupling, techniques for permanently joining many fibers to a thin film waveguide chip are needed ultimately. The first adjustable connector applied double eccentric cylindrical supports permitting the alignment of two single-mode waveguide components, see J. Guttmann et al's article "Optical Fiber-Stripline-Coupler" Applied Optics 14, 1225 (1975). A second approach uses etched Si V-grooves for fiber positioning, see L. P. Boivin "Thin Film Laser to Fiber Coupler" Applied Optics 13, 391 (1974). A flip-chip orientation for the thin film channel waveguides and tapered fibers transverse to the coupling fibers for fine height adjustment is discussed in the article by C. H. Bulmer entitled "High Efficiency Flip-Chip Coupling Between Single-Mode Fibers and LiNbO.sub.3 Channel Waveguides" Applied Physics Letters 37, 351 (1980). These techniques appear limited to one or two fibers per chip facet. Other approaches for attaching fibers permanently to LiNBO.sub.3 channel waveguides have been implemented in varying degrees. A coupler disclosed by J. Noda et al in the article "Single-Mode Optical Waveguide Fiber Coupler" Applied Optics 17, 2092 (1978) holds the fiber in place with a jig attached to the chip carrier. Fine adjustment is accomplished by set screws and a final step of plastic molding fixes the fiber in place. Apparently this approach is not suitable for more than one or two fibers per chip edge owing to the size of the positioning fixture. Another technique used Si V-grooves to support fibers in positions determined by photolithographic masking and preferential chemical etching. The grooves can be defined to the same accuracy as the channel waveguide lithography and the etching has excellent control and reproducibility. In this scheme the accuracy of a fiber core position naturally depends upon the uniformity of the fiber outer diameter and its concentricity with the core. Various methods are used for aligning the Si carrier chip with the waveguide chip and subsequently cementing them. Another coupler disclosed by H. P. Hsu et al in "Multiple Fiber End Fire Coupling With Single-Mode Channel Waveguides" Applied Physics Letters 33, 603 (1978) uses etched V-grooves in Si for both fiber holding and as alignment markers to bring the flip-chip LiNbO.sub.3 waveguides into proper registration before cementing. Fibers are then placed in the grooves, however, permanent attachment was not reported. Another V-groove approach disclosed by O. G. Ramer et al in "Experimental Integrated Optic Circuit Losses and Fiber Pigtailing of Chips" IEEE J. Ouantum Electron. QE-17,970 (1981) epoxies the fibers in the V-grooves and polishes the Si and fiber end surfaces flat together. Then the Si chip is epoxied to a holder which offers micrometer adjustment in the transverse directions after the epoxy has cured. An initial version had an external micropositioner which adjusted the Si chip into place and was removed after the epoxy step. However, this arrangement apparently tended to suffer misalignment from stress which was placed on the Si chip by the curing epoxy. A waveguide switch network is coupled to four output fibers in the arrangement disclosed by M. Kondo et al in "Integrated Optical Switch Matrix for Single-Mode Fiber Networks" IEEE J. Quantum Electronics QE-18, 1759 (1982). The fibers are sandwiched between two Si chips etched with V-grooves and the chips are positioned and epoxied to a mount carrying the switch chip. Mention is made of the difficulty in epoxying the fibers and Si chips for fine positioning, but the details were not presented. In summary the methods of the prior art for permanently attaching multiple fibers depend upon aligning them relative to each other by their outer diameters using the very accurate V-grooves. Thus, the core locations depend upon the fiber qualities of circularity and concentricity. The Si chip is then positioned to align the V-grooves or the fibers in them with the channel waveguides and an adhesive is applied to fix positions. Apparently, the chip positioning can be done well within the required accuracy but stress from the curing epoxy causes misalignment. Thus, a continuing need exists in the state of the art for an optical fiber to channel waveguide coupler that accurately positions end-on couplings for being cemented in place without the creation of misaligning strains.

Patent Drawings (3 sheets)

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