We have realised vertically tapered and antireflection-coated waveguides in InP/InGaAsP with 1.7dB coupling loss and relaxed alignment tolerances to lensed single-mode fibres with spot diameters of 3.8μm. The waveguide tapers are fabricated by a dip-etch process that is well suited for integration with optical waveguide circuits.
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Brenner et al. (1992) studied this question.