Practical implementation of a wide range of integrated optical devices, both passive and active, on lithium nlobate as well as other substrates requires efficient waveguide-fiber couplers. Highly efficient butt coupling has been achieved1 by selecting the waveguide to match appropriately the fiber modal field. However, these waveguides, due to their large optical modal fields, require large drive voltages and, therefore, are not suitable for efficient modulator applications. To overcome this problem, constant-index, variable guide width tapered sections (horns) have been suggested and implemented.2 Another novel approach using variable width as well as index along the taper length has been suggested3 and analyzed.4 We present a simplified version of this approach in which the mode depth and width are increased along the taper by slowly forcing the waveguide close to cutoff at the fiber end of the sample. This is accomplished by keeping the guide width constant and gradually decreasing the index by varying the Ti thickness. This reduces the

© 1987 Optical Society of America

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